Class System

java.lang.Object
java.lang.System
public final class System extends Object
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.
Since:
1.0

  • Field Details

  • Method Details

    • setIn

      public static void setIn(InputStream in)
      Reassigns the "standard" input stream.
      Parameters:
      in - the new standard input stream.
      Since:
      1.1

    • setOut

      public static void setOut(PrintStream out)
      Reassigns the "standard" output stream.
      Parameters:
      out - the new standard output stream
      Since:
      1.1

    • setErr

      public static void setErr(PrintStream err)
      Reassigns the "standard" error output stream.
      Parameters:
      err - the new standard error output stream.
      Since:
      1.1

    • console

      public static Console console()
      Returns the unique Console object associated with the current Java virtual machine, if any.
      Returns:
      The system console, if any, otherwise null.
      Since:
      1.6

    • inheritedChannel

      public static Channel inheritedChannel() throws IOException
      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.

      Returns:
      The inherited channel, if any, otherwise null.
      Throws:
      IOException - If an I/O error occurs
      Since:
      1.5

    • setSecurityManager

      @Deprecated(since="17", forRemoval=true) public static void setSecurityManager(SecurityManager sm)
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method originally set the system-wide Security Manager. Setting a Security Manager is no longer supported. There is no replacement for the Security Manager or this method.
      Throws UnsupportedOperationException. Setting a security manager is not supported.
      Parameters:
      sm - ignored
      Throws:
      UnsupportedOperationException - always
      See Also:

    • getSecurityManager

      @Deprecated(since="17", forRemoval=true) public 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. Setting a Security Manager is no longer supported. There is no replacement for the Security Manager or this method.
      Returns null. Setting a security manager is not supported.
      Returns:
      null
      See Also:

    • currentTimeMillis

      public static long currentTimeMillis()
      Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds.

      See the description of the class Date for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).

      Returns:
      the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
      See Also:

    • nanoTime

      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 origin 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 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.
      Returns:
      the current value of the running Java Virtual Machine's high-resolution time source, in nanoseconds
      Since:
      1.5

    • arraycopy

      public 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. 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:

      • The src argument refers to an object that is not an array.
      • The dest argument refers to an object that is not an array.
      • The src argument and dest argument refer to arrays whose component types are different primitive types.
      • The src argument refers to an array with a primitive component type and the dest argument refers to an array with a reference component type.
      • The 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:

      • The srcPos argument is negative.
      • The destPos argument is negative.
      • The 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.)

      Parameters:
      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.
      Throws:
      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.

    • identityHashCode

      public 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(). The hash code for the null reference is zero.
      Parameters:
      x - object for which the hashCode is to be calculated
      Returns:
      the hashCode
      Since:
      1.1
      See Also:

    • getProperties

      public static Properties getProperties()
      Determines the current system properties.

      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.

      Shows property keys and associated values
      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 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 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.

      Additional locale-related system properties defined by the Default Locale section in the Locale class description may also be obtained with this method.

      API Note:
      Changing a standard system property may have unpredictable results unless otherwise specified. Property values may be cached during initialization or on first use. Setting a standard property after initialization using getProperties(), setProperties(Properties), setProperty(String, String), or clearProperty(String) may not have the desired effect.
      Implementation Note:
      In addition to the standard system properties, the system properties may include the following keys:
      Shows property keys and associated values
      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.
      Returns:
      the system properties
      See Also:

    • lineSeparator

      public static String lineSeparator()
      Returns the system-dependent line separator string. It always returns the same value - the initial value of the system property line.separator.

      On UNIX systems, it returns "\n"; on Microsoft Windows systems it returns "\r\n".

      Returns:
      the system-dependent line separator string
      Since:
      1.7

    • setProperties

      public static void setProperties(Properties props)
      Sets the system properties to the 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.

      API Note:
      Changing a standard system property may have unpredictable results unless otherwise specified. See getProperties for details.
      Parameters:
      props - the new system properties.
      See Also:

    • getProperty

      public static String getProperty(String key)
      Gets the system property indicated by the specified 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.

      API Note:
      Changing a standard system property may have unpredictable results unless otherwise specified. See getProperties for details.
      Parameters:
      key - the name of the system property.
      Returns:
      the string value of the system property, or null if there is no property with that key.
      Throws:
      NullPointerException - if key is null.
      IllegalArgumentException - if key is empty.
      See Also:

    • getProperty

      public static String getProperty(String key, String def)
      Gets the system property indicated by the specified 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.

      Parameters:
      key - the name of the system property.
      def - a default value.
      Returns:
      the string value of the system property, or the default value if there is no property with that key.
      Throws:
      NullPointerException - if key is null.
      IllegalArgumentException - if key is empty.
      See Also:

    • setProperty

      public static String setProperty(String key, String value)
      Sets the system property indicated by the specified key.
      API Note:
      Changing a standard system property may have unpredictable results unless otherwise specified. See getProperties for details.
      Parameters:
      key - the name of the system property.
      value - the value of the system property.
      Returns:
      the previous value of the system property, or null if it did not have one.
      Throws:
      NullPointerException - if key or value is null.
      IllegalArgumentException - if key is empty.
      Since:
      1.2
      See Also:

    • clearProperty

      public static String clearProperty(String key)
      Removes the system property indicated by the specified key.
      API Note:
      Changing a standard system property may have unpredictable results unless otherwise specified. See getProperties method for details.
      Parameters:
      key - the name of the system property to be removed.
      Returns:
      the previous string value of the system property, or null if there was no property with that key.
      Throws:
      NullPointerException - if key is null.
      IllegalArgumentException - if key is empty.
      Since:
      1.5
      See Also:

    • getenv

      public static String getenv(String name)
      Gets the value of the specified environment variable. An environment variable is a system-dependent external named value.

      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.

      Parameters:
      name - the name of the environment variable
      Returns:
      the string value of the variable, or null if the variable is not defined in the system environment
      Throws:
      NullPointerException - if name is null
      See Also:

    • getenv

      public static Map<String,String> getenv()
      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.

      When passing information to a Java subprocess, system properties are generally preferred over environment variables.

      Returns:
      the environment as a map of variable names to values
      Since:
      1.5
      See Also:

    • getLogger

      public static System.Logger getLogger(String name)
      Returns an instance of Logger for the caller's use.
      API Note:
      This method may defer calling the 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.
      Implementation Requirements:
      Instances returned by this method route messages to loggers obtained by calling 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.
      Parameters:
      name - the name of the logger.
      Returns:
      an instance of System.Logger that can be used by the calling class.
      Throws:
      NullPointerException - if name is null.
      IllegalCallerException - if there is no Java caller frame on the stack.
      Since:
      9

    • getLogger

      public static System.Logger getLogger(String name, ResourceBundle bundle)
      Returns a localizable instance of Logger for the caller's use. The returned logger will use the provided resource bundle for message localization.
      API Note:
      This method is intended to be used after the system is fully initialized. This method may trigger the immediate loading and initialization of the 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.
      Implementation Requirements:
      The returned logger will perform message localization as specified by 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.
      Parameters:
      name - the name of the logger.
      bundle - a resource bundle.
      Returns:
      an instance of System.Logger which will use the provided resource bundle for message localization.
      Throws:
      NullPointerException - if name is null or bundle is null.
      IllegalCallerException - if there is no Java caller frame on the stack.
      Since:
      9

    • exit

      public static void exit(int status)
      Initiates the shutdown sequence of the Java Virtual Machine. This method initiates the shutdown sequence (if it is not already initiated) and then blocks indefinitely. This method neither returns nor throws an exception; that is, it does not complete either normally or abruptly.

      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)
      Implementation Note:
      The initiation of the shutdown sequence is logged by Runtime.exit(int).
      Parameters:
      status - exit status.
      See Also:

    • gc

      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()
      See Also:

    • runFinalization

      @Deprecated(since="18", forRemoval=true) public static void runFinalization()
      Deprecated, for removal: This API element is subject to removal in a future version.
      Finalization has been deprecated for removal. See 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.

      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()
      See Java Language Specification:
      12.6 Finalization of Class Instances
      See Also:

    • load

      public static void load(String filename)
      load is a restricted method of the Java platform.
      Programs can only use load when access to restricted methods is enabled.
      Restricted methods are unsafe, and, if used incorrectly, might crash the JVM or result in memory corruption.
      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 Specification for 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)
      Parameters:
      filename - the file to load.
      Throws:
      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.
      External Specifications
      See Also:

    • loadLibrary

      public static void loadLibrary(String libname)
      loadLibrary is a restricted method of the Java platform.
      Programs can only use loadLibrary when access to restricted methods is enabled.
      Restricted methods are unsafe, and, if used incorrectly, might crash the JVM or result in memory corruption.
      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 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)
      Parameters:
      libname - the name of the library.
      Throws:
      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.
      External Specifications
      See Also:

    • mapLibraryName

      public static String mapLibraryName(String libname)
      Maps a library name into a platform-specific string representing a native library.
      Parameters:
      libname - the name of the library.
      Returns:
      a platform-dependent native library name.
      Throws:
      NullPointerException - if libname is null
      Since:
      1.2
      See Also: