Java, executing a method when object's scope e

问题:

I've an object with a certain state. The object is passed around and it's state is temporarly altered. Something like:

public void doSomething(MyObject obj) {
    obj.saveState();
    obj.changeState(...);
    obj.use();
    obj.loadState();
}

In C++ it's possible to use the scope of an object to run some code when constructing and distructing, like

NeatManager(MyObject obj) { obj.saveState(); }
~NeatManager() { obj.loadState(); }

and call it like

void doSomething(MyObject obj) {
    NeatManager mng(obj);
    obj.changeState();
    obj.use();
}

This simplifies the work, because the save/load is binded with the scope of NeatManager object. Is it possible to do something like this in Java? Is there a way to call a method when the object goes out of the scope it's been declared in? I'm not talking about finalize() nor "destruction" (garbage collection), I'm interested on the scope.

Thanks

回答1:

Nope, there's no such thing. The closest is probably a try/finally block:

try
{
    obj.changeState(...);
    obj.use();
}
finally
{
    obj.loadState();
}

This ensures that loadState() gets called even when an Exception is thrown or there's an early return.

回答2:

No, there's nothing like that. The closest you've got is try/finally.

In C# there's the using statement, which executes a Dispose method at the end:

using (Stream x = ...)
{
} // x.Dispose() is called here, in a finally block

There's a possibility that Java 7 will gain something a bit like this, but I don't think anything's been set in stone yet.

回答3:

As an additional note, don't be tempted to use the Object.finalize() method, which runs when an object is GC'd, as you have no control over when the object is collected.

回答4:

Java 7 has try-with-resources and you can extend AutoCloseable class to make use of it.

For Java 6 you may want to see the related question: What is the best way to emulate try-with-resources in Java 6?

回答5:

Short answer: No.

Medium answer: The best practice in this situation is to use a try ... finally block.

Longer answer: C++ doesn't really give you this either: C++ destructors are run on de-allocation. If you fail to deallocate an object and all references to it fall out of scope, the destructor will not be called.

As an aside, if garbage collection in Java was reference counting, then finalizers would implement exactly this behaviour.