java.lang
Class System

java.lang.Object
  |
  +--java.lang.System

public final class System

extends Object

The System class contains several useful class fields and methods. It cannot be instantiated.

Among the facilities provided by the System class are standard input, standard output, and error output streams; access to externally defined "properties"; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.

Since:

JDK1.0

Field Summary

static PrintStream	err The "standard" error output stream.
static InputStream	in The "standard" input stream.
static PrintStream	out The "standard" output stream.

Method Summary

static void	arraycopy(Object src, int src_position, Object dst, int dst_position, int length) Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
static long	currentTimeMillis() Returns the current time in milliseconds.
static void	exit(int status) Terminates the currently running Java Virtual Machine.
static void	gc() Runs the garbage collector.
static Properties	getProperties() Determines the current system properties.
static String	getProperty(String key) Gets the system property indicated by the specified key.
static String	getProperty(String key, String def) Gets the system property indicated by the specified key.
static SecurityManager	getSecurityManager() Gets the system security interface.
static int	identityHashCode(Object x) Returns the same hashcode for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode().
static void	load(String filename) Loads a code file with the specified filename from the local file system as a dynamic library.
static void	loadLibrary(String libname) Loads the system library specified by the libname argument.
static String	mapLibraryName(String libname) Maps a library name into a platform-specific string representing a native library.
static void	runFinalization() Runs the finalization methods of any objects pending finalization.
static void	setErr(PrintStream err) Reassigns the "standard" error output stream.
static void	setIn(InputStream in) Reassigns the "standard" input stream.
static void	setOut(PrintStream out) Reassigns the "standard" output stream.
static void	setProperties(Properties props) Sets the system properties to the Properties argument.
static String	setProperty(String key, String value) Sets the system property indicated by the specified key.
static void	setSecurityManager(SecurityManager s) Sets the System security.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

in

public static final InputStream in

The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.

out

public static final PrintStream out

The "standard" output stream. This stream is already open and ready to accept output data. Typically this stream corresponds to display output or another output destination specified by the host environment or user.

For simple stand-alone Java applications, a typical way to write a line of output data is:

     System.out.println(data)
 

See the println methods in class PrintStream.

See Also:

PrintStream.println(), PrintStream.println(boolean), PrintStream.println(char), PrintStream.println(char[]), PrintStream.println(double), PrintStream.println(float), PrintStream.println(int), PrintStream.println(long), PrintStream.println(java.lang.Object), PrintStream.println(java.lang.String)

err

public static final PrintStream err

The "standard" error output stream. This stream is already open and ready to accept output data.

Typically this stream corresponds to display output or another output destination specified by the host environment or user. By convention, this output stream is used to display error messages or other information that should come to the immediate attention of a user even if the principal output stream, the value of the variable out, has been redirected to a file or other destination that is typically not continuously monitored.

Method Detail

setIn

public static void setIn(InputStream in)

Reassigns the "standard" input stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" input stream.

Parameters:

in - the new standard input stream.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard input stream.

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

setOut

public static void setOut(PrintStream out)

Reassigns the "standard" output stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" output stream.

Parameters:

out - the new standard output stream

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream.

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

setErr

public static void setErr(PrintStream err)

Reassigns the "standard" error output stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" error output stream.

Parameters:

err - the new standard error output stream.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard error output stream.

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

setSecurityManager

public static void setSecurityManager(SecurityManager s)

Sets the System security.

If there is a security manager already installed, this method first calls the security manager's checkPermission method with a RuntimePermission("setSecurityManager") permission to ensure it's ok to replace the existing security manager. This may result in throwing a SecurityException.

Otherwise, the argument is established as the current security manager. If the argument is null and no security manager has been established, then no action is taken and the method simply returns.

Parameters:

s - the security manager.

Throws:

SecurityException - if the security manager has already been set and its checkPermission method doesn't allow it to be replaced.

See Also:

getSecurityManager(), SecurityManager.checkPermission(java.security.Permission), RuntimePermission

getSecurityManager

public static SecurityManager getSecurityManager()

Gets the system security interface.

Returns:

if a security manager has already been established for the current application, then that security manager is returned; otherwise, null is returned.

See Also:

setSecurityManager(java.lang.SecurityManager)

currentTimeMillis

public static long currentTimeMillis()

Returns the current time in milliseconds.

See the description of the class Date for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).

Returns:

the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.

See Also:

Date

arraycopy

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

Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. A subsequence of array components are copied from the source array referenced by src to the destination array referenced by dst. The number of components copied is equal to the length argument. The components at positions srcOffset through srcOffset+length-1 in the source array are copied into positions dstOffset through dstOffset+length-1, respectively, of the destination array.

If the src and dst arguments refer to the same array object, then the copying is performed as if the components at positions srcOffset through srcOffset+length-1 were first copied to a temporary array with length components and then the contents of the temporary array were copied into positions dstOffset through dstOffset+length-1 of the destination array.

If dst is null, then a NullPointerException is thrown.

If src is null, then a NullPointerException is thrown and the destination array is not modified.

Otherwise, if any of the following is true, an ArrayStoreException is thrown and the destination is not modified:

• The src argument refers to an object that is not an array.
• The dst argument refers to an object that is not an array.
• The src argument and dst argument refer to arrays whose component types are different primitive types.
• The src argument refers to an array with a primitive component type and the dst argument refers to an array with a reference component type.
• The src argument refers to an array with a reference component type and the dst argument refers to an array with a primitive component type.

Otherwise, if any of the following is true, an IndexOutOfBoundsException is thrown and the destination is not modified:

• The srcOffset argument is negative.
• The dstOffset argument is negative.
• The length argument is negative.
• srcOffset+length is greater than src.length, the length of the source array.
• dstOffset+length is greater than dst.length, the length of the destination array.

Otherwise, if any actual component of the source array from position srcOffset through srcOffset+length-1 cannot be converted to the component type of the destination array by assignment conversion, an ArrayStoreException is thrown. In this case, let k be the smallest nonnegative integer less than length such that src[srcOffset+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positions srcOffset through srcOffset+k-1 will already have been copied to destination array positions dstOffset through dstOffset+k-1 and no other positions of the destination array will have been modified. (Because of the restrictions already itemized, this paragraph effectively applies only to the situation where both arrays have component types that are reference types.)

Parameters:

src - the source array.

src_position - start position in the source array.

dst - the destination array.

dst_position - pos start position in the destination data.

length - the number of array elements to be copied.

Throws:

IndexOutOfBoundsException - if copying would cause access of data outside array bounds.

ArrayStoreException - if an element in the src array could not be stored into the dest array because of a type mismatch.

NullPointerException - if either src or dst is null.

identityHashCode

public static int identityHashCode(Object x)

Returns the same hashcode for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). The hashcode for the null reference is zero.

Parameters:

x - object for which the hashCode is to be calculated

Returns:

the hashCode

Since:

JDK1.1

getProperties

public static Properties getProperties()

Determines the current system properties.

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The current set of system properties for use by the getProperty(String) method is returned as a Properties object. If there is no current set of system properties, a set of system properties is first created and initialized. This set of system properties always includes values for the following keys:

Key	Description of Associated Value
java.version	Java Runtime Environment version
java.vendor	Java Runtime Environment vendor
java.vendor.url	Java vendor URL
java.home	Java installation directory
java.vm.specification.version	Java Virtual Machine specification version
java.vm.specification.vendor	Java Virtual Machine specification vendor
java.vm.specification.name	Java Virtual Machine specification name
java.vm.version	Java Virtual Machine implementation version
java.vm.vendor	Java Virtual Machine implementation vendor
java.vm.name	Java Virtual Machine implementation name
java.specification.version	Java Runtime Environment specification version
java.specification.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
java.class.path	Java class path
java.ext.dirs	Path of extension directory or directories
os.name	Operating system name
os.arch	Operating system architecture
os.version	Operating system version
file.separator	File separator ("/" on UNIX)
path.separator	Path separator (":" on UNIX)
line.separator	Line separator ("\n" on UNIX)
user.name	User's account name
user.home	User's home directory
user.dir	User's current working directory

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the getProperty(String) operation.

Returns:

the system properties

Throws:

SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.

See Also:

setProperties(java.util.Properties), SecurityException, SecurityManager.checkPropertiesAccess(), Properties

setProperties

public static void setProperties(Properties props)

Sets the system properties to the Properties argument.

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The argument becomes the current set of system properties for use by the getProperty(String) method. If the argument is null, then the current set of system properties is forgotten.

Parameters:

props - the new system properties.

Throws:

SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.

See Also:

getProperties(), Properties, SecurityException, SecurityManager.checkPropertiesAccess()

getProperty

public static String getProperty(String key)

Gets the system property indicated by the specified key.

First, if there is a security manager, its checkPropertyAccess method is called with the key as its argument. This may result in a SecurityException.

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

Parameters:

key - the name of the system property.

Returns:

the string value of the system property, or null if there is no property with that key.

Throws:

SecurityException - if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.

NullPointerException - if key is null.

IllegalArgumentException - if key is empty.

See Also:

setProperty(java.lang.String, java.lang.String), SecurityException, SecurityManager.checkPropertyAccess(java.lang.String), getProperties()

getProperty

public static String getProperty(String key,
                                 String def)

Gets the system property indicated by the specified key.

First, if there is a security manager, its checkPropertyAccess method is called with the key as its argument.

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

Parameters:

key - the name of the system property.

def - a default value.

Returns:

the string value of the system property, or the default value if there is no property with that key.

Throws:

SecurityException - if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.

NullPointerException - if key is null.

IllegalArgumentException - if key is empty.

See Also:

setProperty(java.lang.String, java.lang.String), SecurityManager.checkPropertyAccess(java.lang.String), getProperties()

setProperty

public static String setProperty(String key,
                                 String value)

Sets the system property indicated by the specified key.

First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is set to the given value.

Parameters:

key - the name of the system property.

value - the value of the system property.

Returns:

the previous value of the system property, or null if it did not have one.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow setting of the specified property.

NullPointerException - if key is null.

IllegalArgumentException - if key is empty.

Since:

1.2

See Also:

getProperty(java.lang.String), getProperty(java.lang.String), getProperty(java.lang.String, java.lang.String), PropertyPermission, SecurityManager.checkPermission(java.security.Permission)

exit

public static void exit(int status)

Terminates the currently running Java Virtual Machine. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.

This method calls the exit method in class Runtime. This method never returns normally.

The call System.exit(n) is effectively equivalent to the call:

 Runtime.getRuntime().exit(n)
 

If process model exists, System.exit() must rely on the process exiting to release resources.

If process model does not exist, and a security manager exists, the security manager by default must prohibit calls to System.exit() by throwing SecurityException. Otherwise, the system will hang indefinitely when called,

Parameters:

status - exit status.

Throws:

SecurityException - if a security manager exists and its checkExit method doesn't allow exit with the specified status.

See Also:

Runtime.exit(int)

gc

public static void gc()

Runs the garbage collector.

Calling the gc 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 a best effort to reclaim space from all discarded objects.

The call System.gc() is effectively equivalent to the call:

 Runtime.getRuntime().gc()
 

See Also:

Runtime.gc()

runFinalization

public static 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 call System.runFinalization() is effectively equivalent to the call:

 Runtime.getRuntime().runFinalization()
 

See Also:

Runtime.runFinalization()

load

public static void load(String filename)

Loads a code file with the specified filename from the local file system as a dynamic library. The filename argument must be a complete pathname.

The call System.load(name) is effectively equivalent to the call:

 Runtime.getRuntime().load(name)
 

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:

Runtime.load(java.lang.String), SecurityManager.checkLink(java.lang.String)

loadLibrary

public static void loadLibrary(String libname)

Loads the system library specified by the libname argument. The manner in which a library name is mapped to the actual system library is system dependent.

The call System.loadLibrary(name) is effectively equivalent to the call

 Runtime.getRuntime().loadLibrary(name)
 

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:

Runtime.loadLibrary(java.lang.String), SecurityManager.checkLink(java.lang.String)

mapLibraryName

public static String mapLibraryName(String libname)

Maps a library name into a platform-specific string representing a native library.

Parameters:

libname - the name of the library.

Returns:

a platform-dependent native library name.

Since:

1.2

See Also:

loadLibrary(java.lang.String), ClassLoader.findLibrary(java.lang.String)