Will the following finally clause be executed, if an exception is thrown by the PrintWriter?
try{
PrintWriter out = new PrintWriter(filename);
try {
//output
} finally {
out.close();
}
} catch {
//handle exception
}
If the PrintWriter throws an exception, then the nested try block will never get executed, but why the nested finally clause will still be executed, even it's nested and skipped?
Updates:
I ran some tests, if an exception is thrown before the nested try clause, that nested finally will not be executed.
If the exception is thrown inside the nested try clause, then the inner finally and the outter catch will be executed.
No because the inner try block will not be reached when an exception occurs before and therefore the finally block is not reached either.
Finally block is always executed whether exception is handled or not. Even though their is an error and it reaches to catch block, it will go to finally block to execute the piece of code.
finally block is a block that is used to execute important code such
as closing connection, stream etc.
So, Inside try{} block you placed try and finally, but you asked about the catch of outside try ,thus its not going inside the first try block.That finally wont work.
P.S. : If you put finally something like this:
try{
try{...}
finally{...}
}catch(Exception e){...}
finally{... }
//in case of exception also , the outside finally is going to work.
P.S.: Though you got your answer , but the concept is for reference of other naive programmers
An uglier variant (sometimes generated by the IDE) one sees also:
// *** UGLY
PrintWriter out = null;
try {
out = new PrintWriter(filename);
//output
} catch (IOException e) {
//handle exception
} finally {
if (out != null) {
try {
out.close();
} catch (IOException e2) {
// IGNORE
}
}
}
That explains the code a bit: as close may throw an IOException too, the code becomes cumbersome. Your code still needs nested exceptions.
With try-with-resources this can & should be written as:
try (PrintWriter out = new PrintWriter(filename)) {
//output
} catch (IOException e) {
//handle exception
} // Automatically closes.
And no longer nested exceptions.
The biggest advantage is that you need not catch any exception, and just add a throws IOException in the method signature.
Related
Please correct me if I'm wrong, but, I believe that the statements in the try block get executed first, then, if any exception occurs, the statements in the finally block get executed and then the statements in the catch block get executed. If no exception occurs, then the statements in the finally block are executed once the statements in the try block are executed and the statements in the catch block are skipped.
If my above conception is not mistaken, then I don't understand why this piece of code doesn't work:
// sock is an object of the class Socket
public void run() {
try {
in = sock.getInputStream();
scan = new Scanner(in);
out = sock.getOutputStream();
pw = new PrintWriter(out);
} catch (IOException e) {
e.printStackTrace();
} finally {
sock.close();
}
}
It still says that I need to surround the statement in the finally block with try-catch.
No! the statements in the try block are executed first. Then if any exceptions occurs then the catch block statements are executed. And finally block is executed lastly. finally block gets executed even if an exception occurs in the try block. In other words if no exception occurs then first the try block is executed and then the finally block is executed.
In general If your catch block can catch the Exception thrown by the try block.
try -> finally (If No Exception)
try -> catch -> finally (If Exception)
The statements within the catch block are executed whenever an exception is encountered (which can be caught by the listed catch blocks). The statements within the finally block will always get executed, whether or not an exception was thrown within the try block.
Your sock.close(); statement could potentially throw an IOException, so either you should have another try-catch block wrapping the inner try-catch or add a throws declaration to your run() method (and the caller of the method can have a try-catch block surrounding the call).
EDIT (as per comment):
Statements within catch get executed first, then the statements within finally.
This question already has answers here:
What's the purpose of try-with-resources statements?
(7 answers)
Closed 2 years ago.
I have been looking at code and I have seen try with resources. I have used the standard try-catch statement before and it looks like they do the same thing. So my question is Try With Resources vs Try-Catch what are the differences between those, and which is better.
Here is a try with resources :
objects jar = new objects("brand");
objects can= new objects("brand");
try (FileOutputStream outStream = new FileOutputStream("people.bin")){
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
}
The main point of try-with-resources is to make sure resources are closed reliably without possibly losing information.
When you don't use try-with-resources there's a potential pitfall called exception-masking. When code in a try block throws an exception, and the close method in the finally also throws an exception, the exception thrown by the try block gets lost and the exception thrown in the finally gets propagated. This is usually unfortunate, since the exception thrown on close is something unhelpful while the informative one is the one thrown from within the try block. (So instead of seeing the SQLException that tells you which referential integrity constraint was violated, you're shown something like BrokenPipeException where closing the resource failed.)
This exception-masking is an annoying problem that try-with-resources prevents from happening.
As part of making sure exception-masking wouldn't lose important exception information, when try-with-resources was developed they had to decide what to do with the exceptions thrown from the close method.
With try-with-resources, if the try block throws an exception and the close method also throws an exception, then the exception from the close block gets tacked on to the original exception:
... there are situations where two independent exceptions can be thrown in sibling code blocks, in particular in the try block of a try-with-resources statement and the compiler-generated finally block which closes the resource. In these situations, only one of the thrown exceptions can be propagated. In the try-with-resources statement, when there are two such exceptions, the exception originating from the try block is propagated and the exception from the finally block is added to the list of exceptions suppressed by the exception from the try block. As an exception unwinds the stack, it can accumulate multiple suppressed exceptions.
On the other hand if your code completes normally but the resource you're using throws an exception on close, that exception (which would get suppressed if the code in the try block threw anything) gets thrown. That means that if you have some JDBC code where a ResultSet or PreparedStatement is closed by try-with-resources, an exception resulting from some infrastructure glitch when a JDBC object gets closed can be thrown and can rollback an operation that otherwise would have completed successfully.
Without try-with-resources whether the close method exception gets thrown is up to the application code. If it gets thrown in a finally block when the try block throws an exception, the exception from the finally block will mask the other exception. But the developer has the option of catching the exception thrown on close and not propagating it.
You missed something, the finally block. The try-with-resouces will make it something like,
FileOutputStream outStream = null;
try {
outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
} finally {
if (outStream != null) {
try {
outStream.close();
} catch (Exception e) {
}
}
}
Which means you really wanted something like (never swallow exceptions),
try (FileOutputStream outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);) {
stream.writeObject(jar);
stream.writeObject(can);
// stream.close(); // <-- closed by try-with-resources.
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
} catch(IOException f) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
}
The only difference is that try-resource is adding automatically resource.close();
as you would do in finally block
Any object (either the class or their superclass) that implements java.lang.AutoCloseable or java.io.Closeable
can only be used in try-with-resource clause.
AutoClosable interface is the parent interface and Closable interface extends the AutoClosable interface.AutoClosable interface has method close which throws Exception while Closable interface has method that throws IOException.
We can also have catch and finally block followed by try-with-resource like ordinary try, catch and finally, but catch and finally block only get executed once the resource declared inside the try-with-resource clause is closed.
Succinctly it is syntactic sugar to support the AutoClosable interface and call the close() method for you for any outcome.
I just want to know is it necessary to put catch after try block, or can we use try blocks without a catch block?
You need to put either catch or finally block after try.
try {
}
finally {
}
or
try {
}
catch (Exception e) {
}
is it necessary to put catch after try block ?
Nope, not at all. Its not mandatory to put catch after try block, unless and until the try block is followed by a finally block. Just remember one thing, after try, a catch or a finally or both can work.
we can use try without catch block?
Yes, you can. But that will be a bad practise. Since, you are writing a try block, you should be writing catch block ( for catching the exception) and a good practise to follow it by a finally block.
Yes you can... but you must put a finally block after try. So you can do it like this:
try
{
}
finally
{
}
or
try
{
}
catch(Exception e)
{
}
Yes you can write try without catch. In that case you require finally block. Try requires either catch or finally or both that is at least one catch or finally is compulsory.
try{
// throw exception
} finally{
// do something.
}
But you should avoid this case cause in this case you will loose exception details. So if you don't want to handle it in here then simply throw that exception.
try without a catch block is a syntax error because it makes no sense (unless you also want to use a finally block). The only reason to use try is in order to catch the exception (or do a finally) from within that block
In Java 7 the try-with-resource statement doesn't need catch or finally clause
try(InputStream is = new FileInputStream(..))
{
is.read();
}
Yes you can use finally instead but to be more practical I use "throws Exception" function if I can because using try and catch blocks makes code harder to read.
First thing to remember is that you have to know what the purpose of the try-catch-finally block is.
The try block is used to test the code written inside it. If the code causes an exception, it throws the exception to the catch block.
The catch block is used to handle the thrown exception like, assume that you wrote a code that prompt the user to insert numbers only. But the user inputted a letter, thus the code throw an exception. The exception then would be caught by the catch block. Then the catch block prompt the user to re-input the data. This is what you call exception handling. But if you want to just leave the catch block empty is fine.
You may write try without the catch keyword following it but, you have to write the finally after the try block.
The code in the finally block will always be executed no matter what. You usually write codes in the finally block to close resources opened in the try block like files or database connection.
You can use the try-with-resources in place of the finally block (available in java 8).
So, you can write try followed by catch then followed by finally like the following example :
try{
//code
}
catch(Exception ex){
//code to handle the problem.
}
finally{
//Closing resources etc.
}
Or You can write this :
try{
//code
}
catch(Exception ex){
//code to handle the problem.
}
Or this :
try{
//code
}
finally{
//Closing resources etc.
}
But, you usually would want to handle the problem with the catch block.
I feel puzzle ...
I write a small routine in .jsp. Finally, ResultSet, Statement and Connection are required to be closed. I also write the closing codes in finally { }, but when the page is run, it return error that I didn't catch exception ...
I read some forum. Other people didn't catch any exception in finally { }
Any Hint ?
Sounds like you have the old problem of needing to close() in a finally block but close() throws an exception itself. Try somethig like the following...
ResultSet rs;
try {
// do various stuff
rs = ...;
} finally {
try {
if (rs != null) rs.close();
} catch (SQLException e) {
// do something with exception
}
}
You must catch exceptions in the code finally block. As you must catch exceptions in the catch block. Nested try/catches are a regular thing (albeit ugly).
One important note here is that you could have the exceptions that occur in finally declared in the throws clause of the method. However that would lead to the exception in finally overriding the original exception, which is lost. And you will see, for example, a NullPointerException, rather than FileNotFoundException.
By the way, avoid having code in the JSP file. Place it in a servlet.
finally{} doesn't do any exception catching. A finally{} block exists to make sure that certain code is run, no matter whether the try{} block reached its natural end or if it's jumping temporarily to the finally{} because an exception happened and that finally{} block was along the way. But after the finally{} finishes, the exception goes about its merry business, cavorting its way up the stack and cheerfully crashing your program.
If you want to actually catch the exception and stop it from unwinding the stack further, use catch(){}. But don't use catch blindly- catching an exception you don't actually know how to recover from is much worse than crashing, because now your program isn't working correctly and you don't have an exception stack trace telling you why.
Your ResultSet, Statement, and Connection almost certainly did get closed. And then the exception continued happening and crashed your program anyway, because that had nothing to do with your ResultSet, Statement, and Connection.
What was the actual exception?
Maybe I'm getting old, but what's wrong with catching exceptions in the catch block?
It helps if you say what is in your try block. You are probably not catching appropriate exception or your code in finally throws exception.
It is OK to have finally without catch.
try {
//do some work
}
finally {
//check of state and do clean up. You would have reached here via multiple branches.
}
It more appropriate to catch specific exceptions using catch and then handle specific cleanup there. Use finally for any code that must get executed even when exception happen.
try {
//do some work
}
catch ( RecoverableException1 re1) {
//cleanup
}
catch ( RecoverableException2 re2) {
//cleanup
}
finally {
//check of state and do clean up. You would have reached here via multiple branches.
}
finally{
try{
resultSet.close();
}catch(E e){
}finally{
try{
statement.close();
}catch(E e){
}finally{
conn.close();
}
}
}
How do I use exceptions and exception handling to make my program continue even if an exception occurs while processing certain files in a set of files?
I want my program to work fine for correct files while for those files which cause an exception in program, it should ignore.
Regards,
magggi
for(File f : files){
try {
process(f); // may throw various exceptions
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
}
You have to use the try/catch/finally blocs.
try{
//Sensitive code
} catch(ExceptionType e){
//Handle exceptions of type ExceptionType or its subclasses
} finally {
//Code ALWAYS executed
}
try will allow you to execute sensitive code which could throw an exception.
catch will handle a particular exception (or any subtype of this exception).
finally will help to execute statements even if an exception is thrown and not catched.
In your case
for(File f : getFiles()){
//You might want to open a file and read it
InputStream fis;
//You might want to write into a file
OutputStream fos;
try{
handleFile(f);
fis = new FileInputStream(f);
fos = new FileOutputStream(f);
} catch(IOException e){
//Handle exceptions due to bad IO
} finally {
//In fact you should do a try/catch for each close operation.
//It was just too verbose to be put here.
try{
//If you handle streams, don't forget to close them.
fis.close();
fos.close();
}catch(IOException e){
//Handle the fact that close didn't work well.
}
}
}
Resources :
oracle.com - Lesson: Exceptions
JLS - exceptions
I guess your new to programming as execeptions are a fairly fundermental concept, as problems can happen out of your control and you need to deal with it.
The basic premise is a try catch block.
try
{
//Your code here that causes problems
}
catch(exception ex)
{
//Your code to handle the exception
}
You 'try' your code, and if an exception is raised, you 'catch' it. And do what you need.
There is also an addition to the catch block in that you can add finally{} below it. Basically even if no exception is raised the finally code is still run. You may wonder the point in this, but its often used with streams/file handling etc to close the stream.
Read more on java exceptions here in tutorials written by Sun (now Oracle)- http://download.oracle.com/javase/tutorial/essential/exceptions/
try
{
//Your code here that causes problems
}
catch(exception ex)
{
//Your code to handle the exception
}
finally
{
//Always do this, i.e. try to read a file, catch any errors, always close the file
}
The question you may ask is how do you catch different exceptions, i.e. is it a null reference, is it divide by zero, is it no file found or file not writeable etc. For this you write several different catch blocks under the try, basically one catch for each type of exception, the use of "exception" is basically a catch all statement, and like in stack of if statements if an "exception" is the first catch block it will catch everything, so if you have several catch blocks ensure exception is the last one.
Again, this is a useful but large topic so you need to read up about it.
Since you are doing multiple files, you need to basically do a loop and within the loop is contained the try/catch block.
so even if one file fails, you catch it, but carry on running, the code will then loop around onto the next file unhindered.
just catch the excpetion it may throw and do nothing with it; eat it as people say :)
But at least log it!
Very concise example:
try {
your code...
} catch (Exception e) {
log here
}
Typically, I would have done this.
ArrayList<Entry> allEntries = getAllEntries();
for(Entry eachEntry:allEntries){
try{
//do all your processing for eachEntry
} catch(Exception e{
ignoredEntries.add(eachEntry);
//if concerned, you can store even the specific problem.
} finally{
//In case of resource release
}
}
if(ignoredEntries.size() > 0){
//Handle this scenario, may be display the error to the user
}
FileSystemException may be the specific exception you are looking for.
Although, a better idea for beginners is to catch an exception and print it using
System.out.println(e);
where e is the caught exception.
public class Main
{
public static void main(String args[])
{
int a=10;
try
{
System.out.println(a/0); //Here it is not possible in maths so it goes to catch block
}
catch(ArithmeticException e)
{
System.out.println("Arithmetic Exception");
}
}
}
output:Arithmetic Exception
Exception in java are runtime error which can be handled by the program, the process is called as exception handling. Parent class of exception is Throwable.
Exception : Exception are those runtime error which can be handled by program.
Error : Those runtime error which can’nt handled by the program.
Tools used to handle Exception:
Try
Catch
Finally
Throw
Throws
more