System
public
final
class
System
extends Object
java.lang.Object | |
↳ | java.lang.System |
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.
Summary
Fields | |
---|---|
public
static
final
PrintStream |
err
The "standard" error output stream. |
public
static
final
InputStream |
in
The "standard" input stream. |
public
static
final
PrintStream |
out
The "standard" output stream. |
Public methods | |
---|---|
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 |
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
Properties
|
getProperties()
Determines the current system properties. |
static
String
|
getProperty(String key, String def)
Gets the system property indicated by the specified key. |
static
String
|
getProperty(String key)
Gets the system property indicated by the specified key. |
static
SecurityManager
|
getSecurityManager()
Always returns |
static
String
|
getenv(String name)
Gets the value of the specified environment variable. |
static
Map<String, String>
|
getenv()
Returns an unmodifiable string map view of the current system environment. |
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 |
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)
This method was deprecated in API level 15. 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)
Attempts to set all system properties. |
static
String
|
setProperty(String key, String value)
Sets the system property indicated by the specified key. |
static
void
|
setSecurityManager(SecurityManager s)
Throws |
Inherited methods | |
---|---|
Fields
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.
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 class PrintStream
.
Public methods
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 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 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
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 |
Object : the source array. |
srcPos |
int : starting position in the source array. |
dest |
Object : the destination array. |
destPos |
int : starting position in the destination data. |
length |
int : 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 the src
array could not be stored into the dest array
because of a type mismatch. |
NullPointerException |
if either src or
dest is null . |
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 a PropertyPermission(key, "write")
permission. This may result in a SecurityException being thrown.
If no exception is thrown, the specified property is removed.
Parameters | |
---|---|
key |
String : the name of the system property to be removed. |
Returns | |
---|---|
String |
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 its
checkPropertyAccess method doesn't allow
access to the specified system property. |
NullPointerException |
if key is
null . |
IllegalArgumentException |
if key is empty. |
console
public static Console console ()
Returns the unique Console
object associated
with the current Java virtual machine, if any.
Returns | |
---|---|
Console |
The system console, if any, otherwise null. |
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 | |
---|---|
long |
the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC. |
See also:
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.
The call System.exit(n)
is effectively equivalent to
the call:
Runtime.getRuntime().exit(n)
Parameters | |
---|---|
status |
int : exit status. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its checkExit
method doesn't allow exit with the specified status. |
See also:
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:
At API level 34 and lower, this does nothing unless it is preceded or followed by a runFinalization call. Runtime.getRuntime().gc() always attempts to reclaim space.Runtime.getRuntime().gc()
Calling this indiscriminately is likely to severely degrade performance. Intended primarily for testing.
See also:
getProperties
public static Properties getProperties ()
Determines the current system properties.
The following properties are always provided by the Dalvik VM:
Name | Meaning | Example |
file.separator | File.separator | / |
java.class.path | System class path | . |
java.class.version | (Not useful on Android) | 50.0 |
java.compiler | (Not useful on Android) | Empty |
java.ext.dirs | (Not useful on Android) | Empty |
java.home | Location of the VM on the file system | /apex/com.android.art/ |
java.io.tmpdir | Location of a temporary directory. The location varies by application. See File.createTempFile(String, String) | /data/user/0/com.android.app/cache |
java.library.path | Search path for JNI libraries | /vendor/lib:/system/lib |
java.vendor | Human-readable VM vendor | The Android Project |
java.vendor.url | URL for VM vendor's web site | http://www.android.com/ |
java.version | (Not useful on Android) | 0 |
java.specification.version | VM libraries version | 0.9 |
java.specification.vendor | VM libraries vendor | The Android Project |
java.specification.name | VM libraries name | Dalvik Core Library |
java.vm.version | VM implementation version | 1.2.0 |
java.vm.vendor | VM implementation vendor | The Android Project |
java.vm.name | VM implementation name | Dalvik |
java.vm.specification.version | VM specification version | 0.9 |
java.vm.specification.vendor | VM specification vendor | The Android Project |
java.vm.specification.name | VM specification name | Dalvik Virtual Machine Specification |
line.separator | The system line separator | \n |
os.arch | OS architecture | aarch64 |
os.name | OS (kernel) name | Linux |
os.version | OS (kernel) version | 5.10.98-g6ea688a79989 |
path.separator | See File.pathSeparator | : |
user.dir | Base of non-absolute paths | / |
user.home | (Not useful on Android) | Empty |
user.name | (Not useful on Android) | Empty |
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(java.lang.String)
operation.
Returns | |
---|---|
Properties |
the system properties |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkPropertiesAccess method doesn't allow access
to the system properties. |
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 |
String : the name of the system property. |
def |
String : a default value. |
Returns | |
---|---|
String |
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 its
checkPropertyAccess method doesn't allow
access to the specified system property. |
NullPointerException |
if key is
null . |
IllegalArgumentException |
if key is empty. |
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 |
String : the name of the system property. |
Returns | |
---|---|
String |
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 its
checkPropertyAccess method doesn't allow
access to the specified system property. |
NullPointerException |
if key is
null . |
IllegalArgumentException |
if key is empty. |
getSecurityManager
public static SecurityManager getSecurityManager ()
Always returns null
in Android
Returns | |
---|---|
SecurityManager |
null in Android |
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 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.
Parameters | |
---|---|
name |
String : the name of the environment variable |
Returns | |
---|---|
String |
the string value of the variable, or null
if the variable is not defined in the system environment |
Throws | |
---|---|
NullPointerException |
if name is null |
SecurityException |
if a security manager exists and its
checkPermission
method doesn't allow access to the environment variable
name |
See also:
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 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.
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.
Returns | |
---|---|
Map<String, String> |
the environment as a map of variable names to values |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkPermission
method doesn't allow access to the process environment |
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 : object for which the hashCode is to be calculated |
Returns | |
---|---|
int |
the hashCode |
inheritedChannel
public static Channel inheritedChannel ()
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 default
SelectorProvider
object.
In addition to the network-oriented channels described in
inheritedChannel
, this method may return other kinds of
channels in the future.
Returns | |
---|---|
Channel |
The inherited channel, if any, otherwise null. |
Throws | |
---|---|
IOException |
If an I/O error occurs |
SecurityException |
If a security manager is present and it does not permit access to the channel. |
lineSeparator
public static String lineSeparator ()
Returns the system-dependent line separator string. It always
returns the same value - the initial value of the system property line.separator
.
On UNIX systems, it returns "\n"
; on Microsoft
Windows systems it returns "\r\n"
.
Returns | |
---|---|
String |
the system-dependent line separator string |
load
public static void load (String filename)
Loads the native library specified by the filename argument. The filename argument must be an absolute path name. If the filename argument, when stripped of any platform-specific library prefix, path, and file extension, indicates a library whose name is, for example, L, and a native library called L is statically linked with the VM, then the JNI_OnLoad_L function exported by the library is invoked rather than attempting to load a dynamic library. A filename matching the argument does not have to exist in the file system. See the JNI Specification for more details. Otherwise, the filename argument is mapped to a native library image in an implementation-dependent manner.
The call System.load(name)
is effectively equivalent
to the call:
Runtime.getRuntime().load(name)
Parameters | |
---|---|
filename |
String : the file to load. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkLink method doesn't allow
loading of the specified dynamic library |
UnsatisfiedLinkError |
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 |
loadLibrary
public static void loadLibrary (String libname)
Loads the native library specified by the 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)
Parameters | |
---|---|
libname |
String : the name of the library. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkLink method doesn't allow
loading of the specified dynamic library |
UnsatisfiedLinkError |
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 |
mapLibraryName
public static String mapLibraryName (String libname)
Maps a library name into a platform-specific string representing a native library.
Parameters | |
---|---|
libname |
String : the name of the library. |
Returns | |
---|---|
String |
a platform-dependent native library name. |
Throws | |
---|---|
NullPointerException |
if libname is
null |
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();
one should use t1 - t0 < 0
, not t1 < t0
,
because of the possibility of numerical overflow.
The value returned by this method does not account for elapsed
time during deep sleep. For timekeeping facilities available on
Android see SystemClock
.
Returns | |
---|---|
long |
the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds |
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.
The call System.runFinalization()
is effectively
equivalent to the call:
Runtime.getRuntime().runFinalization()
See also:
runFinalizersOnExit
public static void runFinalizersOnExit (boolean value)
This method was deprecated
in API level 15.
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 |
boolean : indicating enabling or disabling of finalization |
Throws | |
---|---|
SecurityException |
if a security manager exists and its checkExit
method doesn't allow the exit. |
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 a RuntimePermission("setIO")
permission
to see if it's ok to reassign the "standard" error output stream.
Parameters | |
---|---|
err |
PrintStream : the new standard error output stream. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkPermission method doesn't allow
reassigning of the standard error output stream. |
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 a RuntimePermission("setIO")
permission
to see if it's ok to reassign the "standard" input stream.
Parameters | |
---|---|
in |
InputStream : the new standard input stream. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkPermission method doesn't allow
reassigning of the standard input stream. |
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 a RuntimePermission("setIO")
permission
to see if it's ok to reassign the "standard" output stream.
Parameters | |
---|---|
out |
PrintStream : the new standard output stream |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkPermission method doesn't allow
reassigning of the standard output stream. |
setProperties
public static void setProperties (Properties props)
Attempts to set all system properties. Copies all properties from
p
and discards system properties that are read only and cannot
be modified. See getProperty(String)
for a list of such properties.
Parameters | |
---|---|
props |
Properties |
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 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.
Parameters | |
---|---|
key |
String : the name of the system property. |
value |
String : the value of the system property. |
Returns | |
---|---|
String |
the previous value of the system property,
or null if it did not have one. |
Throws | |
---|---|
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. |
setSecurityManager
public static void setSecurityManager (SecurityManager s)
Throws SecurityException
(except in case sm == null
).
Security managers do not provide a secure environment for
executing untrusted code and are unsupported on Android. Untrusted code
cannot be safely isolated within a single VM on Android, so this method
always throws a SecurityException
when passed a non-null SecurityManager
Parameters | |
---|---|
s |
SecurityManager : a security manager |
Throws | |
---|---|
SecurityException |
always, unless sm == null |