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I was reading that Any Exception thrown by finalize method is ignored by GC thread and it will not be propagated further but what is the reason for ignoring Exception by GC.

Also finalization of that object terminates, does that means, that object always remains in memory?

Vishrant
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    Exceptions that propagate out of a `finalize` method are ignored. In essence the `finalize` method is run, and regardless of whether it ends "normally" or with an exception the object is marked for no further attempts at finalization. On the next GC cycle the finalizer is ignored and the object is collected, if it's no longer referenced. (The finalizer could have made it referenced again.) – Hot Licks Mar 25 '14 at 11:54

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It ignores the exception as it has no means of handling it. The purpose of a finalize() method is to be the final cleanup before the object dies. If you want to handle an exception in a finalize() method, you have to add code to do this.

Peter Lawrey
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  • also is it the case that Garbage Collector ignores the `Exception` and finalization of that object terminates, does that means that object always remains in memory? – Vishrant Mar 25 '14 at 11:39
  • @Vishrant The finalizer thread doesn't tell the GC anything so there is nothing for an Exception to mess up as it were. – Peter Lawrey Mar 25 '14 at 11:42
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    @Vishrant The GC works independently of the finalizer thread. When the GC run it just collects objects which are not strongly reachable and don't need to have finalize() called. The finalizer thread just ignores exception/errors by discarding them and pretending they didn't happen. – Peter Lawrey Mar 25 '14 at 12:04
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From Effective Java

In case you are not yet convinced that finalizers should be avoided, here’s another tidbit worth considering: if an uncaught exception is thrown during finalization, the exception is ignored, and finalization of that object terminates [JLS, 12.6]. Uncaught exceptions can leave objects in a corrupt state. If another thread attempts to use such a corrupted object, arbitrary nondeterministic behavior may result. Normally, an uncaught exception will terminate the thread and print a stack trace, but not if it occurs in a finalizer—it won’t even print a warning.

The problem is when uncaught exception is raised inside finalize method then object may stay in a corrupted state. Uncaught exception is ignored in finalize method, but it doesn't mean that GC ignores Exception as your question may suggest.

Jakub H
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The GC itself does not call finalize. Instead, when the GC finds a "finalizable" object to which no strong rooted references exist, it marks it "non-finalizable" and moves it to a queue of objects which need to be finalized immediately and if necessary starts a thread which will process each item in the queue. The queue itself will act as a strong rooted reference to objects whose finalize methods haven't been run, and the execution context of the currently-running finalizer will keep it alive, but once control leaves the finalizer, the object will become eligible for collection unless a strong rooted reference to it has been stored elsewhere.

The finalizer need not actually do anything to enable the GC to collect an object. The GC itself clears the "finalizable" flag when the object gets moved to the queue of objects which need immediate finalization, and the act of pulling an object out of the queue in order to run it will eliminate the queue as a rooted reference. By the time the finalizer executes, its execution context will be the only thing keeping the object alive, so leaving that execution context via any means--exceptional or otherwise--will make the object eligible for garbage collection.

Note that the real purpose of a finalize method is not to have objects do things to themselves, but rather to allow objects to notify outside entities acting on their behalf that their services are no longer required. If a finalizer dies without delivering such notifications, the notifications will not be sent.

supercat
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