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.

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);  
}