1 *java.lang.System* *System* The System class contains several useful class field
3 public final class System
4 extends |java.lang.Object|
6 |java.lang.System_Description|
7 |java.lang.System_Fields|
8 |java.lang.System_Constructors|
9 |java.lang.System_Methods|
11 ================================================================================
13 *java.lang.System_Fields*
14 |java.io.PrintStream_java.lang.System.err|
15 |java.io.InputStream_java.lang.System.in|
16 |java.io.PrintStream_java.lang.System.out|
18 *java.lang.System_Methods*
19 |java.lang.System.arraycopy(Object,int,Object,int,int)|Copies an array from the
20 |java.lang.System.clearProperty(String)|Removes the system property indicated b
21 |java.lang.System.currentTimeMillis()|Returns the current time in milliseconds.
22 |java.lang.System.exit(int)|Terminates the currently running Java Virtual Machi
23 |java.lang.System.gc()|Runs the garbage collector.
24 |java.lang.System.getenv()|Returns an unmodifiable string map view of the curre
25 |java.lang.System.getenv(String)|Gets the value of the specified environment va
26 |java.lang.System.getProperties()|Determines the current system properties.
27 |java.lang.System.getProperty(String)|Gets the system property indicated by the
28 |java.lang.System.getProperty(String,String)|Gets the system property indicated
29 |java.lang.System.getSecurityManager()|Gets the system security interface.
30 |java.lang.System.identityHashCode(Object)|Returns the same hash code for the g
31 |java.lang.System.inheritedChannel()|Returns the channel inherited from the ent
32 |java.lang.System.load(String)|Loads a code file with the specified filename fr
33 |java.lang.System.loadLibrary(String)|Loads the system library specified by the
34 |java.lang.System.mapLibraryName(String)|Maps a library name into a platform-sp
35 |java.lang.System.nanoTime()|Returns the current value of the most precise avai
36 |java.lang.System.runFinalization()|Runs the finalization methods of any object
37 |java.lang.System.runFinalizersOnExit(boolean)|Enable or disable finalization o
38 |java.lang.System.setErr(PrintStream)|Reassigns the "standard" error output str
39 |java.lang.System.setIn(InputStream)|Reassigns the "standard" input stream.
40 |java.lang.System.setOut(PrintStream)|Reassigns the "standard" output stream.
41 |java.lang.System.setProperties(Properties)|Sets the system properties to the P
42 |java.lang.System.setProperty(String,String)|Sets the system property indicated
43 |java.lang.System.setSecurityManager(SecurityManager)|Sets the System security.
45 *java.lang.System_Description*
47 The System class contains several useful class fields and methods. It cannot be
50 Among the facilities provided by the System class are standard input, standard
51 output, and error output streams; access to externally defined properties and
52 environment variables; a means of loading files and libraries; and a utility
53 method for quickly copying a portion of an array.
56 *java.io.PrintStream_java.lang.System.err*
58 The System class contains several useful class fields and methods. It cannot be
61 Among the facilities provided by the System class are standard input, standard
62 output, and error output streams; access to externally defined properties and
63 environment variables; a means of loading files and libraries; and a utility
64 method for quickly copying a portion of an array.
67 *java.io.InputStream_java.lang.System.in*
69 The System class contains several useful class fields and methods. It cannot be
72 Among the facilities provided by the System class are standard input, standard
73 output, and error output streams; access to externally defined properties and
74 environment variables; a means of loading files and libraries; and a utility
75 method for quickly copying a portion of an array.
78 *java.io.PrintStream_java.lang.System.out*
80 The System class contains several useful class fields and methods. It cannot be
83 Among the facilities provided by the System class are standard input, standard
84 output, and error output streams; access to externally defined properties and
85 environment variables; a means of loading files and libraries; and a utility
86 method for quickly copying a portion of an array.
90 *java.lang.System.arraycopy(Object,int,Object,int,int)*
92 public static native void arraycopy(
95 java.lang.Object dest,
99 Copies an array from the specified source array, beginning at the specified
100 position, to the specified position of the destination array. A subsequence of
101 array components are copied from the source array referenced by src to the
102 destination array referenced by dest. The number of components copied is equal
103 to the length argument. The components at positions srcPos through
104 srcPos+length-1 in the source array are copied into positions destPos through
105 destPos+length-1, respectively, of the destination array.
107 If the src and dest arguments refer to the same array object, then the copying
108 is performed as if the components at positions srcPos through srcPos+length-1
109 were first copied to a temporary array with length components and then the
110 contents of the temporary array were copied into positions destPos through
111 destPos+length-1 of the destination array.
113 If dest is null, then a NullPointerException is thrown.
115 If src is null, then a NullPointerException is thrown and the destination array
118 Otherwise, if any of the following is true, an ArrayStoreException is thrown
119 and the destination is not modified:
121 The src argument refers to an object that is not an array. The dest argument
122 refers to an object that is not an array. The src argument and dest argument
123 refer to arrays whose component types are different primitive types. The src
124 argument refers to an array with a primitive component type and the dest
125 argument refers to an array with a reference component type. The src argument
126 refers to an array with a reference component type and the dest argument refers
127 to an array with a primitive component type.
129 Otherwise, if any of the following is true, an IndexOutOfBoundsException is
130 thrown and the destination is not modified:
132 The srcPos argument is negative. The destPos argument is negative. The length
133 argument is negative. srcPos+length is greater than src.length, the length of
134 the source array. destPos+length is greater than dest.length, the length of the
137 Otherwise, if any actual component of the source array from position srcPos
138 through srcPos+length-1 cannot be converted to the component type of the
139 destination array by assignment conversion, an ArrayStoreException is thrown.
140 In this case, let k be the smallest nonnegative integer less than length such
141 that src[srcPos+k] cannot be converted to the component type of the destination
142 array; when the exception is thrown, source array components from positions
143 srcPos through srcPos+k-1 will already have been copied to destination array
144 positions destPos through destPos+k-1 and no other positions of the destination
145 array will have been modified. (Because of the restrictions already itemized,
146 this paragraph effectively applies only to the situation where both arrays have
147 component types that are reference types.)
149 src - the source array.
150 srcPos - starting position in the source array.
151 dest - the destination array.
152 destPos - starting position in the destination data.
153 length - the number of array elements to be copied.
155 *java.lang.System.clearProperty(String)*
157 public static |java.lang.String| clearProperty(java.lang.String key)
159 Removes the system property indicated by the specified key.
161 First, if a security manager exists, its SecurityManager.checkPermission method
162 is called with a PropertyPermission(key, "write") permission. This may result
163 in a SecurityException being thrown. If no exception is thrown, the specified
166 key - the name of the system property to be removed.
168 Returns: the previous string value of the system property, or null if there was no
169 property with that key.
170 *java.lang.System.currentTimeMillis()*
172 public static native long currentTimeMillis()
174 Returns the current time in milliseconds. Note that while the unit of time of
175 the return value is a millisecond, the granularity of the value depends on the
176 underlying operating system and may be larger. For example, many operating
177 systems measure time in units of tens of milliseconds.
179 See the description of the class Date for a discussion of slight discrepancies
180 that may arise between "computer time" and coordinated universal time (UTC).
183 Returns: the difference, measured in milliseconds, between the current time and
184 midnight, January 1, 1970 UTC.
185 *java.lang.System.exit(int)*
187 public static void exit(int status)
189 Terminates the currently running Java Virtual Machine. The argument serves as a
190 status code; by convention, a nonzero status code indicates abnormal
193 This method calls the exit method in class Runtime. This method never returns
196 The call System.exit(n) is effectively equivalent to the call:
198 Runtime.getRuntime().exit(n)
200 status - exit status.
202 *java.lang.System.gc()*
204 public static void gc()
206 Runs the garbage collector.
208 Calling the gc method suggests that the Java Virtual Machine expend effort
209 toward recycling unused objects in order to make the memory they currently
210 occupy available for quick reuse. When control returns from the method call,
211 the Java Virtual Machine has made a best effort to reclaim space from all
214 The call System.gc() is effectively equivalent to the call:
216 Runtime.getRuntime().gc()
219 *java.lang.System.getenv()*
221 public static |java.util.Map| getenv()
223 Returns an unmodifiable string map view of the current system environment. The
224 environment is a system-dependent mapping from names to values which is passed
225 from parent to child processes.
227 If the system does not support environment variables, an empty map is returned.
229 The returned map will never contain null keys or values. Attempting to query
230 the presence of a null key or value will throw a
231 (|java.lang.NullPointerException|) . Attempting to query the presence of a key
232 or value which is not of type (|java.lang.String|) will throw a
233 (|java.lang.ClassCastException|) .
235 The returned map and its collection views may not obey the general contract of
236 the (|java.lang.Object|) and (|java.lang.Object|) methods.
238 The returned map is typically case-sensitive on all platforms.
240 If a security manager exists, its checkPermission(|java.lang.SecurityManager|)
241 method is called with a (|java.lang.RuntimePermission|) ("getenv.*")
242 permission. This may result in a (|java.lang.SecurityException|) being thrown.
244 When passing information to a Java subprocess, system properties are generally
245 preferred over environment variables.
248 Returns: the environment as a map of variable names to values
249 *java.lang.System.getenv(String)*
251 public static |java.lang.String| getenv(java.lang.String name)
253 Gets the value of the specified environment variable. An environment variable
254 is a system-dependent external named value.
256 If a security manager exists, its checkPermission(|java.lang.SecurityManager|)
257 method is called with a (|java.lang.RuntimePermission|) ("getenv."+name)
258 permission. This may result in a (|java.lang.SecurityException|) being thrown.
259 If no exception is thrown the value of the variable name is returned.
261 System properties and environment variables are both conceptually mappings
262 between names and values. Both mechanisms can be used to pass user-defined
263 information to a Java process. Environment variables have a more global effect,
264 because they are visible to all descendants of the process which defines them,
265 not just the immediate Java subprocess. They can have subtly different
266 semantics, such as case insensitivity, on different operating systems. For
267 these reasons, environment variables are more likely to have unintended side
268 effects. It is best to use system properties where possible. Environment
269 variables should be used when a global effect is desired, or when an external
270 system interface requires an environment variable (such as PATH).
272 On UNIX systems the alphabetic case of name is typically significant, while on
273 Microsoft Windows systems it is typically not. For example, the expression
274 System.getenv("FOO").equals(System.getenv("foo")) is likely to be true on
277 name - the name of the environment variable
279 Returns: the string value of the variable, or null if the variable is not defined in the
281 *java.lang.System.getProperties()*
283 public static |java.util.Properties| getProperties()
285 Determines the current system properties.
287 First, if there is a security manager, its checkPropertiesAccess method is
288 called with no arguments. This may result in a security exception.
290 The current set of system properties for use by the (|java.lang.System|) method
291 is returned as a Properties object. If there is no current set of system
292 properties, a set of system properties is first created and initialized. This
293 set of system properties always includes values for the following keys:
295 Key Description of Associated Value java.version Java Runtime Environment
296 version java.vendor Java Runtime Environment vendorjava.vendor.url Java vendor
297 URL java.home Java installation directory java.vm.specification.version Java
298 Virtual Machine specification version java.vm.specification.vendor Java Virtual
299 Machine specification vendor java.vm.specification.name Java Virtual Machine
300 specification name java.vm.version Java Virtual Machine implementation version
301 java.vm.vendor Java Virtual Machine implementation vendor java.vm.name Java
302 Virtual Machine implementation name java.specification.version Java Runtime
303 Environment specification version java.specification.vendor Java Runtime
304 Environment specification vendor java.specification.name Java Runtime
305 Environment specification name java.class.version Java class format version
306 number java.class.path Java class path java.library.path List of paths to
307 search when loading libraries java.io.tmpdir Default temp file path
308 java.compiler Name of JIT compiler to use java.ext.dirs Path of extension
309 directory or directories os.name Operating system name os.arch Operating system
310 architecture os.version Operating system version file.separator File separator
311 ("/" on UNIX) path.separator Path separator (":" on UNIX) line.separator Line
312 separator ("\n" on UNIX) user.name User's account name user.home User's home
313 directory user.dir User's current working directory
315 Multiple paths in a system property value are separated by the path separator
316 character of the platform.
318 Note that even if the security manager does not permit the getProperties
319 operation, it may choose to permit the (|java.lang.System|) operation.
322 Returns: the system properties
323 *java.lang.System.getProperty(String)*
325 public static |java.lang.String| getProperty(java.lang.String key)
327 Gets the system property indicated by the specified key.
329 First, if there is a security manager, its checkPropertyAccess method is called
330 with the key as its argument. This may result in a SecurityException.
332 If there is no current set of system properties, a set of system properties is
333 first created and initialized in the same manner as for the getProperties
336 key - the name of the system property.
338 Returns: the string value of the system property, or null if there is no property with
340 *java.lang.System.getProperty(String,String)*
342 public static |java.lang.String| getProperty(
343 java.lang.String key,
344 java.lang.String def)
346 Gets the system property indicated by the specified key.
348 First, if there is a security manager, its checkPropertyAccess method is called
349 with the key as its argument.
351 If there is no current set of system properties, a set of system properties is
352 first created and initialized in the same manner as for the getProperties
355 key - the name of the system property.
356 def - a default value.
358 Returns: the string value of the system property, or the default value if there is no
359 property with that key.
360 *java.lang.System.getSecurityManager()*
362 public static |java.lang.SecurityManager| getSecurityManager()
364 Gets the system security interface.
367 Returns: if a security manager has already been established for the current application,
368 then that security manager is returned; otherwise, null is
370 *java.lang.System.identityHashCode(Object)*
372 public static native int identityHashCode(java.lang.Object x)
374 Returns the same hash code for the given object as would be returned by the
375 default method hashCode(), whether or not the given object's class overrides
376 hashCode(). The hash code for the null reference is zero.
378 x - object for which the hashCode is to be calculated
381 *java.lang.System.inheritedChannel()*
383 public static |java.nio.channels.Channel| inheritedChannel()
384 throws |java.io.IOException|
386 Returns the channel inherited from the entity that created this Java virtual
389 This method returns the channel obtained by invoking the
390 inheritedChannel(|java.nio.channels.spi.SelectorProvider|) method of the
391 system-wide default (|java.nio.channels.spi.SelectorProvider|) object.
393 In addition to the network-oriented channels described in
394 inheritedChannel(|java.nio.channels.spi.SelectorProvider|) , this method may
395 return other kinds of channels in the future.
398 Returns: The inherited channel, if any, otherwise null.
399 *java.lang.System.load(String)*
401 public static void load(java.lang.String filename)
403 Loads a code file with the specified filename from the local file system as a
404 dynamic library. The filename argument must be a complete path name.
406 The call System.load(name) is effectively equivalent to the call:
408 Runtime.getRuntime().load(name)
410 filename - the file to load.
412 *java.lang.System.loadLibrary(String)*
414 public static void loadLibrary(java.lang.String libname)
416 Loads the system library specified by the libname argument. The manner in which
417 a library name is mapped to the actual system library is system dependent.
419 The call System.loadLibrary(name) is effectively equivalent to the call
421 Runtime.getRuntime().loadLibrary(name)
423 libname - the name of the library.
425 *java.lang.System.mapLibraryName(String)*
427 public static native |java.lang.String| mapLibraryName(java.lang.String libname)
429 Maps a library name into a platform-specific string representing a native
432 libname - the name of the library.
434 Returns: a platform-dependent native library name.
435 *java.lang.System.nanoTime()*
437 public static native long nanoTime()
439 Returns the current value of the most precise available system timer, in
442 This method can only be used to measure elapsed time and is not related to any
443 other notion of system or wall-clock time. The value returned represents
444 nanoseconds since some fixed but arbitrary time (perhaps in the future, so
445 values may be negative). This method provides nanosecond precision, but not
446 necessarily nanosecond accuracy. No guarantees are made about how frequently
447 values change. Differences in successive calls that span greater than
448 approximately 292 years (263 nanoseconds) will not accurately compute elapsed
449 time due to numerical overflow.
451 For example, to measure how long some code takes to execute:
453 long startTime = System.nanoTime(); // ... the code being measured ... long
454 estimatedTime = System.nanoTime() - startTime;
457 Returns: The current value of the system timer, in nanoseconds.
458 *java.lang.System.runFinalization()*
460 public static void runFinalization()
462 Runs the finalization methods of any objects pending finalization.
464 Calling this method suggests that the Java Virtual Machine expend effort toward
465 running the finalize methods of objects that have been found to be discarded
466 but whose finalize methods have not yet been run. When control returns from the
467 method call, the Java Virtual Machine has made a best effort to complete all
468 outstanding finalizations.
470 The call System.runFinalization() is effectively equivalent to the call:
472 Runtime.getRuntime().runFinalization()
475 *java.lang.System.runFinalizersOnExit(boolean)*
477 public static void runFinalizersOnExit(boolean value)
479 Enable or disable finalization on exit; doing so specifies that the finalizers
480 of all objects that have finalizers that have not yet been automatically
481 invoked are to be run before the Java runtime exits. By default, finalization
484 If there is a security manager, its checkExit method is first called with 0 as
485 its argument to ensure the exit is allowed. This could result in a
488 value - indicating enabling or disabling of finalization
490 *java.lang.System.setErr(PrintStream)*
492 public static void setErr(java.io.PrintStream err)
494 Reassigns the "standard" error output stream.
496 First, if there is a security manager, its checkPermission method is called
497 with a RuntimePermission("setIO") permission to see if it's ok to reassign the
498 "standard" error output stream.
500 err - the new standard error output stream.
502 *java.lang.System.setIn(InputStream)*
504 public static void setIn(java.io.InputStream in)
506 Reassigns the "standard" input stream.
508 First, if there is a security manager, its checkPermission method is called
509 with a RuntimePermission("setIO") permission to see if it's ok to reassign the
510 "standard" input stream.
512 in - the new standard input stream.
514 *java.lang.System.setOut(PrintStream)*
516 public static void setOut(java.io.PrintStream out)
518 Reassigns the "standard" output stream.
520 First, if there is a security manager, its checkPermission method is called
521 with a RuntimePermission("setIO") permission to see if it's ok to reassign the
522 "standard" output stream.
524 out - the new standard output stream
526 *java.lang.System.setProperties(Properties)*
528 public static void setProperties(java.util.Properties props)
530 Sets the system properties to the Properties argument.
532 First, if there is a security manager, its checkPropertiesAccess method is
533 called with no arguments. This may result in a security exception.
535 The argument becomes the current set of system properties for use by the
536 (|java.lang.System|) method. If the argument is null, then the current set of
537 system properties is forgotten.
539 props - the new system properties.
541 *java.lang.System.setProperty(String,String)*
543 public static |java.lang.String| setProperty(
544 java.lang.String key,
545 java.lang.String value)
547 Sets the system property indicated by the specified key.
549 First, if a security manager exists, its SecurityManager.checkPermission method
550 is called with a PropertyPermission(key, "write") permission. This may result
551 in a SecurityException being thrown. If no exception is thrown, the specified
552 property is set to the given value.
554 key - the name of the system property.
555 value - the value of the system property.
557 Returns: the previous value of the system property, or null if it did not have one.
558 *java.lang.System.setSecurityManager(SecurityManager)*
560 public static void setSecurityManager(java.lang.SecurityManager s)
562 Sets the System security.
564 If there is a security manager already installed, this method first calls the
565 security manager's checkPermission method with a
566 RuntimePermission("setSecurityManager") permission to ensure it's ok to replace
567 the existing security manager. This may result in throwing a SecurityException.
569 Otherwise, the argument is established as the current security manager. If the
570 argument is null and no security manager has been established, then no action
571 is taken and the method simply returns.
573 s - the security manager.