- java.lang.Object
-
- java.lang.System
-
public final class System extends Object
TheSystem
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
Fields Modifier and Type Field and Description static PrintStream
err
The "standard" error output stream.static InputStream
in
The "standard" input stream.static PrintStream
out
The "standard" output stream.
-
Method Summary
Methods Modifier and Type Method and Description 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 Console
console()
Returns the uniqueConsole
object associated with the current Java virtual machine, if any.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 String
lineSeparator()
Returns the system-dependent line separator string.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 thelibname
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 running Java Virtual Machine's high-resolution time source, 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 theProperties
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.
-
-
-
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 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 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 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 void setOut(PrintStream out)
Reassigns the "standard" output stream.First, if there is a security manager, its
checkPermission
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 void setErr(PrintStream err)
Reassigns the "standard" error output stream.First, if there is a security manager, its
checkPermission
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
-
console
public static Console console()
Returns the uniqueConsole
object associated with the current Java virtual machine, if any.- Returns:
- The system console, if any, otherwise null.
- Since:
- 1.6
-
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 defaultSelectorProvider
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 occursSecurityException
- 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 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 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 itscheckPermission
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 running Java Virtual Machine's high-resolution time source, 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 origin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.
This method provides nanosecond precision, but not necessarily nanosecond resolution (that is, how frequently the value changes) - no guarantees are made except that the resolution is at least as good as that of
currentTimeMillis()
.Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not correctly compute elapsed time due to numerical overflow.
The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.
For example, to measure how long some code takes to execute:
long startTime = System.nanoTime(); // ... the code being measured ... long estimatedTime = System.nanoTime() - startTime;
To compare two nanoTime values
long t0 = System.nanoTime(); ... long t1 = System.nanoTime();
t1 - t0 < 0
, nott1 < t0
, because of the possibility of numerical overflow.- Returns:
- the current value of the running Java Virtual Machine's high-resolution time source, 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 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 the
src
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.If
dest
isnull
, then aNullPointerException
is thrown.If
src
isnull
, then aNullPointerException
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 anddest
argument refer to arrays whose component types are different primitive types. - The
src
argument refers to an array with a primitive component type and thedest
argument refers to an array with a reference component type. - The
src
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, 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 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 position
srcPos
throughsrcPos+length-1
cannot be converted to the component type of the destination array by assignment conversion, anArrayStoreException
is thrown. In this case, let k 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
.
- The
-
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 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 the
getProperties
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
-
lineSeparator
public static String lineSeparator()
Returns the system-dependent line separator string. It always returns the same value - the initial value of the system propertyline.separator
.On UNIX systems, it returns
"\n"
; on Microsoft Windows systems it returns"\r\n"
.
-
setProperties
public static void setProperties(Properties props)
Sets the system properties to theProperties
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 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 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 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 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 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 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 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 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 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,
or
null
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 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 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,
or
null
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 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
permission. This may result in aRuntimePermission
("getenv."+name)SecurityException
being thrown. If no exception is thrown the value of the variablename
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 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, or
null
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 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 typeString
will throw aClassCastException
.The returned map and its collection views may not obey the general contract of the
Object.equals(java.lang.Object)
andObject.hashCode()
methods.The returned map is typically case-sensitive on all platforms.
If a security manager exists, its
checkPermission
method is called with a
permission. This may result in aRuntimePermission
("getenv.*")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 itscheckPermission
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 classRuntime
. 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 itscheckExit
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 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 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 itscheckExit
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 itscheckLink
method doesn't allow loading of the specified dynamic libraryUnsatisfiedLinkError
- if the file does not exist.NullPointerException
- iffilename
isnull
- 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 thelibname
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 callRuntime.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 libraryUnsatisfiedLinkError
- if the library does not exist.NullPointerException
- iflibname
isnull
- 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
- iflibname
isnull
- Since:
- 1.2
- See Also:
loadLibrary(java.lang.String)
,ClassLoader.findLibrary(java.lang.String)
-
-
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