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Java Language Reference

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System

Name

System

Synopsis

Class Name:

java.lang.System

Superclass:

java.lang.Object

Immediate Subclasses:

None

Interfaces Implemented:

None

Availability:

JDK 1.0 or later

Description

The System class provides access to various information about the operating system environment in which a program is running. For example, the System class defines variables that allow access to the standard I/O streams and methods that allow a program to run the garbage collector and stop the Java virtual machine.

All of the variables and methods in the System class are static. In other words, it is not necessary to create an instance of the System class in order to use its variables and methods. In fact, the System class does not define any public constructors, so it cannot be instantiated.

The System class supports the concept of system properties that can be queried and set. The following properties are guaranteed always to be defined:

Property Name

Description

file.encoding

The character encoding for the default locale (Java 1.1 only)

file.encoding.pkg

The package that contains converters between local encodings and Unicode (Java 1.1 only)

file.separator

File separator ('/' on UNIX, ' \' on Windows)

java.class.path

The class path

java.class.version

Java class version number

java.compiler

The just-in-time compiler to use, if any (Java 1.1 only)

java.home

Java installation directory

java.vendor

Java vendor-specific string

java.vendor.url

Java vendor URL

java.version

Java version number

line.separator

Line separator(' \n' on UNIX, ' \r\n' on Windows)

os.arch

Operating system architecture

os.name

Operating system name

os.version

Operating system version

path.separator

Path separator (':' on UNIX, ',' on Windows)

user.dir

User's current working directory when the properties were initialized

user.home

User's home directory

user.language

The two-letter language code of the default locale (Java 1.1 only)

user.name

User's account name

user.region

The two-letter country code of the default locale (Java 1.1 only)

user.timezone

The default time zone (Java 1.1 only)

Additional properties may be defined by the run-time environment. The -D command-line option can be used to define system properties when a program is run.

The Runtime class is related to the System class; it provides access to information about the environment in which a program is running.

Class Summary

public final class java.lang.System extends java.lang.Object {
    // Constants
    public static final PrintStream err;
    public static final InputStream in;
    public static final PrintStream out;
    // Class Methods
    public static void arraycopy(Object src, int srcOffset,
                                 Object dst, int dstOffset, int length);
    public static long currentTimeMillis();
    public static void exit(int status);
    public static void gc();
    public static Properties getProperties();
    public static String getProperty(String key);
    public static String getProperty(String key, String default); 
    public static SecurityManager getSecurityManager();
    public static String getenv(String name);           // Deprecated in 1.1
    public static native int identityHashCode(Object x);   // New in 1.1
    public static void load(String filename);
    public static void loadLibrary(String libname);
    public static void runFinalization();
    public static void runFinalizersOnExit(boolean value); // New in 1.1
    public static void setErr(PrintStream err);            // New in 1.1
    public static void setIn(InputStream in);              // New in 1.1
    public static void setOut(PrintStream out);            // New in 1.1
    public static void setProperties(Properties props);
    public static void setSecurityManager(SecurityManager s);
}

Variables

err

public static final PrintStream err

Description

The standard error stream. In an application environment, this variable refers to a java.io.PrintStream object that is associated with the standard error output for the process running the Java virtual machine. In an applet environment, the PrintStream is likely to be associated with a separate window, although this is not guaranteed.

The value of err can be set using the setErr() method. The value of err can only be set if the currenly installed SecurityManager does not throw a SecurityException when the request is made.

Prior to to Java 1.1, err was not final. It has been made final as of Java 1.1 because the unchecked ability to set err is a security hole.

in

public static final InputStream in

Description

The standard input stream. In an application environment, this variable refers to a java.io.InputStream object that is associated with the standard input for the process running the Java virtual machine.

The value of in can be set using the setIn() method. The value of in can only be set if the currenly installed SecurityManager does not throw a SecurityException when the request is made.

Prior to to Java 1.1, in was not final. It has been made final as of Java 1.1 because the unchecked ability to set in is a security hole.

out

public static final PrintStream out

Description

The standard output stream. In an application environment, this variable refers to a java.io.PrintStream object that is associated with the standard output for the process running the Java virtual machine. In an applet environment, the PrintStream is likely to be associated with a separate window, although this is not guaranteed.

out is the most commonly used of the three I/O streams provided by the System class. Even in GUI-based applications, sending output to this stream can be useful for debugging. The usual idiom for sending output to this stream is:

System.out.println("Some text");

The value of out can be set using the setOut() method. The value of out can only be set if the currenly installed SecurityManager does not throw a SecurityException when the request is made.

Prior to to Java 1.1, out was not final. It has been made final as of Java 1.1 because the unchecked ability to set out is a security hole.

Class Methods

arraycopy

 public static void arraycopy(Object src, int src_position, Object dst, int dst_position, int length) 

Parameters

src

The source array.

src_position

An index into the source array.

dst

The destination array.

dst_position

An index into the destination array.

length

The number of elements to be copied.

Throws

ArrayIndexOutOfBoundsException

If the values of the src_position, dst_position, and length arguments imply accessing either array with an index that is less than zero or an index greater than or equal to the number of elements in the array.

ArrayStoreException

If the type of value stored in the src array cannot be stored in the dst array.

NullPointerException

If src or dst is null.

Description

This method copies a range of array elements from the src array to the dst array. The number of elements that are copied is specified by length. The elements at positions src_position through src_position+length-1 in src are copied to the positions dst_position through dst_position+length-1 in dst, respectively.

If src and dst refer to the same array, the copying is done as if the array elements were first copied to a temporary array and then copied to the destination array.

Before this method does any copying, it performs a number of checks. If either src or dst are null, the method throws a NullPointerException and dst is not modified.

If any of the following conditions are true, the method throws an ArrayStoreException, and dst is not modified:

  • Either src or dst refers to an object that is not an array.

  • src and dst refer to arrays whose element types are different primitive types.

  • src refers to an array that has elements that contain a primitive type, while dst refers to an array that has elements that contain a reference type, or vice versa.

If any of the following conditions are true, the method throws an ArrayIndexOutOfBoundsException, and dst is not modified:

  • srcOffset, dstOffset, or length is negative.

  • srcOffset+length is greater than src.length().

  • dstOffset+length is greater than dst.length().

Otherwise, if an element in the source array being copied cannot be converted to the type of the destination array using the rules of the assignment operator, the method throws an ArrayStoreException when the problem occurs. Since the problem is discovered during the copy operation, the state of the dst array reflects the incomplete copy operation.

currentTimeMillis

public static native long currentTimeMillis()

Returns

The current time as the number of milliseconds since 00:00:00 UTC, January 1, 1970.

Description

This method returns the current time as the number of milliseconds since 00:00:00 UTC, January 1, 1970. It will not overflow until the year 292280995.

The java.util.Date class provides more extensive facilities for dealing with times and dates.

exit

public static void exit(int status)

Parameters

status

The exit status code to use.

Throws

SecurityException

If the checkExit() method of the SecurityManager throws a SecurityException.

Description

This method causes the Java virtual machine to exit with the given status code. This method works by calling the exit() method of the current Runtime object. By convention, a nonzero status code indicates abnormal termination. This method never returns.

gc

public static void gc()

Description

This method causes the Java virtual machine to run the garbage collector in the current thread. This method works by calling the gc() method of the current Runtime object.

The garbage collector finds objects that will never be used again because there are no live references to them. After it finds these objects, the garbage collector frees the storage occupied by these objects.

The garbage collector is normally run continuously in a thread with the lowest possible priority, so that it works intermittently to reclaim storage. The gc() method allows a program to invoke the garbage collector explicitly when necessary.

getProperties

public static Properties getProperties()

Returns

A Properties object that contains the values of all the system properies.

Throws

SecurityException

If the checkPropertiesAccess() method of the SecurityManager throws a SecurityException.

Description

This method returns all of the defined system properties encapsulated in a java.util.Properties object. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed in the description of the System class, some system properties are guaranteed always to be defined.

getProperty

public static String getProperty(String key)

Parameters

key

The name of a system property.

Returns

The value of the named system property or null if the named property is not defined.

Throws

SecurityException

If the checkPropertyAccess() method of the SecurityManager throws a SecurityException.

Description

This method returns the value of the named system property. If there is no definition for the named property, the method returns null. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed in the description of the System class, some system properties are guaranteed always to be defined.

public static String getProperty(String key, String def)

Parameters

key

The name of a system property.

def

A default value for the property.

Returns

The value of the named system property, or the default value if the named property is not defined.

Throws

SecurityException

If the checkPropertyAccess() method of the SecurityManager throws a SecurityException.

Description

This method returns the value of the named system property. If there is no definition for the named property, the method returns the default value as specified by the def parameter. If there are no system properties currently defined, a set of default system properties is created and initialized. As discussed earlier in the description of the System class, some system properties are guaranteed to always be defined.

getSecurityManager

public static SecurityManager getSecurityManager()

Returns

A reference to the installed SecurityManager object or null if there is no SecurityManager object installed.

Description

This method returns a reference to the installed SecurityManager object. If there is no SecurityManager object installed, the method returns null.

getenv

public static String getenv(String name)

Availability

Deprecated as of JDK 1.1

Parameters

name

The name of a system-dependent environment variable.

Returns

The value of the environment variable or null if the variable is not defined.

Description

This method is obsolete; it always throws an error. Use getProperties() and the -D option instead.

identityHashCode

public static native int identityHashCode(Object x)

Availability

New as of JDK 1.1

Parameters

x

An object.

Returns

The identity hashcode value for the specified object.

Description

This method returns the same hashcode value for the specified object as would be returned by the default hashCode() method of Object, regardless of whether or not the object's class overrides hashCode().

load

public void load(String filename)

Parameters

filename

A string that specifies the complete path of the file to be loaded.

Throws

SecurityException

If the checkLink() method of the SecurityManager throws a SecurityException.

UnsatisfiedLinkError

If the method is unsuccessful in loading the specified dynamically linked library.

Description

This method loads the specified dynamically linked library. This method works by calling the load() method of the current Runtime object.

loadLibrary

public void loadLibrary(String libname)

Parameters

libname

A string that specifies the name of a dynamically linked library.

Throws

SecurityException

If the checkLink() method of the SecurityManager throws a SecurityException.

UnsatisfiedLinkError

If the method is unsuccessful in loading the specified dynamically linked library.

Description

This method loads the specified dynamically linked library. It looks for the specified library in a platform-specific way. This method works by calling the loadLibrary() method of the current Runtime object.

runFinalization

public static void runFinalization()

Description

This method causes the Java virtual machine to run the finalize() methods of any objects in the finalization queue in the current thread. This method works by calling the runFinalization() method of the current Runtime object.

When the garbage collector discovers that there are no references to an object, it checks to see if the object has a finalize() method that has never been called. If the object has such a finalize() method, the object is placed in the finalization queue. While there is a reference to the object in the finalization queue, the object is no longer considered garbage collectable.

Normally, the objects in the finalization queue are handled by a separate finalization thread that runs continuously at a very low priority. The finalization thread removes an object from the queue and calls its finalize() method. As long as the finalize() method does not generate a reference to the object, the object again becomes available for garbage collection.

Because the finalization thread runs at a very low priority, there may be a long delay from the time that an object is put on the finalization queue until the time that its finalize() method is called. The runFinalization() method allows a program to run the finalize() methods explicitly. This can be useful when there is a shortage of some resource that is released by a finalize() method.

runFinalizersOnExit

public static void runFinalizersOnExit(boolean value)

Availability

New as of JDK 1.1

Parameters

value

A boolean value that specifies whether or not finalization occurs on exit.

Throws

SecurityException

If the checkExit() method of the SecurityManager throws a SecurityException.

Description

This method specifies whether or not the finalize() methods of all objects that have finalize() methods are run before the Java virtual machine exits. By default, the finalizers are not run on exit. This method works by calling the runFinalizersOnExit() method of the current Runtime object.

setErr

public static void setErr(PrintStream err)

Availability

New as of JDK 1.1

Parameters

err

A PrintStream object to use for the standard error stream.

Throws

SecurityException

If the checkExec() method of the SecurityManager throws a SecurityException.

Description

This method sets the standard error stream to be this PrintStream object.

setIn

public static void setIn(InputStream in)

Availability

New as of JDK 1.1

Parameters

in

A InputStream object to use for the standard input stream.

Throws

SecurityException

If the checkExec() method of the SecurityManager throws a SecurityException.

Description

This method sets the standard input stream to be this InputStream object.

setOut

public static void setOut(PrintStream out)

Availability

New as of JDK 1.1

Parameters

out

A PrintStream object to use for the standard output stream.

Throws

SecurityException

If the checkExec() method of the SecurityManager throws a SecurityException.

Description

This method sets the standard output stream to be this PrintStream object.

setProperties

public static void setProperties(Properties props)

Parameters

props

A reference to a Properties object.

Throws

SecurityException

If the checkPropertiesAccess() method of the SecurityManager throws a SecurityException.

Description

This method replaces the current set of system property definitions with a new set of system property definitions that are encapsulated by the given Properties object. As discussed in the description of the System class, some system properties are guaranteed to always be defined.

setSecurityManager

public static void setSecurityManager(SecurityManager s)

Parameters

s

A reference to a SecurityManager object.

Throws

SecurityException

If a SecurityManager object has already been installed.

Description

This method installs the given SecurityManager object. If s is null, then no SecurityManager object is installed. Once a SecurityManager object is installed, any subsequent calls to this method throw a SecurityException.

Inherited Methods

Method

Inherited From

Method

Inherited From

clone()

Object

equals(Object)

Object

finalize()

Object

getClass()

Object

hashCode()

Object

notify()

Object

notifyAll()

Object

toString()

Object

wait()

Object

wait(long)

Object

wait(long, int)

Object

   

See Also

Assignment Compatibility; Errors; Exceptions; Object; Object Destruction; Process; Runtime; SecurityManager


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