Module java.base
Package java.lang

Class System

  • public final classSystemextendsObject
    TheSystem class contains several useful class fields and methods. It cannot be instantiated. Among the facilities provided by theSystem 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

    • Method Detail

      • setIn

        public static void setIn​(InputStream in)
        Reassigns the "standard" input stream. First, if there is a security manager, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission 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, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission 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, itscheckPermission method is called with aRuntimePermission("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 itscheckPermission 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 uniqueConsole object associated with the current Java virtual machine, if any.
        Returns:
        The system console, if any, otherwisenull.
        Since:
        1.6

      • inheritedChannel

        public static Channel inheritedChannel()                                throwsIOException
        Returns the channel inherited from the entity that created this Java virtual machine. This method returns the channel obtained by invoking theinheritedChannel method of the system-wide defaultSelectorProvider object.

        In addition to the network-oriented channels described ininheritedChannel, this method may return other kinds of channels in the future.

        Returns:
        The inherited channel, if any, otherwisenull.
        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 s)
        Sets the System security. If there is a security manager already installed, this method first calls the security manager'scheckPermission method with aRuntimePermission("setSecurityManager") permission to ensure it's ok to replace the existing security manager. This may result in throwing aSecurityException.

        Otherwise, the argument is established as the current security manager. If the argument isnull and no security manager has been established, then no action is taken and the method simply returns.

        Parameters:
        s - the security manager.
        Throws:
        SecurityException - if the security manager has already been set and itscheckPermission method doesn't allow it to be replaced.
        See Also:
        getSecurityManager(),SecurityManager.checkPermission(java.security.Permission),RuntimePermission

      • getSecurityManager

        public static SecurityManager getSecurityManager()
        Gets the system security interface.
        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 classDate 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 arbitraryorigin 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 ofcurrentTimeMillis().

        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 bysrc to the destination array referenced bydest. The number of components copied is equal to thelength argument. The components at positionssrcPos throughsrcPos+length-1 in the source array are copied into positionsdestPos throughdestPos+length-1, respectively, of the destination array.

        If thesrc anddest arguments refer to the same array object, then the copying is performed as if the components at positionssrcPos throughsrcPos+length-1 were first copied to a temporary array withlength components and then the contents of the temporary array were copied into positionsdestPos throughdestPos+length-1 of the destination array.

        Ifdest isnull, then aNullPointerException is thrown.

        Ifsrc isnull, then aNullPointerException is thrown and the destination array is not modified.

        Otherwise, if any of the following is true, anArrayStoreException is thrown and the destination is not modified:

        • Thesrc argument refers to an object that is not an array.
        • Thedest argument refers to an object that is not an array.
        • Thesrc argument anddest argument refer to arrays whose component types are different primitive types.
        • Thesrc argument refers to an array with a primitive component type and thedest argument refers to an array with a reference component type.
        • Thesrc argument refers to an array with a reference component type and thedest argument refers to an array with a primitive component type.

        Otherwise, if any of the following is true, anIndexOutOfBoundsException is thrown and the destination is not modified:

        • ThesrcPos argument is negative.
        • ThedestPos argument is negative.
        • Thelength argument is negative.
        • srcPos+length is greater thansrc.length, the length of the source array.
        • destPos+length is greater thandest.length, the length of the destination array.

        Otherwise, if any actual component of the source array from positionsrcPos throughsrcPos+length-1 cannot be converted to the component type of the destination array by assignment conversion, anArrayStoreException is thrown. In this case, letk be the smallest nonnegative integer less than length such thatsrc[srcPos+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positionssrcPos throughsrcPos+k-1 will already have been copied to destination array positionsdestPos throughdestPos+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 thesrc array could not be stored into thedest array because of a type mismatch.
        NullPointerException - if eithersrc ordest isnull.

      • 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)

      • getProperties

        public static Properties getProperties()
        Determines the current system properties. First, if there is a security manager, itscheckPropertiesAccess method is called with no arguments. This may result in a security exception.

        The current set of system properties for use by thegetProperty(String) method is returned as aProperties 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 always includes values for the following keys:

        Shows property keys and associated values
        KeyDescription of Associated Value
        java.versionJava Runtime Environment version, which may be interpreted as aRuntime.Version
        java.version.dateJava Runtime Environment version date, in ISO-8601 YYYY-MM-DD format, which may be interpreted as aLocalDate
        java.vendorJava Runtime Environment vendor
        java.vendor.urlJava vendor URL
        java.vendor.versionJava vendor version
        java.homeJava installation directory
        java.vm.specification.versionJava Virtual Machine specification version, whose value is thefeature element of theruntime version
        java.vm.specification.vendorJava Virtual Machine specification vendor
        java.vm.specification.nameJava Virtual Machine specification name
        java.vm.versionJava Virtual Machine implementation version which may be interpreted as aRuntime.Version
        java.vm.vendorJava Virtual Machine implementation vendor
        java.vm.nameJava Virtual Machine implementation name
        java.specification.versionJava Runtime Environment specification version, whose value is thefeature element of theruntime version
        java.specification.maintenance.versionJava Runtime Environment specification maintenance version, may be interpreted as a positive integer(optional, see below)
        java.specification.vendorJava Runtime Environment specification vendor
        java.specification.nameJava Runtime Environment specification name
        java.class.versionJava class format version number
        java.class.pathJava class path (refer toClassLoader.getSystemClassLoader() for details)
        java.library.pathList of paths to search when loading libraries
        java.io.tmpdirDefault temp file path
        java.compilerName of JIT compiler to use
        os.nameOperating system name
        os.archOperating system architecture
        os.versionOperating system version
        file.separatorFile separator ("/" on UNIX)
        path.separatorPath separator (":" on UNIX)
        line.separatorLine separator ("\n" on UNIX)
        user.nameUser's account name
        user.homeUser's home directory
        user.dirUser's current working directory

        Thejava.specification.maintenance.version property is defined if the specification implemented by this runtime at the time of its construction had undergone amaintenance 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 thegetProperties operation, it may choose to permit thegetProperty(String) operation.

        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 usinggetProperties(),setProperties(Properties),setProperty(String, String), orclearProperty(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
        KeyDescription of Associated Value
        jdk.module.pathThe application module path
        jdk.module.upgrade.pathThe upgrade module path
        jdk.module.mainThe module name of the initial/main module
        jdk.module.main.classThe main class name of the initial module
        Returns:
        the system properties
        Throws:
        SecurityException - if a security manager exists and itscheckPropertiesAccess method doesn't allow access to the system properties.
        See Also:
        setProperties(java.util.Properties),SecurityException,SecurityManager.checkPropertiesAccess(),Properties

      • lineSeparator

        public static String lineSeparator()
        Returns the system-dependent line separator string. It always returns the same value - the initial value of thesystem propertyline.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 theProperties argument. First, if there is a security manager, itscheckPropertiesAccess 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 thegetProperty(String) method. If the argument isnull, then the current set of system properties is forgotten.

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

      • 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, itscheckPermission method is called with aRuntimePermission("getenv."+name) permission. This may result in aSecurityException being thrown. If no exception is thrown the value of the variablename is returned.

        System properties andenvironment 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 asPATH).

        On UNIX systems the alphabetic case ofname is typically significant, while on Microsoft Windows systems it is typically not. For example, the expressionSystem.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, ornull if the variable is not defined in the system environment
        Throws:
        NullPointerException - ifname isnull
        SecurityException - if a security manager exists and itscheckPermission method doesn't allow access to the environment variablename
        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 aNullPointerException. Attempting to query the presence of a key or value which is not of typeString will throw aClassCastException.

        The returned map and its collection views may not obey the general contract of theObject.equals(java.lang.Object) andObject.hashCode() methods.

        The returned map is typically case-sensitive on all platforms.

        If a security manager exists, itscheckPermission method is called with aRuntimePermission("getenv.*") permission. This may result in aSecurityException 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 itscheckPermission 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 ofLogger for the caller's use.
        API Note:
        This method may defer calling theLoggerFinder.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 callingLoggerFinder.getLogger(name, module), wheremodule is the caller's module. In cases whereSystem.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 systemLoggerFinder 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 ofSystem.Logger that can be used by the calling class.
        Throws:
        NullPointerException - ifname isnull.
        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 ofLogger 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 theSystem.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 usinggetLogger(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 byLoggerFinder.getLocalizedLogger(name, bundle, module), wheremodule is the caller's module. In cases whereSystem.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 systemLoggerFinder 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 ofSystem.Logger which will use the provided resource bundle for message localization.
        Throws:
        NullPointerException - ifname isnull orbundle isnull.
        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 theexit method in classRuntime. This method never returns normally.

        The callSystem.exit(n) is effectively equivalent to the call:

         Runtime.getRuntime().exit(n)

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

      • gc

        public static void gc()
        Runs the garbage collector. Calling thegc 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 callSystem.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 thefinalize methods of objects that have been found to be discarded but whosefinalize 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 callSystem.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 callSystem.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 itscheckLink 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 - iffilename isnull
        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 thelibname argument. Thelibname argument must not contain any platform specific prefix, file extension or path. If a native library calledlibname 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 callSystem.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 itscheckLink 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 - iflibname isnull
        See Also:
        Runtime.loadLibrary(java.lang.String),SecurityManager.checkLink(java.lang.String)