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 and environment
variables; a means of loading files and libraries; and a utility
method for quickly copying a portion of an array.
author: unascribed version: 1.165, 06/11/07 since: JDK1.0
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 by src to the destination array
referenced by dest.
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. Parameters: def - a default value.
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 .
native 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().
mapLibraryName(String libname) Maps a library name into a platform-specific string representing
a native library.
Parameters: libname - the name of the library.
native 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).
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.
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.
public static void
setErr(PrintStream err) Reassigns the "standard" error output stream.
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.
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.
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.
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:
native public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)(Code)
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 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:
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 and dest 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 dest argument refers to an array
with a reference component type.
The 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:
The srcPos argument is negative.
The destPos argument is negative.
The 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.)
Parameters: src - the source array. Parameters: srcPos - starting position in the source array. Parameters: dest - the destination array. Parameters: destPos - starting position in the destination data. Parameters: length - the number of array elements to be copied. exception: IndexOutOfBoundsException - if copying would causeaccess of data outside array bounds. exception: ArrayStoreException - if an element in the srcarray could not be stored into the dest arraybecause of a type mismatch. exception: NullPointerException - if either src ordest is null.
Removes the system property indicated by the specified 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.
Returns the unique
java.io.Console Console object associated
with the current Java virtual machine, if any.
The system console, if any, otherwise null. since: 1.6
currentTimeMillis
native public static long currentTimeMillis()(Code)
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).
the difference, measured in milliseconds, betweenthe current time and midnight, January 1, 1970 UTC. See Also:java.util.Date
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.
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 its checkExitmethod doesn't allow exit with the specified status. See Also:java.lang.Runtime.exit(int)
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:
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
System.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
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.
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. Parameters: def - a default value. the string value of the system property,or the default value if there is no property with that key. exception: SecurityException - if a security manager exists and itscheckPropertyAccess method doesn't allowaccess to the specified system property. exception: NullPointerException - if key isnull. exception: IllegalArgumentException - if key is empty. See Also:System.setProperty See Also:java.lang.SecurityManager.checkPropertyAccess(java.lang.String) See Also:java.lang.System.getProperties
Gets the system security interface.
if a security manager has already been established for thecurrent application, then that security manager is returned;otherwise, null is returned. See Also:System.setSecurityManager
Gets the value of the specified environment variable. An
environment variable is a system-dependent external named
value.
If a security manager exists, its
SecurityManager.checkPermission checkPermission method is called with a
RuntimePermission ("getenv."+name)
permission. This may result in a
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.
Parameters: name - the name of the environment variable the string value of the variable, or nullif the variable is not defined in the system environment throws: NullPointerException - if name is null throws: SecurityException - if a security manager exists and itsSecurityManager.checkPermission checkPermissionmethod doesn't allow access to the environment variablename See Also:System.getenv() See Also:ProcessBuilder.environment
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 type
String will throw a
ClassCastException .
The returned map and its collection views may not obey the
general contract of the
Object.equals and
Object.hashCode methods.
The returned map is typically case-sensitive on all platforms.
native public static int identityHashCode(Object x)(Code)
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 the hashCode since: JDK1.1
In addition to the network-oriented channels described in
java.nio.channels.spi.SelectorProvider.inheritedChannelinheritedChannel , this method may return other kinds of
channels in the future.
The inherited channel, if any, otherwise null. throws: IOException - If an I/O error occurs throws: SecurityException - If a security manager is present and it does notpermit access to the channel. since: 1.5
public static void loadLibrary(String libname)(Code)
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.
The call System.loadLibrary(name) is effectively
equivalent to the call
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
System.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.
the current value of the running Java Virtual Machine'shigh-resolution time source, in nanoseconds since: 1.5
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.
The call System.runFinalization() is effectively
equivalent to the call:
public static void runFinalizersOnExit(boolean value)(Code)
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 its checkExitmethod doesn't allow the exit. See Also:java.lang.Runtime.exit(int) See Also:java.lang.Runtime.gc See Also:java.lang.SecurityManager.checkExit(int) since: JDK1.1
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.
Parameters: err - the new standard error output stream. throws: SecurityException - if a security manager exists and itscheckPermission method doesn't allowreassigning of the standard error output stream. See Also:SecurityManager.checkPermission See Also:java.lang.RuntimePermission since: JDK1.1
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.
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.
Parameters: out - the new standard output stream throws: SecurityException - if a security manager exists and itscheckPermission method doesn't allowreassigning of the standard output stream. See Also:SecurityManager.checkPermission See Also:java.lang.RuntimePermission since: JDK1.1
Sets the system property indicated by the specified 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 set to the given
value.
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.
Parameters: s - the security manager. exception: SecurityException - if the security manager has alreadybeen set and its checkPermission methoddoesn't allow it to be replaced. See Also:System.getSecurityManager See Also:SecurityManager.checkPermission See Also:java.lang.RuntimePermission
