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## 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.
`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 system.
`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.
`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 constists 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)`, `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 Win32. 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)`

### 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)`.
  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
  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.

  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
  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.

  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
  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.

  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
  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.

  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.
    `NullPointerException` - if `cmdarray` is `null`.
    `IndexOutOfBoundsException` - if `cmdarray` is an empty array (has length `0`).
    `IOException` - if an I/O error occurs
  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.

  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.
  See Also:
    `Process`, `SecurityException`, `SecurityManager.checkExec(java.lang.String)`

### freeMemory

    public long freeMemory()

  Returns the amount of free memory in the system. 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.

### 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 Java Virtual Machine has made its best effort to recycle all discarded objects.

  The name `gc` stands for "garbage collector". The Java 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 hte 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 Java Virtual Machine has made a best effort to complete all outstanding finalizations.

  The Java 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 Java 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 Java 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 Java 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 Java 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 pathname. 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 rathan 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)`

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