The Java virtual machine shuts down in response to two kinds of events:

• The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or

• The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.

A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.

Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.

Once the shutdown sequence has begun it is impossible to register a new shutdown hook or de-register a previously-registered hook. Attempting either of these operations will cause an IllegalStateException to be thrown.

Shutdown hooks run at a delicate time in the life cycle of a virtual machine and should therefore be coded defensively. They should, in particular, be written to be thread-safe and to avoid deadlocks insofar as possible. They should also not rely blindly upon services that may have registered their own shutdown hooks and therefore may themselves in the process of shutting down. Attempts to use other thread-based services such as the AWT event-dispatch thread, for example, may lead to deadlocks.

Shutdown hooks should also finish their work quickly. When a program invokes exit the expectation is that the virtual machine will promptly shut down and exit. When the virtual machine is terminated due to user logoff or system shutdown the underlying operating system may only allow a fixed amount of time in which to shut down and exit. It is therefore inadvisable to attempt any user interaction or to perform a long-running computation in a shutdown hook.

Uncaught exceptions are handled in shutdown hooks just as in any other thread, by invoking the uncaughtException method of the thread's ThreadGroup object. The default implementation of this method prints the exception's stack trace to System#err and terminates the thread; it does not cause the virtual machine to exit or halt.

In rare circumstances the virtual machine may abort, that is, stop running without shutting down cleanly. This occurs when the virtual machine is terminated externally, for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows. The virtual machine may also abort if a native method goes awry by, for example, corrupting internal data structures or attempting to access nonexistent memory. If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run.

hook	An initialized but unstarted Thread object
Throws	IllegalArgumentException: If the specified hook has already been registered, or if it can be determined that the hook is already running or has already been run
Throws	IllegalStateException: If the virtual machine is already in the process of shutting down
Throws	SecurityException: If a security manager is present and it denies RuntimePermission("shutdownHooks")
since	1.3
See also	removeShutdownHook, halt(int), exit(int)

This value may change during a particular invocation of the virtual machine. Applications that are sensitive to the number of available processors should therefore occasionally poll this property and adjust their resource usage appropriately.

This is a convenience method. An invocation of the form exec(command) behaves in exactly the same way as the invocation exec(command, null, null).

command	a specified system command.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If command is null
Throws	IllegalArgumentException: If command is empty
See also	exec(String[], String[], File), java.lang.ProcessBuilder

This is a convenience method. An invocation of the form exec(command, envp) behaves in exactly the same way as the invocation exec(command, envp, null).

command	a specified system command.
envp	array of strings, each element of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If command is null, or one of the elements of envp is null
Throws	IllegalArgumentException: If command is empty
See also	exec(String[], String[], File), java.lang.ProcessBuilder

This is a convenience method. An invocation of the form exec(command, envp, dir) behaves in exactly the same way as the invocation exec(cmdarray, envp, dir), where cmdarray is an array of all the tokens in command.

More precisely, the command string is broken into tokens using a StringTokenizer created by the call new StringTokenizer(command) with no further modification of the character categories. The tokens produced by the tokenizer are then placed in the new string array cmdarray, in the same order.

command	a specified system command.
envp	array of strings, each element of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process.
dir	the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If command is null, or one of the elements of envp is null
Throws	IllegalArgumentException: If command is empty
since	1.3
See also	java.lang.ProcessBuilder

This is a convenience method. An invocation of the form exec(cmdarray) behaves in exactly the same way as the invocation exec(cmdarray, null, null).

cmdarray	array containing the command to call and its arguments.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If cmdarray is null, or one of the elements of cmdarray is null
Throws	IndexOutOfBoundsException: If cmdarray is an empty array (has length 0)
See also	java.lang.ProcessBuilder

This is a convenience method. An invocation of the form exec(cmdarray, envp) behaves in exactly the same way as the invocation exec(cmdarray, envp, null).

cmdarray	array containing the command to call and its arguments.
envp	array of strings, each element of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If cmdarray is null, or one of the elements of cmdarray is null, or one of the elements of envp is null
Throws	IndexOutOfBoundsException: If cmdarray is an empty array (has length 0)
See also	java.lang.ProcessBuilder

Given an array of strings cmdarray, representing the tokens of a command line, and an array of strings envp, representing "environment" variable settings, this method creates a new process in which to execute the specified command.

This method checks that cmdarray is a valid operating system command. Which commands are valid is system-dependent, but at the very least the command must be a non-empty list of non-null strings.

If envp is null, the subprocess inherits the environment settings of the current process.

ProcessBuilder#start() is now the preferred way to start a process with a modified environment.

The working directory of the new subprocess is specified by dir. If dir is null, the subprocess inherits the current working directory of the current process.

If a security manager exists, its checkExec method is invoked with the first component of the array cmdarray as its argument. This may result in a SecurityException being thrown.

Starting an operating system process is highly system-dependent. Among the many things that can go wrong are:

• The operating system program file was not found.
• Access to the program file was denied.
• The working directory does not exist.

In such cases an exception will be thrown. The exact nature of the exception is system-dependent, but it will always be a subclass of IOException.

cmdarray	array containing the command to call and its arguments.
envp	array of strings, each element of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process.
dir	the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process.
return	A new Process object for managing the subprocess
Throws	SecurityException: If a security manager exists and its checkExec method doesn't allow creation of the subprocess
Throws	IOException: If an I/O error occurs
Throws	NullPointerException: If cmdarray is null, or one of the elements of cmdarray is null, or one of the elements of envp is null
Throws	IndexOutOfBoundsException: If cmdarray is an empty array (has length 0)
since	1.3
See also	java.lang.ProcessBuilder

The virtual machine's shutdown sequence consists of two phases. In the first phase all registered shutdown hooks, if any, are started in some unspecified order and allowed to run concurrently until they finish. In the second phase all uninvoked finalizers are run if finalization-on-exit has been enabled. Once this is done the virtual machine halts.

If this method is invoked after the virtual machine has begun its shutdown sequence then if shutdown hooks are being run this method will block indefinitely. If shutdown hooks have already been run and on-exit finalization has been enabled then this method halts the virtual machine with the given status code if the status is nonzero; otherwise, it blocks indefinitely.

The System.exit method is the conventional and convenient means of invoking this method.

status	Termination status. By convention, a nonzero status code indicates abnormal termination.
Throws	SecurityException: If a security manager is present and its checkExit method does not permit exiting with the specified status
See also	java.lang.SecurityException, checkExit(int), addShutdownHook, removeShutdownHook, runFinalizersOnExit, halt(int)

The name gc stands for "garbage collector". The virtual machine performs this recycling process automatically as needed, in a separate thread, even if the gc method is not invoked explicitly.

The method System#gc() is the conventional and convenient means of invoking this method.

This method should be used with extreme caution. Unlike the exit method, this method does not cause shutdown hooks to be started and does not run uninvoked finalizers if finalization-on-exit has been enabled. If the shutdown sequence has already been initiated then this method does not wait for any running shutdown hooks or finalizers to finish their work.

status	Termination status. By convention, a nonzero status code indicates abnormal termination. If the exit (equivalently, System.exit) method has already been invoked then this status code will override the status code passed to that method.
Throws	SecurityException: If a security manager is present and its checkExit method does not permit an exit with the specified status
since	1.3
See also	exit, addShutdownHook, removeShutdownHook

First, if there is a security manager, its checkLink method is called with the filename as its argument. This may result in a security exception.

This is similar to the method #loadLibrary(String), but it accepts a general file name as an argument rather than just a library name, allowing any file of native code to be loaded.

The method System#load(String) is the conventional and convenient means of invoking this method.

filename	the file to load.
Throws	SecurityException: if a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library
Throws	UnsatisfiedLinkError: if the file does not exist.
Throws	NullPointerException: if filename is null
See also	getRuntime(), java.lang.SecurityException, checkLink(java.lang.String)

First, if there is a security manager, its checkLink method is called with the libname as its argument. This may result in a security exception.

The method System#loadLibrary(String) is the conventional and convenient means of invoking this method. If native methods are to be used in the implementation of a class, a standard strategy is to put the native code in a library file (call it LibFile) and then to put a static initializer:

 static { System.loadLibrary("LibFile"); }
 

If this method is called more than once with the same library name, the second and subsequent calls are ignored.

libname	the name of the library.
Throws	SecurityException: if a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library
Throws	UnsatisfiedLinkError: if the library does not exist.
Throws	NullPointerException: if libname is null
See also	java.lang.SecurityException, checkLink(java.lang.String)

hook	the hook to remove
return	true if the specified hook had previously been registered and was successfully de-registered, false otherwise.
Throws	IllegalStateException: If the virtual machine is already in the process of shutting down
Throws	SecurityException: If a security manager is present and it denies RuntimePermission("shutdownHooks")
since	1.3
See also	addShutdownHook, exit(int)

The virtual machine performs the finalization process automatically as needed, in a separate thread, if the runFinalization method is not invoked explicitly.

The method System#runFinalization() is the conventional and convenient means of invoking this method.

See also

finalize()

If there is a security manager, its checkExit method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.

value	true to enable finalization on exit, false to disable
deprecated	This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.
Throws	SecurityException: if a security manager exists and its checkExit method doesn't allow the exit.
since	JDK1.1
See also	exit(int), gc(), checkExit(int)

Note that the amount of memory required to hold an object of any given type may be implementation-dependent.

return

the total amount of memory currently available for current and future objects, measured in bytes.

If the boolean argument is false, this method causes the virtual machine to stop performing the detailed instruction trace it is performing.

on	true to enable instruction tracing; false to disable this feature.

Calling this method with argument false suggests that the virtual machine cease emitting per-call debugging information.

on	true to enable instruction tracing; false to disable this feature.

If the argument is already a localized stream, it may be returned as the result.

in	InputStream to localize
return	a localized input stream
deprecated	As of JDK 1.1, the preferred way to translate a byte stream in the local encoding into a character stream in Unicode is via the InputStreamReader and BufferedReader classes.
See also	java.io.InputStream, BufferedReader(java.io.Reader), InputStreamReader(java.io.InputStream)

If the argument is already a localized stream, it may be returned as the result.

deprecated	As of JDK 1.1, the preferred way to translate a Unicode character stream into a byte stream in the local encoding is via the OutputStreamWriter, BufferedWriter, and PrintWriter classes.
out	OutputStream to localize
return	a localized output stream
See also	java.io.OutputStream, BufferedWriter(java.io.Writer), OutputStreamWriter(java.io.OutputStream), PrintWriter(java.io.OutputStream)