Is it possible to handle all Errors in Java?

Question

I know that Error is Throwable (so it can be handled) and Error handling isn't best practice but suppose we have a requirement to catch all Error.

It is possible in general? I mean what about Errors which can appear in daemon threads? They will crash jvm and I won't know about it.

And can I miss Error even if I surround main(String... args) with try-catch?

Answer 1

You can catch the uncaught exceptions via Thread.setUncaughtExceptionHandler()

Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.

Uncaught exception handling is controlled first by the thread, then by the thread's ThreadGroup object and finally by the default uncaught exception handler. If the thread does not have an explicit uncaught exception handler set, and the thread's thread group (including parent thread groups) does not specialize its uncaughtException method, then the default handler's uncaughtException method will be invoked.

Whether that's a good idea is another question (!). It may be that you simply want to clear up resources, shut down connections etc., log the issue and/or alert the user. If you do have critical issues like OutOfMemoryErrors then there's little else you can do.

Answer 2

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Now, since Error does indeed extends Throwable, it is possible to catch 'any' error via a simple try-catch statement as follows:

try{
    //Error causing code here...
}catch(Error e){
    //Handle error here...
}

However, since errors come in different forms, a thrown error may or may not be accompanied by a change in the jvm's behaviour to cause unexpected results.

Consider the Error OutOfMemoryError. When this Error is thrown, the JVM's heap may be so full that any type of handling code results in another OutOfMemoryError being thrown, voiding any attempt of recovery.

Furthermore, even when running on the "primary" thread (the one the application starts in), a error could cause a JVM to crash before every throwing a Error.

Looking at the description of VirtualMachineError (of which Errors such as OutOfMemoryError, StackOverflowError, InternalError, etc are subclasses of) we see:

Thrown to indicate that the Java Virtual Machine is broken or has run out of resources necessary for it to continue operating.

The javadoc itself states that the jvm no longer can continue operation normally much less allow a programmer to just 'handle' them away.

On the other hand, errors such as UnsatisfiedLinkError generally do not cause a problem with the jvm straight away and can generally be handled (whether this is good practice is debatable). I personally have once used a structure that involves handling a UnsatisfiedLinkError to determine the correct library to load for JNI.

Now, can all errors be handled? Theoretically yes, if we assume that the JVM can continue operation perfectly despite claiming to have failed fatally... With that in regard, practically only a small subset of errors are handleable. Whether or not these small subset of errors should be handled is also a highly controversial topic.

Answer 3

You can handle exceptions, but you can't handle errors.