Class java.lang.System - JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

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Class java.lang.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.

err: The "standard" error output stream.
in: The "standard" input stream.
out: The "standard" output stream.

arraycopy(Object, int, Object, int, int): Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
currentTimeMillis(): Returns the current time in milliseconds.
exit(int): Terminates the currently running Java Virtual Machine.
gc(): Runs the garbage collector.
getenv(String): Gets an environment variable. Deprecated.
getProperties(): Determines the current system properties.
getProperty(String): Gets the system property indicated by the specified key.
getProperty(String, String): Gets the system property indicated by the specified key.
getSecurityManager(): Gets the system security interface.
identityHashCode(Object): 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().
load(String): Loads the specified filename as a dynamic library.
loadLibrary(String): Loads the system library specified by the libname argument.
runFinalization(): Runs the finalization methods of any objects pending finalization.
runFinalizersOnExit(boolean): 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.
setErr(PrintStream): Reassigns the "standard" error output stream.
setIn(InputStream): Reassigns the "standard" input stream.
setOut(PrintStream): Reassigns the "standard" output stream.
setProperties(Properties): Sets the system properties to the Properties argument.
setSecurityManager(SecurityManager): Sets the System security.

 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:: println, println, println, println, println, println, println, println, println, println

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.

setIn

 public static void setIn(InputStream in)

Reassigns the "standard" input stream.

setOut

 public static void setOut(PrintStream out)

Reassigns the "standard" output stream.

setErr

 public static void setErr(PrintStream err)

Reassigns the "standard" error output stream.

setSecurityManager

 public static void setSecurityManager(SecurityManager s)

Sets the System security. If a security manager has already been established for the currently running Java application, a SecurityException is thrown. 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.

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.

currentTimeMillis

 public static native 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 native 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 argument array.

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 ArrayIndexOutOfBoundsException 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.

Parameters:: src: - the source array.; srcpos - start position in the source array.; dest - the destination array.; destpos - start position in the destination data.; length - the number of array elements to be copied.
Throws: ArrayIndexOutOfBoundsException: if copying would cause access of data outside array bounds.
Throws: ArrayStoreException: if an element in the src array could not be stored into the dest array because of a type mismatch.

identityHashCode

 public static native 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.

getProperties

 public static Properties getProperties()

Determines the current system properties.

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 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 version number

java.vendor Java vendor-specific string

java.vendor.url Java vendor URL

java.home Java installation directory

java.class.version Java class format version number

java.class.path Java class path

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

Key	Description of Associated Value
`java.version`	Java version number
`java.vendor`	Java vendor-specific string
`java.vendor.url`	Java vendor URL
`java.home`	Java installation directory
`java.class.version`	Java class format version number
`java.class.path`	Java class path
`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

Throws: SecurityException: if the current thread cannot access the system properties.
See Also:: SecurityException, 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 method. If the argument is null, then the current set of system properties is forgotten.

Parameters:: props - the new system properties.
Throws: SecurityException: if the current thread cannot set the system properties.
See Also:: SecurityException, 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 system exception.

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 the current thread cannot access the system properties or the specified property.
See Also:: SecurityException, checkPropertyAccess, 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 the current thread cannot access the system properties or the specified property.
See Also:: checkPropertyAccess, getProperties

getenv

 public static String getenv(String name)

Note: getenv() is deprecated.

Gets an environment variable. An environment variable is a system dependent external variable that has a string value.

Parameters:: the - name of the environment variable.
Returns:: the value of the variable, or null if the variable is not defined.

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.

Parameters:: status - exit status.
Throws: SecurityException: if the current thread cannot exit with the specified status.
See Also:: exit

 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 unused objects.

See Also:: 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.

See Also:: runFinalization

runFinalizersOnExit

 public static void runFinalizersOnExit(boolean value)

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.

See Also:: exit, gc

load

 public static void load(String filename)

Loads the specified filename as a dynamic library. The filename argument must be a complete pathname.

This method calls the load method in class Runtime.

Parameters:: filename - the file to load.
Throws: SecurityException: if the current thread cannot load the specified dynamic library.
Throws: UnsatisfiedLinkError: if the file does not exist.
See Also:: load

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.

Parameters:: libname - the name of the library.
Throws: SecurityException: if the current thread cannot load the specified dynamic library.
Throws: UnsatisfiedLinkError: if the library does not exist.
See Also:: loadLibrary

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