System

System

java.lang

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Java 1.0

This class defines a platform-independent interface to system facilities, including system properties and system input and output streams. All methods and variables of this class are static, and the class cannot be instantiated . Because the methods defined by this class are low-level system methods, most require special permissions and cannot be executed by untrusted code.

getProperty( ) looks up a named property on the system properties list, returning the optionally specified default value if no property definition is found. getProperties( ) returns the entire properties list. setProperties( ) sets a Properties object on the properties list. In Java 1.2 and later, setProperty( ) sets the value of a system property. In Java 5.0, you can clear a property setting with clearProperty( ) . The following table lists system properties that are always defined. Untrusted code may be unable to read some or all of these properties. Additional properties can be defined using the -D option when invoking the Java interpreter.

Property name	Description
file.separator	Platform directory separator character
path .separator	Platform path separator character
line.separator	Platform line separator character(s)
user .name	Current user s account name
user.home	Home directory of current user
user.dir	The current working directory
java.class.path	Where classes are loaded from
java.class.version	Version of the Java class file format
java.compiler	The name of the just-in-time compiler
java.ext.dirs	Path to directories that hold extensions
java.home	The directory Java is installed in
java.io.tmpdir	The directory that temporary files are written to
java.library.path	Directories to search for native libraries
java.specification.version	Version of the Java API specification
java.specification.vendor	Vendor of the Java API specification
java.specification.name	Name of the Java API specification
java.version	Version of the Java API implementation
java.vendor	Vendor of this Java API implementation
java.vendor.url	URL of the vendor of this Java API implementation
java.vm.specification.version	Version of the Java VM specification
java.vm.specification.vendor	Vendor of the Java VM specification
java.vm.specification.name	Name of the Java VM specification
java.vm.version	Version of the Java VM implementation
java.vm.vendor	Vendor of the Java VM implementation
java.vm.name	Name of the Java VM implementation
os.name	Name of the host operating system
os.arch	Host operating system architecture
os.version	Version of the host operating system

The in , out , and err fields hold the standard input, output, and error streams for the system. These fields are frequently used in calls such as System.out.println( ) . In Java 1.1, setIn( ) , setOut( ) , and setErr( ) allow these streams to be redirected.

System also defines various other useful static methods. exit( ) causes the Java VM to exit. arraycopy( ) efficiently copies an array or a portion of an array into a destination array. currentTimeMillis( ) returns the current time in milliseconds since midnight GMT, January 1, 1970 GMT. In Java 5.0, nanoTime( ) returns a time in nanoseconds. Unlike currentTimeMillis( ) this time is not relative to any fixed point and so is useful only for elapsed time computations .

getenv( ) returns the value of a platform-dependent environment variable, or (in Java 5.0) returns a Map of all environment variables. The one-argument version of getenv( ) was previously deprecated but has been restored in Java 5.0.

identityHashCode( ) computes the hashcode for an object in the same way that the default Object.hashCode( ) method does. It does this regardless of whether or how the hashCode( ) method has been overridden.

In Java 5.0, inheritedChannel( ) returns a java.nio.channels.Channel object that represents a network connection passed to the Java process by the invoking process. This allows Java programs to be used with the Unix inetd daemon, for example.

load( ) and loadLibrary( ) can read libraries of native code into the system. mapLibraryName( ) converts a system-independent library name into a system-dependent library filename. Finally, getSecurityManager( ) and setSecurityManager( ) get and set the system SecurityManager object responsible for the system security policy.

See also Runtime , which defines several other methods that provide low-level access to system facilities.

 public final class  System  {  // No Constructor   // Public Constants  public static final java.io.PrintStream  err  ;        public static final java.io.InputStream  in  ;        public static final java.io.PrintStream  out  ;  // Public Class Methods  public static void  arraycopy  (Object  src  , int  srcPos  , Object  dest  , int  destPos  ,       int  length  );  native   5.0  public static String  clearProperty  (String  key  );        public static long  currentTimeMillis  ( );  native  public static void  exit  (int  status  );        public static void  gc  ( );  5.0  public static java.util.Map<String,String>  getenv  ( );        public static String  getenv  (String  name  );        public static java.util.Properties  getProperties  ( );        public static String  getProperty  (String  key  );        public static String  getProperty  (String  key  , String  def  );        public static SecurityManager  getSecurityManager  ( );  1.1  public static int  identityHashCode  (Object  x  );  native   5.0  public static java.nio.channels.Channel  inheritedChannel  ( ) throws java.io.IOException;        public static void  load  (String  filename  );        public static void  loadLibrary  (String  libname  );  1.2  public static String  mapLibraryName  (String  libname  );  native   5.0  public static long  nanoTime  ( );  native  public static void  runFinalization  ( );  1.1  public static void  setErr  (java.io.PrintStream  err  );  1.1  public static void  setIn  (java.io.InputStream  in  );  1.1  public static void  setOut  (java.io.PrintStream  out  );        public static void  setProperties  (java.util.Properties  props  );  1.2  public static String  setProperty  (String  key  , String  value  );        public static void  setSecurityManager  (SecurityManager  s  );  // Deprecated Public Methods   1.1#  public static void  runFinalizersOnExit  (boolean  value  );   }