Java Thread: Run method cannot throw checked exception

Question

In Java thread, the 'run' method cannot throw a 'checked exception'. I came across this in the Core Java (vol 1) book. Can someone please explain the reasoning behind it?

Answer 1

Can someone please explain the reasoning behind it?

Yes, because any exception you throw in run method will be carefully ignored by JVM. Thus, throwing it there is probably a mistake (unless you have specific exception handler for the thread, see the docs about that). No reason to incite potentially erroneous behaviour.

Or, with an example.

 class MyThread extends Thread {      public void run() {          throw new RuntimeException();      }  }  ...  new MyThread().start(); // here thread dies silently with no visible effects at all 

edit

Why can't the parent thread 'catch' the exception from the spawned 'child' thread?

@chaotic3quilibrium has already noted in his comment why not: because parent thread has likely moved on already.

new MyThread().start(); // launch thread and forget  // 1000 lines of code further... i = i + 1; // would you like exception from child thread to be propagated here? 

Answer 2

What would catch the exception and handle it? Let's assume that the run method could throw a checked exception. Then you could write code like this:

Thread myThread = new Thread(aRunnable); try{     myThread.start(); } catch(Exception e){    e.printStackTrace(); } //do other stuff 

BUT once you call myThread.start, the new thread is started in the background and the current thread continues and exits the try-catch and does other stuff. So if myThread did throw an exception later on, you can't catch it!

What you need to do is deal with the exception within the run method and then probably have a way of notifying another object that this thread failed.