I am using a thread-safe queue class and have a problem with the insert method I've defined. A Buffer stores an array (elementData) that uses the start/end variables to know where to add/delete stuff from the queue. It's thread-safe, so it uses synchronized methods so I can have multiple threads refer to the same buffer.
public class Buffer<T> {
private T[] elementData;
private int elementCount;
private int start;
private int end;
// Additional fields
// Code to instantiate a Buffer, other methods (e.g. delete)
public synchronized void insert(T t) throws InterruptedException {
while (elementCount == elementData.length) {
wait();
}
end = (end + 1) % elementData.length;
elementData[end] = t;
elementCount++;
notifyAll();
}
public static void main(String[] args) {
Buffer<Integer> b = new Buffer();
b.insert(3);
}
}
Here's my understanding of the situation. When a method such as insert is called, we want to be able to throw an exception that could happen when the main method or some other thread gets called and tries to perform insert while it's suspended. But what I don't understand is why I get this unreported exception. I thought that having a "throws InterruptedException" after the method would be sufficient. Do I need a "try" block? My attempts with try blocks have all failed, so I'm a little stumped as to how to fix this error.
Also, I'm aware that I don't have any actual threads running. I'll do those once I can fix this unreported exception. :) Thanks to anyone who can help.
Buffer.java:56: unreported exception java.lang.InterruptedException; must be caught or declared to be thrown
b.insert(3);
The compile exception is because your insert method could throw an InterruptedException (even if you're not throwing it on purpose), so every method that calls it must use a try/catch block, even if the error never arises:
public static void main(String[] args) {
Buffer<Integer> b = new Buffer();
try {
b.insert(3);
} catch(InterruptedException ie) {
//error handling
e.printStackTrace();
}
}
Related
I am trying to stop a long running method after 10 seconds of execution, so far i followed the timer instructions on baeldung.
https://www.baeldung.com/java-stop-execution-after-certain-time#1-using-a-timer
When the method is a simple call to a thread sleep it works, but when I call my function with sub methods it doesn't stop.
My implementation looks like this:
class TimeOutTask extends TimerTask {
private Thread t;
private Timer timer;
TimeOutTask(Thread t, Timer timer){
this.t = t;
this.timer = timer;
}
public void run() {
if (t != null && t.isAlive()) {
t.interrupt();
timer.cancel();
}
}
}
class Execution implements Runnable {
private String carpeta;
private Experiment exp;
public Execution(String carpeta, Experiment exp) {
this.carpeta = carpeta;
this.exp = exp;
}
#Override
public void run() {
try {
while (!Thread.currentThread().isInterrupted()) {
exp.executeExperiment(carpeta);
}
} catch (InterruptedException e) {
System.out.println("Fin de ejecución por tiempo");
}
}
}
And the way I am calling this execution is throught the executeTimedExperiment method
public Experiment() {
this.cases = new ArrayList<>();
}
private void executeTimedExperiment(String carpeta){
Thread t = new Thread(new Execution(carpeta,this));
Timer timer = new Timer();
timer.schedule(new TimeOutTask(t, timer), 10000);
t.start();
}
private void executeExperiment(String carpeta) throws InterruptedException {
String[] files = getFiles(carpeta);
Arrays.sort(files);
for (String file : files) {
executeCase(carpeta, file);
}
}
private boolean executeCase(String carpeta, String file) {
Graph g = readDataToGraph(carpeta + "/" + file);
Solution s = new ExactSolutionGenerator().ExactSolution(g);
addNewCase(file, s);
}
The executeExperiment method is the long running and I marked it with InterruptedException but the compiler tells me the exception is never throw.
What happens now when I execute it is that it runs normally without stoppping.
I am not sure if I need to add InterruptedException to the submethods or something else, but I would like to not touch the submethods if possible.
Thanks in advance.
You will need to do more than add throws InterruptedException to all of those ‘submethods’ (and your own methods). The body of each of those methods must be altered to properly respond to interrupts.
It is not possible to arbitrarily stop running code. Interrupts are cooperative—they only mean something if the thread being interrupted pays attention to them.
Your run() method does this properly: by placing the entire loop inside a try/catch, any InterruptedException will cause the loop to terminate and thus the thread will terminate.
But the methods it calls must do the same thing. Your run method calls executeExperiment, which does this:
String[] files = getFiles(carpeta);
I don’t know how long that method takes, but if it takes any significant amount of time at all (more than a fraction of a second), it needs to be capable of throwing InterruptedException in the middle of the file reading.
executeExperiment also calls executeCase, which calls the ‘submethods’ readDataToGraph, ExactSolution, and addNewCase. As above, each of those methods which takes more than a fraction of a second needs to respond to an interrupt by throw InterruptedException. So, I’m afraid you will need to modify them.
An example would be:
private Graph readDataToGraph(String filename)
throws InterruptedException {
Graph graph = new Graph();
try (BufferedReader reader = Files.newBufferedReader(Path.of(filename))) {
String line;
while ((line = reader.readLine()) != null) {
graph.addData(convertDataToGraphEntry(line));
if (Thread.interrupted()) {
throw new InterruptedException();
}
}
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}
Compiler tells you the exception is never throw is beacuse your executeExperiment method is uninterruptable(Unlike some blocking methods, e.g. Object#wait), so thread.interrupt does not make the thread executing this method receive an InterruptedException.
Maybe you need to check whether the current thread has been interrupted every time you iterate files in your executeExperiment method, if it is, then throw an InterruptedException.(But this may still be inaccurate, because the executeCase method may be executed for a long time.)
In the Code written below although i have not caught the ArithmeticException,yet the exception is handled automatically and with finally Block, the content of main() method is successfully Executed. Whereas if i remove the return statement from finally and make demo as returning void then the program after executing finally block throws MainThread Exception..why is it so?
public class FinallyDemo {
int demo() {
try {
int a=5/0;
}
finally {
System.out.println("Finally Executed");
return 10;
}
}
public static void main(String s[]) {
int a=new FinallyDemo().demo();
System.out.println("Exception Handled");
}
}
Because you return from the finally block, the exception is silently disposed. You should never return from a finally block! (Well, almost always never).
From the Java Language Specification:
If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and reason R is discarded).
This also means if you threw a different exception, like an IllegalStateException, from the finally block, the original exception would also be discarded.
Is it possible to make an exception that is optional to be caught?
In other words, an exception that can either:
be caught in a try-catch block
or skipped if no try-catch block exists for it
To visualize, I have a ReportedException, which is just a plain subclass of RuntimeException, and want to be able to catch it when it's needed:
try
{
hideWindow();
}
catch (ReportedException ex)
{
// Window could not be hidden.
// Exception has already been caught and logged by parseInput(),
// and now we're going to do something more.
printAdditionalErrorMessage();
}
Note: I edited the above example to better fit my question.
or skip catching if the result is irrelevant:
hideWindow(); // We don't care if there was an error or not.
openAnotherWindow();
I know I can leave the catch block empty and have the same thing as above, but I use ReportedException very often and it would make my code highly unreadable.
If it's impossible (I suspect it is), what alternative/walkaround would you recommend?
P.S. The method names used in the examples are just foo's and bar's.
EDIT: I know I don't need to catch RuntimeExceptions. What I want is to ignore them if they occur.
Exceptions should be used for exceptional situations.
From your example, if the window not being hidden is a typical event, it shouldn't throw an exception. If that is your function, then use a return value to indicate whether it was successful instead of throwing an exception. Then you can safely ignore the return value when you don't care if it succeeded or not.
If you do not have control over that method, then you can wrap it in another method that catches the exception and turns it into a return value. E.g.
private boolean tryHideWindow() {
try {
hideWindow();
}
catch (ReportedException ex) {
return false;
}
return true;
}
If you need some parameters of the exception to determine what to do, then you could return the exception instead.
private static class MyReturnType {
private final Throwable thrown;
private final OrigRtnType returnVal;
public MyReturnType(Throwable thrown) {
this.thrown = thrown;
this.returnVal = null;
}
public MyReturnType(OrigRtnType returnVal) {
this.thrown = null;
this.returnVal = returnVal
}
public boolean wasExceptionThrown() {
return thrown != null;
}
}
private MyReturnType tryHideWindow() {
try {
OrigRtnType returnVal = hideWindow();
}
catch (ReportedException ex) {
return new MyReturnType(ex);
}
return new MyReturnType(returnVal);
}
This is an answer to your question, but it is not necessarily a good idea. As others will doubless comment, using exceptions for program flow is less than ideal.
I'm a little fuzzy on how to use ThreadLocal (and there are apt to be some other tupos), but something like this:
public class IgnorableException {
static class DontIgnoreCount {
int count;
}
// Thread local variable containing each thread's ID
private static final ThreadLocal<DontIgnoreCount> dontIgnoreCount =
new ThreadLocal<DontIgnoreCount>();
static void incrementDontIgnore() {
DontIgnoreCount counter = dontIgnoreCount.get();
if (counter == null) {
counter = new DontIgnoreCount();
dontIgnoreCount.set(counter);
}
counter.count++;
}
static void decrementDontIgnore() {
DontIgnoreCount counter = dontIgnoreCount.get();
// Must not be null here
counter.count--;
static bool shouldSignal() {
DontIgnoreCount counter = dontIgnoreCount.get();
return counter.count > 0;
}
}
To use, invoke DontIgnoreCount.incrementIgnoreCount() early in try range, and DontIgnoreCount.decrementIgnoreCount() late in finally range.
When signalling an exception that follows this protocol, only signal it if shouldSignal returns true.
void iWannaCatchException() {
try {
IgnornableException.incrementDontIgnore();
int x = someOptionallySignallingMethod();
}
catch (...) {
...
}
finally {
IgnorableException.decrementDontIgnore();
}
}
void iDontWannaCatchException() {
int x = someOptionallySignallingMethod();
}
int someOptionallySignallingMethod() {
if (somethingBad) {
if (IgnorableException.shouldSignal()) {
throw new BadException();
}
}
return 42;
}
Note that not shown above are any throws clauses you'd have to add to keep the compiler happy. This mechanism would not remove the need for those.
You could also inplement a delegate/observer scheme, replacing the simple counter with a stack of observer objects, and pass a message to the observer vs throwing the exception. But this, by itself (without coupled exceptions/try ranges) would not allow blowing away the stack to the appropriate recovery point.
It sounds like you want to use exceptions for flow control, rather than for reporting truly exceptional cases.
Using exceptions for flow control is typically frowned upon. The common approach is to return a success/failure indication as the return value of the function.
You can use something like this:
try{
hideWindow();
}catch (ReportedException ex){
// ingore
}catch (NullPointerException ex){
killWindow();
}finally {
//to do something more.
}
I am new to java, and to make clear of "System.out", i read relevant java source code, then find something i cannot understand.
First the source code of "System.out":
public final static PrintStream out = nullPrintStream();
then i went to nullPrintStream
private static PrintStream nullPrintStream() throws NullPointerException {
if (currentTimeMillis() > 0) {
return null;
}
throw new NullPointerException();
}
My question is: the program may throw a NullPointerException in the function nullPrintStream(), and we needn't to catch the exception in public final static PrintStream out = nullPrintStream();? To make clear of it, i wrote some test codes in Eclipse as follows:
package MainPackage;
public class Src {
private static int throwException() throws Exception{
int m = 1;
if(m == 0) {
throw new Exception();
}
return 0;
}
public static final int aTestObject = throwException(); <==Here i got an error
public static void main(String args[]) {
}
}
Just like i think, i got an error Unhandled exception type Exception, but why System.out is OK without doing with the NullPointerException?
Java has a special class of Exceptions called RuntimeExceptions. They all extend the RuntimeException object, which in turn extends the Exception object. The special thing about a RuntimeException (as opposed to a regular exception) is that it does not need to be explicitly thrown. Several different exceptions fit into this category, such as IllegalArgumentException, IllegalStateException etc...
The advantage of using RTE when you are coding is that you do not need to cover your code with a lot of try/catch/throws statements, especially if the exceptions are expected to be extremely rare and unlikely. Additionally, if you have a general mechanism in place for catching RTE, this will also help make sure your app deals with expection conditions cleanly.
That being said, RTEs can be much more difficult to deal with, as it is not obvious from the signature that a particular class or method will throw that type of exception. Consequently, they are not always a good idea for APIs, unless they are well documented.
A NullPointerException is a RuntimeException, and consequently, does not need to be explicitly declared in the method signature.
NullPointerException is a RuntimeException - it doesn't need to be explicitly caught.
if you make your method do this, it won't bomb on compile:
private static int throwException() throws Exception{
int m = 1;
if(m == 0) {
throw new RuntimeException();
}
return 0;
}
if i adhere to throw Exception() in private static int throwException() , how should i modify public static final int aTestObject = throwException();
You can need to intialise the value in a static block and catch the exception there.
public static final int aTestObject;
static {
try {
aTestObject = throwException(); <==Here i got an error
} catch (Exception e) {
throw new AssertionError(e);
}
}
Given this code:
public class TwoThreads {
static Thread laurel, hardy;
public static void main(String[] args) {
laurel = new Thread() {
public void run() {
System.out.println("A");
try {
hardy.sleep(1000);
} catch (Exception e) {
System.out.println("B");
}
System.out.println("C");
}
};
hardy = new Thread() {
public void run() {
System.out.println("D");
try {
laurel.wait();
} catch (Exception e) {
System.out.println("E");
}
System.out.println("F");
}
};
laurel.start();
hardy.start();
}
}
The output includes:
A C D E and F
I'm puzzled about why F is included, given that an IllegalMonitorStateException is thrown when wait() is called outside of synchronized code. Why is the print statement of F reached? I believe that the thread stack blows then, but then the program should pass control to its main stack.
Is this correct?
You are catching the exception in the block that prints "E" effectively swallowing it. The code will then continue on to print "F". Blocks that simply catch exceptions and do nothing else are dangerous for this reason.
you catch the exception so control goes to the catch block then continues executing the code after the try/catch.
In the code above as long as their are non application fatal errors after "D" is printed, "F" will always be printed as all catchable errors are handled.
If there are no thread hangs, this behaviour will be consistent.
Add a boolean check to "F" which is suppressed if there is an error thrown and that will give you the desired behaviour.
As a side note, you're calling Thread's static sleep method on objects, which does something other than you might expect. You shouldn't call static class methods on instances for this reason.
(As for why F is printed, the other guys are correct)
I wonder how you know IllegalMonitorStateException is being thrown. You are consuming any Exception and not doing something like e.printStackTrace();.