java中的System.exit（0）

Question

I am writing an application program using swing. I need to exit from the application by clicking the JButton for that can i use System.exit() or should i use some other methods, which is the best practice. If calling System.exit() is not best practice then tell the reason and tell the alternative way to exit from the application.

Answer 1

In general, calling System.exit(...) anywhere but the "main" method of an application can be problematic for (at least) the following reasons.

• It is an impediment to reusing your code.

• It makes unit testing hard. For example, if your code calls System.exit when your JUnit tests exercise some error handling, that it the end of your test sequence!

The specific case of calling System.exit(...) in the button listener for the "exit" button is not so bad. You are unlikely to want reuse the button listener somewhere that doesn't also require this behaviour. Also, you can probably figure out a work-around for the unit testing conundrum; eg don't unit test that particular method!

However, I think I'd still try to get the exit happen a different way. For example, have the main method launch everything and then block on a CountDownLatch, then have the button listener decrement the latch. When the main method unblocks, it executes the relevant shutdown code and returns or exits as appropriate.

Answer 2

Personnaly, I do consider as best practice to let application exit by itself : if you write a main class with a main(String[] args) , with empty code, running it will exit silently from Java program.

But, in most times, like most of developpers, I rely upon System.exit(/ an exit code /) , especially for Swing applications, where the Swing EDT will run endlessly, as justkt wrote.. The known drawxback of this method is that most of applciation code is not called, what I avoid by setting a shutdown hook by calling Runtime.addShutdownHook(Thread) , which will allow me to clean application (close threads, and so on).

Answer 3

If you need to set an exit code for your application, you have to use System.exit (I think).

But when you do not need a specific code (or 0 is fine), then I would much rather have the JVM terminate "naturally", which happens when there are no more (non-daemon) threads left.

Usually, you would just reach the end of your main method.

I would be careful with System.exit because there may be other threads that still need to do something, and exiting without shutting them down properly may cause damage. For example the embedded database might still need to flush its buffers.

If you know about these threads, you can usually arrange for them to end gracefully. If you do not know what threads are running in your program, you have bigger problems...

Answer 4

Generally speaking the only time you need to do a System.exit(n) is if you want to exit a command line program with an error code so that anything that is monitoring the program can detect the error code.

This is most useful for example, if you are writing a program designed to be run from a shell script.

As other replies say though, if you are using threads then you must make every effort to shut them down gracefully before considering a System.exit(n)

Answer 5

System.exit can be bad at least when you have multiple threads or resources that need to be closed properly (that live outside JVM). If your application isn't exiting when you think it should then you should find out what the problem is and not just call System.exit().

The only reason for calling System.exit() would be to give some none standard exit code.