java.lang
Class Runtime

java.lang.Object   
  java.lang.Runtime

public class Runtime
extends Object   

Every Java application has a single instance of class Runtime that allows the application to interface with the environment in which the application is running. The current runtime can be obtained from the getRuntime method.

An application cannot create its own instance of this class.

Since:

JDK1.0

See Also:

getRuntime()

Method Summary

void	addShutdownHook (Thread hook) Registers a new virtual-machine shutdown hook.
int	availableProcessors () Returns the number of processors available to the Java virtual machine.
Process	exec (String command) Executes the specified string command in a separate process.
Process	exec (String [] cmdarray) Executes the specified command and arguments in a separate process.
Process	exec (String [] cmdarray, String [] envp) Executes the specified command and arguments in a separate process with the specified environment.
Process	exec (String [] cmdarray, String [] envp, File dir) Executes the specified command and arguments in a separate process with the specified environment and working directory.
Process	exec (String command, String [] envp) Executes the specified string command in a separate process with the specified environment.
Process	exec (String command, String [] envp, File dir) Executes the specified string command in a separate process with the specified environment and working directory.
void	exit (int status) Terminates the currently running Java virtual machine by initiating its shutdown sequence.
long	freeMemory () Returns the amount of free memory in the Java Virtual Machine.
void	gc () Runs the garbage collector.
InputStream	getLocalizedInputStream (InputStream in) 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.
OutputStream	getLocalizedOutputStream (OutputStream out) 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.
static Runtime	getRuntime () Returns the runtime object associated with the current Java application.
void	halt (int status) Forcibly terminates the currently running Java virtual machine.
void	load (String filename) Loads the specified filename as a dynamic library.
void	loadLibrary (String libname) Loads the dynamic library with the specified library name.
long	maxMemory () Returns the maximum amount of memory that the Java virtual machine will attempt to use.
boolean	removeShutdownHook (Thread hook) De-registers a previously-registered virtual-machine shutdown hook.
void	runFinalization () Runs the finalization methods of any objects pending finalization.
static void	runFinalizersOnExit (boolean value) 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.
long	totalMemory () Returns the total amount of memory in the Java virtual machine.
void	traceInstructions (boolean on) Enables/Disables tracing of instructions.
void	traceMethodCalls (boolean on) Enables/Disables tracing of method calls.

Methods inherited from class java.lang.Object

clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait

Method Detail

getRuntime

public static Runtime    getRuntime()

Returns the runtime object associated with the current Java application. Most of the methods of class Runtime are instance methods and must be invoked with respect to the current runtime object.

Returns:

the Runtime object associated with the current Java application.

exit

public void exit(int status)

Terminates the currently running Java virtual machine by initiating its shutdown sequence. This method never returns normally. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.

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.

Parameters:

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:

SecurityException , SecurityManager.checkExit(int) , addShutdownHook(java.lang.Thread) , removeShutdownHook(java.lang.Thread) , runFinalizersOnExit(boolean) , halt(int)

addShutdownHook

public void addShutdownHook(Thread    hook)

Registers a new virtual-machine shutdown hook.

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.

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.

Parameters:

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

IllegalStateException - If the virtual machine is already in the process of shutting down

SecurityException - If a security manager is present and it denies RuntimePermission ("shutdownHooks")

Since:

1.3

See Also:

removeShutdownHook(java.lang.Thread) , halt(int) , exit(int)

removeShutdownHook

public boolean removeShutdownHook(Thread    hook)

De-registers a previously-registered virtual-machine shutdown hook.

Parameters:

hook - the hook to remove

Returns:

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

SecurityException - If a security manager is present and it denies RuntimePermission ("shutdownHooks")

Since:

1.3

See Also:

addShutdownHook(java.lang.Thread) , exit(int)

halt

public void halt(int status)

Forcibly terminates the currently running Java virtual machine. This method never returns normally.

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.

Parameters:

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(int) , addShutdownHook(java.lang.Thread) , removeShutdownHook(java.lang.Thread)

runFinalizersOnExit

@Deprecated
public static void runFinalizersOnExit(boolean value)

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.

Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

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.

Parameters:

value - true to enable finalization on exit, false to disable

Throws:

SecurityException - if a security manager exists and its checkExit method doesn't allow the exit.

Since:

JDK1.1

See Also:

exit(int) , gc() , SecurityManager.checkExit(int)

exec

public Process    exec(String    command)
             throws IOException   

Executes the specified string command in a separate process.

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).

Parameters:

command - a specified system command.

Returns:

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

IOException - If an I/O error occurs

NullPointerException - If command is null

IllegalArgumentException - If command is empty

See Also:

exec(String[], String[], File) , ProcessBuilder

exec

public Process    exec(String    command,
                    String   [] envp)
             throws IOException   

Executes the specified string command in a separate process with the specified environment.

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).

Parameters:

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.

Returns:

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

IOException - If an I/O error occurs

NullPointerException - If command is null, or one of the elements of envp is null

IllegalArgumentException - If command is empty

See Also:

exec(String[], String[], File) , ProcessBuilder

exec

public Process    exec(String    command,
                    String   [] envp,
                    File    dir)
             throws IOException   

Executes the specified string command in a separate process with the specified environment and working directory.

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.

Parameters:

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.

Returns:

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

IOException - If an I/O error occurs

NullPointerException - If command is null, or one of the elements of envp is null

IllegalArgumentException - If command is empty

Since:

1.3

See Also:

ProcessBuilder

exec

public Process    exec(String   [] cmdarray)
             throws IOException   

Executes the specified command and arguments in a separate process.

This is a convenience m