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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 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 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 null in Android

static String getenv(String name)

Gets the value of the specified environment variable.

static Map<StringString> 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 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 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)

This method was deprecated in API level 1. 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 sm)

Throws SecurityException (except in case sm == null).

Inherited methods

From class java.lang.Object

Fields

err

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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.

See also:

Public methods

arraycopy

Added in API level 1
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 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 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

Added in API level 1
String clearProperty (String key)

Removes the system property indicated by the specified key.

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
NullPointerException if key is null.
IllegalArgumentException if key is empty.

See also:

console

Added in API level 9
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

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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:

getProperties

Added in API level 1
Properties getProperties ()

Determines the current system properties.

The following properties are always provided by the Dalvik VM:

Name Meaning Example
file.separator 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 /system
java.io.tmpdir See createTempFile(String, String) /sdcard
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 armv7l
os.name OS (kernel) name Linux
os.version OS (kernel) version 2.6.32.9-g103d848
path.separator See 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.

Returns
Properties the system properties

See also:

getProperty

Added in API level 1
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
NullPointerException if key is null.
IllegalArgumentException if key is empty.

See also:

getProperty

Added in API level 1
String getProperty (String key)

Gets the system property indicated by the specified key. 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
NullPointerException if key is null.
IllegalArgumentException if key is empty.

See also:

getSecurityManager

Added in API level 1
SecurityManager getSecurityManager ()

Always returns null in Android

Returns
SecurityManager null in Android

getenv

Added in API level 1
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 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 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

Added in API level 1
Map<StringString> 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 equals(Object) and hashCode() methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv.*") permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

Returns
Map<StringString> 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

See also:

identityHashCode

Added in API level 1
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

Added in API level 1
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

Added in API level 19
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

load

Added in API level 1
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 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 the file does not exist.
NullPointerException if filename is null

See also:

loadLibrary

Added in API level 1
void loadLibrary (String libname)

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

 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 the library does not exist.
NullPointerException if libname is null

See also:

mapLibraryName

Added in API level 1
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

See also:

nanoTime

Added in API level 1
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.

Returns
long the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds

runFinalization

Added in API level 1
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

Added in API level 1
void runFinalizersOnExit (boolean value)

This method was deprecated in API level 1.
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.

See also:

setErr

Added in API level 1
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.

See also:

setIn

Added in API level 1
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.

See also:

setOut

Added in API level 1
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.

See also:

setProperties

Added in API level 1
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

Added in API level 1
String setProperty (String key, 
                String value)

Sets the system property indicated by the specified key.

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
NullPointerException if key or value is null.
IllegalArgumentException if key is empty.

See also:

setSecurityManager

Added in API level 1
void setSecurityManager (SecurityManager sm)

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
sm SecurityManager: a security manager
Throws
SecurityException always, unless sm == null
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