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.
Nested Classes
static interface
System.Logger
instances log messages that will be routed to the underlying logging framework the
LoggerFinder
uses.
static class
The LoggerFinder service is responsible for creating, managing, and configuring loggers to the underlying framework it uses.
Fields
The "standard" error output stream.
The "standard" input stream.
The "standard" output stream.
static void
Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
Removes the system property indicated by the specified key.
Returns the unique
Console
object associated with the current Java virtual machine, if any.
static long
Returns the current time in milliseconds.
static void
Terminates the currently running Java Virtual Machine.
static void
Runs the garbage collector in the Java Virtual Machine.
Returns an unmodifiable string map view of the current system environment.
Gets the value of the specified environment variable.
Returns an instance of
Logger
for the caller's use.
Returns a localizable instance of
Logger
for the caller's use.
Determines the current system properties.
Gets the system property indicated by the specified key.
Gets the system property indicated by the specified key.
Deprecated, for removal: This API element is subject to removal in a future version.static int
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().
Returns the channel inherited from the entity that created this Java virtual machine.
Returns the system-dependent line separator string.
static void
Loads the native library specified by the filename argument.
static void
Loads the native library specified by the libname argument.
Maps a library name into a platform-specific string representing a native library.
static long
Returns the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds.
static void
Runs the finalization methods of any objects pending finalization.
static void
Reassigns the "standard" error output stream.
static void
Reassigns the "standard" input stream.
static void
Reassigns the "standard" output stream.
static void
Sets the system properties to the Properties argument.
Sets the system property indicated by the specified key.
static void
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitThe "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. In case this stream is wrapped in a
InputStreamReader
,
Console.charset()
should be used for the charset, or consider using
Console.reader()
.
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. The encoding used in the conversion from characters to bytes is equivalent to
Console.charset()
if the
Console
exists,
Charset.defaultCharset()
otherwise.
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.
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)Console.charset()Charset.defaultCharset()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. The encoding used in the conversion from characters to bytes is equivalent to Console.charset() if the Console exists, Charset.defaultCharset() otherwise.
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.
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.
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.
out - the new standard output stream
SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream.
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.
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.
Returns the unique
Console
object associated with the current Java virtual machine, if any.
null.
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.
null.
IOException - If an I/O error occurs
SecurityException - If a security manager is present and it does not permit access to the channel.
Sets the system-wide security manager. 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.
java.security.manager set to the special token "disallow" then the setSecurityManager method cannot be used to set a security manager.
sm - the security manager or null
SecurityException - if the security manager has already been set and its checkPermission method doesn't allow it to be replaced
UnsupportedOperationException - if sm is non-null and a security manager is not allowed to be set dynamically
Gets the system-wide security manager.
null is returned.
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()
Returns the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds. 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
origintime (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 of currentTimeMillis().
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 elapsedNanos = System.nanoTime() - startTime;
To compare elapsed time against a timeout, use
if (System.nanoTime() - startTime >= timeoutNanos) ...
instead of
if (System.nanoTime() >= startTime + timeoutNanos) ...
because of the possibility of numerical overflow.
Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. A subsequence of array components are copied from the source array referenced by
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.
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(). The hash code for the null reference is zero.
x - object for which the hashCode is to be calculated
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 includes a value for each of the following keys unless the description of the associated value indicates that the value is optional.
java.version Java Runtime Environment version, which may be interpreted as a Runtime.Version java.version.date Java Runtime Environment version date, in ISO-8601 YYYY-MM-DD format, which may be interpreted as a LocalDate java.vendor Java Runtime Environment vendor java.vendor.url Java vendor URL java.vendor.version Java vendor version (optional) java.home Java installation directory java.vm.specification.version Java Virtual Machine specification version, whose value is the feature element of the runtime 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 which may be interpreted as a Runtime.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, whose value is the feature element of the runtime version java.specification.maintenance.version Java Runtime Environment specification maintenance version, may be interpreted as a positive integer (optional, see below) 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 (refer to ClassLoader.getSystemClassLoader() for details) 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 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 native.encoding Character encoding name derived from the host environment and/or the user's settings. Setting this system property has no effect.
The java.specification.maintenance.version property is defined if the specification implemented by this runtime at the time of its construction had undergone a maintenance 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 the getProperties operation, it may choose to permit the getProperty(String) operation.
getProperties(), setProperties(Properties), setProperty(String, String), or clearProperty(String) may not have the desired effect.
jdk.module.path The application module path jdk.module.upgrade.path The upgrade module path jdk.module.main The module name of the initial/main module jdk.module.main.class The main class name of the initial module
SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
()
Returns the system-dependent line separator string. It always returns the same value - the initial value of the
system propertyline.separator
.
On UNIX systems, it returns "\n"; on Microsoft Windows systems it returns "\r\n".
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.
props - the new system properties.
SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
Gets the system property indicated by the specified 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.
Gets the system property indicated by the specified key. 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.
Sets the system property indicated by the specified 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 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 or value is null.
IllegalArgumentException - if key is empty.
Removes the system property indicated by the specified 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.
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 its checkPropertyAccess method doesn't allow access to the specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.
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.
name - the name of the environment variable
null if the variable is not defined in the system environment
NullPointerException - if name is null
SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the environment variable name
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.
SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the process environment
Returns an instance of
Logger
for the caller's use.
LoggerFinder.getLogger method to create an actual logger supplied by the logging backend, for instance, to allow loggers to be obtained during the system initialization time.
LoggerFinder.getLogger(name, module), where module is the caller's module. In cases where System.getLogger is called from a context where there is no caller frame on the stack (e.g when called directly from a JNI attached thread), IllegalCallerException is thrown. To obtain a logger in such a context, use an auxiliary class that will implicitly be identified as the caller, or use the system LoggerFinder to obtain a logger instead. Note that doing the latter may eagerly initialize the underlying logging system.
name - the name of the logger.
System.Logger that can be used by the calling class.
NullPointerException - if name is null.
IllegalCallerException - if there is no Java caller frame on the stack.
Returns a localizable instance of
Logger
for the caller's use. The returned logger will use the provided resource bundle for message localization.
System.LoggerFinder service, which may cause issues if the Java Runtime is not ready to initialize the concrete service implementation yet. System classes which may be loaded early in the boot sequence and need to log localized messages should create a logger using getLogger(java.lang.String) and then use the log methods that take a resource bundle as parameter.
LoggerFinder.getLocalizedLogger(name, bundle, module), where module is the caller's module. In cases where System.getLogger is called from a context where there is no caller frame on the stack (e.g when called directly from a JNI attached thread), IllegalCallerException is thrown. To obtain a logger in such a context, use an auxiliary class that will implicitly be identified as the caller, or use the system LoggerFinder to obtain a logger instead. Note that doing the latter may eagerly initialize the underlying logging system.
name - the name of the logger.
bundle - a resource bundle.
System.Logger which will use the provided resource bundle for message localization.
NullPointerException - if name is null or bundle is null.
IllegalCallerException - if there is no Java caller frame on the stack.
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)
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()
Runs the garbage collector in the Java Virtual Machine.
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 reuse by the Java Virtual Machine. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all unused objects. There is no guarantee that this effort will recycle any particular number of unused objects, reclaim any particular amount of space, or complete at any particular time, if at all, before the method returns or ever. There is also no guarantee that this effort will determine the change of reachability in any particular number of objects, or that any particular number of Reference objects will be cleared and enqueued.
The call System.gc() is effectively equivalent to the call:
Runtime.getRuntime().gc()
public static void runFinalization()
Runs the finalization methods of any objects pending finalization. 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()
Loads the native library specified by the filename argument. The filename argument must be an absolute path name. If the filename argument, when stripped of any platform-specific library prefix, path, and file extension, indicates a library whose name is, for example, L, and a native library called L is statically linked with the VM, then the JNI_OnLoad_L function exported by the library is invoked rather than attempting to load a dynamic library. A filename matching the argument does not have to exist in the file system. See the
JNI Specificationfor more details. Otherwise, the filename argument is mapped to a native library image in an implementation-dependent manner.
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 library
UnsatisfiedLinkError - 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 - if filename is null
Loads the native library specified by the
libname
argument. The
libname
argument must not contain any platform specific prefix, file extension or path. If a native library called
libname
is statically linked with the VM, then the JNI_OnLoad_
libname
function exported by the library is invoked. See the
JNI Specificationfor 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 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 library
UnsatisfiedLinkError - 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 - if libname is null
Maps a library name into a platform-specific string representing a native library.
libname - the name of the library.
NullPointerException - if libname is null
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