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As far as I understand, a finally block always executes. Some people have argued that there is no purpose to finally block, because if there is an exception or there isn't, the code after the last catch block gets executed anyways.

However, the argument against this is that if there is an exception thrown in a catch block, there are no subsequent catch blocks to catch that exception. So by putting resource cleanup code in a finally block, you ensure that resources will be released in the event that an exception is thrown in a catch block.

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By using a finally block, you can clean up any resources that are allocated in a try block, and you can run code even if an exception occurs in the try block. Typically, the statements of a finally block run when control leaves a try statement. The transfer of control can occur as a result of normal execution, of execution of a break, continue, goto, or return statement, or of propagation of an exception out of the try statement.

One could copy the finally code before every return or throw, and wrap catch blocks within their own try/catch to allow for the possibility of an accidental exception occurring, but it's far easier to forgo all that and simply use a finally block.

BTW, one thing I wish language designers would include would be an exception argument to the finally block, to deal with the case where one needs to clean up after an exception but still wants it to percolate up the call stack (e.g. one could wrap the code for a constructor in such a construct, and Dispose the object under construction if the constructor was going to exit with an exception).

finally is a syntactic sugar to allow DRY principle in try-catch pattern. Exception is usually thrown if the library code has not enough information to handle some state and wants the client code to solve it. If you don't have library-client code separation, you can handle everything by if instead of try.

As @supercat points out, the finally block is executed also if try block exits via return. I suggest you avoid this bad habit and have only one return in every function (maybe some early exists at the very beginning of functions). The reasons for single-return functions are:

The finally() method of Promise instances schedules a function to be called when the promise is settled (either fulfilled or rejected). It immediately returns an equivalent Promise object, allowing you to chain calls to other promise methods.

Returns an equivalent Promise. If the handler throws an error or returns a rejected promise, the promise returned by finally() will be rejected with that value instead. Otherwise, the return value of the handler does not affect the state of the original promise.

Note: A throw (or returning a rejected promise) in the finally callback still rejects the returned promise. For example, both Promise.reject(3).finally(() => { throw 99; }) and Promise.reject(3).finally(() => Promise.reject(99)) reject the returned promise with the reason 99.

It seems like lots of people think it's a bad thing to do as described in 'Don't return in a finally clause'. Scratching a little deeper, I also found 'Java's return doesn't always' which shows some pretty horrible examples of other types of flow control in finally blocks.

What happened is that the exception was thrown down in some other code. It was being caught and logged and rethrown within the somethingThatThrewAnException() method. But the exception wasn't being propagated up past problemMethod(). After a LONG time of looking at this we finally tracked it down to the return method. The return method in the finally block was basically stopping the exception that happened in the try block from propagating up even though it wasn't caught.

javac will warn of return in finally if you use the -Xlint:finally. Originally javac emitted no warnings - if something is wrong with the code, it should fail to compile. Unfortunately backwards compatibility means that unanticipated ingenious foolishness cannot be prohibited.

Adding control structures and returns to finally{} blocks are just another example of "just because you can" abuses which are scattered throughout virtually all development languages. Jason was right in suggesting it could easily become a maintenance nightmare - the arguments against early returns from functions apply more-so to this case of "late returns".

Anything that affects the return of the function should lie in the try{} block. Even if you had a method whereby you checked an external state, did a time consuming operation, then checked that state again in case it became invalid, you would still want the second check inside the try{} - if it sat inside finally{} and the long operation failed, you would then be checking that state a second time needlessly.

One interesting point for me was to see how Groovy deals with implicit returns. In Groovy it is possible to "return" from a method simply leaving a value at the end (without return). What do you think happens, if you uncomment the runningThreads.remove(..) line in the finally statement - will this overwrite the regular return value ("OK") and cover the exception?!

Pass a handler that will be called regardless of this promise's fate. Returns a new promise chained from this promise. There are special semantics for .finally in that the final value cannot be modified from the handler.

Now the animation is hidden but, unless it throws an exception, the function has no effect on the fulfilled or rejected value of the returned promise. This is similar to how the synchronous finally keyword behaves.

If the handler function passed to .finally returns a promise, the promise returned by .finally will not be settled until the promise returned by the handler is settled. If the handler fulfills its promise, the returned promise will be fulfilled or rejected with the original value. If the handler rejects its promise, the returned promise will be rejected with the handler's value. This is similar to throwing an exception in a synchronous finally block, causing the original value or exception to be forgotten. This delay can be useful if the actions performed by the handler are done asynchronously. For example:

Some languages (such as C++ and early versions of PHP) don't support the finally part of a try ... catch ... finally construct. Is finally ever necessary? Because the code in it always runs, why wouldn't/shouldn't I just place that code after a try ... catch block without a finally clause? Why use one? (I'm looking for a reason/motivation for using/not using finally, not a reason to do away with 'catch' or why it's legal to do so.)

In this example, the Cleanup() function never runs, because an exception gets thrown in the catch clause and the next highest up catch in the call stack will catch that. Using a finally block removes this risk, and makes the code cleaner to boot.

Most of the other languages provide some form of garbage collection instead. While there are things about garbage collection that are controversial (e.g., its efficiency relative to other methods of memory management) one thing generally isn't: the exact time when an object will be "cleaned up" by the garbage collector is not tied directly to the scope of the object. This prevents its use when cleanup needs to be deterministic either when it's simply required for correct operation, or when dealing with resources so precious that their cleanup most not be delayed arbitrarily. try/finally provides a way for such languages to deal with those situations that require that deterministic cleanup.

Although I've called it "the Java approach" above, try/finally and similar mechanisms under other names aren't entirely restricted to Java. For one prominent example, most (all?) of the .NET languages (e.g., C#) provide the same.

Recent iterations of both Java and C# also provide something of a halfway point between "classic" Java and C++ in this regard. In C#, an object that wants to automate its cleanup can implement the IDisposable interface, which provides a Dispose method that's (at least vaguely) similar to a C++ destructor. While this can be used via a try/finally like in Java, C# automates the task a little more with a using statement that lets you define resources that will be created as a scope is entered, and destroyed when the scope is exited. Though still well short of the level of automation and certainty provided by C++, this is still a substantial improvement over Java. In particular, the class designer can centralize the details of how to dispose of the class in its implementation of IDisposable. All that's left for the client programmer is the lesser burden of writing a using statement to assure that the IDisposable interface will be used when it should be. In Java 7 and newer, the names have been changed to protect the guilty, but the basic idea is basically identical.

What this method does have to do, though, it to clean up after itself, so that the "Outside World" never needs to know anything about the mess that it got itself into. The finally clause does just that - no matter how the called methods behave, the finally clause will be executed "on the way out" of the method (and the same is true for every finally clause between the point at which the Exception is thrown and the eventual catch clause that handles it); each one is run as the call stack "unwinds". ff782bc1db