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.

@author
unascribed
@version
1.149, 06/02/04
@since
JDK1.0
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 "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.
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.

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.

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
srcthe source array.
srcPosstarting position in the source array.
destthe destination array.
destPosstarting position in the destination data.
lengththe number of array elements to be copied.
Throws
IndexOutOfBoundsExceptionif copying would cause access of data outside array bounds.
ArrayStoreExceptionif an element in the src array could not be stored into the dest array because of a type mismatch.
NullPointerExceptionif either src or dest is null.
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.

Parameters
keythe name of the system property to be removed.
Return
the previous string value of the system property, or null if there was no property with that key.
Throws
SecurityExceptionif a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
NullPointerExceptionif key is null.
IllegalArgumentExceptionif key is empty.
@since
1.5
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).

Return
the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

Parameters
objthe reference object with which to compare.
Return
true if this object is the same as the obj argument; false otherwise.
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
statusexit status.
Throws
SecurityException if a security manager exists and its checkExit method doesn't allow exit with the specified status.
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()
 
Returns the runtime class of an object. That Class object is the object that is locked by static synchronized methods of the represented class.
Return
The java.lang.Class object that represents the runtime class of the object. The result is of type {@code Class} where X is the erasure of the static type of the expression on which getClass is called.
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 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.

Return
the environment as a map of variable names to values
Throws
SecurityException if a security manager exists and its {@link SecurityManager#checkPermission checkPermission} method doesn't allow access to the process environment
@since
1.5
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 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 always includes values for the following keys:
Key Description of Associated Value
java.version Java Runtime Environment version
java.vendor Java Runtime Environment vendor
java.vendor.url Java vendor URL
java.home Java installation directory
java.vm.specification.version Java Virtual Machine specification 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
java.vm.vendor Java Virtual Machine implementation vendor
java.vm.name Java Virtual Machine implementation name
java.specification.version Java Runtime Environment specification 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
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
java.ext.dirs Path of extension directory or directories
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

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 operation.

Return
the system properties
Throws
SecurityExceptionif a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
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.

Parameters
keythe name of the system property.
Return
the string value of the system property, or null if there is no property with that key.
Throws
SecurityExceptionif a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
NullPointerExceptionif key is null.
IllegalArgumentExceptionif key is empty.
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.

Parameters
keythe name of the system property.
defa default value.
Return
the string value of the system property, or the default value if there is no property with that key.
Throws
SecurityExceptionif a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
NullPointerExceptionif key is null.
IllegalArgumentExceptionif key is empty.
Gets the system security interface.
Return
if a security manager has already been established for the current application, then that security manager is returned; otherwise, null is returned.
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Return
a hash code value for this object.
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
xobject for which the hashCode is to be calculated
Return
the hashCode
@since
JDK1.1
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 java.nio.channels.spi.SelectorProvider object.

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

Return
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
Loads a code file with the specified filename from the local file system as a dynamic library. The filename argument must be a complete path name.

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

 Runtime.getRuntime().load(name)
 
Parameters
filenamethe file to load.
Throws
SecurityExceptionif a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library
UnsatisfiedLinkErrorif the file does not exist.
NullPointerExceptionif filename is null
Loads the system library specified by the libname argument. The manner in which a library name is mapped to the actual system library is system dependent.

The call System.loadLibrary(name) is effectively equivalent to the call

 Runtime.getRuntime().loadLibrary(name)
 
Parameters
libnamethe name of the library.
Throws
SecurityExceptionif a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library
UnsatisfiedLinkErrorif the library does not exist.
NullPointerExceptionif libname is null
Maps a library name into a platform-specific string representing a native library.
Parameters
libnamethe name of the library.
Return
a platform-dependent native library name.
Throws
NullPointerExceptionif libname is null
@since
1.2
Returns the current value of the most precise available system timer, 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 time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not accurately compute elapsed time due to numerical overflow.

For example, to measure how long some code takes to execute:

   long startTime = System.nanoTime();
   // ... the code being measured ...
   long estimatedTime = System.nanoTime() - startTime;
 
Return
The current value of the system timer, in nanoseconds.
@since
1.5
Wakes up a single thread that is waiting on this object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. A thread waits on an object's monitor by calling one of the wait methods.

The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.

This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:

  • By executing a synchronized instance method of that object.
  • By executing the body of a synchronized statement that synchronizes on the object.
  • For objects of type Class, by executing a synchronized static method of that class.

Only one thread at a time can own an object's monitor.

Throws
IllegalMonitorStateExceptionif the current thread is not the owner of this object's monitor.
Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait methods.

The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

Throws
IllegalMonitorStateExceptionif the current thread is not the owner of this object's monitor.
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()
 
Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

If there is a security manager, its checkExit method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.

Parameters
valueindicating enabling or disabling of finalization
Throws
SecurityException if a security manager exists and its checkExit method doesn't allow the exit.
@deprecated
This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.
@since
JDK1.1
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
errthe 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
JDK1.1
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
inthe 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
JDK1.1
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
outthe 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
JDK1.1
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 method. If the argument is null, then the current set of system properties is forgotten.

Parameters
propsthe new system properties.
Throws
SecurityExceptionif a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
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.

Parameters
keythe name of the system property.
valuethe value of the system property.
Return
the previous value of the system property, or null if it did not have one.
Throws
SecurityExceptionif a security manager exists and its checkPermission method doesn't allow setting of the specified property.
NullPointerExceptionif key or value is null.
IllegalArgumentExceptionif key is empty.
@since
1.2
Sets the System security.

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.

Parameters
sthe security manager.
Throws
SecurityExceptionif the security manager has already been set and its checkPermission method doesn't allow it to be replaced.
Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())
 
Return
a string representation of the object.
Causes current thread to wait until another thread invokes the method or the method for this object. In other words, this method behaves exactly as if it simply performs the call wait(0).

The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.

As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:

     synchronized (obj) {
         while (<condition does not hold>)
             obj.wait();
         ... // Perform action appropriate to condition
     }
 
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
Throws
IllegalMonitorStateExceptionif the current thread is not the owner of the object's monitor.
InterruptedExceptionif another thread interrupted the current thread before or while the current thread was waiting for a notification. The interrupted status of the current thread is cleared when this exception is thrown.
Causes current thread to wait until either another thread invokes the method or the method for this object, or a specified amount of time has elapsed.

The current thread must own this object's monitor.

This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Thread T becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:

  • Some other thread invokes the notify method for this object and thread T happens to be arbitrarily chosen as the thread to be awakened.
  • Some other thread invokes the notifyAll method for this object.
  • Some other thread interrupts thread T.
  • The specified amount of real time has elapsed, more or less. If timeout is zero, however, then real time is not taken into consideration and the thread simply waits until notified.
The thread T is then removed from the wait set for this object and re-enabled for thread scheduling. It then competes in the usual manner with other threads for the right to synchronize on the object; once it has gained control of the object, all its synchronization claims on the object are restored to the status quo ante - that is, to the situation as of the time that the wait method was invoked. Thread T then returns from the invocation of the wait method. Thus, on return from the wait method, the synchronization state of the object and of thread T is exactly as it was when the wait method was invoked.

A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:

     synchronized (obj) {
         while (<condition does not hold>)
             obj.wait(timeout);
         ... // Perform action appropriate to condition
     }
 
(For more information on this topic, see Section 3.2.3 in Doug Lea's "Concurrent Programming in Java (Second Edition)" (Addison-Wesley, 2000), or Item 50 in Joshua Bloch's "Effective Java Programming Language Guide" (Addison-Wesley, 2001).

If the current thread is interrupted by another thread while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.

Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

Parameters
timeoutthe maximum time to wait in milliseconds.
Throws
IllegalArgumentExceptionif the value of timeout is negative.
IllegalMonitorStateExceptionif the current thread is not the owner of the object's monitor.
InterruptedExceptionif another thread interrupted the current thread before or while the current thread was waiting for a notification. The interrupted status of the current thread is cleared when this exception is thrown.
Causes current thread to wait until another thread invokes the method or the method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.

This method is similar to the wait method of one argument, but it allows finer control over the amount of time to wait for a notification before giving up. The amount of real time, measured in nanoseconds, is given by:

 1000000*timeout+nanos

In all other respects, this method does the same thing as the method of one argument. In particular, wait(0, 0) means the same thing as wait(0).

The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:

  • Another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method.
  • The timeout period, specified by timeout milliseconds plus nanos nanoseconds arguments, has elapsed.

The thread then waits until it can re-obtain ownership of the monitor and resumes execution.

As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:

     synchronized (obj) {
         while (<condition does not hold>)
             obj.wait(timeout, nanos);
         ... // Perform action appropriate to condition
     }
 
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
Parameters
timeoutthe maximum time to wait in milliseconds.
nanosadditional time, in nanoseconds range 0-999999.
Throws
IllegalArgumentExceptionif the value of timeout is negative or the value of nanos is not in the range 0-999999.
IllegalMonitorStateExceptionif the current thread is not the owner of this object's monitor.
InterruptedExceptionif another thread interrupted the current thread before or while the current thread was waiting for a notification. The interrupted status of the current thread is cleared when this exception is thrown.