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 originally returned the system-wide Security Manager. |
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 |
Restricted. Loads the native library specified by the filename argument. |
static void |
loadLibrary |
Restricted. 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 originally set the system-wide Security Manager. |
public static final InputStream in
stdin.encoding property to convert input bytes to character data.System.in within the object that handles character encoding. After this is done, subsequent reading should use only the wrapper object; continuing to operate directly on System.in results in unspecified behavior. Here are two common examples. Using an InputStreamReader:
new InputStreamReader(System.in, System.getProperty("stdin.encoding"));
Scanner: new Scanner(System.in, System.getProperty("stdin.encoding"));
For handling interactive input, consider using Console.
public static final PrintStream out
stdout.encoding. 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 stderr.encoding.
public static void setIn(InputStream in)
in - the new standard input stream.public static void setOut(PrintStream out)
out - the new standard output streampublic static void setErr(PrintStream err)
err - the new 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 occurs@Deprecated(since="17", forRemoval=true) public static void setSecurityManager(SecurityManager sm)
UnsupportedOperationException. Setting a security manager is not supported.sm - ignoredUnsupportedOperationException - always@Deprecated(since="17", forRemoval=true) public static SecurityManager getSecurityManager()
null. Setting a security manager is not supported.nullpublic 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) ...if (System.nanoTime() >= startTime + timeoutNanos) ...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()
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 the user's settings. Setting this system property on the command line has no effect. |
stdin.encoding | Character encoding name for System.in. The Java runtime can be started with the system property set to UTF-8. Starting it with the property set to another value results in unspecified behavior. |
stdout.encoding | Character encoding name for System.out and System.console(). The Java runtime can be started with the system property set to UTF-8. Starting it with the property set to another value results in unspecified 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 results in unspecified 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.
Additional locale-related system properties defined by the Default Locale section in the Locale class description may also be obtained with this method.
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 results in unspecified behavior. |
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. 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.public static String getProperty(String key)
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.NullPointerException - if key is null.IllegalArgumentException - if key is empty.public static String getProperty(String key, String def)
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.NullPointerException - if key is null.IllegalArgumentException - if key is empty.public static String setProperty(String key, String value)
key - the name of the system property.value - the value of the system property.null if it did not have one.NullPointerException - if key or value is null.IllegalArgumentException - if key is empty.public static String clearProperty(String key)
key - the name of the system property to be removed.null if there was no property with that key.NullPointerException - if key is null.IllegalArgumentException - if key is empty.public static String getenv(String name)
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
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(Object) and Object.hashCode() methods.
The returned map is typically case-sensitive on all platforms.
When passing information to a Java subprocess, system properties are generally preferred over environment variables.
public 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.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)
load is a restricted method of the Java platform. load when access to restricted methods is enabled. The call System.load(name) is effectively equivalent to the call:
Runtime.getRuntime().load(name)
filename - the file to load.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
IllegalCallerException - if the caller is in a module that does not have native access enabled.public static void loadLibrary(String libname)
loadLibrary is a restricted method of the Java platform. loadLibrary when access to restricted methods is enabled.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.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
IllegalCallerException - if the caller is in a module that does not have native access enabled.public static String mapLibraryName(String libname)
libname - the name of the library.NullPointerException - if libname is null
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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).
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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/System.html