问题
Is there any condition where finally might not run in java? Thanks.
回答1:
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...
回答2:
System.exit shuts down the Virtual Machine.
Terminates the currently running Java Virtual Machine. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.
This method calls the
exitmethod in classRuntime. This method never returns normally.
try {
System.out.println("hello");
System.exit(0);
}
finally {
System.out.println("bye");
} // try-finally
"bye" does not print out in above code.
回答3:
Just to expand on what others have said, anything that does not cause something like the JVM exiting will incur the finally block. So the following method:
public static int Stupid() {
try {
return 0;
}
finally {
return 1;
}
}
will strangely both compile and return 1.
回答4:
Related to System.exit, there are also certain types of catastrophic failure where a finally block may not execute. If the JVM runs out of memory entirely, it may just exit without catch or finally happening.
Specifically, I remember a project where we foolishly tried to use
catch (OutOfMemoryError oome) {
// do stuff
}
This didn't work because the JVM had no memory left for executing the catch block.
回答5:
try { for (;;); } finally { System.err.println("?"); }
In that case the finally will not execute (unless the deprecated Thread.stop is called, or an equivalent, say, through a tools interface).
回答6:
The Sun tutorial has been wrongly quoted here in this thread.
Note: If the JVM exits while the try or catch code is being executed, then the finally block will not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block will not execute even though the application as a whole continues.
If you look into sun tutorial closely for finally block, it doesn't say "will not execute" but "may not execute" Here is the correct description
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.
The apparent reason for this behavior is, call to system.exit() is processed in a runtime system thread which may take time to shutdown the jvm, meanwhile thread scheduler can ask finally to execute. So finally is designed to always execute, but if you are shutting down jvm, it may happen that jvm shuts down prior to finally getting being executed.
回答7:
Also if a deadlock/livelock happens inside try block.
Here's the code that demonstrates it:
public class DeadLocker {
private static class SampleRunnable implements Runnable {
private String threadId;
private Object lock1;
private Object lock2;
public SampleRunnable(String threadId, Object lock1, Object lock2) {
super();
this.threadId = threadId;
this.lock1 = lock1;
this.lock2 = lock2;
}
@Override
public void run() {
try {
synchronized (lock1) {
System.out.println(threadId + " inside lock1");
Thread.sleep(1000);
synchronized (lock2) {
System.out.println(threadId + " inside lock2");
}
}
} catch (Exception e) {
} finally {
System.out.println("finally");
}
}
}
public static void main(String[] args) throws Exception {
Object ob1 = new Object();
Object ob2 = new Object();
Thread t1 = new Thread(new SampleRunnable("t1", ob1, ob2));
Thread t2 = new Thread(new SampleRunnable("t2", ob2, ob1));
t1.start();
t2.start();
}
}
This code produces the following output:
t1 inside lock1
t2 inside lock1
and "finally" never gets printed
回答8:
Here are some conditions which can bypass a finally block:
- If the JVM exits while the try or catch code is being executed, then the finally block may not execute.
- Normal Shutdown - this occurs either when the last non-daemon thread exits OR when Runtime.exit()
- When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads, it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned finally blocks are not executed, stacks are not unwound the JVM just exits. Daemon threads should be used sparingly few processing activities can be safely abandoned at any time with no cleanup. In particular, it is dangerous to use daemon threads for tasks that might perform any sort of I/O. Daemon threads are best saved for "housekeeping" tasks, such as a background thread that periodically removes expired entries from an in-memory cache.
Last non-daemon thread exits example:
public class TestDaemon {
private static Runnable runnable = new Runnable() {
@Override
public void run() {
try {
while (true) {
System.out.println("Is alive");
Thread.sleep(10);
// throw new RuntimeException();
}
} catch (Throwable t) {
t.printStackTrace();
} finally {
System.out.println("This will never be executed.");
}
}
};
public static void main(String[] args) throws InterruptedException {
Thread daemon = new Thread(runnable);
daemon.setDaemon(true);
daemon.start();
Thread.sleep(100);
// daemon.stop();
System.out.println("Last non-daemon thread exits.");
}
}
Output:
Is alive
Is alive
Is alive
Is alive
Is alive
Is alive
Is alive
Is alive
Is alive
Is alive
Last non-daemon thread exits.
Is alive
Is alive
Is alive
Is alive
Is alive
回答9:
In following cases, finally block will not be executed :-
- When
System.exit(0)is invoked fromtryblock. - When JVM runs out of memory
- When your java process is killed forcefully from task mgr or console
- Deadlock condition in your
tryblock - When your machine shuts down due power failure
There may also be other fringe cases, where finally block will not be executed.
回答10:
There are two ways to stop finally block code execution:
1. Use System.exit();
2. If somehow execution control don't reach to try block.
See:
public class Main
{
public static void main (String[]args)
{
if(true){
System.out.println("will exceute");
}else{
try{
System.out.println("result = "+5/0);
}catch(ArithmeticException e){
System.out.println("will not exceute");
}finally{
System.out.println("will not exceute");
}
}
}
}
回答11:
I've come across a very specific case of the finally block not executing related specifically to the play framework.
I was surprised to find that the finally block in this controller action code only got called after an Exception, but never when the call actually succeeded.
try {
InputStream is = getInputStreamMethod();
renderBinary(is, "out.zip");
catch (Exception e) {
e.printStackTrace();
} finally {
cleanUp();
}
Perhaps the thread is terminated or something when renderBinary() is called. I would suspect the same thing happens for other render() calls, but I didn't verify it.
I solved the problem by moving the renderBinary() to after the try/catch. Further investigation revealed that play provides an @Finally annotation to create a method that gets executed after a controller action executes. The caveat here is that this will get called after the execution of ANY action in the controller, so it may not always be a good choice.
回答12:
//If ArithmeticException Occur Inner finally would not be executed
class Temp
{
public static void main(String[] s)
{
try
{
int x = 10/s.length;
System.out.println(x);
try
{
int z[] = new int[s.length];
z[10] = 1000;
}catch(ArrayIndexOutOfBoundsException e)
{
System.out.println(e);
}
finally
{
System.out.println("Inner finally");
}
}
catch(ArithmeticException e)
{
System.out.println(e);
}
finally
{
System.out.println("Outer Finally");
}
System.out.println("Remaining Code");
}
}
来源：https://stackoverflow.com/questions/464098/does-a-finally-block-always-run