java.lang.System
System
class contains a number of useful class variables and class methods.
It cannot be instantiated. Among the facilities provided by the System
class are
standard input, 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.
public final classSystem
{ public static InputStreamin
; public static PrintStreamout
; public static PrintStreamerr
; public static SecurityManagergetSecurityManager
(); public static voidsetSecurityManager
(SecurityManager s) throws SecurityException; public static longcurrentTimeMillis
(); public static PropertiesgetProperties
() throws SecurityException; public static voidsetProperties
(Properties props) throws SecurityException; public static StringgetProperty
(String key) throws SecurityException; public static StringgetProperty
(String key, String defaults) throws SecurityException; public static voidexit
(int status) throws SecurityException; public static voidgc
(); public static voidrunFinalization
(); public static voidload
(String filename) throws SecurityException, UnsatisfiedLinkError; public static voidloadLibrary
(String libname) throws SecurityException, UnsatisfiedLinkError; public static voidarraycopy
(Object src, int srcOffset, Object dst, int dstOffset, int length)
throws NullPointerException, ArrayStoreException, IndexOutOfBoundsException; }
20.18.1 public static InputStream
in
;
The initial value of this variable is a "standard" input stream, already open and
ready to supply input data. Typically, this corresponds to keyboard input or
another input source specified by the host environment or user. Note that this field
is not final
, so its value may be updated if necessary.
20.18.2 public static PrintStream
out
;
The initial value of this variable is a "standard" output stream, already open and
ready to accept output data. Typically, this corresponds to display output or
another output destination specified by the host environment or user. Note that this
field is not final
, so its value may be updated if necessary.
For simple Java applications, a typical way to write a line of output data is:
System.out.println(data)See the
println
method of class PrintStream
(§22.22).20.18.3 public static PrintStream
err
;
The initial value of this variable is a "standard" error output stream, already open
and ready to accept output data. Typically, this 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. Note that this field is not final
,
so its value may be updated if necessary.
20.18.4 public static SecurityManager
getSecurityManager
()
If a security manager has already been established for the currently running Java
system, a reference to that security manager is returned. Otherwise, null
is
returned.
20.18.5 public static void
setSecurityManager
(SecurityManager s)
throws SecurityException
If a security manager has already been established for the currently running Java
system, 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 normally.
20.18.6 public static long
currentTimeMillis
()
Returns the difference, measured in milliseconds, between the current time and
the standard base time known as "the epoch," 00:00:00 GMT on January 1, 1970.
See the description of the class Date
(§21.3) for a discussion of slight discrepancies
that may arise between "computer time" and UTC (Coordinated Universal
Time).
20.18.7 public static Properties
getProperties
()
throws SecurityException
First, if there is a security manager, its checkPropertiesAccess
method
(§20.17.15) is called with no arguments.
The current set of system properties for use by the getProperty
method is returned as a Properties object (§21.6). 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 classpath os.name Operating system name os.arch Operating system architecture os.version Operating system version file.separator File separator (Note that even if the security manager does not permit the/
on UNIX) path.separator Path separator (:
on UNIX) line.separator Line separator (\n
on UNIX) user.name User account name user.home User home directory user.dir User's current working directory
getProperties
operation, it may choose to permit the getProperty
operation (§20.18.9).20.18.8 public static void
setProperties
(Properties props)
throws SecurityException
First, if there is a security manager, its checkPropertiesAccess
method
(§20.17.15) is called with no arguments.
The argument becomes the current set of system properties for use by the getProperty
method. See the class Properties
(§21.6). If the argument is null
, then the current set of system properties is forgotten.
20.18.9 public static String
getProperty
(String key)
throws SecurityException
First, if there is a security manager, its checkPropertyAccess
method
(§20.17.16) 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 (§20.18.7).
The system property value associated with the specified key
string is returned. If there is no property with that key, then null
is returned.
20.18.10 public static String
getProperty
(String key,
String defaults)
throws SecurityException
First, if there is a security manager, its checkPropertyAccess
method
(§20.17.16) 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 (§20.18.7).
The system property value associated with the specified key
string is returned. If there is no property with that key, then the argument defaults
is returned.
20.18.11 public static void
exit
(int status)
throws SecurityException
This method 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 never returns normally.
The call System.exit(n)
is effectively equivalent to the call:
Runtime.getRuntime().exit(n)For a more complete description, see the
exit
method of class Runtime
(§20.16.2).
20.18.12 public static void
gc
()
Calling this method suggests that the Java Virtual Machine expend effort toward recycling discarded 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 recycle all discarded objects.
The call System.gc()
is effectively equivalent to the call:
Runtime.getRuntime().gc()For a more complete description, see the
gc
method of class Runtime
(§20.16.9).
20.18.13 public static void
runFinalization
()
Calling this method suggests that the Java Virtual Machine expend effort toward running the finalization methods of objects that have been found to be discarded but whose finalization 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
()
For a more complete description, see the runFinalization
method of class
Runtime
(§20.16.10).
20.18.14 public static void
load
(String filename)
throws SecurityException, UnsatisfiedLinkError
This method loads a code file with the specified file name from the local file system.
The call System.load(name)
is effectively equivalent to the call:
Runtime.getRuntime().load(name)For a more complete description, see the
load
method of class Runtime
(§20.16.13).
20.18.15 public static void
loadLibrary
(String libname)
throws SecurityException, UnsatisfiedLinkError
This method loads a library code file with the specified library name from the local file system.
The call System.loadLibrary(name)
is effectively equivalent to the call
Runtime.getRuntime().loadLibrary
(name)
For a more complete description, see the loadLibrary
method of class Runtime
(§20.16.14).
20.18.16 public static void
arraycopy
(Object src, int srcOffset,
Object dst, int dstOffset, int length)
throws NullPointerException, ArrayStoreException,
IndexOutOfBoundsException
A subsequence of array components is 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 the positions srcOffset
through srcOffset+length-1
in the source array are copied into the positions
dstOffset
through dstOffset+length-1
, respectively, of the destination array.
If the src
and dst
arguments refer to the same array object, then copying is performed as if the components of the source array at positions srcOffset
through srcOffset+length-1
were first copied to a temporary array of length length
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, then an ArrayStoreException
is thrown and the destination is not modified:
src
argument refers to an object that is not an array.
dst
argument refers to an object that is not an array.
src
argument and dst
argument refer to arrays whose component types are different primitive types.
src
argument refers to an array of primitive component type and the dst
argument refers to an array of reference component type.
src
argument refers to an array of reference component type and the dst
argument refers to an array of primitive component type.
IndexOutOfBoundsException
is thrown and the destination is not modified:
srcOffset
argument is negative.
dstOffset
argument is negative.
length
argument is negative.
srcOffset+length
is greater than src.length
, the length of the src
array.
dstOffset+length
is greater than dst.length
, the length of the dst
array.
srcOffset
through srcOffset+length-1
cannot be converted to the component type of the destination array by assignment conversion, then 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, the source array components from positions srcOffset
through srcOffset+
k-1
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.)java.lang.Runnable
Runnable
interface should be implemented by any class whose instances are
intended to be executed by a new thread. All that is required of such a class is that
it implement a method of no arguments called run
.
public interfaceRunnable
{ public abstract voidrun
(); }
20.19.1 public abstract void
run
()
The general contract of the method run
is that it may take any action whatsoever.
If an object implementing interface Runnable
is used to create a thread (§20.20), then starting the thread will (normally) lead to the invocation of the object's run
method in that separately executing thread.
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Java Language Specification (HTML generated by Suzette Pelouch on February 24, 1998)
Copyright © 1996 Sun Microsystems, Inc.
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