Runtime

See also:

• getRuntime()

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 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.

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.

exec

public Process exec (String[] cmdarray)

Executes the specified command and arguments in a separate process.

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).

exec

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

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

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).

exec

public Process exec (String command)

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).

exec

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

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).

exec

public 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.

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.

A minimal set of system dependent environment variables may be required to start a process on some operating systems. As a result, the subprocess may inherit additional environment variable settings beyond those in the specified environment.

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.

If the operating system does not support the creation of processes, an UnsupportedOperationException will be thrown.

exec

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

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 {@link 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.

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.

All registered shutdown hooks, if any, are started in some unspecified order and allowed to run concurrently until they finish. Once this is done the virtual machine halts.

If this method is invoked after all shutdown hooks have already been run and the status is nonzero then this method halts the virtual machine with the given status code. Otherwise, this method blocks indefinitely.

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

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.

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.

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.

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. If the shutdown sequence has already been initiated then this method does not wait for any running shutdown hooks to finish their work.

load

public void load (String filename)

Loads the native library specified by the filename argument. The filename argument must be an absolute path name. (for example Runtime.getRuntime().load("/home/avh/lib/libX11.so");). If the filename argument, when stripped of any platform-specific library prefix, path, and file extension, indicates a library whose name is, for example, L, and a native library called L is statically linked with the VM, then the JNI_OnLoad_L function exported by the library is invoked rather than attempting to load a dynamic library. A filename matching the argument does not have to exist in the file system. See the JNI Specification for more details. Otherwise, the filename argument is mapped to a native library image in an implementation-dependent manner.

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(java.lang.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.

loadLibrary

public void loadLibrary (String libname)

Loads the native library specified by the libname argument. The libname argument must not contain any platform specific prefix, file extension or path. If a native library called libname is statically linked with the VM, then the JNI_OnLoad_libname function exported by the library is invoked. See the JNI Specification for more details. Otherwise, the libname argument is loaded from a system library location and mapped to a native library image in an implementation- dependent 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"); }
 

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

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.

removeShutdownHook

public boolean removeShutdownHook (Thread hook)

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

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.

runFinalizersOnExit

public static void runFinalizersOnExit (boolean value)

This method was deprecated in API level 15.
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.

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.

traceInstructions

public void traceInstructions (boolean on)

This method was deprecated in API level 34.
This method was intended to control instruction tracing. It has been superseded by ART-specific tracing mechanisms.

Not implemented, does nothing.

traceMethodCalls

public void traceMethodCalls (boolean on)

This method was deprecated in API level 34.
This method was intended to control method call tracing. It has been superseded by ART-specific tracing mechanisms.

Not implemented, does nothing.

Calling this method on Android Lollipop or later (API level >= 21) with true argument will cause it to throw an UnsupportedOperationException.