Module java.base
Package java.lang

Class 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

    • Nested Class Summary

      Nested Classes 
      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.

    • Field Summary

      Fields 
      Modifier and Type Field Description
      static PrintStream err
      The "standard" error output stream.
      static InputStream in
      The "standard" input stream.
      static PrintStream out
      The "standard" output stream.

    • Method Detail

      • setIn

        public static void setIn​(InputStream in)
        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.
        Parameters:
        in - the new standard input stream.
        Throws:
        SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard input stream.
        Since:
        1.1
        See Also:
        SecurityManager.checkPermission(java.security.Permission), RuntimePermission

      • setOut

        public static void setOut​(PrintStream out)
        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.
        Parameters:
        out - the new standard output stream
        Throws:
        SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream.
        Since:
        1.1
        See Also:
        SecurityManager.checkPermission(java.security.Permission), RuntimePermission

      • setErr

        public static void setErr​(PrintStream err)
        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.
        Parameters:
        err - the new standard error output stream.
        Throws:
        SecurityException - if a security manager exists and its checkPermission method doesn't allow reassigning of the standard error output stream.
        Since:
        1.1
        See Also:
        SecurityManager.checkPermission(java.security.Permission), RuntimePermission

      • 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
        SecurityException - If a security manager is present and it does not permit access to the channel.
        Since:
        1.5

      • setSecurityManager

        public static void setSecurityManager​(SecurityManager sm)
        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.

        Implementation Note:
        In the JDK implementation, if the Java virtual machine is started with the system property java.security.manager set to the special token "disallow" then the setSecurityManager method cannot be used to set a security manager.
        Parameters:
        sm - the security manager or null
        Throws:
        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
        See Also:
        getSecurityManager(), SecurityManager.checkPermission(java.security.Permission), RuntimePermission

      • getSecurityManager

        public static SecurityManager getSecurityManager()
        Gets the system-wide security manager.
        Returns:
        if a security manager has already been established for the current application, then that security manager is returned; otherwise, null is returned.
        See Also:
        setSecurityManager(java.lang.SecurityManager)

      • 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:
        Date

      • 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:
        Object.hashCode(), Objects.hashCode(Object)

      • 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. 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.

        API Note:
        Changing a standard system property may have unpredictable results unless otherwise specified. See getProperties for details.
        Parameters:
        props - the new system properties.
        Throws:
        SecurityException - if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
        See Also:
        getProperties(), Properties, SecurityException, SecurityManager.checkPropertiesAccess()

      • getProperty

        public static String getProperty​(String key)
        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.

        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:
        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.
        See Also:
        setProperty(java.lang.String, java.lang.String), SecurityException, SecurityManager.checkPropertyAccess(java.lang.String), getProperties()

      • 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.

        If a security manager exists, its checkPermission method is called with a {@link 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.

        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
        SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the environment variable name
        See Also:
        getenv(), ProcessBuilder.environment()

      • 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.

        If a security manager exists, its checkPermission method is called with a {@link 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.

        Returns:
        the environment as a map of variable names to values
        Throws:
        SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the process environment
        Since:
        1.5
        See Also:
        getenv(String), ProcessBuilder.environment()

      • 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)
        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)

        Parameters:
        status - exit status.
        Throws:
        SecurityException - if a security manager exists and its checkExit method doesn't allow exit with the specified status.
        See Also:
        Runtime.exit(int)

      • gc

        public static void gc()
        Runs the garbage collector. 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 quick reuse. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all discarded objects.

        The call System.gc() is effectively equivalent to the call: 

         Runtime.getRuntime().gc()

        See Also:
        Runtime.gc()

      • runFinalization

        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()

        See Also:
        Runtime.runFinalization()

      • load

        public static void load​(String filename)
        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:
        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
        See Also:
        Runtime.load(java.lang.String), SecurityManager.checkLink(java.lang.String)

      • loadLibrary

        public static void loadLibrary​(String libname)
        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:
        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
        See Also:
        Runtime.loadLibrary(java.lang.String), SecurityManager.checkLink(java.lang.String)