How to set UncaughtExceptionHandler for a thread group

Question

Suppose I have 5 threads belonging to a thread group named "Fruits-group".

How can I assign UncaughtExceptionHandler to all threads of Fruits-group at one go?

I know we can define a global UncaughtExceptionHandler for all threads. What I am looking for is assigning a UncaughtExceptionHandler to an entire thread-group?

Answer 1

TL;DR Subclass the ThreadGroup and override the uncaughtException() method.

A ThreadGroup is an UncaughtExceptionHandler, implementing the uncaughtException(Thread t, Throwable e) method:

Called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception, and the thread does not have a specific Thread.UncaughtExceptionHandler installed.

The uncaughtException method of ThreadGroup does the following:

• If this thread group has a parent thread group, the uncaughtException method of that parent is called with the same two arguments.
• Otherwise, this method checks to see if there is a default uncaught exception handler installed, and if so, its uncaughtException method is called with the same two arguments.
• Otherwise, this method determines if the Throwable argument is an instance of ThreadDeath. If so, nothing special is done. Otherwise, a message containing the thread's name, as returned from the thread's getName method, and a stack backtrace, using the Throwable's printStackTrace method, is printed to the standard error stream.

Applications can override this method in subclasses of ThreadGroup to provide alternative handling of uncaught exceptions.

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UPDATE

If you want to be able to set an UncaughtExceptionHandler for the ThreadGroup, you can create a delegating subclass:

public class ExceptionHandlingThreadGroup extends ThreadGroup {
    private UncaughtExceptionHandler uncaughtExceptionHandler;
    
    public ExceptionHandlingThreadGroup(String name) {
        super(name);
    }
    public ExceptionHandlingThreadGroup(ThreadGroup parent, String name) {
        super(parent, name);
    }
    
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return this.uncaughtExceptionHandler;
    }
    public void setUncaughtExceptionHandler(UncaughtExceptionHandler uncaughtExceptionHandler) {
        this.uncaughtExceptionHandler = uncaughtExceptionHandler;
    }
    
    @Override
    public void uncaughtException(Thread t, Throwable e) {
        if (this.uncaughtExceptionHandler != null)
            this.uncaughtExceptionHandler.uncaughtException(t, e);
        else
            super.uncaughtException(t, e);
    }
}

In general, though, it would likely be better to just implement the exception handling logic directly in the subclass, but this way, you can use an existing UncaughtExceptionHandler implementation.