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 and environment variables; 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 srcPos, Object dest, int destPos, int length) Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
static String	clearProperty(String key) Removes the system property indicated by the specified key.
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 Map<String,String>	getenv() Returns an unmodifiable string map view of the current system environment.
static String	getenv(String name) Gets the value of the specified environment variable.
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 hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode().
static Channel	inheritedChannel() Returns the channel inherited from the entity that created this Java virtual machine.
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 long	nanoTime() Returns the current value of the most precise available system timer, in nanoseconds.
static void	runFinalization() Runs the finalization methods of any objects pending finalization.
static void	runFinalizersOnExit(boolean value) Deprecated. This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.
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

inheritedChannel

public static Channel inheritedChannel()
                                throws IOException

Returns the channel inherited from the entity that created this Java virtual machine.

This method returns the channel obtained by invoking the inheritedChannel method of the system-wide default SelectorProvider object.

In addition to the network-oriented channels described in inheritedChannel, this method may return other kinds of channels in the future.

Returns:

The inherited channel, if any, otherwise null.

Throws:

IOException - If an I/O error occurs

SecurityException - If a security manager is present and it does not permit access to the channel.

Since:

1.5

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. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of 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

nanoTime

public static long nanoTime()

Returns the current value of the most precise available system timer, in nanoseconds.

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292 years (2⁶³ nanoseconds) will not accurately compute elapsed time due to numerical overflow.

For example, to measure how long some code takes to execute:

   long startTime = System.nanoTime();
   // ... the code being measured ...
   long estimatedTime = System.nanoTime() - startTime;
 

Returns:

The current value of the system timer, in nanoseconds.

Since:

1.5

arraycopy

public static void arraycopy(Object src,
                             int srcPos,
                             Object dest,
                             int destPos,
                             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 dest. The number of components copied is equal to the length argument. The components at positions srcPos through srcPos+length-1 in the source array are copied into positions destPos through destPos+length-1, respectively, of the destination array.

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

If dest 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 dest argument refers to an object that is not an array.
• The src argument and dest 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 dest argument refers to an array with a reference component type.
• The src argument refers to an array with a reference component type and the dest 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 srcPos argument is negative.
• The destPos argument is negative.
• The length argument is negative.
• srcPos+length is greater than src.length, the length of the source array.
• destPos+length is greater than dest.length, the length of the destination array.

Otherwise, if any actual component of the source array from position srcPos through srcPos+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[srcPos+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positions srcPos through srcPos+k-1 will already have been copied to destination array positions destPos through destPos+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.

srcPos - starting position in the source array.

dest - the destination array.

destPos - starting 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 dest is null.

identityHashCode

public static int identityHashCode(Object x)

Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). The hash code 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.library.path	List of paths to search when loading libraries
java.io.tmpdir	Default temp file path
java.compiler	Name of JIT compiler to use
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

Multiple paths in a system property value are separated by the path separator character of the platform.

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 or value 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)

clearProperty

public static String clearProperty(String key)

Removes 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 removed.

Parameters:

key - the name of the system property to be removed.

Returns:

the previous string value of the system property, or null if there was 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.

Since:

1.5

See Also:

getProperty(java.lang.String), setProperty(java.lang.String, java.lang.String), Properties, SecurityException, SecurityManager.checkPropertiesAccess()

getenv

public static String getenv(String name)

Gets the value of the specified environment variable. An environment variable is a system-dependent external named value.

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv."+name) permission. This may result in a SecurityException being thrown. If no exception is thrown the value of the variable name is returned.

System properties and environment variables are both conceptually mappings between names and values. Both mechanisms can be used to pass user-defined information to a Java process. Environment variables have a more global effect, because they are visible to all descendants of the process which defines them, not just the immediate Java subprocess. They can have subtly different semantics, such as case insensitivity, on different operating systems. For these reasons, environment variables are more likely to have unintended side effects. It is best to use system properties where possible. Environment variables should be used when a global effect is desired, or when an external system interface requires an environment variable (such as PATH).

On UNIX systems the alphabetic case of name is typically significant, while on Microsoft Windows systems it is typically not. For example, the expression System.getenv("FOO").equals(System.getenv("foo")) is likely to be true on Microsoft Windows.

Parameters:

name - the name of the environment variable

Returns:

the string value of the variable, or null if the variable is not defined in the system environment

Throws:

NullPointerException - if name is null

SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the environment variable name

See Also:

getenv(), ProcessBuilder.environment()

getenv

public static Map<String,String> getenv()

Returns an unmodifiable string map view of the current system environment. The environment is a system-dependent mapping from names to values which is passed from parent to child processes.

If the system does not support environment variables, an empty map is returned.

The returned map will never contain null keys or values. Attempting to query the presence of a null key or value will throw a NullPointerException. Attempting to query the presence of a key or value which is not of type String will throw a ClassCastException.

The returned map and its collection views may not obey the general contract of the Object.equals(java.lang.Object) and Object.hashCode() methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv.*") permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

Returns:

the environment as a map of variable names to values

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the process environment

Since:

1.5

See Also:

getenv(String), ProcessBuilder.environment()

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)
 

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()

runFinalizersOnExit

@Deprecated
public static void runFinalizersOnExit(boolean value)

Deprecated. This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.

Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

If there is a security manager, its checkExit method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.

Parameters:

value - indicating enabling or disabling of finalization

Throws:

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

Since:

JDK1.1

See Also:

Runtime.exit(int), Runtime.gc(), SecurityManager.checkExit(int)

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 path name.

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.

NullPointerException - if filename is null

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.

NullPointerException - if libname is null

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.

Throws:

NullPointerException - if libname is null

Since:

1.2

See Also:

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