java.lang.Object java.lang.System
public final class 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.
Field Summary | |
---|---|
static PrintStream |
err
The "standard" error output stream. |
static InputStream |
in
The "standard" input stream. |
static PrintStream |
out
The "standard" output stream. |
Method Summary | |
---|---|
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 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 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) Gets the system property indicated by the specified key. |
static String |
getProperty
(
String
key,
String
def) Gets the system property indicated by the specified key. |
static SecurityManager |
getSecurityManager
() Gets the system security interface. |
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 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 most precise available system timer, in nanoseconds. |
static void |
runFinalization
() Runs the finalization methods of any objects pending finalization. |
static void |
runFinalizersOnExit
(boolean value) Deprecated. 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) Sets the system properties to the Properties argument. |
static String |
setProperty
(
String
key,
String
value) Sets the system property indicated by the specified 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 |
---|
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.
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.
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.
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.
public static Channel inheritedChannel() throws IOException
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.
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.
public static SecurityManager getSecurityManager()
public static 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 long nanoTime()
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 time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292 years (2 63 nanoseconds) will not accurately compute elapsed time due to numerical overflow.
For example, to measure how long some code takes to execute:
long startTime = System.nanoTime(); // ... the code being measured ... long estimatedTime = System.nanoTime() - startTime;
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
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:
Otherwise, if any of the following is true, an IndexOutOfBoundsException is thrown and the destination is not modified:
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.)
public static int identityHashCode(Object x)
public 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.
public static void setProperties(Properties props)
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.
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.
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.
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.
public static String clearProperty(String key)
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 removed.
public static String getenv(String name)
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.
public static Map<String,String> getenv()
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.
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)
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()
@Deprecated 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.
public static void load(String filename)
The call System.load(name) is effectively equivalent to the call:
Runtime.getRuntime().load(name)
public static void loadLibrary(String libname)
The call System.loadLibrary(name) is effectively equivalent to the call
Runtime.getRuntime().loadLibrary(name)
public static String mapLibraryName(String libname)