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

Version:

1.57, 05/03/00

Author:

unascribed

See Also:

getRuntime()

Method Summary

Methods

Modifier and Type	Method and Description
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 cmd, 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.
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.
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.

If process model exists, Runtime.exit() must rely on the process exiting to release resources.

If process model does not exist, and a security manager exists, the security manager by default must prohibit calls to Runtime.exit() by throwing SecurityException. Otherwise, the system will hang indefinitely when called.

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), 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.

If process model exists, Runtime.halt() must rely on the process exiting to release resources.

If process model does not exist, and a security manager exists, the security manager by default must prohibit calls to Runtime.halt() by throwing SecurityException. Otherwise, the system will hang indefinitely when called.

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)

exec

public Process exec(String command)
             throws IOException

Executes the specified string command in a separate process.

The command argument is parsed into tokens and then executed as a command in a separate process. The token parsing is done by a StringTokenizer created by the call:

 new StringTokenizer(command)
 

with no further modifications of the character categories. This method has exactly the same effect as exec(command, null).

If process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

command - a specified system command.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

IOException - if an I/O error occurs

NullPointerException - if command is null

IllegalArgumentException - if command is empty

See Also:

exec(java.lang.String, java.lang.String[]), SecurityManager.checkExec(java.lang.String)

exec

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

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

This method breaks the command string into tokens and creates a new array cmdarray containing the tokens in the order that they were produced by the string tokenizer; it then performs the call exec(cmdarray, envp). The token parsing is done by a StringTokenizer created by the call:

 new StringTokenizer(command)
 

with no further modification of the character categories.

The environment variable settings are specified by envp. If envp is null, the subprocess inherits the environment settings of the current process.

If process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

cmd - a specified system command.

envp - array of strings, each element of which has environment variable settings in format name=value.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

IOException - if an I/O error occurs

NullPointerException - if cmd is null

IllegalArgumentException - if cmd is empty

See Also:

exec(java.lang.String[]), exec(java.lang.String[], java.lang.String[]), SecurityManager.checkExec(java.lang.String)

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 method breaks the command string into tokens and creates a new array cmdarray containing the tokens in the order that they were produced by the string tokenizer; it then performs the call exec(cmdarray, envp). The token parsing is done by a StringTokenizer created by the call:

 new StringTokenizer(command)
 

with no further modification of the character categories.

The environment variable settings are specified by envp. If envp is null, the subprocess inherits the environment settings of the current process.

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 process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

command - a specified system command.

envp - array of strings, each element of which has environment variable settings in format name=value.

dir - the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

IOException - if an I/O error occurs

NullPointerException - if command is null

IllegalArgumentException - if command is empty

Since:

1.3

See Also:

exec(java.lang.String[], java.lang.String[], File), SecurityManager.checkExec(java.lang.String)

exec

public Process exec(String[] cmdarray)
             throws IOException

Executes the specified command and arguments in a separate process.

The command specified by the tokens in cmdarray is executed as a command in a separate process. This has exactly the same effect as exec(cmdarray, null).

If there is a security manager, its checkExec method is called with the first component of the array cmdarray as its argument. This may result in a security exception.

If process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

cmdarray - array containing the command to call and its arguments.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

IOException - if an I/O error occurs

NullPointerException - if cmdarray is null

IndexOutOfBoundsException - if cmdarray is an empty array (has length 0).

See Also:

exec(java.lang.String[], java.lang.String[]), SecurityManager.checkExec(java.lang.String)

exec

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

Executes the specified command and arguments in a separate process with the specified environment.

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.

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

If process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

cmdarray - array containing the command to call and its arguments.

envp - array of strings, each element of which has environment variable settings in format name=value.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

IOException - if an I/O error occurs

NullPointerException - if cmdarray is null

IndexOutOfBoundsException - if cmdarray is an empty array (has length 0).

See Also:

Process, SecurityException, SecurityManager.checkExec(java.lang.String)

exec

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

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

If there is a security manager, its checkExec method is called with the first component of the array cmdarray as its argument. This may result in a security exception.

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.

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

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 process model exists, Runtime.exec() must behave as it does in the J2SE specification.

If process model does not exist, Runtime.exec() and all java.lang.Process methods must throw SecurityException.

Parameters:

cmdarray - array containing the command to call and its arguments.

envp - array of strings, each element of which has environment variable settings in format name=value.

dir - the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process.

Returns:

a Process object for managing the subprocess.

Throws:

SecurityException - if a security manager exists and its checkExec method doesn't allow creation of a subprocess.

NullPointerException - if cmdarray is null

IndexOutOfBoundsException - if cmdarray is an empty array (has length 0).

IOException - if an I/O error occurs.

Since:

1.3

See Also:

Process, SecurityException, SecurityManager.checkExec(java.lang.String)

availableProcessors

public int availableProcessors()

Returns the number of processors available to the Java virtual machine.

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.

Returns:

the maximum number of processors available to the virtual machine; never smaller than one

freeMemory

public long freeMemory()

Returns the amount of free memory in the Java Virtual Machine. Calling the gc method may result in increasing the value returned by freeMemory.

Returns:

an approximation to the total amount of memory currently available for future allocated objects, measured in bytes.

totalMemory

public long totalMemory()

Returns the total amount of memory in the Java virtual machine. The value returned by this method may vary over time, depending on the host environment.

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

Returns:

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

maxMemory

public long maxMemory()

Returns the maximum amount of memory that the Java virtual machine will attempt to use. If there is no inherent limit then the value Long.MAX_VALUE will be returned.

Returns:

the maximum amount of memory that the virtual machine will attempt to use, measured in bytes

gc

public void gc()

Runs the garbage collector. Calling this method suggests that the Java virtual machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the virtual machine has made its best effort to recycle all discarded objects.

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.

runFinalization

public void runFinalization()

Runs the finalization methods of any objects pending finalization. Calling this method suggests that the Java virtual machine expend effort toward running the finalize methods of objects that have been found to be discarded but whose finalize methods have not yet been run. When control returns from the method call, the virtual machine has made a best effort to complete all outstanding finalizations.

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:

Object.finalize()

traceInstructions

public void traceInstructions(boolean on)

Enables/Disables tracing of instructions. If the boolean argument is true, this method suggests that the Java virtual machine emit debugging information for each instruction in the virtual machine as it is executed. The format of this information, and the file or other output stream to which it is emitted, depends on the host environment. The virtual machine may ignore this request if it does not support this feature. The destination of the trace output is system dependent.

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

Parameters:

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

traceMethodCalls

public void traceMethodCalls(boolean on)

Enables/Disables tracing of method calls. If the boolean argument is true, this method suggests that the Java virtual machine emit debugging information for each method in the virtual machine as it is called. The format of this information, and the file or other output stream to which it is emitted, depends on the host environment. The virtual machine may ignore this request if it does not support this feature.

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

Parameters:

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

load

public void load(String filename)

Loads the specified filename as a dynamic library. The filename argument must be a complete path name. From java_g it will automagically insert "_g" before the ".so" (for example Runtime.getRuntime().load("/home/avh/lib/libX11.so");).

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.

Parameters:

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

UnsatisfiedLinkError - if the file does not exist.

See Also:

getRuntime(), SecurityException, SecurityManager.checkLink(java.lang.String)

loadLibrary

public void loadLibrary(String libname)

Loads the dynamic library with the specified library name. A file containing native code is loaded from the local file system from a place where library files are conventionally obtained. The details of this process are implementation-dependent. The mapping from a library name to a specific filename is done in a system-specific manner.

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"); }
 

within the class declaration. When the class is loaded and initialized, the necessary native code implementation for the native methods will then be loaded as well.

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

Parameters:

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

UnsatisfiedLinkError - if the library does not exist.

See Also:

SecurityException, SecurityManager.checkLink(java.lang.String)