Class System

  java.lang.System

1.0

Field Detail

The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user. In case this stream is wrapped in a InputStreamReader, Console.charset() should be used for the charset, or consider using Console.reader().

The "standard" output stream. This stream is already open and ready to accept output data. Typically this stream corresponds to display output or another output destination specified by the host environment or user. The encoding used in the conversion from characters to bytes is equivalent to Console.charset() if the Console exists, Charset.defaultCharset() otherwise.

For simple stand-alone Java applications, a typical way to write a line of output data is:

     System.out.println(data)
 

See the println methods in class PrintStream.

The "standard" error output stream. This stream is already open and ready to accept output data.

Typically this stream corresponds to display output or another output destination specified by the host environment or user. By convention, this output stream is used to display error messages or other information that should come to the immediate attention of a user even if the principal output stream, the value of the variable out, has been redirected to a file or other destination that is typically not continuously monitored. The encoding used in the conversion from characters to bytes is equivalent to Console.charset() if the Console exists, Charset.defaultCharset() otherwise.

Method Detail

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. Throws: Since: See Also:

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. Throws: Since: See Also:

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. Throws: Since: See Also:

Returns the unique Console object associated with the current Java virtual machine, if any. Since:

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.

Throws: Since:

Deprecated, for removal. This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. Consequently, this method is also deprecated and subject to removal. There is no replacement for the Security Manager or this method. Throws: See Also:

Deprecated, for removal. This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. Consequently, this method is also deprecated and subject to removal. There is no replacement for the Security Manager or this method. See Also:

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).

See Also:

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. Since:

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.)

Throws:

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. Returns: Since: See Also:

Determines the current system properties. First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The current set of system properties for use by the getProperty(String) method is returned as a Properties object. If there is no current set of system properties, a set of system properties is first created and initialized. This set of system properties includes a value for each of the following keys unless the description of the associated value indicates that the value is optional.

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.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
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
java.compiler	Name of JIT compiler to use
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.

Multiple paths in a system property value are separated by the path separator character of the platform.

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the getProperty(String) operation.

Throws: See Also:

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".

Since:

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.

Throws: See Also:

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.

Returns: Throws: See Also:

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.

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.

Returns: Throws: See Also:

Sets the system property indicated by the specified key. First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is set to the given value. Returns: Throws: Since: See Also:

Removes the system property indicated by the specified key. First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is removed. Returns: Throws: Since: See Also:

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 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.

Returns: Throws: See Also:

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(Object) and Object.hashCode() methods.

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

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv.*") permission. This may result in a SecurityException being thrown.

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

Throws: Since: See Also:

Returns an instance of Logger for the caller's use. Returns: Throws: Since:

Returns a localizable instance of Logger for the caller's use. The returned logger will use the provided resource bundle for message localization. Returns: Throws: Since:

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)
 

Throws: See Also:

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()
 

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()
 

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)
 

Throws: See Also:

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)
 

Throws: See Also:

Maps a library name into a platform-specific string representing a native library. Returns: Throws: Since: See Also:

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