java.lang

Class System


public final class System
extends Object

System represents system-wide resources; things that represent the general environment. As such, all methods are static.
Since:
1.0

Field Summary

static PrintStream
err
The standard output PrintStream.
static InputStream
in
The standard InputStream.
static PrintStream
out
The standard output PrintStream.

Method Summary

static Map
String> getenv()
Returns an unmodifiable view of the system environment variables.
static void
arraycopy(Object src, int srcStart, Object dest, int destStart, int len)
Copy one array onto another from src[srcStart] ...
static String
clearProperty(String key)
Remove a single system property by name.
static Console
console()
static long
currentTimeMillis()
Get the current time, measured in the number of milliseconds from the beginning of Jan.
static void
exit(int status)
Terminate the Virtual Machine.
static void
gc()
Calls the garbage collector.
static Properties
getProperties()
Get all the system properties at once.
static String
getProperty(String key)
Get a single system property by name.
static String
getProperty(String key, String def)
Get a single system property by name.
static SecurityManager
getSecurityManager()
Get the current SecurityManager.
static String
getenv(String name)
Gets the value of an environment variable.
static int
identityHashCode(Object o)
Get a hash code computed by the VM for the Object.
static Channel
inheritedChannel()
Returns the inherited channel of the VM.
static String
lineSeparator()
Returns the system-dependent line separator.
static void
load(String filename)
Load a code file using its explicit system-dependent filename.
static void
loadLibrary(String libname)
Load a library using its explicit system-dependent filename.
static String
mapLibraryName(String libname)
Convert a library name to its platform-specific variant.
static long
nanoTime()
Returns the current value of a nanosecond-precise system timer.
static void
runFinalization()
Runs object finalization on pending objects.
static void
runFinalizersOnExit(boolean finalizeOnExit)
Deprecated. never rely on finalizers to do a clean, thread-safe, mop-up from your code
static void
setErr(PrintStream err)
Set err to a new PrintStream.
static void
setIn(InputStream in)
Set in to a new InputStream.
static void
setOut(PrintStream out)
Set out to a new PrintStream.
static void
setProperties(Properties properties)
Set all the system properties at once.
static String
setProperty(String key, String value)
Set a single system property by name.
static void
setSecurityManager(SecurityManager sm)
Set the current SecurityManager.

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

err

public static final PrintStream err
The standard output PrintStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using setErr(PrintStream) through some hefty VM magic.

This corresponds to the C stderr and C++ cerr variables, which typically output error messages to the screen, but may be used to pipe output to other processes or files. That should all be transparent to you, however.


in

public static final InputStream in
The standard InputStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using setIn(InputStream) through some hefty VM magic.

This corresponds to the C stdin and C++ cin variables, which typically input from the keyboard, but may be used to pipe input from other processes or files. That should all be transparent to you, however.


out

public static final PrintStream out
The standard output PrintStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using setOut(PrintStream) through some hefty VM magic.

This corresponds to the C stdout and C++ cout variables, which typically output normal messages to the screen, but may be used to pipe output to other processes or files. That should all be transparent to you, however.

Method Details

String> getenv

public static MapString> getenv()
Returns an unmodifiable view of the system environment variables. If the underlying system does not support environment variables, an empty map is returned.

The returned map is read-only and does not accept queries using null keys or values, or those of a type other than String. Attempts to modify the map will throw an UnsupportedOperationException, while attempts to pass in a null value will throw a NullPointerException. Types other than String throw a ClassCastException.

As the returned map is generated using data from the underlying platform, it may not comply with the equals() and hashCode() contracts. It is also likely that the keys of this map will be case-sensitive.

Use of this method may require a security check for the RuntimePermission "getenv.*".

Returns:
a map of the system environment variables.
Throws:
SecurityException - if the checkPermission method of an installed security manager prevents access to the system environment variables.
Since:
1.5

arraycopy

public static void arraycopy(Object src,
                             int srcStart,
                             Object dest,
                             int destStart,
                             int len)
Copy one array onto another from src[srcStart] ... src[srcStart+len-1] to dest[destStart] ... dest[destStart+len-1]. First, the arguments are validated: neither array may be null, they must be of compatible types, and the start and length must fit within both arrays. Then the copying starts, and proceeds through increasing slots. If src and dest are the same array, this will appear to copy the data to a temporary location first. An ArrayStoreException in the middle of copying will leave earlier elements copied, but later elements unchanged.
Parameters:
src - the array to copy elements from
srcStart - the starting position in src
dest - the array to copy elements to
destStart - the starting position in dest
len - the number of elements to copy
Throws:
NullPointerException - if src or dest is null
ArrayStoreException - if src or dest is not an array, if they are not compatible array types, or if an incompatible runtime type is stored in dest
IndexOutOfBoundsException - if len is negative, or if the start or end copy position in either array is out of bounds

clearProperty

public static String clearProperty(String key)
Remove a single system property by name. A security check may be performed, checkPropertyAccess(key, "write").
Parameters:
key - the name of the system property to remove
Returns:
the previous value, or null
Throws:
SecurityException - if permission is denied
NullPointerException - if key is null
IllegalArgumentException - if key is ""
Since:
1.5

console

public static Console console()

currentTimeMillis

public static long currentTimeMillis()
Get the current time, measured in the number of milliseconds from the beginning of Jan. 1, 1970. This is gathered from the system clock, with any attendant incorrectness (it may be timezone dependent).
Returns:
the current time
See Also:
Date

exit

public static void exit(int status)
Terminate the Virtual Machine. This just calls Runtime.getRuntime().exit(status), and never returns. Obviously, a security check is in order, checkExit.
Parameters:
status - the exit status; by convention non-zero is abnormal
Throws:
SecurityException - if permission is denied

gc

public static void gc()
Calls the garbage collector. This is only a hint, and it is up to the implementation what this hint suggests, but it usually causes a best-effort attempt to reclaim unused memory from discarded objects. This calls Runtime.getRuntime().gc().
See Also:
Runtime.gc()

getProperties

public static Properties getProperties()
Get all the system properties at once. A security check may be performed, checkPropertiesAccess. Note that a security manager may allow getting a single property, but not the entire group.

The required properties include:

In addition, gnu defines several other properties, where ? stands for each character in '0' through '9':
Returns:
the system properties, will never be null
Throws:
SecurityException - if permission is denied

getProperty

public static String getProperty(String key)
Get a single system property by name. A security check may be performed, checkPropertyAccess(key).
Parameters:
key - the name of the system property to get
Returns:
the property, or null if not found
Throws:
SecurityException - if permission is denied
NullPointerException - if key is null
IllegalArgumentException - if key is ""

getProperty

public static String getProperty(String key,
                                 String def)
Get a single system property by name. A security check may be performed, checkPropertyAccess(key).
Parameters:
key - the name of the system property to get
def - the default
Returns:
the property, or def if not found
Throws:
SecurityException - if permission is denied
NullPointerException - if key is null
IllegalArgumentException - if key is ""

getSecurityManager

public static SecurityManager getSecurityManager()
Get the current SecurityManager. If the SecurityManager has not been set yet, then this method returns null.
Returns:
the current SecurityManager, or null

getenv

public static String getenv(String name)
Gets the value of an environment variable.
Parameters:
name - the name of the environment variable
Returns:
the string value of the variable or null when the environment variable is not defined.
Throws:
NullPointerException -
SecurityException - if permission is denied
Since:
1.5

identityHashCode

public static int identityHashCode(Object o)
Get a hash code computed by the VM for the Object. This hash code will be the same as Object's hashCode() method. It is usually some convolution of the pointer to the Object internal to the VM. It follows standard hash code rules, in that it will remain the same for a given Object for the lifetime of that Object.
Parameters:
o - the Object to get the hash code for
Returns:
the VM-dependent hash code for this Object
Since:
1.1

inheritedChannel

public static Channel inheritedChannel()
            throws IOException
Returns the inherited channel of the VM. This wraps the inheritedChannel() call of the system's default SelectorProvider.
Returns:
the inherited channel of the VM
Throws:
IOException - If an I/O error occurs
SecurityException - If an installed security manager denies access to RuntimePermission("inheritedChannel")
Since:
1.5

lineSeparator

public static String lineSeparator()
Returns the system-dependent line separator.
Returns:
the system-dependent line separator.

load

public static void load(String filename)
Load a code file using its explicit system-dependent filename. A security check may be performed, checkLink. This just calls Runtime.getRuntime().load(filename).

The library is loaded using the class loader associated with the class associated with the invoking method.

Parameters:
filename - the code file to load
Throws:
SecurityException - if permission is denied

loadLibrary

public static void loadLibrary(String libname)
Load a library using its explicit system-dependent filename. A security check may be performed, checkLink. This just calls Runtime.getRuntime().load(filename).

The library is loaded using the class loader associated with the class associated with the invoking method.

Parameters:
libname - the library file to load
Throws:
SecurityException - if permission is denied

mapLibraryName

public static String mapLibraryName(String libname)
Convert a library name to its platform-specific variant.
Parameters:
libname - the library name, as used in loadLibrary
Returns:
the platform-specific mangling of the name
Since:
1.2

nanoTime

public static long nanoTime()
Returns the current value of a nanosecond-precise system timer. The value of the timer is an offset relative to some arbitrary fixed time, which may be in the future (making the value negative). This method is useful for timing events where nanosecond precision is required. This is achieved by calling this method before and after the event, and taking the difference betweent the two times:

long startTime = System.nanoTime();
... event code ...
long endTime = System.nanoTime();
long duration = endTime - startTime;

Note that the value is only nanosecond-precise, and not accurate; there is no guarantee that the difference between two values is really a nanosecond. Also, the value is prone to overflow if the offset exceeds 2^63.

Returns:
the time of a system timer in nanoseconds.
Since:
1.5

runFinalization

public static void runFinalization()
Runs object finalization on pending objects. This is only a hint, and it is up to the implementation what this hint suggests, but it usually causes a best-effort attempt to run finalizers on all objects ready to be reclaimed. This calls Runtime.getRuntime().runFinalization().

runFinalizersOnExit

public static void runFinalizersOnExit(boolean finalizeOnExit)

Deprecated. never rely on finalizers to do a clean, thread-safe, mop-up from your code

Tell the Runtime whether to run finalization before exiting the JVM. This is inherently unsafe in multi-threaded applications, since it can force initialization on objects which are still in use by live threads, leading to deadlock; therefore this is disabled by default. There may be a security check, checkExit(0). This calls Runtime.runFinalizersOnExit().
Parameters:
finalizeOnExit - whether to run finalizers on exit
Throws:
SecurityException - if permission is denied
Since:
1.1

setErr

public static void setErr(PrintStream err)
Set err to a new PrintStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").
Parameters:
err - the new PrintStream
Throws:
SecurityException - if permission is denied
Since:
1.1

setIn

public static void setIn(InputStream in)
Set in to a new InputStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").
Parameters:
in - the new InputStream
Throws:
SecurityException - if permission is denied
Since:
1.1

setOut

public static void setOut(PrintStream out)
Set out to a new PrintStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").
Parameters:
out - the new PrintStream
Throws:
SecurityException - if permission is denied
Since:
1.1

setProperties

public static void setProperties(Properties properties)
Set all the system properties at once. A security check may be performed, checkPropertiesAccess. Note that a security manager may allow setting a single property, but not the entire group. An argument of null resets the properties to the startup default.
Parameters:
properties - the new set of system properties
Throws:
SecurityException - if permission is denied

setProperty

public static String setProperty(String key,
                                 String value)
Set a single system property by name. A security check may be performed, checkPropertyAccess(key, "write").
Parameters:
key - the name of the system property to set
value - the new value
Returns:
the previous value, or null
Throws:
SecurityException - if permission is denied
NullPointerException - if key is null
IllegalArgumentException - if key is ""
Since:
1.2

setSecurityManager

public static void setSecurityManager(SecurityManager sm)
Set the current SecurityManager. If a security manager already exists, then RuntimePermission("setSecurityManager") is checked first. Since this permission is denied by the default security manager, setting the security manager is often an irreversible action. Spec Note: Don't ask me, I didn't write it. It looks pretty vulnerable; whoever gets to the gate first gets to set the policy. There is probably some way to set the original security manager as a command line argument to the VM, but I don't know it.
Parameters:
sm - the new SecurityManager
Throws:
SecurityException - if permission is denied

System.java -- useful methods to interface with the system Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2007, 2012 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.