[Chapter 10] Runtime

#

| Chapter 10 The java.lang Package |

# Runtime

## Name

Runtime

## Synopsis

Class Name:

  `java.lang.Runtime`

Superclass:

  `java.lang.Object`

Immediate Subclasses:

  None

Interfaces Implemented:

  None

Availability:

  JDK 1.0 or later

## Description

The `Runtime` class provides access to various information about the environment in which a program is running. The Java run-time environment creates a single instance of this class that is associated with a program. The `Runtime` class does not have any public constructors, so a program cannot create its own instances of the class. A program must call the `getRuntime()` method to get a reference to the current `Runtime` object.

Information about operating system features is accessible through the `System` class.

## Class Summary

    public class java.lang.Runtime extends java.lang.Object {
        // Class Methods
        public static Runtime getRuntime();
        public static void runFinalizersOnExit(boolean value);  // New in 1.1
        // Instance Methods
        public Process exec(String command);
        public Process exec(String command, String envp[]);
        public Process exec(String cmdarray[]);
        public Process exec(String cmdarray[], String envp[]);
        public void exit(int status);
        public native long freeMemory();
        public native void gc();
        public InputStream
               getLocalizedInputStream(InputStream in);      // Deprecated in 1.1
        public OutputStream
               getLocalizedOutputStream(OutputStream out);   // Deprecated in 1.1
        public synchronized void load(String filename);
        public synchronized void loadLibrary(String libname);
        public native void runFinalization();
        public native long totalMemory();
        public native void traceInstructions(boolean on);
        public native void traceMethodCalls(boolean on);
    }

## Class Methods

### getRuntime

#### `public static Runtime getRuntime()`

Returns

  A reference to the current `Runtime` object.

Description

  This method returns a reference to the current `Runtime` object. Because the other methods of the `Runtime` class are not `static`, a program must call this method first in order to get a reference to a `Runtime` object that can be used in calling the other methods.

### runFinalizersOnExit

#### `public static void runFinalizersOnExit(boolean value)`

Availability

  New as of JDK 1.1

Parameters

  `value`

    A `boolean` value that specifies whether or not finalization occurs on exit.

Throws

  `SecurityException`

    If the `checkExit()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method specifies whether or not the `finalize()` methods of all objects that have `finalize()` methods are run before the Java virtual machine exits. By default, the finalizers are not run on exit.

## Instance Methods

### exec

#### `public Process exec(String command) throws IOException`

Parameters

  `command`

    A string that contains the name of an external command and any arguments to be passed to it.

Returns

  A `Process` object that controls the process started by this method.

Throws

  `IOException`

    If there is a problem finding or accessing the specified external command.

  `SecurityException`

    If the `checkExec()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method starts a new process to execute the given external command. The standard input, standard output, and standard error streams from the process are redirected to `OutputStream` and `InputStream` objects that are accessible through the `Process` object returned by this method.

  Calling this method is equivalent to:

      exec(command, null)

####

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

Parameters

  `command`

    A string that contains the name of an external command and any arguments to be passed to it.

  `envp`

    An array of strings that specifies the values for the environment variables of the new process. Each `String` in the array should be of the form variableName =value. If `envp` is `null`, the values of the environment variables in the current process are copied to the new process.

Returns

  A `Process` object that controls the process started by this method.

Throws

  `IOException`

    If there is a problem finding or accessing the specified external command.

  `SecurityException`

    If the `checkExec()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method starts a new process to execute the given external command. The standard input, standard output, and standard error streams from the process are redirected to `OutputStream` and `InputStream` objects that are accessible through the `Process` object returned by this method.

  The method parses the `command` string into words that are separated by whitespace. It creates a `String` object for each word and places word `String` objects into an array. If that array is called `commandArray`, calling this method is equivalent to:

      exec(commmandArray, envp)

####

     public Process exec(String[] commandArray) throws IOException

Parameters

  `commandArray`

    An array of strings that contains separate strings for the name of an external command and any arguments to be passed to it. The first string in the array must be the command name.

Returns

  A `Process` object that controls the process started by this method.

Throws

  `IOException`

    If there is a problem finding or accessing the specified external command.

  `SecurityException`

    If the `checkExec()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method starts a new process to execute the given external command. The standard input, standard output, and standard error streams from the process are redirected to `OutputStream` and `InputStream` objects that are accessible through the `Process` object returned by this method.

  Calling this method is equivalent to:

      exec(commandArray, null)

####

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

Parameters

  `commandArray`

    An array of strings that contains separate strings for the name of an external command and any arguments to be passed to it. The first string in the array must be the command name.

  `envp`

    An array of strings that specifies the values for the environment variables of the new process. Each `String` in the array should be of the form variableName =value. If `envp` is `null`, the values of the environment variables in the current process are copied to the new process.

Returns

  A `Process` object that controls the process started by this method.

Throws

  `IOException`

    If there is a problem finding or accessing the specified external command.

  `SecurityException`

    If the `checkExec()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method starts a new process to execute the given external command. The standard input, standard output, and standard error streams from the process are redirected to `OutputStream` and `InputStream` objects that are accessible through the `Process` object returned by this method.

### exit

#### `public void exit(int status)`

Parameters

  `status`

    The exit status code to use.

Throws

  `SecurityException`

    If the `checkExit()` method of the `SecurityManager` throws a `SecurityException`.

Description

  This method causes the Java virtual machine to exit with the given status code. By convention, a nonzero status code indicates abnormal termination. This method never returns.

### freeMemory

#### `public native long freeMemory()`

Returns

  An estimate of the number of free bytes in system memory.

Description

  This method returns an estimate of the number of free bytes in system memory. The value returned by this method is always less than the value returned by `totalMemory()`. Additional memory may be freed by calling the `gc()` method.

### gc

#### `public native void gc()`

Description

  This method causes the Java virtual machine to run the garbage collector in the current thread.

  The garbage collector finds objects that will never be used again because there are no live references to them. After it finds these objects, the garbage collector frees the storage occupied by these objects.

  The garbage collector is normally run continuously in a thread with the lowest possible priority, so that it works intermittently to reclaim storage. The `gc()` method allows a program to invoke the garbage collector explicitly when necessary.

### getLocalizedInputStream

#### `public InputStream getLocalizedInputStream(InputStream in)`

Availability

  Deprecated as of JDK 1.1

Parameters

  `in`

    An `InputStream` object that is to be localized.

Returns

  The localized `InputStream`.

Description

  This method returns an `InputStream` object that converts characters from the local character set to Unicode. For example, if the `InputStream` uses an 8-bit character set with values less than 128 representing Cyrillic letters, this method maps those characters to the corresponding Unicode characters in the range `'\u0400'` to `'\u04FF'`.

  This method is deprecated as of JDK 1.1. You should instead use the new `InputStreamReader` and `BufferedReader` classes to convert characters from the local character set to Unicode.

### getLocalizedOutputStream

#### `public OutputStream getLocalizedOutputStream(OutputStream out)`

Availability

  Deprecated as of JDK 1.1

Parameters

  `out`

    An `OutputStream` object that is to be localized.

Returns

  The localized `OutputStream`.

Description

  This method returns an `OutputStream` object that converts characters from Unicode to the local character set. For example, if the local character set is an 8-bit character set with values less than 128 representing Cyrillic letters, this method maps Unicode characters in the range `'\u0400'` to `'\u04FF'` to the appropriate characters in the local character set.

  This method is deprecated as of JDK 1.1. You should instead use the new `OutputStreamWriter` and `BufferedWriter` classes to convert characters from Unicode to the local character set.

### load

#### `public synchronized void load(String filename)`

Parameters

  `filename`

    A string that specifies the complete path of the file to be loaded.

Throws

  `SecurityException`

    If the `checkLink()` method of the `SecurityManager` throws a `SecurityException`.

  `UnsatisfiedLinkError`

    If the method is unsuccessful in loading the specified dynamically linked library.

Description

  This method loads the specified dynamically linked library.

  It is often more convenient to call the `load()` method of the `System` class because it does not require getting a `Runtime` object.

### loadLibrary

#### `public synchronized void loadLibrary(String libname)`

Parameters

  `libname`

    A string that specifies the name of a dynamically linked library.

Throws

  `SecurityException`

    If the `checkLink()` method of the `SecurityManager` throws a `SecurityException`.

  `UnsatisfiedLinkError`

    If the method is unsuccessful in loading the specified dynamically linked library.

Description

  This method loads the specified dynamically linked library. It looks for the specified library in a platform-specific way.

  It is often more convenient to call the `loadLibrary()` method of the `System` class because it does not require getting a `Runtime` object.

### runFinalization

#### `public native void runFinalization()`

Description

  This method causes the Java virtual machine to run the `finalize()` methods of any objects in the finalization queue in the current thread.

  When the garbage collector discovers that there are no references to an object, it checks to see if the object has a `finalize()` method that has never been called. If the object has such a `finalize()` method, the object is placed in the finalization queue. While there is a reference to the object in the finalization queue, the object is no longer considered garbage-collectable.

  Normally, the objects in the finalization queue are handled by a separate finalization thread that runs continuously at a very low priority. The finalization thread removes an object from the queue and calls its `finalize()` method. As long as the `finalize()` method does not generate a reference to the object, the object again becomes available for garbage collection.

  Because the finalization thread runs at a very low priority, there may be a long delay from the time that an object is put on the finalization queue until the time that its `finalize()` method is called. The `runFinalization()` method allows a program to run the `finalize()` methods explicitly. This can be useful when there is a shortage of some resource that is released by a `finalize()` method.

### totalMemory

#### `public native long totalMemory()`

Returns

  The total number of bytes in system memory.

Description

  This method returns the total number of bytes in system memory in the Java virtual machine. The total includes the number of bytes of memory being used by allocated objects, as well as the number of free bytes available for allocating additional objects. An estimate of the number of free bytes in system memory is available through the `freeMemory()` method.

### traceInstructions

#### `public native void traceInstructions(boolean on)`

Parameters

  `on`

    A `boolean` value that specifies if instructions are to be traced. `true` if instructions are to be traced; otherwise `false`.

Description

  This method controls whether or not the Java virtual machine outputs a detailed trace of each instruction that is executed. The `boolean` parameter causes tracing to be turned on or off. The tracing of instructions is only possible in a Java virtual machine that was compiled with the tracing option enabled. Production releases of the Java virtual machine are generally not compiled with tracing enabled.

### traceMethodCalls

#### `public native void traceMethodCalls(boolean on)`

Parameters

  `on`

    A `boolean` value that specifies if method calls are to be traced. `true` if instructions are to be traced; otherwise `false`.

Description

  This method controls whether or not the Java virtual machine outputs a detailed trace of each method that is invoked. The `boolean` parameter causes tracing to be turned on or off. The tracing of instructions is only possible in a Java virtual machine that was compiled with the tracing option enabled. Production releases of the Java virtual machine are generally not compiled with tracing enabled.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clone()` | `Object` | `equals(Object)` | `Object`
`finalize()` | `Object` | `getClass()` | `Object`
`hashCode()` | `Object` | `notify()` | `Object`
`notifyAll()` | `Object` | `toString()` | `Object`
`wait()` | `Object` | `wait(long)` | `Object`
`wait(long, int)` | `Object` |  |

## See Also

Errors; Exceptions; Object; Object Destruction; Process; SecurityManager; System

Runnable |  | SecurityManager