public final class System extends Object
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.Modifier and Type | Class | Description |
---|---|---|
static interface |
System.Logger |
System.Logger instances log messages that will be routed to the underlying logging framework the LoggerFinder uses. |
static class |
System.LoggerFinder |
The LoggerFinder service is responsible for creating, managing, and configuring loggers to the underlying framework it uses. |
Modifier and Type | Field | Description |
---|---|---|
static final PrintStream |
err |
The "standard" error output stream. |
static final InputStream |
in |
The "standard" input stream. |
static final PrintStream |
out |
The "standard" output stream. |
Modifier and Type | Method | Description |
---|---|---|
static void |
arraycopy |
Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. |
static String |
clearProperty |
Removes the system property indicated by the specified key. |
static Console |
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 |
Initiates the shutdown sequence of the Java Virtual Machine. |
static void |
gc() |
Runs the garbage collector in the Java Virtual Machine. |
static Map |
getenv() |
Returns an unmodifiable string map view of the current system environment. |
static String |
getenv |
Gets the value of the specified environment variable. |
static System.Logger |
getLogger |
Returns an instance of Logger for the caller's use. |
static System.Logger |
getLogger |
Returns a localizable instance of Logger for the caller's use. |
static Properties |
getProperties() |
Determines the current system properties. |
static String |
getProperty |
Gets the system property indicated by the specified key. |
static String |
getProperty |
Gets the system property indicated by the specified key. |
static SecurityManager |
getSecurityManager() |
Deprecated, for removal: This API element is subject to removal in a future version. This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. |
static int |
identityHashCode |
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 String |
lineSeparator() |
Returns the system-dependent line separator string. |
static void |
load |
Loads the native library specified by the filename argument. |
static void |
loadLibrary |
Loads the native library specified by the libname argument. |
static String |
mapLibraryName |
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() |
Deprecated, for removal: This API element is subject to removal in a future version. Finalization has been deprecated for removal. |
static void |
setErr |
Reassigns the "standard" error output stream. |
static void |
setIn |
Reassigns the "standard" input stream. |
static void |
setOut |
Reassigns the "standard" output stream. |
static void |
setProperties |
Sets the system properties to the Properties argument. |
static String |
setProperty |
Sets the system property indicated by the specified key. |
static void |
setSecurityManager |
Deprecated, for removal: This API element is subject to removal in a future version. This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. |
public static final InputStream in
InputStreamReader
, Console.charset()
should be used for the charset, or consider using Console.reader()
.public static final PrintStream out
Console.charset()
if the Console
exists, stdout.encoding 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
.
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. The encoding used in the conversion from characters to bytes is equivalent to Console.charset()
if the Console
exists, stderr.encoding otherwise.
public static void setIn(InputStream in)
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)
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)
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 Console console()
Console
object associated with the current Java virtual machine, if any.null
.public static Channel inheritedChannel() throws IOException
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 occursSecurityException
- If a security manager is present and it does not permit access to the channel.@Deprecated(since="17", forRemoval=true) public static void setSecurityManager(SecurityManager sm)
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
not set or set to the special token "disallow
" then the setSecurityManager
method cannot be used to set a security manager. See the following section of the SecurityManager
class specification for more details.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 replacedUnsupportedOperationException
- if sm
is non-null and a security manager is not allowed to be set dynamically@Deprecated(since="17", forRemoval=true) public static SecurityManager getSecurityManager()
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()
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.public static 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 int identityHashCode(Object x)
x
- object for which the hashCode is to be calculatedpublic static Properties getProperties()
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.
Key | Description of Associated Value |
---|---|
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 |
Latest Java class file format version recognized by the Java runtime as "MAJOR.MINOR" where MAJOR and MINOR are both formatted as decimal integers |
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 |
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. |
stdout.encoding | Character encoding name for System.out . The Java runtime can be started with the system property set to UTF-8 , starting it with the property set to another value leads to undefined behavior. |
stderr.encoding | Character encoding name for System.err . The Java runtime can be started with the system property set to UTF-8 , starting it with the property set to another value leads to undefined behavior. |
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.Key | Description of Associated Value |
---|---|
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 |
file.encoding | The name of the default charset, defaults to UTF-8 . The property may be set on the command line to the value UTF-8 or COMPAT . If set on the command line to the value COMPAT then the value is replaced with the value of the native.encoding property during startup. Setting the property to a value other than UTF-8 or COMPAT leads to unspecified behavior. |
SecurityException
- if a security manager exists and its checkPropertiesAccess
method doesn't allow access to the system properties.public 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, 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)
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)
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)
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.public static String clearProperty(String key)
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.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.
name
- the name of the environment variablenull
if the variable is not defined in the system environmentNullPointerException
- if name
is null
SecurityException
- if a security manager exists and its checkPermission
method doesn't allow access to the environment variable name
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.
SecurityException
- if a security manager exists and its checkPermission
method doesn't allow access to the process environmentpublic static System.Logger getLogger(String name)
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.public static System.Logger getLogger(String name, ResourceBundle bundle)
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)
The argument serves as a status code. By convention, a nonzero status code indicates abnormal termination.
The call System.exit(n)
is effectively equivalent to the call:
Runtime.getRuntime().exit(n)
Runtime.exit(int)
.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 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()
@Deprecated(since="18", forRemoval=true) public static void runFinalization()
Object.finalize()
for background information and details about migration options. When running in a JVM in which finalization has been disabled or removed, no objects will be pending finalization, so this method does nothing.
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 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 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
public static void loadLibrary(String libname)
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 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 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 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
public static String mapLibraryName(String libname)
libname
- the name of the library.NullPointerException
- if libname
is null
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/System.html