compact1, compact2, compact3

java.lang

Class System

java.lang.Object

java.lang.System

public final class System
extends Object

System contains static fields and methods that permit access to properties of the current environment. This includes standard input, output and error streams, environment variables and security manager.

Since:

1.0

Field Summary

Fields

Modifier and Type	Field and Description
static PrintStream	err The "standard" error output stream.
static InputStream	in The "standard" input stream.
static PrintStream	out The "standard" output stream.

Method Summary

Modifier and Type	Method and Description
static void	arraycopy(Object src, int sp, Object dst, int dp, int length) arraycopy copies element from one array to another array.
static String	clearProperty(String key) clearProperty removes a given property
static Console	console() Returns the unique Console object associated with the current Java virtual machine, if any.
static long	currentTimeMillis() setProp(b75b89c0,b75b89a0) currentTimeMillis returns the current time measured from 00:00:00 1 Jan 1970 in milliseconds.
static void	exit(int status) Terminates the currently running Java Virtual Machine.
static void	gc() gc indicates to the VM that it may be a good time to run the garbage collector.
static Map<String,String>	getenv() Returns an unmodifiable string map view of the current system environment.
static String	getenv(String name) Gets the value of the specified environment variable.
static Properties	getProperties() Determines the current system properties.
static String	getProperty(String key) getProperty returns the value of a given property.
static String	getProperty(String key, String def) getProperty returns the value of a given property, or a default value if the property is not set.
static SecurityManager	getSecurityManager() getSecurityManager returns the currently installed security manager.
static int	identityHashCode(Object x) identityHashCode returns the same hash code that is returned by the not-overwritten Object.hashCode() function.
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) load a library or system file.
static void	loadLibrary(String libname) loadLibrary loads a system library with the given name into the system.
static String	mapLibraryName(String libname) mapLibraryName maps a library name to the corresponding platform-specific native library name
static long	nanoTime() nanoTime returns a raw high resolution time value.
static void	runFinalization() runFinalization runs the finalize() method of all objects that are pending for finalization.
static void	runFinalizersOnExit(boolean value) Deprecated. running the finalizers is unsafe, therefore this method is deprecated.
static void	setErr(PrintStream err) Reassigns the "standard" error output stream.
static void	setIn(InputStream in) Reassigns the "standard" input stream.
static void	setOut(PrintStream out) Reassigns the "standard" output stream.
static void	setProperties(Properties props) Sets the system properties to the Properties argument.
static String	setProperty(String key, String value) setProperty sets property for a given key.
static void	setSecurityManager(SecurityManager s) Sets the System security.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

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.

See Also:

PrintStream.println(), PrintStream.println(boolean), PrintStream.println(char), PrintStream.println(char[]), PrintStream.println(double), PrintStream.println(float), PrintStream.println(int), PrintStream.println(long), PrintStream.println(java.lang.Object), PrintStream.println(java.lang.String)

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.

Method Detail

identityHashCode

public static int identityHashCode(Object x)

identityHashCode returns the same hash code that is returned by the not-overwritten Object.hashCode() function. This hashcode value is guaranteed never to change during the lifetime of an object.

For JamaicaVM, on a 32-bit system, this function returns the object's memory address modified with shift and XOR operations. The hashcode is unique for every object with JamaicaVM.

Parameters:

x - an object or null

Returns:

a hashcode value for x, or 0 if x is null.

Since:

1.1

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.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard input stream.

Parameters:

in - the new standard input stream.

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

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.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream.

Parameters:

out - the new standard output stream

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

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.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard error output stream.

Parameters:

err - the new standard error output stream.

Since:

JDK1.1

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

console

public static Console console()

Returns the unique Console object associated with the current Java virtual machine, if any.

Returns:

The system console, if any, otherwise null.

Since:

1.6

inheritedChannel

public static Channel inheritedChannel()
                                throws IOException

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.

Throws:

IOException - If an I/O error occurs

SecurityException - If a security manager is present and it does not permit access to the channel.

Returns:

The inherited channel, if any, otherwise null.

Since:

1.5

setSecurityManager

public static void setSecurityManager(SecurityManager s)

Sets the System security.

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.

Throws:

SecurityException - if the security manager has already been set and its checkPermission method doesn't allow it to be replaced.

Parameters:

s - the security manager.

See Also:

getSecurityManager(), SecurityManager.checkPermission(java.security.Permission), RuntimePermission

getSecurityManager

public static SecurityManager getSecurityManager()

getSecurityManager returns the currently installed security manager.

Returns:

the current security manager or null if none.

arraycopy

public static void arraycopy(Object src,
                             int sp,
                             Object dst,
                             int dp,
                             int length)

arraycopy copies element from one array to another array.

src and dst must be arrays E[] of a common element type E that is one of boolean, byte, short, char, int, float, long, double or Object.

ensure

   (for all i=0..length-1: dst[dp+i] == old src[sp+i]);
 

Throws:

NullPointerException - if src or dst are null. No modifications are made in this case.

ArrayStoreException - if src or dst is not an array or if src and dst are arrays of different primitive element types or if one of src and dst is an array of references and the other array is an array of primitive types. No modifications are made in these cases.

ArrayStoreException - if src and dst are reference arrays and one element of src[sp+k] cannot be stored into dst[dp+k]. In this case, elements from src[sp+0..sp+k-1] will have been copied to dst[dp+0..dp+k-1], or, if src==dst and dp>sp, then src[sp+k+1..sp+length-1] will haven been copied to dst[dp+k+1..dp+length-1].

IndexOutOfBoundsException - if sp<0 || dp<0 || length<0 || sp+length>src.length || dp+length>dst.length.

Parameters:

src - a source array

sp - start index in src

dst - (out!) the destination array

dp - start index in dst

length - the number of elements to be copied.

currentTimeMillis

public static long currentTimeMillis()

setProp(b75b89c0,b75b89a0) currentTimeMillis returns the current time measured from 00:00:00 1 Jan 1970 in milliseconds.

The underlying system may have a granularity that is less than this function.

Returns:

the current time in milliseconds.

getProperties

public static Properties getProperties()

Determines the current system properties.

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The current set of system properties for use by the getProperty(String) method is returned as a Properties object. If there is no current set of system properties, a set of system properties is first created and initialized. This set of system properties always includes values for the following keys:

Key	Description of Associated Value
java.version	Java Runtime Environment version
java.vendor	Java Runtime Environment vendor
java.vendor.url	Java vendor URL
java.home	Java installation directory
java.vm.specification.version	Java Virtual Machine specification version
java.vm.specification.vendor	Java Virtual Machine specification vendor
java.vm.specification.name	Java Virtual Machine specification name
java.vm.version	Java Virtual Machine implementation version
java.vm.vendor	Java Virtual Machine implementation vendor
java.vm.name	Java Virtual Machine implementation name
java.specification.version	Java Runtime Environment specification version
java.specification.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
java.class.path	Java class path
java.library.path	List of paths to search when loading libraries
java.io.tmpdir	Default temp file path
java.compiler	Name of JIT compiler to use
java.ext.dirs	Path of extension directory or directories Deprecated. This property, and the mechanism which implements it, may be removed in a future release.
os.name	Operating system name
os.arch	Operating system architecture
os.version	Operating system version
file.separator	File separator ("/" on UNIX)
path.separator	Path separator (":" on UNIX)
line.separator	Line separator ("\n" on UNIX)
user.name	User's account name
user.home	User's home directory
user.dir	User's current working directory

Multiple paths in a system property value are separated by the path separator character of the platform.

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the getProperty(String) operation.

Throws:

SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.

Returns:

the system properties

See Also:

setProperties(java.util.Properties), SecurityException, SecurityManager.checkPropertiesAccess(), Properties

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:

the system-dependent line separator string

Since:

1.7

setProperties

public static void setProperties(Properties props)

Sets the system properties to the Properties argument.

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The argument becomes the current set of system properties for use by the getProperty(String) method. If the argument is null, then the current set of system properties is forgotten.

Throws:

SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.

Parameters:

props - the new system properties.

See Also:

getProperties(), Properties, SecurityException, SecurityManager.checkPropertiesAccess()

getProperty

public static String getProperty(String key)

getProperty returns the value of a given property.

Throws:

IllegalArgumentException - if key is ""

NullPointerException - if key is null

SecurityException - if a security manager sm is installed and sm.checkPropertyAccess(key) throws this exception.

Parameters:

key - the property name

Returns:

the value of the property with name key, or null if this property was not set.

getProperty

public static String getProperty(String key,
                                 String def)

getProperty returns the value of a given property, or a default value if the property is not set.

Throws:

IllegalArgumentException - if key is ""

NullPointerException - if key is null

SecurityException - if a security manager sm is installed and sm.checkPropertyAccess(key) throws this exception.

Parameters:

key - the property name

Returns:

the value of the property with name key, or def if this property was not set.

setProperty

public static String setProperty(String key,
                                 String value)

setProperty sets property for a given key.

Throws:

IllegalArgumentException - if key is ""

NullPointerException - if key or value is null

SecurityException - if a security manager sm is installed and sm.checkPermission(new PropertyPermission(key,"write")) throws this exception.

Parameters:

key - the property key

value - the new property value

Returns:

the old value of the property with name key or null if it was not set.

Since:

1.2

clearProperty

public static String clearProperty(String key)

clearProperty removes a given property

Throws:

IllegalArgumentException - if key is ""

NullPointerException - if key is null

SecurityException - if a security manager sm is installed and sm.checkPermission(new PropertyPermission(key,"write")) throws this exception.

Parameters:

key - the property key

Returns:

the old value of the property with name key or null if it was not set.

Since:

1.5

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 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.

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

Parameters:

name - the name of the environment variable

Returns:

the string value of the variable, or null if the variable is not defined in the system environment

See Also:

getenv(), ProcessBuilder.environment()

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(java.lang.Object) and Object.hashCode() methods.

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

If a security manager exists, its checkPermission method is called with a 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.

Throws:

SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the process environment

Returns:

the environment as a map of variable names to values

Since:

1.5

See Also:

getenv(String), ProcessBuilder.environment()

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)
 

Throws:

SecurityException - if a security manager exists and its checkExit method doesn't allow exit with the specified status.

Parameters:

status - exit status.

See Also:

Runtime.exit(int)

gc

public static void gc()

gc indicates to the VM that it may be a good time to run the garbage collector.

For JamaicaVM, a call to Runtime.gc() will perform at least one full garbage collection cycle and at most two full garbage collection cycles. A call to gc() may therefore require a significant amount of time.

Note that running the garbage collector alone does not ensure that memory that is occupied by objects that have a finalize() method will be reclaimed: Their memory will be held until the finalize() method has been executed and the object is found to be unreachable after a second GC cycle. To ensure that all finalizers are executed and the memory of finalized objects is freed, three calls are required as follows:

   Runtime.getRuntime().gc();  // find garbage
   Runtime.getRuntime().runFinalization(); // execute finalizers
   Runtime.getRuntime().gc();  // recycle finalized objects.
 

Any weak, soft or phantom references used in JamaicaVM will be updated also via the runFinalization(), i.e., an object to which a weak, soft or phantom reference exist can only be reclaimed when runFinalization() is called either by the finalizer thread or by an application thread,

To increase the overall system performance, it may be wise to run a low-priority thread that calls gc() and runFinalization() perpetually:

   while (true)
     {
       Runtime.getRuntime().gc();
       Runtime.getRuntime().runFinalization();
     }
 

This will use all free CPU time to keep the amount of allocated memory as low as possible, such that memory allocation overhead for all higher-priority threads is minimized.

This function is short hand for Runtime.getRuntime().gc().

runFinalization

public static void runFinalization()

runFinalization runs the finalize() method of all objects that are pending for finalization. The finalize methods will be executed within the current thread.

Running the finalizers will permit the garbage collector to reclaim the memory occupied by these objects. However, the memory cannot be reclaimed immediately after execution of finalize(). Instead, since finalize() may have made the object reachable again, the garbage collector first has to show that no new references to the object have been generated. So in the worst case, the garbage collector needs to complete a full GC cycle before the memory can be reclaimed.

In JamaicaVM, runFinalization is wrapped into HeapMemory.executeInArea(), such that it can be called from scoped or immortal memory as well.

This function is short hand for Runtime.getRuntime().runFinalization().

runFinalizersOnExit

@Deprecated
public static void runFinalizersOnExit(boolean value)

Deprecated. running the finalizers is unsafe, therefore this method is deprecated.

Set flag if finalizers should be run in exit of the JVM.

For JamaicaVM, the finalizer of boot classes are not run even if this routine was called with a true argument. The reason is that these finalizers would do things like closing System.out, which would make it very hard to do anything useful afterwards.

This function is equivalent to Runtime.runFinalizersOnExit().

Throws:

SecurityException - if a security manager is installed and its checkExit(0) method throws this exception.

Parameters:

value - flag

Since:

1.1

load

public static void load(String filename)

load a library or system file.

If a security manager sm is installed, sm.checkLink(filename) is called to check the permission to load this library.

Repeated attempts to load the same library will have not effect.

This function is short hand for Runtime.getRuntime().load(String).

Throws:

NullPointerException - if filename is null

UnsatisfiedLinkError - if the library could not be loaded from the specified file.

SecurityException - if a security manager sm is installed and sm.checkLink(filename) throws this exception.

Parameters:

filename - absolute pathname of the library to loaded.

loadLibrary

public static void loadLibrary(String libname)

loadLibrary loads a system library with the given name into the system. The place where library are loaded from (e.g., the library path) and the exact file name (pre- and suffixes etc.) are system dependent.

If a security manager sm is installed, sm.checkLink(filename) is called to check the permission to load this library.

Repeated calls of this method have no effect, any library is loaded only once.

This function is short hand for Runtime.getRuntime().loadLibrary(String).

Throws:

NullPointerException - if libname is null

UnsatisfiedLinkError - if the library with the specified name could not be loaded.

SecurityException - if a security manager sm is installed and sm.checkLink(libname) throws this exception.

Parameters:

libname - the library name, excluding path, suffix etc.

mapLibraryName

public static String mapLibraryName(String libname)

mapLibraryName maps a library name to the corresponding platform-specific native library name

Throws:

NullPointerException - if libname is null.

Parameters:

libname - the library name, e.g. "m"

Returns:

the native library name, e.g. "libm.so", never null

Since:

1.2

nanoTime

public static long nanoTime()

nanoTime returns a raw high resolution time value. The result is not relative to any specific basis and is intended to measure intervals only. The result typically has a high precision for measuring short time intervals, but a low accuracy for longer intervals.

On systems with dynamically changing CPU clock speeds, the accuracy may vary dramatically with the changes in CPU clock.

On an overflow of the CPU cycle counter, the result of nanoTime() will drop suddenly from a high positive value to a low negative value. This occurs rarely, e.g., on a 3GHz system with a 64 bit cycle counter, an overflow occurs after 97 years (assuming an initial counter value of 0). When this happens, the difference between the result of nanoTime() and that of a call to nanoTime() before the CPU cycle counter overflow will not be a measure of the time between these two calls.

On a system with a cycle speed of less then 1GHz, an overflow of the nano seconds counter may occur before the CPU cycle counter overflows. In the case of an overflow of the result of nanoTime() will represent the lower 64 bits of the correct nano seconds, i.e. the difference between two consecutive calls will be correct unless the time passed between two calls exceeds about 292 years.

Returns:

a raw time value in nanoseconds. 0 on systems that don't permit access to CPU cycle counter.

Since:

1.5