1. Overview
In this tutorial, we’ll look into System.exit(), Runtime.getRuntime().halt(), and how these two methods compare with each other.
2. System.exit()
The System.exit() method stops the running Java Virtual Machine. But, before stopping the JVM, it calls the shutdown sequence, also known as an orderly shutdown. Please refer to this article to learn more about adding shutdown hooks.
The shutdown sequence of JVM first invokes all registered shutdown hooks and waits for them to complete. Then, it runs all uninvoked finalizers if finalization-on-exit is enabled. Finally, it halts the JVM.
This method, in fact, calls the Runtime.getRuntime().exit() method internally. It takes an integer status code as an argument and has a void return type:
public static void exit(int status)
If the status code is nonzero, it indicates that the program stopped abnormally.
3. Runtime.getRuntime().halt()
The Runtime class allows an application to interact with the environment in which the application is running.
It has a halt method that can be used to forcibly terminate the running JVM.
Unlike the exit method, this method does not trigger the JVM shutdown sequence. Therefore, neither the shutdown hooks or the finalizers are executed when we call the halt method.
This method is non-static and has a similar signature to System.exit():
public void halt(int status)
Similar to exit, the non-zero status code in this method also indicates abnormal termination of the program.
4. Example
Now, let’s see an example of exit and halt methods, with the help of a shutdown hook.
To keep it simple, we’ll create a Java class and register a shutdown hook in a static block. Also, we’ll create two methods; the first calls the exit method and the second calls the halt method:
public class JvmExitAndHaltDemo {
private static Logger LOGGER = LoggerFactory.getLogger(JvmExitAndHaltDemo.class);
static {
Runtime.getRuntime()
.addShutdownHook(new Thread(() -> {
LOGGER.info("Shutdown hook initiated.");
}));
}
public void processAndExit() {
process();
LOGGER.info("Calling System.exit().");
System.exit(0);
}
public void processAndHalt() {
process();
LOGGER.info("Calling Runtime.getRuntime().halt().");
Runtime.getRuntime().halt(0);
}
private void process() {
LOGGER.info("Process started.");
}
}
So, to test the exit method first, let’s create a test case:
@Test
public void givenProcessComplete_whenExitCalled_thenTriggerShutdownHook() {
jvmExitAndHaltDemo.processAndExit();
}
Let’s now run the test case and see that the shutdown hook is called:
12:48:43.156 [main] INFO com.baeldung.exitvshalt.JvmExitAndHaltDemo - Process started.
12:48:43.159 [main] INFO com.baeldung.exitvshalt.JvmExitAndHaltDemo - Calling System.exit().
12:48:43.160 [Thread-0] INFO com.baeldung.exitvshalt.JvmExitAndHaltDemo - Shutdown hook initiated.
Similarly, we’ll create a test case for the halt method:
@Test
public void givenProcessComplete_whenHaltCalled_thenDoNotTriggerShutdownHook() {
jvmExitAndHaltDemo.processAndHalt();
}
Now, we can run this test case also and see that the shutdown hook is not called:
12:49:16.839 [main] INFO com.baeldung.exitvshalt.JvmExitAndHaltDemo - Process started.
12:49:16.842 [main] INFO com.baeldung.exitvshalt.JvmExitAndHaltDemo - Calling Runtime.getRuntime().halt().
5. When to Use exit and halt
As we’ve seen earlier, the System.exit() method triggers the shutdown sequence of JVM, whereas the Runtime.getRuntime().halt() terminates the JVM abruptly.
We can also do this by using operating system commands. For example, we can use SIGINT or Ctrl+C to trigger the orderly shutdown like System.exit() and SIGKILL to kill the JVM process abruptly.
Therefore, we rarely need to use these methods. Having said that, we may need to use the exit method when we need the JVM to run the registered shutdown hooks or return a specific status code to the caller, like with a shell script.
However, it is important to note that the shutdown hook may cause a deadlock, if not designed properly. Consequently, the exit method can get blocked as it waits until the registered shutdown hooks finish. So, a possible way to take care of this is to use the halt method to force JVM to halt, in case exit blocks.
Finally, an application can also restrict these methods from accidental use. Both these methods call the checkExit method of the SecurityManager class. So, to disallow the exit and halt operations, an application can create a security policy using the SecurityManager class and throw the SecurityException from the checkExit method.
6. Conclusion
In this tutorial, we’ve looked into the System.exit() and Runtime.getRuntime().halt() methods with the help of an example. Moreover, we’ve also talked about the usage and best practices of these methods.
As usual, the complete source code of this article is available over on Github.