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It seems there are different ways to read and write data of files in Java.
I want to read ASCII data from a file. What are the possible ways and their differences?
My favorite way to read a small file is to use a BufferedReader and a StringBuilder. It is very simple and to the point (though not particularly effective, but good enough for most cases):
BufferedReader br = new BufferedReader(new FileReader("file.txt"));
try {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
sb.append(line);
sb.append(System.lineSeparator());
line = br.readLine();
}
String everything = sb.toString();
} finally {
br.close();
}
Some has pointed out that after Java 7 you should use try-with-resources (i.e. auto close) features:
try(BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
sb.append(line);
sb.append(System.lineSeparator());
line = br.readLine();
}
String everything = sb.toString();
}
When I read strings like this, I usually want to do some string handling per line anyways, so then I go for this implementation.
Though if I want to actually just read a file into a String, I always use Apache Commons IO with the class IOUtils.toString() method. You can have a look at the source here:
http://www.docjar.com/html/api/org/apache/commons/io/IOUtils.java.html
FileInputStream inputStream = new FileInputStream("foo.txt");
try {
String everything = IOUtils.toString(inputStream);
} finally {
inputStream.close();
}
And even simpler with Java 7:
try(FileInputStream inputStream = new FileInputStream("foo.txt")) {
String everything = IOUtils.toString(inputStream);
// do something with everything string
}
ASCII is a TEXT file so you would use Readers for reading. Java also supports reading from a binary file using InputStreams. If the files being read are huge then you would want to use a BufferedReader on top of a FileReader to improve read performance.
Go through this article on how to use a Reader
I'd also recommend you download and read this wonderful (yet free) book called Thinking In Java
In Java 7:
new String(Files.readAllBytes(...))
(docs)
or
Files.readAllLines(...)
(docs)
In Java 8:
Files.lines(..).forEach(...)
(docs)
The easiest way is to use the Scanner class in Java and the FileReader object. Simple example:
Scanner in = new Scanner(new FileReader("filename.txt"));
Scanner has several methods for reading in strings, numbers, etc... You can look for more information on this on the Java documentation page.
For example reading the whole content into a String:
StringBuilder sb = new StringBuilder();
while(in.hasNext()) {
sb.append(in.next());
}
in.close();
outString = sb.toString();
Also if you need a specific encoding you can use this instead of FileReader:
new InputStreamReader(new FileInputStream(fileUtf8), StandardCharsets.UTF_8)
Here is a simple solution:
String content = new String(Files.readAllBytes(Paths.get("sample.txt")));
Or to read as list:
List<String> content = Files.readAllLines(Paths.get("sample.txt"))
Here's another way to do it without using external libraries:
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
public String readFile(String filename)
{
String content = null;
File file = new File(filename); // For example, foo.txt
FileReader reader = null;
try {
reader = new FileReader(file);
char[] chars = new char[(int) file.length()];
reader.read(chars);
content = new String(chars);
reader.close();
} catch (IOException e) {
e.printStackTrace();
} finally {
if(reader != null){
reader.close();
}
}
return content;
}
I had to benchmark the different ways. I shall comment on my findings but, in short, the fastest way is to use a plain old BufferedInputStream over a FileInputStream. If many files must be read then three threads will reduce the total execution time to roughly half, but adding more threads will progressively degrade performance until making it take three times longer to complete with twenty threads than with just one thread.
The assumption is that you must read a file and do something meaningful with its contents. In the examples here is reading lines from a log and count the ones which contain values that exceed a certain threshold. So I am assuming that the one-liner Java 8 Files.lines(Paths.get("/path/to/file.txt")).map(line -> line.split(";")) is not an option.
I tested on Java 1.8, Windows 7 and both SSD and HDD drives.
I wrote six different implementations:
rawParse: Use BufferedInputStream over a FileInputStream and then cut lines reading byte by byte. This outperformed any other single-thread approach, but it may be very inconvenient for non-ASCII files.
lineReaderParse: Use a BufferedReader over a FileReader, read line by line, split lines by calling String.split(). This is approximatedly 20% slower that rawParse.
lineReaderParseParallel: This is the same as lineReaderParse, but it uses several threads. This is the fastest option overall in all cases.
nioFilesParse: Use java.nio.files.Files.lines()
nioAsyncParse: Use an AsynchronousFileChannel with a completion handler and a thread pool.
nioMemoryMappedParse: Use a memory-mapped file. This is really a bad idea yielding execution times at least three times longer than any other implementation.
These are the average times for reading 204 files of 4 MB each on an quad-core i7 and SSD drive. The files are generated on the fly to avoid disk caching.
rawParse 11.10 sec
lineReaderParse 13.86 sec
lineReaderParseParallel 6.00 sec
nioFilesParse 13.52 sec
nioAsyncParse 16.06 sec
nioMemoryMappedParse 37.68 sec
I found a difference smaller than I expected between running on an SSD or an HDD drive being the SSD approximately 15% faster. This may be because the files are generated on an unfragmented HDD and they are read sequentially, therefore the spinning drive can perform nearly as an SSD.
I was surprised by the low performance of the nioAsyncParse implementation. Either I have implemented something in the wrong way or the multi-thread implementation using NIO and a completion handler performs the same (or even worse) than a single-thread implementation with the java.io API. Moreover the asynchronous parse with a CompletionHandler is much longer in lines of code and tricky to implement correctly than a straight implementation on old streams.
Now the six implementations followed by a class containing them all plus a parametrizable main() method that allows to play with the number of files, file size and concurrency degree. Note that the size of the files varies plus minus 20%. This is to avoid any effect due to all the files being of exactly the same size.
rawParse
public void rawParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
overrunCount = 0;
final int dl = (int) ';';
StringBuffer lineBuffer = new StringBuffer(1024);
for (int f=0; f<numberOfFiles; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
FileInputStream fin = new FileInputStream(fl);
BufferedInputStream bin = new BufferedInputStream(fin);
int character;
while((character=bin.read())!=-1) {
if (character==dl) {
// Here is where something is done with each line
doSomethingWithRawLine(lineBuffer.toString());
lineBuffer.setLength(0);
}
else {
lineBuffer.append((char) character);
}
}
bin.close();
fin.close();
}
}
public final void doSomethingWithRawLine(String line) throws ParseException {
// What to do for each line
int fieldNumber = 0;
final int len = line.length();
StringBuffer fieldBuffer = new StringBuffer(256);
for (int charPos=0; charPos<len; charPos++) {
char c = line.charAt(charPos);
if (c==DL0) {
String fieldValue = fieldBuffer.toString();
if (fieldValue.length()>0) {
switch (fieldNumber) {
case 0:
Date dt = fmt.parse(fieldValue);
fieldNumber++;
break;
case 1:
double d = Double.parseDouble(fieldValue);
fieldNumber++;
break;
case 2:
int t = Integer.parseInt(fieldValue);
fieldNumber++;
break;
case 3:
if (fieldValue.equals("overrun"))
overrunCount++;
break;
}
}
fieldBuffer.setLength(0);
}
else {
fieldBuffer.append(c);
}
}
}
lineReaderParse
public void lineReaderParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
String line;
for (int f=0; f<numberOfFiles; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
FileReader frd = new FileReader(fl);
BufferedReader brd = new BufferedReader(frd);
while ((line=brd.readLine())!=null)
doSomethingWithLine(line);
brd.close();
frd.close();
}
}
public final void doSomethingWithLine(String line) throws ParseException {
// Example of what to do for each line
String[] fields = line.split(";");
Date dt = fmt.parse(fields[0]);
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCount++;
}
lineReaderParseParallel
public void lineReaderParseParallel(final String targetDir, final int numberOfFiles, final int degreeOfParalelism) throws IOException, ParseException, InterruptedException {
Thread[] pool = new Thread[degreeOfParalelism];
int batchSize = numberOfFiles / degreeOfParalelism;
for (int b=0; b<degreeOfParalelism; b++) {
pool[b] = new LineReaderParseThread(targetDir, b*batchSize, b*batchSize+b*batchSize);
pool[b].start();
}
for (int b=0; b<degreeOfParalelism; b++)
pool[b].join();
}
class LineReaderParseThread extends Thread {
private String targetDir;
private int fileFrom;
private int fileTo;
private DateFormat fmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
private int overrunCounter = 0;
public LineReaderParseThread(String targetDir, int fileFrom, int fileTo) {
this.targetDir = targetDir;
this.fileFrom = fileFrom;
this.fileTo = fileTo;
}
private void doSomethingWithTheLine(String line) throws ParseException {
String[] fields = line.split(DL);
Date dt = fmt.parse(fields[0]);
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCounter++;
}
#Override
public void run() {
String line;
for (int f=fileFrom; f<fileTo; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
try {
FileReader frd = new FileReader(fl);
BufferedReader brd = new BufferedReader(frd);
while ((line=brd.readLine())!=null) {
doSomethingWithTheLine(line);
}
brd.close();
frd.close();
} catch (IOException | ParseException ioe) { }
}
}
}
nioFilesParse
public void nioFilesParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
for (int f=0; f<numberOfFiles; f++) {
Path ph = Paths.get(targetDir+filenamePreffix+String.valueOf(f)+".txt");
Consumer<String> action = new LineConsumer();
Stream<String> lines = Files.lines(ph);
lines.forEach(action);
lines.close();
}
}
class LineConsumer implements Consumer<String> {
#Override
public void accept(String line) {
// What to do for each line
String[] fields = line.split(DL);
if (fields.length>1) {
try {
Date dt = fmt.parse(fields[0]);
}
catch (ParseException e) {
}
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCount++;
}
}
}
nioAsyncParse
public void nioAsyncParse(final String targetDir, final int numberOfFiles, final int numberOfThreads, final int bufferSize) throws IOException, ParseException, InterruptedException {
ScheduledThreadPoolExecutor pool = new ScheduledThreadPoolExecutor(numberOfThreads);
ConcurrentLinkedQueue<ByteBuffer> byteBuffers = new ConcurrentLinkedQueue<ByteBuffer>();
for (int b=0; b<numberOfThreads; b++)
byteBuffers.add(ByteBuffer.allocate(bufferSize));
for (int f=0; f<numberOfFiles; f++) {
consumerThreads.acquire();
String fileName = targetDir+filenamePreffix+String.valueOf(f)+".txt";
AsynchronousFileChannel channel = AsynchronousFileChannel.open(Paths.get(fileName), EnumSet.of(StandardOpenOption.READ), pool);
BufferConsumer consumer = new BufferConsumer(byteBuffers, fileName, bufferSize);
channel.read(consumer.buffer(), 0l, channel, consumer);
}
consumerThreads.acquire(numberOfThreads);
}
class BufferConsumer implements CompletionHandler<Integer, AsynchronousFileChannel> {
private ConcurrentLinkedQueue<ByteBuffer> buffers;
private ByteBuffer bytes;
private String file;
private StringBuffer chars;
private int limit;
private long position;
private DateFormat frmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public BufferConsumer(ConcurrentLinkedQueue<ByteBuffer> byteBuffers, String fileName, int bufferSize) {
buffers = byteBuffers;
bytes = buffers.poll();
if (bytes==null)
bytes = ByteBuffer.allocate(bufferSize);
file = fileName;
chars = new StringBuffer(bufferSize);
frmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
limit = bufferSize;
position = 0l;
}
public ByteBuffer buffer() {
return bytes;
}
#Override
public synchronized void completed(Integer result, AsynchronousFileChannel channel) {
if (result!=-1) {
bytes.flip();
final int len = bytes.limit();
int i = 0;
try {
for (i = 0; i < len; i++) {
byte by = bytes.get();
if (by=='\n') {
// ***
// The code used to process the line goes here
chars.setLength(0);
}
else {
chars.append((char) by);
}
}
}
catch (Exception x) {
System.out.println(
"Caught exception " + x.getClass().getName() + " " + x.getMessage() +
" i=" + String.valueOf(i) + ", limit=" + String.valueOf(len) +
", position="+String.valueOf(position));
}
if (len==limit) {
bytes.clear();
position += len;
channel.read(bytes, position, channel, this);
}
else {
try {
channel.close();
}
catch (IOException e) {
}
consumerThreads.release();
bytes.clear();
buffers.add(bytes);
}
}
else {
try {
channel.close();
}
catch (IOException e) {
}
consumerThreads.release();
bytes.clear();
buffers.add(bytes);
}
}
#Override
public void failed(Throwable e, AsynchronousFileChannel channel) {
}
};
FULL RUNNABLE IMPLEMENTATION OF ALL CASES
https://github.com/sergiomt/javaiobenchmark/blob/master/FileReadBenchmark.java
Here are the three working and tested methods:
Using BufferedReader
package io;
import java.io.*;
public class ReadFromFile2 {
public static void main(String[] args)throws Exception {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
BufferedReader br = new BufferedReader(new FileReader(file));
String st;
while((st=br.readLine()) != null){
System.out.println(st);
}
}
}
Using Scanner
package io;
import java.io.File;
import java.util.Scanner;
public class ReadFromFileUsingScanner {
public static void main(String[] args) throws Exception {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
Scanner sc = new Scanner(file);
while(sc.hasNextLine()){
System.out.println(sc.nextLine());
}
}
}
Using FileReader
package io;
import java.io.*;
public class ReadingFromFile {
public static void main(String[] args) throws Exception {
FileReader fr = new FileReader("C:\\Users\\pankaj\\Desktop\\test.java");
int i;
while ((i=fr.read()) != -1){
System.out.print((char) i);
}
}
}
Read the entire file without a loop using the Scanner class
package io;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class ReadingEntireFileWithoutLoop {
public static void main(String[] args) throws FileNotFoundException {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
Scanner sc = new Scanner(file);
sc.useDelimiter("\\Z");
System.out.println(sc.next());
}
}
The methods within org.apache.commons.io.FileUtils may also be very handy, e.g.:
/**
* Reads the contents of a file line by line to a List
* of Strings using the default encoding for the VM.
*/
static List readLines(File file)
I documented 15 ways to read a file in Java and then tested them for speed with various file sizes - from 1 KB to 1 GB and here are the top three ways to do this:
java.nio.file.Files.readAllBytes()
Tested to work in Java 7, 8, and 9.
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
public class ReadFile_Files_ReadAllBytes {
public static void main(String [] pArgs) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
File file = new File(fileName);
byte [] fileBytes = Files.readAllBytes(file.toPath());
char singleChar;
for(byte b : fileBytes) {
singleChar = (char) b;
System.out.print(singleChar);
}
}
}
java.io.BufferedReader.readLine()
Tested to work in Java 7, 8, 9.
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class ReadFile_BufferedReader_ReadLine {
public static void main(String [] args) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
FileReader fileReader = new FileReader(fileName);
try (BufferedReader bufferedReader = new BufferedReader(fileReader)) {
String line;
while((line = bufferedReader.readLine()) != null) {
System.out.println(line);
}
}
}
}
java.nio.file.Files.lines()
This was tested to work in Java 8 and 9 but won't work in Java 7 because of the lambda expression requirement.
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.stream.Stream;
public class ReadFile_Files_Lines {
public static void main(String[] pArgs) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
File file = new File(fileName);
try (Stream linesStream = Files.lines(file.toPath())) {
linesStream.forEach(line -> {
System.out.println(line);
});
}
}
}
What do you want to do with the text? Is the file small enough to fit into memory? I would try to find the simplest way to handle the file for your needs. The FileUtils library is very handle for this.
for(String line: FileUtils.readLines("my-text-file"))
System.out.println(line);
Below is a one-liner of doing it in the Java 8 way. Assuming text.txt file is in the root of the project directory of the Eclipse.
Files.lines(Paths.get("text.txt")).collect(Collectors.toList());
The most intuitive method is introduced in Java 11 Files.readString
import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;
public class App {
public static void main(String args[]) throws IOException {
String content = Files.readString(Paths.get("D:\\sandbox\\mvn\\my-app\\my-app.iml"));
System.out.print(content);
}
}
PHP has this luxury for decades! ☺
The buffered stream classes are much more performant in practice, so much so that the NIO.2 API includes methods that specifically return these stream classes, in part to encourage you always to use buffered streams in your application.
Here is an example:
Path path = Paths.get("/myfolder/myfile.ext");
try (BufferedReader reader = Files.newBufferedReader(path)) {
// Read from the stream
String currentLine = null;
while ((currentLine = reader.readLine()) != null)
//do your code here
} catch (IOException e) {
// Handle file I/O exception...
}
You can replace this code
BufferedReader reader = Files.newBufferedReader(path);
with
BufferedReader br = new BufferedReader(new FileReader("/myfolder/myfile.ext"));
I recommend this article to learn the main uses of Java NIO and IO.
Using BufferedReader:
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
BufferedReader br;
try {
br = new BufferedReader(new FileReader("/fileToRead.txt"));
try {
String x;
while ( (x = br.readLine()) != null ) {
// Printing out each line in the file
System.out.println(x);
}
}
catch (IOException e) {
e.printStackTrace();
}
}
catch (FileNotFoundException e) {
System.out.println(e);
e.printStackTrace();
}
This is basically the exact same as Jesus Ramos' answer, except with File instead of FileReader plus iteration to step through the contents of the file.
Scanner in = new Scanner(new File("filename.txt"));
while (in.hasNext()) { // Iterates each line in the file
String line = in.nextLine();
// Do something with line
}
in.close(); // Don't forget to close resource leaks
... throws FileNotFoundException
Probably not as fast as with buffered I/O, but quite terse:
String content;
try (Scanner scanner = new Scanner(textFile).useDelimiter("\\Z")) {
content = scanner.next();
}
The \Z pattern tells the Scanner that the delimiter is EOF.
The most simple way to read data from a file in Java is making use of the File class to read the file and the Scanner class to read the content of the file.
public static void main(String args[])throws Exception
{
File f = new File("input.txt");
takeInputIn2DArray(f);
}
public static void takeInputIn2DArray(File f) throws Exception
{
Scanner s = new Scanner(f);
int a[][] = new int[20][20];
for(int i=0; i<20; i++)
{
for(int j=0; j<20; j++)
{
a[i][j] = s.nextInt();
}
}
}
PS: Don't forget to import java.util.*; for Scanner to work.
You can use readAllLines and the join method to get whole file content in one line:
String str = String.join("\n",Files.readAllLines(Paths.get("e:\\text.txt")));
It uses UTF-8 encoding by default, which reads ASCII data correctly.
Also you can use readAllBytes:
String str = new String(Files.readAllBytes(Paths.get("e:\\text.txt")), StandardCharsets.UTF_8);
I think readAllBytes is faster and more precise, because it does not replace new line with \n and also new line may be \r\n. It is depending on your needs which one is suitable.
I don't see it mentioned yet in the other answers so far. But if "Best" means speed, then the new Java I/O (NIO) might provide the fastest preformance, but not always the easiest to figure out for someone learning.
http://download.oracle.com/javase/tutorial/essential/io/file.html
Guava provides a one-liner for this:
import com.google.common.base.Charsets;
import com.google.common.io.Files;
String contents = Files.toString(filePath, Charsets.UTF_8);
Cactoos give you a declarative one-liner:
new TextOf(new File("a.txt")).asString();
This might not be the exact answer to the question. It's just another way of reading a file where you do not explicitly specify the path to your file in your Java code and instead, you read it as a command-line argument.
With the following code,
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
public class InputReader{
public static void main(String[] args)throws IOException{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String s="";
while((s=br.readLine())!=null){
System.out.println(s);
}
}
}
just go ahead and run it with:
java InputReader < input.txt
This would read the contents of the input.txt and print it to the your console.
You can also make your System.out.println() to write to a specific file through the command line as follows:
java InputReader < input.txt > output.txt
This would read from input.txt and write to output.txt.
For JSF-based Maven web applications, just use ClassLoader and the Resources folder to read in any file you want:
Put any file you want to read in the Resources folder.
Put the Apache Commons IO dependency into your POM:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-io</artifactId>
<version>1.3.2</version>
</dependency>
Use the code below to read it (e.g. below is reading in a .json file):
String metadata = null;
FileInputStream inputStream;
try {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
inputStream = (FileInputStream) loader
.getResourceAsStream("/metadata.json");
metadata = IOUtils.toString(inputStream);
inputStream.close();
}
catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return metadata;
You can do the same for text files, .properties files, XSD schemas, etc.
try {
File f = new File("filename.txt");
Scanner r = new Scanner(f);
while (r.hasNextLine()) {
String data = r.nextLine();
JOptionPane.showMessageDialog(data);
}
r.close();
} catch (FileNotFoundException ex) {
JOptionPane.showMessageDialog("Error occurred");
ex.printStackTrace();
}
Use Java kiss if this is about simplicity of structure:
import static kiss.API.*;
class App {
void run() {
String line;
try (Close in = inOpen("file.dat")) {
while ((line = readLine()) != null) {
println(line);
}
}
}
}
import java.util.stream.Stream;
import java.nio.file.*;
import java.io.*;
class ReadFile {
public static void main(String[] args) {
String filename = "Test.txt";
try(Stream<String> stream = Files.lines(Paths.get(filename))) {
stream.forEach(System.out:: println);
} catch (IOException e) {
e.printStackTrace();
}
}
}
Just use java 8 Stream.
In case you have a large file you can use Apache Commons IO to process the file iteratively without exhausting the available memory.
try (LineIterator it = FileUtils.lineIterator(theFile, "UTF-8")) {
while (it.hasNext()) {
String line = it.nextLine();
// do something with line
}
}
try (Stream<String> stream = Files.lines(Paths.get(String.valueOf(new File("yourFile.txt"))))) {
stream.forEach(System.out::println);
} catch (IOException e) {
e.printStackTrace();
}
new File(<path_name>)
Creates a new File instance by converting the given pathname string into an abstract pathname. If the given string is the empty string, then the result is the empty abstract pathname.
Params:
pathname – A pathname string
Throws:
NullPointerException – If the pathname argument is null
Files.lines returns a stream of String
Stream<String> stream = Files.lines(Paths.get(String.valueOf(new File("yourFile.txt"))))
can throw nullPointerExcetion , FileNotFoundException so, keepint it inside try will take care of Exception in runtime
stream.forEach(System.out::println);
This is used to iterate over the stream and print in console
If you have different use case you can provide your custome function to manipulate the stream of lines
My new favorite approach to simply read a whole text file from a BufferedReader input goes:
String text = input.lines().collect(Collectors.joining(System.lineSeparator())));
This will read the whole file by adding new line (lineSeparator) behind each line. Without the separator it would join all lines together as one.
This appears to have existed since Java 8.
For Android developers ending up here (who use Kotlin):
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val text = myFileUrl.readText() // Not recommended for huge files
println(text)
Other solution:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
val lines = file.readLines() // Not recommended for huge files
lines.forEach(::println)
Another good solution which can be used for huge files as well:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
file
.bufferedReader()
.lineSequence()
.forEach(::println)
Or:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
file.useLines { lines ->
lines.forEach(::println)
}
Notes:
The vegetables.txt file should be in your classpath (for example, in src/main/resources directory)
The above solutions all treat the file encodings as UTF-8 by default. You can specify your desired encoding as the argument for the functions.
The above solutions do not need any further action like closing the files or readers. They are automatically taken care of by the Kotlin standard library.
Okay guys, I have a file with some HEX values as well as a program that take this values with a byte[] in order to convert some hex values and then reconvert it to a file.
The problem is that when I reconvert de byte array to a file some hex values are modified, and I don't find the problem.
If you see any possible mistake don't hesitate.
As you can see I have a test.sav file, here it is:
And this is the product of the program, the two files are different and they should be the same because any change has been made:
Here is the code:
public class Test {
public static File file;
public static String hex;
public static byte[] mext;
public static byte[] bytearray;
public static void main(String[] args) throws IOException {
file = new File("C:\\Users\\Roman\\Desktop\\test.sav");
StringBuilder sb = new StringBuilder();
FileInputStream fin = null;
try {
fin = new FileInputStream(file);
bytearray = new byte[(int)file.length()];
fin.read(bytearray);
for(byte bytev : bytearray){
sb.append(String.format("%02X", bytev));
}
System.out.println(sb);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {}
//replaceMax(); <-- I deduced that conversion is not the problem
save(); // THIS IS THE IMPORTANT PART
}
public static void save() throws IOException{
PrintWriter pw = new PrintWriter("C:\\Users\\Roman\\Desktop\\test2.sav");
pw.write("");
pw.close();
FileWriter fw = new FileWriter(new File("C:\\Users\\Roman\\Desktop\\test2.sav"));
BufferedWriter out = new BufferedWriter(fw);
out.write(new String(bytearray, "ASCII"));
out.close();
}
}
You are reading data from a binary file and then trying to write it out as a character stream. Furthermore you're forcing it to use ASCII (a 7 bit character set) as the character encoding.
Try altering the save method to use:
FileOutputStream output = new FileOutputStream("C:\\Users\\Roman\\Desktop\\test2.sav");
try {
output.write(bytearray);
} finally {
output.close();
}
This will avoid character (re)encoding issues.
It seems there are different ways to read and write data of files in Java.
I want to read ASCII data from a file. What are the possible ways and their differences?
My favorite way to read a small file is to use a BufferedReader and a StringBuilder. It is very simple and to the point (though not particularly effective, but good enough for most cases):
BufferedReader br = new BufferedReader(new FileReader("file.txt"));
try {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
sb.append(line);
sb.append(System.lineSeparator());
line = br.readLine();
}
String everything = sb.toString();
} finally {
br.close();
}
Some has pointed out that after Java 7 you should use try-with-resources (i.e. auto close) features:
try(BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
sb.append(line);
sb.append(System.lineSeparator());
line = br.readLine();
}
String everything = sb.toString();
}
When I read strings like this, I usually want to do some string handling per line anyways, so then I go for this implementation.
Though if I want to actually just read a file into a String, I always use Apache Commons IO with the class IOUtils.toString() method. You can have a look at the source here:
http://www.docjar.com/html/api/org/apache/commons/io/IOUtils.java.html
FileInputStream inputStream = new FileInputStream("foo.txt");
try {
String everything = IOUtils.toString(inputStream);
} finally {
inputStream.close();
}
And even simpler with Java 7:
try(FileInputStream inputStream = new FileInputStream("foo.txt")) {
String everything = IOUtils.toString(inputStream);
// do something with everything string
}
ASCII is a TEXT file so you would use Readers for reading. Java also supports reading from a binary file using InputStreams. If the files being read are huge then you would want to use a BufferedReader on top of a FileReader to improve read performance.
Go through this article on how to use a Reader
I'd also recommend you download and read this wonderful (yet free) book called Thinking In Java
In Java 7:
new String(Files.readAllBytes(...))
(docs)
or
Files.readAllLines(...)
(docs)
In Java 8:
Files.lines(..).forEach(...)
(docs)
The easiest way is to use the Scanner class in Java and the FileReader object. Simple example:
Scanner in = new Scanner(new FileReader("filename.txt"));
Scanner has several methods for reading in strings, numbers, etc... You can look for more information on this on the Java documentation page.
For example reading the whole content into a String:
StringBuilder sb = new StringBuilder();
while(in.hasNext()) {
sb.append(in.next());
}
in.close();
outString = sb.toString();
Also if you need a specific encoding you can use this instead of FileReader:
new InputStreamReader(new FileInputStream(fileUtf8), StandardCharsets.UTF_8)
Here is a simple solution:
String content = new String(Files.readAllBytes(Paths.get("sample.txt")));
Or to read as list:
List<String> content = Files.readAllLines(Paths.get("sample.txt"))
Here's another way to do it without using external libraries:
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
public String readFile(String filename)
{
String content = null;
File file = new File(filename); // For example, foo.txt
FileReader reader = null;
try {
reader = new FileReader(file);
char[] chars = new char[(int) file.length()];
reader.read(chars);
content = new String(chars);
reader.close();
} catch (IOException e) {
e.printStackTrace();
} finally {
if(reader != null){
reader.close();
}
}
return content;
}
I had to benchmark the different ways. I shall comment on my findings but, in short, the fastest way is to use a plain old BufferedInputStream over a FileInputStream. If many files must be read then three threads will reduce the total execution time to roughly half, but adding more threads will progressively degrade performance until making it take three times longer to complete with twenty threads than with just one thread.
The assumption is that you must read a file and do something meaningful with its contents. In the examples here is reading lines from a log and count the ones which contain values that exceed a certain threshold. So I am assuming that the one-liner Java 8 Files.lines(Paths.get("/path/to/file.txt")).map(line -> line.split(";")) is not an option.
I tested on Java 1.8, Windows 7 and both SSD and HDD drives.
I wrote six different implementations:
rawParse: Use BufferedInputStream over a FileInputStream and then cut lines reading byte by byte. This outperformed any other single-thread approach, but it may be very inconvenient for non-ASCII files.
lineReaderParse: Use a BufferedReader over a FileReader, read line by line, split lines by calling String.split(). This is approximatedly 20% slower that rawParse.
lineReaderParseParallel: This is the same as lineReaderParse, but it uses several threads. This is the fastest option overall in all cases.
nioFilesParse: Use java.nio.files.Files.lines()
nioAsyncParse: Use an AsynchronousFileChannel with a completion handler and a thread pool.
nioMemoryMappedParse: Use a memory-mapped file. This is really a bad idea yielding execution times at least three times longer than any other implementation.
These are the average times for reading 204 files of 4 MB each on an quad-core i7 and SSD drive. The files are generated on the fly to avoid disk caching.
rawParse 11.10 sec
lineReaderParse 13.86 sec
lineReaderParseParallel 6.00 sec
nioFilesParse 13.52 sec
nioAsyncParse 16.06 sec
nioMemoryMappedParse 37.68 sec
I found a difference smaller than I expected between running on an SSD or an HDD drive being the SSD approximately 15% faster. This may be because the files are generated on an unfragmented HDD and they are read sequentially, therefore the spinning drive can perform nearly as an SSD.
I was surprised by the low performance of the nioAsyncParse implementation. Either I have implemented something in the wrong way or the multi-thread implementation using NIO and a completion handler performs the same (or even worse) than a single-thread implementation with the java.io API. Moreover the asynchronous parse with a CompletionHandler is much longer in lines of code and tricky to implement correctly than a straight implementation on old streams.
Now the six implementations followed by a class containing them all plus a parametrizable main() method that allows to play with the number of files, file size and concurrency degree. Note that the size of the files varies plus minus 20%. This is to avoid any effect due to all the files being of exactly the same size.
rawParse
public void rawParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
overrunCount = 0;
final int dl = (int) ';';
StringBuffer lineBuffer = new StringBuffer(1024);
for (int f=0; f<numberOfFiles; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
FileInputStream fin = new FileInputStream(fl);
BufferedInputStream bin = new BufferedInputStream(fin);
int character;
while((character=bin.read())!=-1) {
if (character==dl) {
// Here is where something is done with each line
doSomethingWithRawLine(lineBuffer.toString());
lineBuffer.setLength(0);
}
else {
lineBuffer.append((char) character);
}
}
bin.close();
fin.close();
}
}
public final void doSomethingWithRawLine(String line) throws ParseException {
// What to do for each line
int fieldNumber = 0;
final int len = line.length();
StringBuffer fieldBuffer = new StringBuffer(256);
for (int charPos=0; charPos<len; charPos++) {
char c = line.charAt(charPos);
if (c==DL0) {
String fieldValue = fieldBuffer.toString();
if (fieldValue.length()>0) {
switch (fieldNumber) {
case 0:
Date dt = fmt.parse(fieldValue);
fieldNumber++;
break;
case 1:
double d = Double.parseDouble(fieldValue);
fieldNumber++;
break;
case 2:
int t = Integer.parseInt(fieldValue);
fieldNumber++;
break;
case 3:
if (fieldValue.equals("overrun"))
overrunCount++;
break;
}
}
fieldBuffer.setLength(0);
}
else {
fieldBuffer.append(c);
}
}
}
lineReaderParse
public void lineReaderParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
String line;
for (int f=0; f<numberOfFiles; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
FileReader frd = new FileReader(fl);
BufferedReader brd = new BufferedReader(frd);
while ((line=brd.readLine())!=null)
doSomethingWithLine(line);
brd.close();
frd.close();
}
}
public final void doSomethingWithLine(String line) throws ParseException {
// Example of what to do for each line
String[] fields = line.split(";");
Date dt = fmt.parse(fields[0]);
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCount++;
}
lineReaderParseParallel
public void lineReaderParseParallel(final String targetDir, final int numberOfFiles, final int degreeOfParalelism) throws IOException, ParseException, InterruptedException {
Thread[] pool = new Thread[degreeOfParalelism];
int batchSize = numberOfFiles / degreeOfParalelism;
for (int b=0; b<degreeOfParalelism; b++) {
pool[b] = new LineReaderParseThread(targetDir, b*batchSize, b*batchSize+b*batchSize);
pool[b].start();
}
for (int b=0; b<degreeOfParalelism; b++)
pool[b].join();
}
class LineReaderParseThread extends Thread {
private String targetDir;
private int fileFrom;
private int fileTo;
private DateFormat fmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
private int overrunCounter = 0;
public LineReaderParseThread(String targetDir, int fileFrom, int fileTo) {
this.targetDir = targetDir;
this.fileFrom = fileFrom;
this.fileTo = fileTo;
}
private void doSomethingWithTheLine(String line) throws ParseException {
String[] fields = line.split(DL);
Date dt = fmt.parse(fields[0]);
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCounter++;
}
#Override
public void run() {
String line;
for (int f=fileFrom; f<fileTo; f++) {
File fl = new File(targetDir+filenamePreffix+String.valueOf(f)+".txt");
try {
FileReader frd = new FileReader(fl);
BufferedReader brd = new BufferedReader(frd);
while ((line=brd.readLine())!=null) {
doSomethingWithTheLine(line);
}
brd.close();
frd.close();
} catch (IOException | ParseException ioe) { }
}
}
}
nioFilesParse
public void nioFilesParse(final String targetDir, final int numberOfFiles) throws IOException, ParseException {
for (int f=0; f<numberOfFiles; f++) {
Path ph = Paths.get(targetDir+filenamePreffix+String.valueOf(f)+".txt");
Consumer<String> action = new LineConsumer();
Stream<String> lines = Files.lines(ph);
lines.forEach(action);
lines.close();
}
}
class LineConsumer implements Consumer<String> {
#Override
public void accept(String line) {
// What to do for each line
String[] fields = line.split(DL);
if (fields.length>1) {
try {
Date dt = fmt.parse(fields[0]);
}
catch (ParseException e) {
}
double d = Double.parseDouble(fields[1]);
int t = Integer.parseInt(fields[2]);
if (fields[3].equals("overrun"))
overrunCount++;
}
}
}
nioAsyncParse
public void nioAsyncParse(final String targetDir, final int numberOfFiles, final int numberOfThreads, final int bufferSize) throws IOException, ParseException, InterruptedException {
ScheduledThreadPoolExecutor pool = new ScheduledThreadPoolExecutor(numberOfThreads);
ConcurrentLinkedQueue<ByteBuffer> byteBuffers = new ConcurrentLinkedQueue<ByteBuffer>();
for (int b=0; b<numberOfThreads; b++)
byteBuffers.add(ByteBuffer.allocate(bufferSize));
for (int f=0; f<numberOfFiles; f++) {
consumerThreads.acquire();
String fileName = targetDir+filenamePreffix+String.valueOf(f)+".txt";
AsynchronousFileChannel channel = AsynchronousFileChannel.open(Paths.get(fileName), EnumSet.of(StandardOpenOption.READ), pool);
BufferConsumer consumer = new BufferConsumer(byteBuffers, fileName, bufferSize);
channel.read(consumer.buffer(), 0l, channel, consumer);
}
consumerThreads.acquire(numberOfThreads);
}
class BufferConsumer implements CompletionHandler<Integer, AsynchronousFileChannel> {
private ConcurrentLinkedQueue<ByteBuffer> buffers;
private ByteBuffer bytes;
private String file;
private StringBuffer chars;
private int limit;
private long position;
private DateFormat frmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public BufferConsumer(ConcurrentLinkedQueue<ByteBuffer> byteBuffers, String fileName, int bufferSize) {
buffers = byteBuffers;
bytes = buffers.poll();
if (bytes==null)
bytes = ByteBuffer.allocate(bufferSize);
file = fileName;
chars = new StringBuffer(bufferSize);
frmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
limit = bufferSize;
position = 0l;
}
public ByteBuffer buffer() {
return bytes;
}
#Override
public synchronized void completed(Integer result, AsynchronousFileChannel channel) {
if (result!=-1) {
bytes.flip();
final int len = bytes.limit();
int i = 0;
try {
for (i = 0; i < len; i++) {
byte by = bytes.get();
if (by=='\n') {
// ***
// The code used to process the line goes here
chars.setLength(0);
}
else {
chars.append((char) by);
}
}
}
catch (Exception x) {
System.out.println(
"Caught exception " + x.getClass().getName() + " " + x.getMessage() +
" i=" + String.valueOf(i) + ", limit=" + String.valueOf(len) +
", position="+String.valueOf(position));
}
if (len==limit) {
bytes.clear();
position += len;
channel.read(bytes, position, channel, this);
}
else {
try {
channel.close();
}
catch (IOException e) {
}
consumerThreads.release();
bytes.clear();
buffers.add(bytes);
}
}
else {
try {
channel.close();
}
catch (IOException e) {
}
consumerThreads.release();
bytes.clear();
buffers.add(bytes);
}
}
#Override
public void failed(Throwable e, AsynchronousFileChannel channel) {
}
};
FULL RUNNABLE IMPLEMENTATION OF ALL CASES
https://github.com/sergiomt/javaiobenchmark/blob/master/FileReadBenchmark.java
Here are the three working and tested methods:
Using BufferedReader
package io;
import java.io.*;
public class ReadFromFile2 {
public static void main(String[] args)throws Exception {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
BufferedReader br = new BufferedReader(new FileReader(file));
String st;
while((st=br.readLine()) != null){
System.out.println(st);
}
}
}
Using Scanner
package io;
import java.io.File;
import java.util.Scanner;
public class ReadFromFileUsingScanner {
public static void main(String[] args) throws Exception {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
Scanner sc = new Scanner(file);
while(sc.hasNextLine()){
System.out.println(sc.nextLine());
}
}
}
Using FileReader
package io;
import java.io.*;
public class ReadingFromFile {
public static void main(String[] args) throws Exception {
FileReader fr = new FileReader("C:\\Users\\pankaj\\Desktop\\test.java");
int i;
while ((i=fr.read()) != -1){
System.out.print((char) i);
}
}
}
Read the entire file without a loop using the Scanner class
package io;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class ReadingEntireFileWithoutLoop {
public static void main(String[] args) throws FileNotFoundException {
File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
Scanner sc = new Scanner(file);
sc.useDelimiter("\\Z");
System.out.println(sc.next());
}
}
The methods within org.apache.commons.io.FileUtils may also be very handy, e.g.:
/**
* Reads the contents of a file line by line to a List
* of Strings using the default encoding for the VM.
*/
static List readLines(File file)
I documented 15 ways to read a file in Java and then tested them for speed with various file sizes - from 1 KB to 1 GB and here are the top three ways to do this:
java.nio.file.Files.readAllBytes()
Tested to work in Java 7, 8, and 9.
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
public class ReadFile_Files_ReadAllBytes {
public static void main(String [] pArgs) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
File file = new File(fileName);
byte [] fileBytes = Files.readAllBytes(file.toPath());
char singleChar;
for(byte b : fileBytes) {
singleChar = (char) b;
System.out.print(singleChar);
}
}
}
java.io.BufferedReader.readLine()
Tested to work in Java 7, 8, 9.
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class ReadFile_BufferedReader_ReadLine {
public static void main(String [] args) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
FileReader fileReader = new FileReader(fileName);
try (BufferedReader bufferedReader = new BufferedReader(fileReader)) {
String line;
while((line = bufferedReader.readLine()) != null) {
System.out.println(line);
}
}
}
}
java.nio.file.Files.lines()
This was tested to work in Java 8 and 9 but won't work in Java 7 because of the lambda expression requirement.
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.stream.Stream;
public class ReadFile_Files_Lines {
public static void main(String[] pArgs) throws IOException {
String fileName = "c:\\temp\\sample-10KB.txt";
File file = new File(fileName);
try (Stream linesStream = Files.lines(file.toPath())) {
linesStream.forEach(line -> {
System.out.println(line);
});
}
}
}
What do you want to do with the text? Is the file small enough to fit into memory? I would try to find the simplest way to handle the file for your needs. The FileUtils library is very handle for this.
for(String line: FileUtils.readLines("my-text-file"))
System.out.println(line);
Below is a one-liner of doing it in the Java 8 way. Assuming text.txt file is in the root of the project directory of the Eclipse.
Files.lines(Paths.get("text.txt")).collect(Collectors.toList());
The most intuitive method is introduced in Java 11 Files.readString
import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;
public class App {
public static void main(String args[]) throws IOException {
String content = Files.readString(Paths.get("D:\\sandbox\\mvn\\my-app\\my-app.iml"));
System.out.print(content);
}
}
PHP has this luxury for decades! ☺
The buffered stream classes are much more performant in practice, so much so that the NIO.2 API includes methods that specifically return these stream classes, in part to encourage you always to use buffered streams in your application.
Here is an example:
Path path = Paths.get("/myfolder/myfile.ext");
try (BufferedReader reader = Files.newBufferedReader(path)) {
// Read from the stream
String currentLine = null;
while ((currentLine = reader.readLine()) != null)
//do your code here
} catch (IOException e) {
// Handle file I/O exception...
}
You can replace this code
BufferedReader reader = Files.newBufferedReader(path);
with
BufferedReader br = new BufferedReader(new FileReader("/myfolder/myfile.ext"));
I recommend this article to learn the main uses of Java NIO and IO.
Using BufferedReader:
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
BufferedReader br;
try {
br = new BufferedReader(new FileReader("/fileToRead.txt"));
try {
String x;
while ( (x = br.readLine()) != null ) {
// Printing out each line in the file
System.out.println(x);
}
}
catch (IOException e) {
e.printStackTrace();
}
}
catch (FileNotFoundException e) {
System.out.println(e);
e.printStackTrace();
}
This is basically the exact same as Jesus Ramos' answer, except with File instead of FileReader plus iteration to step through the contents of the file.
Scanner in = new Scanner(new File("filename.txt"));
while (in.hasNext()) { // Iterates each line in the file
String line = in.nextLine();
// Do something with line
}
in.close(); // Don't forget to close resource leaks
... throws FileNotFoundException
Probably not as fast as with buffered I/O, but quite terse:
String content;
try (Scanner scanner = new Scanner(textFile).useDelimiter("\\Z")) {
content = scanner.next();
}
The \Z pattern tells the Scanner that the delimiter is EOF.
The most simple way to read data from a file in Java is making use of the File class to read the file and the Scanner class to read the content of the file.
public static void main(String args[])throws Exception
{
File f = new File("input.txt");
takeInputIn2DArray(f);
}
public static void takeInputIn2DArray(File f) throws Exception
{
Scanner s = new Scanner(f);
int a[][] = new int[20][20];
for(int i=0; i<20; i++)
{
for(int j=0; j<20; j++)
{
a[i][j] = s.nextInt();
}
}
}
PS: Don't forget to import java.util.*; for Scanner to work.
You can use readAllLines and the join method to get whole file content in one line:
String str = String.join("\n",Files.readAllLines(Paths.get("e:\\text.txt")));
It uses UTF-8 encoding by default, which reads ASCII data correctly.
Also you can use readAllBytes:
String str = new String(Files.readAllBytes(Paths.get("e:\\text.txt")), StandardCharsets.UTF_8);
I think readAllBytes is faster and more precise, because it does not replace new line with \n and also new line may be \r\n. It is depending on your needs which one is suitable.
I don't see it mentioned yet in the other answers so far. But if "Best" means speed, then the new Java I/O (NIO) might provide the fastest preformance, but not always the easiest to figure out for someone learning.
http://download.oracle.com/javase/tutorial/essential/io/file.html
Guava provides a one-liner for this:
import com.google.common.base.Charsets;
import com.google.common.io.Files;
String contents = Files.toString(filePath, Charsets.UTF_8);
Cactoos give you a declarative one-liner:
new TextOf(new File("a.txt")).asString();
This might not be the exact answer to the question. It's just another way of reading a file where you do not explicitly specify the path to your file in your Java code and instead, you read it as a command-line argument.
With the following code,
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
public class InputReader{
public static void main(String[] args)throws IOException{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String s="";
while((s=br.readLine())!=null){
System.out.println(s);
}
}
}
just go ahead and run it with:
java InputReader < input.txt
This would read the contents of the input.txt and print it to the your console.
You can also make your System.out.println() to write to a specific file through the command line as follows:
java InputReader < input.txt > output.txt
This would read from input.txt and write to output.txt.
For JSF-based Maven web applications, just use ClassLoader and the Resources folder to read in any file you want:
Put any file you want to read in the Resources folder.
Put the Apache Commons IO dependency into your POM:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-io</artifactId>
<version>1.3.2</version>
</dependency>
Use the code below to read it (e.g. below is reading in a .json file):
String metadata = null;
FileInputStream inputStream;
try {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
inputStream = (FileInputStream) loader
.getResourceAsStream("/metadata.json");
metadata = IOUtils.toString(inputStream);
inputStream.close();
}
catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return metadata;
You can do the same for text files, .properties files, XSD schemas, etc.
try {
File f = new File("filename.txt");
Scanner r = new Scanner(f);
while (r.hasNextLine()) {
String data = r.nextLine();
JOptionPane.showMessageDialog(data);
}
r.close();
} catch (FileNotFoundException ex) {
JOptionPane.showMessageDialog("Error occurred");
ex.printStackTrace();
}
Use Java kiss if this is about simplicity of structure:
import static kiss.API.*;
class App {
void run() {
String line;
try (Close in = inOpen("file.dat")) {
while ((line = readLine()) != null) {
println(line);
}
}
}
}
import java.util.stream.Stream;
import java.nio.file.*;
import java.io.*;
class ReadFile {
public static void main(String[] args) {
String filename = "Test.txt";
try(Stream<String> stream = Files.lines(Paths.get(filename))) {
stream.forEach(System.out:: println);
} catch (IOException e) {
e.printStackTrace();
}
}
}
Just use java 8 Stream.
In case you have a large file you can use Apache Commons IO to process the file iteratively without exhausting the available memory.
try (LineIterator it = FileUtils.lineIterator(theFile, "UTF-8")) {
while (it.hasNext()) {
String line = it.nextLine();
// do something with line
}
}
try (Stream<String> stream = Files.lines(Paths.get(String.valueOf(new File("yourFile.txt"))))) {
stream.forEach(System.out::println);
} catch (IOException e) {
e.printStackTrace();
}
new File(<path_name>)
Creates a new File instance by converting the given pathname string into an abstract pathname. If the given string is the empty string, then the result is the empty abstract pathname.
Params:
pathname – A pathname string
Throws:
NullPointerException – If the pathname argument is null
Files.lines returns a stream of String
Stream<String> stream = Files.lines(Paths.get(String.valueOf(new File("yourFile.txt"))))
can throw nullPointerExcetion , FileNotFoundException so, keepint it inside try will take care of Exception in runtime
stream.forEach(System.out::println);
This is used to iterate over the stream and print in console
If you have different use case you can provide your custome function to manipulate the stream of lines
My new favorite approach to simply read a whole text file from a BufferedReader input goes:
String text = input.lines().collect(Collectors.joining(System.lineSeparator())));
This will read the whole file by adding new line (lineSeparator) behind each line. Without the separator it would join all lines together as one.
This appears to have existed since Java 8.
For Android developers ending up here (who use Kotlin):
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val text = myFileUrl.readText() // Not recommended for huge files
println(text)
Other solution:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
val lines = file.readLines() // Not recommended for huge files
lines.forEach(::println)
Another good solution which can be used for huge files as well:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
file
.bufferedReader()
.lineSequence()
.forEach(::println)
Or:
val myFileUrl = object{}.javaClass.getResource("/vegetables.txt")
val file = File(myFileUrl.toURI())
file.useLines { lines ->
lines.forEach(::println)
}
Notes:
The vegetables.txt file should be in your classpath (for example, in src/main/resources directory)
The above solutions all treat the file encodings as UTF-8 by default. You can specify your desired encoding as the argument for the functions.
The above solutions do not need any further action like closing the files or readers. They are automatically taken care of by the Kotlin standard library.
Main:
package main;
import racreader.RAFReader;
public class RandomAccessFile {
public static void main(String[] args) {
if (args.length != 2) {
System.err.println("Wrong arguments length");
System.exit(1);
}
try {
RAFReader reader = new RAFReader (args[0]);
try {
String output = reader.readUTF(Integer.parseInt(args[1]));
System.out.print(output);
} catch (Exception e) {
System.err.println(e.toString());
} finally {
reader.close();
}
} catch (Exception e) {
System.err.println(e.toString());
}
}
}
RAFReader:
package racreader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
public class RAFReader {
private final String fileName;
private final RandomAccessFile reader;
public RAFReader(String fileName) throws FileNotFoundException {
this.fileName = fileName;
this.reader = openFile();
}
private RandomAccessFile openFile() throws FileNotFoundException {
RandomAccessFile reader = new RandomAccessFile(fileName, "r");
return reader;
}
public String readUTF(int offset) throws IOException {
reader.seek(offset);
String output = reader.readUTF();
return output;
}
public void close() throws IOException {
reader.close();
}
}
The problem is in EOFException in every file (even encoded in UTF8) and every offset. Why?
UPD: I try to get my program working with file with this content:
Это тест UTF-8 чтения
It works fine only if offset = 0. Any other offset throws EOFException.
The readUTF()/writeUTF() methods from RandomAccesFile use conventions for writing Java String objects, which are not necessarily honored by UTF encoded text files. readUTF() was not meant to be used for reading arbitrary text file, which was not originally written by using RandomAccesFile.writeUTF().
As method Javadocs specify, readUTF() assumes that the first two bytes it reads contain the number of bytes in the following string. This is the case if the string was written to file by the pairing writeUTF() method, but in case of the text file this will throw intermittent EOFException, since the first two bytes will contain actual characters from the string.
In your case, a different set of classes can solve the problem. Consider rewriting RAFReader class using InputStreamReader:
public String readUTF(int offset) throws IOException {
FileInputStream is = new FileInputStream(fileName);
Reader fileReader = new InputStreamReader(is, "UTF-8");
StringBuilder stringBuilder = new StringBuilder();
fileReader.skip(offset);
int charsRead;
char buf[] = new char[256];
//Read until there is no more characters to read.
while ((charsRead = fileReader.read(buf)) > 0) {
stringBuilder.append(buf, 0, charsRead);
}
fileReader.close();
return stringBuilder.toString();
}
If using RandomAccesFile is a must, you can use input stream which wraps the RandomAccesFile. The simplest way to do it is through FileChannel, encapsulated by RandomAccesFile:
InputStream is = Channels.newInputStream(reader.getChannel());
EOFException - if this file reaches the end before reading all the
bytes.
One possible way in which you got EOFException is at line
reader.seek(offset);
String output = reader.readUTF();
Maybe the offset value is high for file length. Try with offset = 0 and check if you get EOF or not.
My Sample Code
String line = null;
RandomAccessFile file = new RandomAccessFile("D:/mahtew.txt", "rw");
System.out.println(file.getFilePointer());
while((line = file.readLine()) != null){
System.out.println(line);
System.out.println(file.getFilePointer());
if(line.contains("Text to be appended with")){
file.seek(file.getFilePointer());
file.write(" new text has been appended".getBytes());
break;
}
}
file.close();
demo.txt before execution
one two three
Text to be appended with
five six seven
eight nine ten
demo.txt after execution
one two three
Text to be appended with
new text has been appendedten
Also i tried using setLength to change length of file before new text is appended. But still some text is getting trimmed from output file. Any help will be appreciated
Thanks
Mathew
RandomAccessFile
A random access file behaves like a large array of bytes stored in the
file system.
In fact it does not care about shifting the array elements in the case of write operations (only the pointer is advanced). Such an operation overwrites existing values:
Output operations write bytes starting at the file pointer and advance
the file pointer past the bytes written.
When you seek to the file's byte location and writes data, the bytes will be overwritten.
Which is why you get an output like this.
Just imagine editing something in notepad with the insert key pressed. It will replace instead of inserting the new data in between. Same thing's happening here.
EDIT:
You should actually do what Eel is suggesting if you want to edit the file content.
Or you can get the rest of the file and add it to the modified data and write to the file in order to avoid the loss, but that will get ugly and complicated real fast. Not to mention performance penalties.
Understand that when you write with a RAF, you over-write data which was previously held at the file pointer location. If you want to insert text into a file, I suggest that you not use a RAF but rather simply read the text of the file into a String or ArrayList<String> or StringBuilder, using a File held by a FileReader wrapped in a BufferedReader or a File wrapped in a Scanner, alter the Strings or StringBuilder held in memory, and then write the altered data to the new file using a FileWriter wrapped in a PrintWriter.
e.g.,
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class AppendLine {
private static final String FILE_PATH = "src/tetris/mahtew.txt";
private static final String MARKER_LINE = "Text to be appended with";
private static final String TEXT_TO_ADD = "new text has been appended";
public static void main(String[] args) {
List<String> fileLines = new ArrayList<String>();
Scanner scanner = null;
try {
scanner = new Scanner(new File(FILE_PATH));
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
fileLines.add(line);
if (line.trim().equalsIgnoreCase(MARKER_LINE)) {
fileLines.add(TEXT_TO_ADD);
}
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} finally {
if (scanner != null) {
scanner.close();
}
}
PrintWriter pw = null;
try {
pw = new PrintWriter(new File(FILE_PATH));
for (String line : fileLines) {
pw.println(line);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} finally {
if (pw != null) {
pw.close();
}
}
}
}
You can use RandomAccessFile in Java to achieve this on one condition:
The size of each line has to be fixed otherwise, when new string is written back, it might override the string in the next line.
Therefore, in my example, I set the line length as 100 and padding with space string when creating the file and writing back to the file.
So in order to allow update, you need to set the length of line a little larger than the longest length of the line in this file.
public class RandomAccessFileUtil {
public static final long RECORD_LENGTH = 100;
public static final String EMPTY_STRING = " ";
public static final String CRLF = "\n";
public static final String PATHNAME = "/home/mjiang/JM/mahtew.txt";
/**
* one two three
Text to be appended with
five six seven
eight nine ten
*
*
* #param args
* #throws IOException
*/
public static void main(String[] args) throws IOException
{
String starPrefix = "Text to be appended with";
String replacedString = "new text has been appended";
RandomAccessFile file = new RandomAccessFile(new File(PATHNAME), "rw");
String line = "";
while((line = file.readLine()) != null)
{
if(line.startsWith(starPrefix))
{
file.seek(file.getFilePointer() - RECORD_LENGTH - 1);
file.writeBytes(replacedString);
}
}
}
public static void createFile() throws IOException
{
RandomAccessFile file = new RandomAccessFile(new File(PATHNAME), "rw");
String line1 = "one two three";
String line2 = "Text to be appended with";
String line3 = "five six seven";
String line4 = "eight nine ten";
file.writeBytes(paddingRight(line1));
file.writeBytes(CRLF);
file.writeBytes(paddingRight(line2));
file.writeBytes(CRLF);
file.writeBytes(paddingRight(line3));
file.writeBytes(CRLF);
file.writeBytes(paddingRight(line4));
file.writeBytes(CRLF);
file.close();
System.out.println(String.format("File is created in [%s]", PATHNAME));
}
public static String paddingRight(String source)
{
StringBuilder result = new StringBuilder(100);
if(source != null)
{
result.append(source);
for (int i = 0; i < RECORD_LENGTH - source.length(); i++)
{
result.append(EMPTY_STRING);
}
}
return result.toString();
}
}
If replaced string is too long, the strings after the line matched with input will be replaced. It seems you have to read file, modify it and write back to old or new file.
Of course, you have options to use multi-threading programming and Java 7's new IO features to improve performance.