[Chapter 12] System

#

| Chapter 12 The java.lang Package |

# System

## Name

System

## Synopsis

Class Name:

  `java.lang.System`

Superclass:

  `java.lang.Object`

Immediate Subclasses:

  None

Interfaces Implemented:

  None

Availability:

  JDK 1.0 or later

## Description

The `System` class provides access to various information about the operating system environment in which a program is running. For example, the `System` class defines variables that allow access to the standard I/O streams and methods that allow a program to run the garbage collector and stop the Java virtual machine.

All of the variables and methods in the `System` class are `static`. In other words, it is not necessary to create an instance of the `System` class in order to use its variables and methods. In fact, the `System` class does not define any `public` constructors, so it cannot be instantiated.

The `System` class supports the concept of system properties that can be queried and set. The following properties are guaranteed always to be defined:

Property Name | Description
--- | ---
`file.encoding` | The character encoding for the default locale (Java 1.1 only)
`file.encoding.pkg` | The package that contains converters between local encodings and Unicode (Java 1.1 only)
`file.separator` | File separator (`'/'` on UNIX, `' \'` on Windows)
`java.class.path` | The class path
`java.class.version` | Java class version number
`java.compiler` | The just-in-time compiler to use, if any (Java 1.1 only)
`java.home` | Java installation directory
`java.vendor` | Java vendor-specific string
`java.vendor.url` | Java vendor URL
`java.version` | Java version number
`line.separator` | Line separator(`' \n'` on UNIX, `' \r\n'` on Windows)
`os.arch` | Operating system architecture
`os.name` | Operating system name
`os.version` | Operating system version
`path.separator` | Path separator (`':'` on UNIX, `','` on Windows)
`user.dir` | User's current working directory when the properties were initialized
`user.home` | User's home directory
`user.language` | The two-letter language code of the default locale (Java 1.1 only)
`user.name` | User's account name
`user.region` | The two-letter country code of the default locale (Java 1.1 only)
`user.timezone` | The default time zone (Java 1.1 only)

Additional properties may be defined by the run-time environment. The `-D` command-line option can be used to define system properties when a program is run.

The `Runtime` class is related to the `System` class; it provides access to information about the environment in which a program is running.

## Class Summary

    public final class java.lang.System extends java.lang.Object {
        // Constants
        public static final PrintStream err;
        public static final InputStream in;
        public static final PrintStream out;
        // Class Methods
        public static void arraycopy(Object src, int srcOffset,
                                     Object dst, int dstOffset, int length);
        public static long currentTimeMillis();
        public static void exit(int status);
        public static void gc();
        public static Properties getProperties();
        public static String getProperty(String key);
        public static String getProperty(String key, String default);
        public static SecurityManager getSecurityManager();
        public static String getenv(String name);           // Deprecated in 1.1
        public static native int identityHashCode(Object x);   // New in 1.1
        public static void load(String filename);
        public static void loadLibrary(String libname);
        public static void runFinalization();
        public static void runFinalizersOnExit(boolean value); // New in 1.1
        public static void setErr(PrintStream err);            // New in 1.1
        public static void setIn(InputStream in);              // New in 1.1
        public static void setOut(PrintStream out);            // New in 1.1
        public static void setProperties(Properties props);
        public static void setSecurityManager(SecurityManager s);
    }

## Variables

### err

#### `public static final PrintStream err`

Description

  The standard error stream. In an application environment, this variable refers to a `java.io.PrintStream` object that is associated with the standard error output for the process running the Java virtual machine. In an applet environment, the `PrintStream` is likely to be associated with a separate window, although this is not guaranteed.

  The value of `err` can be set using the `setErr()` method. The value of `err` can only be set if the currenly installed `SecurityManager` does not throw a `SecurityException` when the request is made.

  Prior to to Java 1.1, `err` was not `final`. It has been made `final` as of Java 1.1 because the unchecked ability to set `err` is a security hole.

### in

#### `public static final InputStream in`

Description

  The standard input stream. In an application environment, this variable refers to a `java.io.InputStream` object that is associated with the standard input for the process running the Java virtual machine.

  The value of `in` can be set using the `setIn()` method. The value of `in` can only be set if the currenly installed `SecurityManager` does not throw a `SecurityException` when the request is made.

  Prior to to Java 1.1, `in` was not `final`. It has been made `final` as of Java 1.1 because the unchecked ability to set `in` is a security hole.

### out

#### `public static final PrintStream out`

Description

  The standard output stream. In an application environment, this variable refers to a `java.io.PrintStream` object that is associated with the standard output for the process running the Java virtual machine. In an applet environment, the `PrintStream` is likely to be associated with a separate window, although this is not guaranteed.

  `out` is the most commonly used of the three I/O streams provided by the `System` class. Even in GUI-based applications, sending output to this stream can be useful for debugging. The usual idiom for sending output to this stream is:

      System.out.println("Some text");

  The value of `out` can be set using the `setOut()` method. The value of `out` can only be set if the currenly installed `SecurityManager` does not throw a `SecurityException` when the request is made.

  Prior to to Java 1.1, `out` was not `final`. It has been made `final` as of Java 1.1 because the unchecked ability to set `out` is a security hole.

## Class Methods

### arraycopy

####

     public static void arraycopy(Object src, int src_position, Object dst, int dst_position, int length)

Parameters

  `src`

    The source array.

  `src_position`

    An index into the source array.

  `dst`

    The destination array.

  `dst_position`

    An index into the destination array.

  `length`

    The number of elements to be copied.

Throws

  `ArrayIndexOutOfBoundsException`

    If the values of the `src_position`, `dst_position`, and `length` arguments imply accessing either array with an index that is less than zero or an index greater than or equal to the number of elements in the array.

  `ArrayStoreException`

    If the type of value stored in the `src` array cannot be stored in the `dst` array.

  `NullPointerException`

    If `src` or `dst` is `null`.

Description

  This method copies a range of array elements from the `src` array to the `dst` array. The number of elements that are copied is specified by `length`. The elements at positions `src_position` through `src_position+length-1` in `src` are copied to the positions `dst_position` through `dst_position+length-1` in `dst`, respectively.

  If `src` and `dst` refer to the same array, the copying is done as if the array elements were first copied to a temporary array and then copied to the destination array.

  Before this method does any copying, it performs a number of checks. If either `src` or `dst` are `null`, the method throws a `NullPointerException` and `dst` is not modified.

  If any of the following conditions are true, the method throws an `ArrayStoreException`, and `dst` is not modified:

  - Either `src` or `dst` refers to an object that is not an array.

  - `src` and `dst` refer to arrays whose element types are different primitive types.

  - `src` refers to an array that has elements that contain a primitive type, while `dst` refers to an array that has elements that contain a reference type, or vice versa.

  If any of the following conditions are true, the method throws an `ArrayIndexOutOfBoundsException`, and `dst` is not modified:

  - `srcOffset`, `dstOffset`, or `length` is negative.

  - `srcOffset+length` is greater than `src.length()`.

  - `dstOffset+length` is greater than `dst.length()`.

  Otherwise, if an element in the source array being copied cannot be converted to the type of the destination array using the rules of the assignment operator, the method throws an `ArrayStoreException` when the problem occurs. Since the problem is discovered during the copy operation, the state of the `dst` array reflects the incomplete copy operation.

### currentTimeMillis

#### `public static native long currentTimeMillis()`

Returns

  The current time as the number of milliseconds since 00:00:00 UTC, January 1, 1970.

Description

  This method returns the current time as the number of milliseconds since 00:00:00 UTC, January 1, 1970. It will not overflow until the year 292280995.

  The `java.util.Date` class provides more extensive facilities for dealing with times and dates.

### exit

#### `public static 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. This method works by calling the `exit()` method of the current `Runtime` object. By convention, a nonzero status code indicates abnormal termination. This method never returns.

### gc

#### `public static void gc()`

Description

  This method causes the Java virtual machine to run the garbage collector in the current thread. This method works by calling the `gc()` method of the current `Runtime` object.

  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.

### getProperties

#### `public static Properties getProperties()`

Returns

  A `Properties` object that contains the values of all the system properies.

Throws

  `SecurityException`

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

Description

  This method returns all of the defined system properties encapsulated in a `java.util.Properties` object. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed in the description of the `System` class, some system properties are guaranteed always to be defined.

### getProperty

#### `public static String getProperty(String key)`

Parameters

  `key`

    The name of a system property.

Returns

  The value of the named system property or `null` if the named property is not defined.

Throws

  `SecurityException`

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

Description

  This method returns the value of the named system property. If there is no definition for the named property, the method returns `null`. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed in the description of the `System` class, some system properties are guaranteed always to be defined.

#### `public static String getProperty(String key, String def)`

Parameters

  `key`

    The name of a system property.

  `def`

    A default value for the property.

Returns

  The value of the named system property, or the default value if the named property is not defined.

Throws

  `SecurityException`

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

Description

  This method returns the value of the named system property. If there is no definition for the named property, the method returns the default value as specified by the `def` parameter. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed earlier in the description of the `System` class, some system properties are guaranteed to always be defined.

### getSecurityManager

#### `public static SecurityManager getSecurityManager()`

Returns

  A reference to the installed `SecurityManager` object or `null` if there is no `SecurityManager` object installed.

Description

  This method returns a reference to the installed `SecurityManager` object. If there is no `SecurityManager` object installed, the method returns `null`.

### getenv

#### `public static String getenv(String name)`

Availability

  Deprecated as of JDK 1.1

Parameters

  `name`

    The name of a system-dependent environment variable.

Returns

  The value of the environment variable or `null` if the variable is not defined.

Description

  This method is obsolete; it always throws an error. Use `getProperties()` and the `-D` option instead.

### identityHashCode

#### `public static native int identityHashCode(Object x)`

Availability

  New as of JDK 1.1

Parameters

  `x`

    An object.

Returns

  The identity hashcode value for the specified object.

Description

  This method returns the same hashcode value for the specified object as would be returned by the default `hashCode()` method of `Object`, regardless of whether or not the object's class overrides `hashCode()`.

### load

#### `public 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. This method works by calling the `load()` method of the current `Runtime` object.

### loadLibrary

#### `public 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. This method works by calling the `loadLibrary()` method of the current `Runtime` object.

### runFinalization

#### `public static 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. This method works by calling the `runFinalization()` method of the current `Runtime` object.

  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.

### 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. This method works by calling the `runFinalizersOnExit()` method of the current `Runtime` object.

### setErr

#### `public static void setErr(PrintStream err)`

Availability

  New as of JDK 1.1

Parameters

  `err`

    A `PrintStream` object to use for the standard error stream.

Throws

  `SecurityException`

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

Description

  This method sets the standard error stream to be this `PrintStream` object.

### setIn

#### `public static void setIn(InputStream in)`

Availability

  New as of JDK 1.1

Parameters

  `in`

    A `InputStream` object to use for the standard input stream.

Throws

  `SecurityException`

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

Description

  This method sets the standard input stream to be this `InputStream` object.

### setOut

#### `public static void setOut(PrintStream out)`

Availability

  New as of JDK 1.1

Parameters

  `out`

    A `PrintStream` object to use for the standard output stream.

Throws

  `SecurityException`

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

Description

  This method sets the standard output stream to be this `PrintStream` object.

### setProperties

#### `public static void setProperties(Properties props)`

Parameters

  `props`

    A reference to a `Properties` object.

Throws

  `SecurityException`

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

Description

  This method replaces the current set of system property definitions with a new set of system property definitions that are encapsulated by the given `Properties` object. As discussed in the description of the `System` class, some system properties are guaranteed to always be defined.

### setSecurityManager

#### `public static void setSecurityManager(SecurityManager s)`

Parameters

  `s`

    A reference to a `SecurityManager` object.

Throws

  `SecurityException`

    If a `SecurityManager` object has already been installed.

Description

  This method installs the given `SecurityManager` object. If `s` is `null`, then no `SecurityManager` object is installed. Once a `SecurityManager` object is installed, any subsequent calls to this method throw a `SecurityException`.

## 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

`ArrayIndexOutOfBoundsException`, `ArrayStoreException`, `InputStream`, `NullPointerException`, `Object`, `PrintStream`, `Process`, `Runtime`, `SecurityException`, `SecurityManager`, `UnsatisfiedLinkError`

StringIndexOutOfBoundsException |  | Thread