public final class System extends Object
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.
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.
|
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 unique
Console 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 the native library specified by the filename argument.
|
static void |
loadLibrary(String libname)
Loads the native 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 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 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.
|
public static final InputStream in
public static final PrintStream out
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
.
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)
public static final PrintStream err
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.
public static void setIn(InputStream in)
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.
in
- the new standard input stream.SecurityException
- if a security manager exists and its
checkPermission
method doesn't allow
reassigning of the standard input stream.SecurityManager.checkPermission(java.security.Permission)
,
RuntimePermission
public static void setOut(PrintStream out)
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.
out
- the new standard output streamSecurityException
- if a security manager exists and its
checkPermission
method doesn't allow
reassigning of the standard output stream.SecurityManager.checkPermission(java.security.Permission)
,
RuntimePermission
public static void setErr(PrintStream err)
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.
err
- the new standard error output stream.SecurityException
- if a security manager exists and its
checkPermission
method doesn't allow
reassigning of the standard error output stream.SecurityManager.checkPermission(java.security.Permission)
,
RuntimePermission
public static Console console()
Console
object associated
with the current Java virtual machine, if any.public static Channel inheritedChannel() throws IOException
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.
IOException
- If an I/O error occursSecurityException
- If a security manager is present and it does not
permit access to the channel.public static void setSecurityManager(SecurityManager s)
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.
s
- the security manager.SecurityException
- if the security manager has already
been set and its checkPermission
method
doesn't allow it to be replaced.getSecurityManager()
,
SecurityManager.checkPermission(java.security.Permission)
,
RuntimePermission
public static SecurityManager getSecurityManager()
null
is returned.setSecurityManager(java.lang.SecurityManager)
public static long currentTimeMillis()
See the description of the class Date
for
a discussion of slight discrepancies that may arise between
"computer time" and coordinated universal time (UTC).
Date
public static long nanoTime()
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();
one should use t1 - t0 < 0
, not t1 < t0
,
because of the possibility of numerical overflow.public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
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:
src
argument refers to an object that is not an
array.
dest
argument refers to an object that is not an
array.
src
argument and dest
argument refer
to arrays whose component types are different primitive types.
src
argument refers to an array with a primitive
component type and the dest
argument refers to an array
with a reference component type.
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:
srcPos
argument is negative.
destPos
argument is negative.
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.)
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.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
.public static int identityHashCode(Object x)
x
- object for which the hashCode is to be calculatedpublic static Properties getProperties()
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 Deprecated. This property, and the mechanism which implements it, may be removed in a future release. |
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.
SecurityException
- if a security manager exists and its
checkPropertiesAccess
method doesn't allow access
to the system properties.setProperties(java.util.Properties)
,
SecurityException
,
SecurityManager.checkPropertiesAccess()
,
Properties
public static String lineSeparator()
line.separator
.
On UNIX systems, it returns "\n"
; on Microsoft
Windows systems it returns "\r\n"
.
public static void setProperties(Properties props)
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.
props
- the new system properties.SecurityException
- if a security manager exists and its
checkPropertiesAccess
method doesn't allow access
to the system properties.getProperties()
,
Properties
,
SecurityException
,
SecurityManager.checkPropertiesAccess()
public static String getProperty(String 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.
key
- the name of the system property.null
if there is no property with that key.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.setProperty(java.lang.String, java.lang.String)
,
SecurityException
,
SecurityManager.checkPropertyAccess(java.lang.String)
,
getProperties()
public static String getProperty(String key, String def)
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.
key
- the name of the system property.def
- a default value.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.setProperty(java.lang.String, java.lang.String)
,
SecurityManager.checkPropertyAccess(java.lang.String)
,
getProperties()
public static String setProperty(String key, String value)
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.
key
- the name of the system property.value
- the value of the system property.null
if it did not have one.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.getProperty(java.lang.String)
,
getProperty(java.lang.String)
,
getProperty(java.lang.String, java.lang.String)
,
PropertyPermission
,
SecurityManager.checkPermission(java.security.Permission)
public static String clearProperty(String 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.
key
- the name of the system property to be removed.null
if there was no property with that key.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.getProperty(java.lang.String)
,
setProperty(java.lang.String, java.lang.String)
,
Properties
,
SecurityException
,
SecurityManager.checkPropertiesAccess()
public static String getenv(String name)
If a security manager exists, its
checkPermission
method is called with a
permission. This may result in a RuntimePermission
("getenv."+name)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.
name
- the name of the environment variablenull
if the variable is not defined in the system environmentNullPointerException
- if name
is null
SecurityException
- if a security manager exists and its
checkPermission
method doesn't allow access to the environment variable
name
getenv()
,
ProcessBuilder.environment()
public static Map<String,String> getenv()
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
permission. This may result in a RuntimePermission
("getenv.*")SecurityException
being
thrown.
When passing information to a Java subprocess, system properties are generally preferred over environment variables.
SecurityException
- if a security manager exists and its
checkPermission
method doesn't allow access to the process environmentgetenv(String)
,
ProcessBuilder.environment()
public static void exit(int status)
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)
status
- exit status.SecurityException
- if a security manager exists and its checkExit
method doesn't allow exit with the specified status.Runtime.exit(int)
public static void gc()
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()
Runtime.gc()
public static void runFinalization()
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()
Runtime.runFinalization()
@Deprecated public static void runFinalizersOnExit(boolean value)
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.
value
- indicating enabling or disabling of finalizationSecurityException
- if a security manager exists and its checkExit
method doesn't allow the exit.Runtime.exit(int)
,
Runtime.gc()
,
SecurityManager.checkExit(int)
public static void load(String filename)
The call System.load(name)
is effectively equivalent
to the call:
Runtime.getRuntime().load(name)
filename
- the file to load.SecurityException
- if a security manager exists and its
checkLink
method doesn't allow
loading of the specified dynamic libraryUnsatisfiedLinkError
- if either the filename is not an
absolute path name, the native library is not statically
linked with the VM, or the library cannot be mapped to
a native library image by the host system.NullPointerException
- if filename
is
null
Runtime.load(java.lang.String)
,
SecurityManager.checkLink(java.lang.String)
public static void loadLibrary(String libname)
libname
argument. The libname
argument must not contain any platform
specific prefix, file extension or path. If a native library
called libname
is statically linked with the VM, then the
JNI_OnLoad_libname
function exported by the library is invoked.
See the JNI Specification for more details.
Otherwise, the libname argument is loaded from a system library
location and mapped to a native library image in an implementation-
dependent manner.
The call System.loadLibrary(name)
is effectively
equivalent to the call
Runtime.getRuntime().loadLibrary(name)
libname
- the name of the library.SecurityException
- if a security manager exists and its
checkLink
method doesn't allow
loading of the specified dynamic libraryUnsatisfiedLinkError
- if either the libname argument
contains a file path, the native library is not statically
linked with the VM, or the library cannot be mapped to a
native library image by the host system.NullPointerException
- if libname
is
null
Runtime.loadLibrary(java.lang.String)
,
SecurityManager.checkLink(java.lang.String)
public static String mapLibraryName(String libname)
libname
- the name of the library.NullPointerException
- if libname
is
null
loadLibrary(java.lang.String)
,
ClassLoader.findLibrary(java.lang.String)
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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