public final classSystemextendsObject
System class contains several useful class fields and methods. It cannot be instantiated.Among the facilities provided by theSystem 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.
| Modifier and Type | Field | Description |
|---|---|---|
staticPrintStream | err | The "standard" error output stream. |
staticInputStream | in | The "standard" input stream. |
staticPrintStream | out | The "standard" output stream. |
| Modifier and Type | Method | Description |
|---|---|---|
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. |
staticString | clearProperty(String key) | Removes the system property indicated by the specified key. |
staticConsole | console() | Returns the unique Console object associated with the current Java virtual machine, if any. |
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. |
staticMap<String,String> | getenv() | Returns an unmodifiable string map view of the current system environment. |
staticString | getenv(String name) | Gets the value of the specified environment variable. |
staticProperties | getProperties() | Determines the current system properties. |
staticString | getProperty(String key) | Gets the system property indicated by the specified key. |
staticString | getProperty(String key,String def) | Gets the system property indicated by the specified key. |
staticSecurityManager | 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(). |
staticChannel | inheritedChannel() | Returns the channel inherited from the entity that created this Java virtual machine. |
staticString | lineSeparator() | Returns the system-dependent line separator string. |
static void | load(String filename) | Loads the native library specified by the filename argument. |
static void | loadLibrary(String libname) | Loads the native library specified by the libname argument. |
staticString | 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 running Java Virtual Machine's high-resolution time source, in nanoseconds. |
static void | runFinalization() | Runs the finalization methods of any objects pending finalization. |
static void | runFinalizersOnExit(boolean value) | Deprecated. This method was originally designed to enable or disable running finalizers on exit. Running finalizers on exit was disabled by default. If enabled, then the finalizers of all objects whose finalizers had not yet been automatically invoked were to be run before the Java runtime exits. That behavior 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. |
staticString | setProperty(String key,String value) | Sets the system property indicated by the specified key. |
static void | setSecurityManager(SecurityManager s) | Sets the System security. |
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 theprintln methods in classPrintStream.
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 variableout, has been redirected to a file or other destination that is typically not continuously monitored.
public static void setIn(InputStream in)
First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard input stream.SecurityManager.checkPermission(java.security.Permission),RuntimePermissionpublic static void setOut(PrintStream out)
First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard output stream.SecurityManager.checkPermission(java.security.Permission),RuntimePermissionpublic static void setErr(PrintStream err)
First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission method doesn't allow reassigning of the standard error output stream.SecurityManager.checkPermission(java.security.Permission),RuntimePermissionpublic static Console console()
Console object associated with the current Java virtual machine, if any.public static Channel inheritedChannel() throwsIOException
This method returns the channel obtained by invoking theinheritedChannel method of the system-wide defaultSelectorProvider object.
In addition to the network-oriented channels described ininheritedChannel, this method may return other kinds of channels in the future.
IOException - If an I/O error occursSecurityException - If a security manager is present and it does not permit access to the channel.public static void setSecurityManager(SecurityManager s)
If there is a security manager already installed, this method first calls the security manager'scheckPermission method with aRuntimePermission("setSecurityManager") permission to ensure it's ok to replace the existing security manager. This may result in throwing aSecurityException.
Otherwise, the argument is established as the current security manager. If the argument isnull 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 itscheckPermission method doesn't allow it to be replaced.getSecurityManager(),SecurityManager.checkPermission(java.security.Permission),RuntimePermissionpublic static SecurityManager getSecurityManager()
null is returned.setSecurityManager(java.lang.SecurityManager)public static long currentTimeMillis()
See the description of the classDate for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).
Datepublic 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 arbitraryorigin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.
This method provides nanosecond precision, but not necessarily nanosecond resolution (that is, how frequently the value changes) - no guarantees are made except that the resolution is at least as good as that ofcurrentTimeMillis().
Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not correctly compute elapsed time due to numerical overflow.
The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.
For example, to measure how long some code takes to execute:
long startTime = System.nanoTime(); // ... the code being measured ... long estimatedTime = System.nanoTime() - startTime;To compare two nanoTime values
long t0 = System.nanoTime(); ... long t1 = System.nanoTime();t1 - t0 < 0, nott1 < t0, because of the possibility of numerical overflow.public static void arraycopy(Object src, int srcPos,Object dest, int destPos, int length)
src to the destination array referenced bydest. The number of components copied is equal to thelength argument. The components at positionssrcPos throughsrcPos+length-1 in the source array are copied into positionsdestPos throughdestPos+length-1, respectively, of the destination array. If thesrc anddest arguments refer to the same array object, then the copying is performed as if the components at positionssrcPos throughsrcPos+length-1 were first copied to a temporary array withlength components and then the contents of the temporary array were copied into positionsdestPos throughdestPos+length-1 of the destination array.
Ifdest isnull, then aNullPointerException is thrown.
Ifsrc isnull, then aNullPointerException is thrown and the destination array is not modified.
Otherwise, if any of the following is true, anArrayStoreException 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 anddest argument refer to arrays whose component types are different primitive types.src argument refers to an array with a primitive component type and thedest argument refers to an array with a reference component type.src argument refers to an array with a reference component type and thedest argument refers to an array with a primitive component type. Otherwise, if any of the following is true, anIndexOutOfBoundsException is thrown and the destination is not modified:
srcPos argument is negative.destPos argument is negative.length argument is negative.srcPos+length is greater thansrc.length, the length of the source array.destPos+length is greater thandest.length, the length of the destination array. Otherwise, if any actual component of the source array from positionsrcPos throughsrcPos+length-1 cannot be converted to the component type of the destination array by assignment conversion, anArrayStoreException is thrown. In this case, letk be the smallest nonnegative integer less than length such thatsrc[srcPos+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positionssrcPos throughsrcPos+k-1 will already have been copied to destination array positionsdestPos throughdestPos+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 thesrc array could not be stored into thedest array because of a type mismatch.NullPointerException - if eithersrc ordest isnull.public static int identityHashCode(Object x)
x - object for which the hashCode is to be calculatedpublic static Properties getProperties()
First, if there is a security manager, itscheckPropertiesAccess method is called with no arguments. This may result in a security exception.
The current set of system properties for use by thegetProperty(String) method is returned as aProperties 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.specification.maintenance.version | Java Runtime Environment specification maintenance version, may be interpreted as a positive integer(optional, see below) |
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 directoriesDeprecated.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 |
Thejava.specification.maintenance.version property is defined if the specification implemented by this runtime at the time of its construction had undergone amaintenance release. When defined, its value identifies that maintenance release. To indicate the first maintenance release this property will have the value"1", to indicate the second maintenance release this property will have the value"2", and so on.
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 thegetProperties operation, it may choose to permit thegetProperty(String) operation.
SecurityException - if a security manager exists and itscheckPropertiesAccess method doesn't allow access to the system properties.setProperties(java.util.Properties),SecurityException,SecurityManager.checkPropertiesAccess(),Propertiespublic static String lineSeparator()
line.separator.On UNIX systems, it returns"\n"; on Microsoft Windows systems it returns"\r\n".
public static void setProperties(Properties props)
Properties argument. First, if there is a security manager, itscheckPropertiesAccess 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 thegetProperty(String) method. If the argument isnull, then the current set of system properties is forgotten.
props - the new system properties.SecurityException - if a security manager exists and itscheckPropertiesAccess method doesn't allow access to the system properties.getProperties(),Properties,SecurityException,SecurityManager.checkPropertiesAccess()public static String getProperty(String key)
First, if there is a security manager, itscheckPropertyAccess 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 thegetProperties method.
key - the name of the system property.null if there is no property with that key.SecurityException - if a security manager exists and itscheckPropertyAccess method doesn't allow access to the specified system property.NullPointerException - ifkey isnull.IllegalArgumentException - ifkey is empty.setProperty(java.lang.String, java.lang.String),SecurityException,SecurityManager.checkPropertyAccess(java.lang.String),getProperties()public static String getProperty(String key,String def)
First, if there is a security manager, itscheckPropertyAccess method is called with thekey 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 thegetProperties method.
key - the name of the system property.def - a default value.SecurityException - if a security manager exists and itscheckPropertyAccess method doesn't allow access to the specified system property.NullPointerException - ifkey isnull.IllegalArgumentException - ifkey is empty.setProperty(java.lang.String, java.lang.String),SecurityManager.checkPropertyAccess(java.lang.String),getProperties()public static String setProperty(String key,String value)
First, if a security manager exists, itsSecurityManager.checkPermission method is called with aPropertyPermission(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 itscheckPermission method doesn't allow setting of the specified property.NullPointerException - ifkey orvalue isnull.IllegalArgumentException - ifkey is empty.getProperty(java.lang.String),getProperty(java.lang.String),getProperty(java.lang.String, java.lang.String),PropertyPermission,SecurityManager.checkPermission(java.security.Permission)public static String clearProperty(String key)
First, if a security manager exists, itsSecurityManager.checkPermission method is called with aPropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is removed.
key - the name of the system property to be removed.null if there was no property with that key.SecurityException - if a security manager exists and itscheckPropertyAccess method doesn't allow access to the specified system property.NullPointerException - ifkey isnull.IllegalArgumentException - ifkey is empty.getProperty(java.lang.String),setProperty(java.lang.String, java.lang.String),Properties,SecurityException,SecurityManager.checkPropertiesAccess()public static String getenv(String name)
If a security manager exists, itscheckPermission method is called with aRuntimePermission("getenv."+name)SecurityException being thrown. If no exception is thrown the value of the variablename is returned.
System properties andenvironment 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 asPATH).
On UNIX systems the alphabetic case ofname is typically significant, while on Microsoft Windows systems it is typically not. For example, the expressionSystem.getenv("FOO").equals(System.getenv("foo")) is likely to be true on Microsoft Windows.
name - the name of the environment variablenull if the variable is not defined in the system environmentNullPointerException - ifname isnullSecurityException - if a security manager exists and itscheckPermission method doesn't allow access to the environment variablenamegetenv(),ProcessBuilder.environment()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 aNullPointerException. Attempting to query the presence of a key or value which is not of typeString will throw aClassCastException.
The returned map and its collection views may not obey the general contract of theObject.equals(java.lang.Object) andObject.hashCode() methods.
The returned map is typically case-sensitive on all platforms.
If a security manager exists, itscheckPermission method is called with aRuntimePermission("getenv.*")SecurityException being thrown.
When passing information to a Java subprocess,system properties are generally preferred over environment variables.
SecurityException - if a security manager exists and itscheckPermission method doesn't allow access to the process environmentgetenv(String),ProcessBuilder.environment()public static void exit(int status)
This method calls theexit method in classRuntime. This method never returns normally.
The callSystem.exit(n) is effectively equivalent to the call:
Runtime.getRuntime().exit(n)
status - exit status.SecurityException - if a security manager exists and itscheckExit method doesn't allow exit with the specified status.Runtime.exit(int)public static void gc()
Calling thegc 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 callSystem.gc() is effectively equivalent to the call:
Runtime.getRuntime().gc()
Runtime.gc()public static void runFinalization()
Calling this method suggests that the Java Virtual Machine expend effort toward running thefinalize methods of objects that have been found to be discarded but whosefinalize 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 callSystem.runFinalization() is effectively equivalent to the call:
Runtime.getRuntime().runFinalization()
Runtime.runFinalization()@Deprecatedpublic static void runFinalizersOnExit(boolean value)
UnsupportedOperationException.The callSystem.runFinalizersOnExit() is effectively equivalent to the call:
Runtime.runFinalizersOnExit()
value - ignoredRuntime.runFinalizersOnExit(boolean)public static void load(String filename)
The callSystem.load(name) is effectively equivalent to the call:
Runtime.getRuntime().load(name)
filename - the file to load.SecurityException - if a security manager exists and itscheckLink method doesn't allow loading of the specified dynamic libraryUnsatisfiedLinkError - if either the filename is not an absolute path name, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system.NullPointerException - iffilename isnullRuntime.load(java.lang.String),SecurityManager.checkLink(java.lang.String)public static void loadLibrary(String libname)
libname argument. Thelibname argument must not contain any platform specific prefix, file extension or path. If a native library calledlibname is statically linked with the VM, then the JNI_OnLoad_libname function exported by the library is invoked. See the JNI Specification for more details. Otherwise, the libname argument is loaded from a system library location and mapped to a native library image in an implementation- dependent manner. The callSystem.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 itscheckLink method doesn't allow loading of the specified dynamic libraryUnsatisfiedLinkError - if either the libname argument contains a file path, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system.NullPointerException - iflibname isnullRuntime.loadLibrary(java.lang.String),SecurityManager.checkLink(java.lang.String)public static String mapLibraryName(String libname)
libname - the name of the library.NullPointerException - iflibname isnullloadLibrary(java.lang.String),ClassLoader.findLibrary(java.lang.String)