Because of the nature of threads, you cannot catch exceptions that escape from threads. Once the exception escapes the run() method, it is propagated to the console unless special steps are taken.
public class ExceptionThread implements Runnable {
public void run() {
throw new RuntimeException();
}
public static void main(String[] args) {
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new ExceptionThread());
System.out.println("ccccccc");
}
} ///:~
output:
ccccccc
Exception in thread "pool-1-thread-1" java.lang.RuntimeException
at ExceptionThread.run(ExceptionThread.java:7)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
The main thread cannot be caught.
Even the try catch in main doesn't work:
public class NaiveExceptionHandling {
public static void main(String[] args) {
try {
ExecutorService exec =
Executors.newCachedThreadPool();
exec.execute(new ExceptionThread());
System.out.println("xxxxxxxxxxx");
} catch(RuntimeException ue) {
// This statement will NOT execute!
System.out.println("Exception has been handled!");
}
}
output:
Exception in thread "pool-1-thread-1" java.lang.RuntimeException
at ExceptionThread.run(ExceptionThread.java:7)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
To solve this problem, we need to modify the way Executors spawn threads.
import java.util.concurrent.*;
class ExceptionThread2 implements Runnable {
public void run() {
Thread t = Thread.currentThread();
System.out.println("run() by " + t);
System.out.println(
"eh = " + t.getUncaughtExceptionHandler());
throw new RuntimeException();
}
}
class MyUncaughtExceptionHandler implements
Thread.UncaughtExceptionHandler {
public void uncaughtException(Thread t, Throwable e) {
System.out.println("caught " + e);
}
}
class HandlerThreadFactory implements ThreadFactory {
public Thread newThread(Runnable r) {
System.out.println(this + " creating new Thread");
Thread t = new Thread(r);
System.out.println("created " + t);
t.setUncaughtExceptionHandler(
new MyUncaughtExceptionHandler());
System.out.println(
"eh = " + t.getUncaughtExceptionHandler());
return t;
}
}
public class CaptureUncaughtException {
public static void main(String[] args) {
ExecutorService exec = Executors.newCachedThreadPool(
new HandlerThreadFactory());
exec.execute(new ExceptionThread2());
}
} /* Output: (90% match)
HandlerThreadFactory@de6ced creating new Thread
created Thread[Thread-0,5,main]
eh = MyUncaughtExceptionHandler@1fb8ee3
run() by Thread[Thread-0,5,main]
eh = MyUncaughtExceptionHandler@1fb8ee3
caught java.lang.RuntimeException
*///:~