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"; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.
Field Detail |
public static final InputStream in
public static final PrintStream out
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 static final PrintStream err
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.
private static Properties props
Constructor Detail |
private System()
Method Detail |
public static void setIn(InputStream in)
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.
in
- the new standard input stream.SecurityException
- if a security manager exists and its checkPermission
method doesn't allow reassigning of the standard input stream.public static void setOut(PrintStream out)
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.
out
- the new standard output streamSecurityException
- if a security manager exists and its checkPermission
method doesn't allow reassigning of the standard output stream.public static void setErr(PrintStream err)
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.
err
- the new standard error output stream.SecurityException
- if a security manager exists and its checkPermission
method doesn't allow reassigning of the standard error output stream.public static void setSecurityManager(SecurityManager s)
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.
s
- the security manager.SecurityException
- if the security manager has already been set and its checkPermission
method doesn't allow it to be replaced.public static SecurityManager getSecurityManager()
null
is returned.public static native long currentTimeMillis()
See the description of the class Date
for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).
public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
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:
src
argument refers to an object that is not an array. dest
argument refers to an object that is not an array. src
argument and dest
argument refer to arrays whose component types are different primitive types. src
argument refers to an array with a primitive component type and the dest
argument refers to an array with a reference component type. 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:
srcPos
argument is negative. destPos
argument is negative. 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.)
src
- the source array.srcPos
- starting position in the source array.dest
- the destination array.destPos
- starting position in the destination data.length
- the number of array elements to be copied.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
.public static native int identityHashCode(Object x)
x
- object for which the hashCode is to be calculatedpublic static Properties getProperties()
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 |
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.
SecurityException
- if a security manager exists and its checkPropertiesAccess
method doesn't allow access to the system properties.public static void setProperties(Properties props)
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.
props
- the new system properties.SecurityException
- if a security manager exists and its checkPropertiesAccess
method doesn't allow access to the system properties.public static String getProperty(String 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.
key
- the name of the system property.null
if there is no property with that key.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.public static String getProperty(String key, String def)
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.
key
- the name of the system property.def
- a default value.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.public static String setProperty(String key, String value)
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.
key
- the name of the system property.value
- the value of the system property.null
if it did not have one.SecurityException
- if a security manager exists and its checkPermission
method doesn't allow setting of the specified property.NullPointerException
- if key
is null
.IllegalArgumentException
- if key
is empty.public static String getenv(String name)
java.lang.System.getProperty
methods and the corresponding get
TypeName methods of the Boolean
, Integer
, and Long
primitive types. For example: String classPath = System.getProperty("java.class.path",".");
if (Boolean.getBoolean("myapp.exper.mode")) enableExpertCommands();
name
- of the environment variablenull
if the variable is not defined.public static void exit(int status)
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)
status
- exit status.SecurityException
- if a security manager exists and its checkExit
method doesn't allow exit with the specified status.public static void gc()
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()
public static void runFinalization()
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()
public static void runFinalizersOnExit(boolean value)
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.
value
- indicating enabling or disabling of finalizationSecurityException
- if a security manager exists and its checkExit
method doesn't allow the exit.public static void load(String filename)
The call System.load(name)
is effectively equivalent to the call:
Runtime.getRuntime().load(name)
filename
- the file to load.SecurityException
- if a security manager exists and its checkLink
method doesn't allow loading of the specified dynamic libraryUnsatisfiedLinkError
- if the file does not exist.public static void loadLibrary(String libname)
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)
libname
- the name of the library.SecurityException
- if a security manager exists and its checkLink
method doesn't allow loading of the specified dynamic libraryUnsatisfiedLinkError
- if the library does not exist.public static native String mapLibraryName(String libname)
libname
- the name of the library.private static InputStream nullInputStream()
private static void initializeSystemClass()