Does a Finally Block Always Get Executed in Java

Does a finally block always get executed in Java?

Yes, finally will be called after the execution of the try or catch code blocks.

The only times finally won't be called are:

1. If you invoke System.exit()
2. If you invoke Runtime.getRuntime().halt(exitStatus)
3. If the JVM crashes first
4. If the JVM reaches an infinite loop (or some other non-interruptable, non-terminating statement) in the try or catch block
5. If the OS forcibly terminates the JVM process; e.g., kill -9 <pid> on UNIX
6. If the host system dies; e.g., power failure, hardware error, OS panic, et cetera
7. If the finally block is going to be executed by a daemon thread and all other non-daemon threads exit before finally is called

Does a finally block always get executed in Java?

Yes, finally will be called after the execution of the try or catch code blocks.

The only times finally won't be called are:

1. If you invoke System.exit()
2. If you invoke Runtime.getRuntime().halt(exitStatus)
3. If the JVM crashes first
4. If the JVM reaches an infinite loop (or some other non-interruptable, non-terminating statement) in the try or catch block
5. If the OS forcibly terminates the JVM process; e.g., kill -9 <pid> on UNIX
6. If the host system dies; e.g., power failure, hardware error, OS panic, et cetera
7. If the finally block is going to be executed by a daemon thread and all other non-daemon threads exit before finally is called

Does a finally block always run?

from the Sun Tutorials

Note: If the JVM exits while the try
or catch code is being executed, then
the finally block may not execute.
Likewise, if the thread executing the
try or catch code is interrupted or
killed, the finally block may not
execute even though the application as
a whole continues.

I don't know of any other ways the finally block wouldn't execute...

Is there possibility that a finally block might not execute?

If the JVM exits while the try or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.

Source: java.sun.com: Java Tutorial: The finally Block

Does a finally block run even if you throw a new Exception?

Yes, the finally blocks always runs... except when:

• The thread running the try-catch-finally block is killed or interrupted
• You use System.exit(0);
• The underlying VM is destroyed in some other way
• The underlying hardware is unusable in some way

Additionally, if a method in your finally block throws an uncaught exception, then nothing after that will be executed (i.e. the exception will be thrown as it would in any other code). A very common case where this happens is java.sql.Connection.close().

As an aside, I am guessing that the code sample you have used is merely an example, but be careful of putting actual logic inside a finally block. The finally block is intended for resource clean-up (closing DB connections, releasing file handles etc), not for must-run logic. If it must-run do it before the try-catch block, away from something that could throw an exception, as your intention is almost certainly functionally the same.

When finally is executed?

The finally block always executes when the try block exits (click).

when does the finally block not execute while try or catch block is interrupted

Good answers can be found here.

Besides a System.exit(), the finally block will not run if the JVM crashes for some reason (e.g. infinite loop in your try block).

As far as the thread itself, only if it is stopped using the stop() method (or suspend() without resume()) will the finally block not be executed. A call to interrupt() will still result in the finally block being executed.

It's also worth noting that, since any return statement in the finally block will override any returns or exception throws in the try/catch blocks, program behavior can quickly become erratic and hard to debug if this happens to you. Not really anything worth taking precaution against (as it only happens in extremely rare cases), but worth being aware of so you can recognize it when it happens.

why is the statement in finally block executed despite using return in try block

Because that's what it's designed to do. It is not like catch (...) in C++ (which only executes if not caught elsewhere).

finally can be very useful in cleaning up resources: it is one way that Java introduces RAII-type features that are more prevalent in other languages like C++ (via destructors). Java more naturally performs clean-up via the garbage collector but finally permits an alternative approach.

Does the finally block execute if the thread running the function is interrupted?

According to the Java Tutorials, "if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues."

Here's the full passage:

The finally block always executes when the try block exits. This
ensures that the finally block is executed even if an unexpected
exception occurs. But finally is useful for more than just exception
handling — it allows the programmer to avoid having cleanup code
accidentally bypassed by a return, continue, or break. Putting cleanup
code in a finally block is always a good practice, even when no
exceptions are anticipated.

Note: If the JVM exits while the try or catch code is being executed, then
the finally block may not execute. Likewise, if the thread executing
the try or catch code is interrupted or killed, the finally block may
not execute even though the application as a whole continues.

class Thread1 implements Runnable {

    @Override
    public void run() {
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("finally executed");
        }
    }
}

...

t1.start();
t1.interrupt();

It prints - finally executed

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