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Class System

java.lang.Objectjava.lang.System

public final classSystem
extendsObject

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

Among the facilities provided by theSystem 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

in

public static finalInputStreamin

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 finalPrintStreamout

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 theprintln methods in classPrintStream.

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 finalPrintStreamerr

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 variableout, has been redirected to a file or other destination that is typically not continuously monitored.

setIn

public static voidsetIn(InputStream in)

Reassigns the "standard" input stream.

First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard input stream.

Since:

JDK1.1

See Also:

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

setOut

public static voidsetOut(PrintStream out)

Reassigns the "standard" output stream.

First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard output stream.

Since:

JDK1.1

See Also:

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

setErr

public static voidsetErr(PrintStream err)

Reassigns the "standard" error output stream.

First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard error output stream.

Since:

JDK1.1

See Also:

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

inheritedChannel

public staticChannelinheritedChannel()                                throwsIOException

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

This method returns the channel obtained by invoking theinheritedChannel method of the system-wide defaultSelectorProvider object.

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

Returns:

The inherited channel, if any, otherwisenull.

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 voidsetSecurityManager(SecurityManager s)

Sets the System security.

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

Otherwise, the argument is established as the current security manager. If the argument isnull 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 itscheckPermission method doesn't allow it to be replaced.

See Also:

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

getSecurityManager

public staticSecurityManagergetSecurityManager()

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 longcurrentTimeMillis()

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 classDate 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 longnanoTime()

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 voidarraycopy(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 bysrc to the destination array referenced bydest. The number of components copied is equal to thelength argument. The components at positionssrcPos throughsrcPos+length-1 in the source array are copied into positionsdestPos throughdestPos+length-1, respectively, of the destination array.

If thesrc anddest arguments refer to the same array object, then the copying is performed as if the components at positionssrcPos throughsrcPos+length-1 were first copied to a temporary array withlength components and then the contents of the temporary array were copied into positionsdestPos throughdestPos+length-1 of the destination array.

Ifdest isnull, then aNullPointerException is thrown.

Ifsrc isnull, then aNullPointerException is thrown and the destination array is not modified.

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

• Thesrc argument refers to an object that is not an array.
• Thedest argument refers to an object that is not an array.
• Thesrc argument anddest argument refer to arrays whose component types are different primitive types.
• Thesrc argument refers to an array with a primitive component type and thedest argument refers to an array with a reference component type.
• Thesrc argument refers to an array with a reference component type and thedest argument refers to an array with a primitive component type.

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

• ThesrcPos argument is negative.
• ThedestPos argument is negative.
• Thelength argument is negative.
• srcPos+length is greater thansrc.length, the length of the source array.
• destPos+length is greater thandest.length, the length of the destination array.

Otherwise, if any actual component of the source array from positionsrcPos throughsrcPos+length-1 cannot be converted to the component type of the destination array by assignment conversion, anArrayStoreException is thrown. In this case, letk be the smallest nonnegative integer less than length such thatsrc[srcPos+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positionssrcPos throughsrcPos+k-1 will already have been copied to destination array positionsdestPos throughdestPos+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 thesrc array could not be stored into thedest array because of a type mismatch.

NullPointerException - if eithersrc ordest isnull.

identityHashCode

public static intidentityHashCode(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 staticPropertiesgetProperties()

Determines the current system properties.

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

The current set of system properties for use by thegetProperty(String) method is returned as aProperties 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 thegetProperties operation, it may choose to permit thegetProperty(String) operation.

Returns:

the system properties

Throws:

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

See Also:

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

setProperties

public static voidsetProperties(Properties props)

Sets the system properties to theProperties argument.

First, if there is a security manager, itscheckPropertiesAccess 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 thegetProperty(String) method. If the argument isnull, then the current set of system properties is forgotten.

Parameters:

props - the new system properties.

Throws:

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

See Also:

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

getProperty

public staticStringgetProperty(String key)

Gets the system property indicated by the specified key.

First, if there is a security manager, itscheckPropertyAccess 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 thegetProperties method.

Parameters:

key - the name of the system property.

Returns:

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

Throws:

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

NullPointerException - ifkey isnull.

IllegalArgumentException - ifkey is empty.

See Also:

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

getProperty

public staticStringgetProperty(String key,String def)

Gets the system property indicated by the specified key.

First, if there is a security manager, itscheckPropertyAccess method is called with thekey 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 thegetProperties 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 itscheckPropertyAccess method doesn't allow access to the specified system property.

NullPointerException - ifkey isnull.

IllegalArgumentException - ifkey is empty.

See Also:

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

setProperty

public staticStringsetProperty(String key,String value)

Sets the system property indicated by the specified key.

First, if a security manager exists, itsSecurityManager.checkPermission method is called with aPropertyPermission(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, ornull if it did not have one.

Throws:

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

NullPointerException - ifkey orvalue isnull.

IllegalArgumentException - ifkey 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 staticStringclearProperty(String key)

Removes the system property indicated by the specified key.

First, if a security manager exists, itsSecurityManager.checkPermission method is called with aPropertyPermission(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, ornull if there was no property with that key.

Throws:

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

NullPointerException - ifkey isnull.

IllegalArgumentException - ifkey is empty.

Since:

1.5

See Also:

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

getenv

public staticStringgetenv(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, itscheckPermission method is called with aRuntimePermission("getenv."+name) permission. This may result in aSecurityException being thrown. If no exception is thrown the value of the variablename is returned.

System properties andenvironment 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 asPATH).

On UNIX systems the alphabetic case ofname is typically significant, while on Microsoft Windows systems it is typically not. For example, the expressionSystem.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, ornull if the variable is not defined in the system environment

Throws:

NullPointerException - ifname isnull

SecurityException - if a security manager exists and itscheckPermission method doesn't allow access to the environment variablename

See Also:

getenv(),ProcessBuilder.environment()

getenv

public staticMap<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 aNullPointerException. Attempting to query the presence of a key or value which is not of typeString will throw aClassCastException.

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

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

If a security manager exists, itscheckPermission method is called with aRuntimePermission("getenv.*") permission. This may result in aSecurityException 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 itscheckPermission method doesn't allow access to the process environment

Since:

1.5

See Also:

getenv(String),ProcessBuilder.environment()

exit

public static voidexit(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 theexit method in classRuntime. This method never returns normally.

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

 Runtime.getRuntime().exit(n)

Parameters:

status - exit status.

Throws:

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

See Also:

Runtime.exit(int)

gc

public static voidgc()

Runs the garbage collector.

Calling thegc 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 callSystem.gc() is effectively equivalent to the call:

 Runtime.getRuntime().gc()

See Also:

Runtime.gc()

runFinalization

public static voidrunFinalization()

Runs the finalization methods of any objects pending finalization.

Calling this method suggests that the Java Virtual Machine expend effort toward running thefinalize methods of objects that have been found to be discarded but whosefinalize 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 callSystem.runFinalization() is effectively equivalent to the call:

 Runtime.getRuntime().runFinalization()

See Also:

Runtime.runFinalization()

runFinalizersOnExit

public static voidrunFinalizersOnExit(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, itscheckExit 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 itscheckExit method doesn't allow the exit.

Since:

JDK1.1

See Also:

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

load

public static voidload(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 callSystem.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 itscheckLink method doesn't allow loading of the specified dynamic library

UnsatisfiedLinkError - if the file does not exist.

NullPointerException - iffilename isnull

See Also:

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

loadLibrary

public static voidloadLibrary(String libname)

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

The callSystem.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 itscheckLink method doesn't allow loading of the specified dynamic library

UnsatisfiedLinkError - if the library does not exist.

NullPointerException - iflibname isnull

See Also:

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

mapLibraryName

public staticStringmapLibraryName(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 - iflibname isnull

Since:

1.2

See Also:

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