java.lang
Class ThreadGroup

java.lang.Object
  java.lang.ThreadGroup

public class ThreadGroup

extends Object

A thread group represents a set of threads. In addition, a thread group can also include other thread groups. The thread groups form a tree in which every thread group except the initial thread group has a parent.

A thread is allowed to access information about its own thread group, but not to access information about its thread group's parent thread group or any other thread groups.

Since:

JDK1.0

Author:

unascribed

Field Summary

(package private) boolean	daemon
(package private) boolean	destroyed
(package private) ThreadGroup[]	groups
(package private) int	maxPriority
(package private) String	name
(package private) int	ngroups
(package private) int	nthreads
(package private) ThreadGroup	parent
(package private) Thread[]	threads
(package private) boolean	vmAllowSuspension

Constructor Summary

private	ThreadGroup() Creates an empty Thread group that is not in any Thread group.
	ThreadGroup(String name) Constructs a new thread group.
	ThreadGroup(ThreadGroup parent, String name) Creates a new thread group.

Method Summary

int	activeCount() Returns an estimate of the number of active threads in this thread group.
int	activeGroupCount() Returns an estimate of the number of active groups in this thread group.
(package private) void	add(Thread t) Adds the specified Thread to this group.
private void	add(ThreadGroup g) Adds the specified Thread group to this group.
boolean	allowThreadSuspension(boolean b) Deprecated. The definition of this call depends on suspend(), which is deprecated. Further, the behavior of this call was never specified.
void	checkAccess() Determines if the currently running thread has permission to modify this thread group.
void	destroy() Destroys this thread group and all of its subgroups.
int	enumerate(Thread[] list) Copies into the specified array every active thread in this thread group and its subgroups.
int	enumerate(Thread[] list, boolean recurse) Copies into the specified array every active thread in this thread group.
private int	enumerate(Thread[] list, int n, boolean recurse)
int	enumerate(ThreadGroup[] list) Copies into the specified array references to every active subgroup in this thread group.
int	enumerate(ThreadGroup[] list, boolean recurse) Copies into the specified array references to every active subgroup in this thread group.
private int	enumerate(ThreadGroup[] list, int n, boolean recurse)
int	getMaxPriority() Returns the maximum priority of this thread group.
String	getName() Returns the name of this thread group.
ThreadGroup	getParent() Returns the parent of this thread group.
void	interrupt() Interrupts all threads in this thread group.
boolean	isDaemon() Tests if this thread group is a daemon thread group.
boolean	isDestroyed() Tests if this thread group has been destroyed.
void	list() Prints information about this thread group to the standard output.
(package private) void	list(PrintStream out, int indent)
boolean	parentOf(ThreadGroup g) Tests if this thread group is either the thread group argument or one of its ancestor thread groups.
(package private) void	remove(Thread t) Removes the specified Thread from this group.
private void	remove(ThreadGroup g) Removes the specified Thread group from this group.
void	resume() Deprecated. This method is used solely in conjunction with Thread.suspend and ThreadGroup.suspend, both of which have been deprecated, as they are inherently deadlock-prone. See Thread.suspend() for details.
void	setDaemon(boolean daemon) Changes the daemon status of this thread group.
void	setMaxPriority(int pri) Sets the maximum priority of the group.
void	stop() Deprecated. This method is inherently unsafe. See Thread.stop() for details.
private boolean	stopOrSuspend(boolean suspend) Helper method: recursively stops or suspends (as directed by the boolean argument) all of the threads in this thread group and its subgroups, except the current thread.
void	suspend() Deprecated. This method is inherently deadlock-prone. See Thread.suspend() for details.
String	toString() Returns a string representation of this Thread group.
void	uncaughtException(Thread t, Throwable e) Called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

parent

ThreadGroup parent

name

String name

maxPriority

int maxPriority

destroyed

boolean destroyed

daemon

boolean daemon

vmAllowSuspension

boolean vmAllowSuspension

nthreads

int nthreads

threads

Thread[] threads

ngroups

int ngroups

groups

ThreadGroup[] groups

Constructor Detail

ThreadGroup

private ThreadGroup()

Creates an empty Thread group that is not in any Thread group. This method is used to create the system Thread group.

ThreadGroup

public ThreadGroup(String name)

Constructs a new thread group. The parent of this new group is the thread group of the currently running thread.

The checkAccess method of the parent thread group is called with no arguments; this may result in a security exception.

Parameters:

name - the name of the new thread group.

Throws:

SecurityException - if the current thread cannot create a thread in the specified thread group.

Since:

JDK1.0

See Also:

checkAccess()

ThreadGroup

public ThreadGroup(ThreadGroup parent,
                   String name)

Creates a new thread group. The parent of this new group is the specified thread group.

The checkAccess method of the parent thread group is called with no arguments; this may result in a security exception.

Parameters:

parent - the parent thread group.

name - the name of the new thread group.

Throws:

NullPointerException - if the thread group argument is null.

SecurityException - if the current thread cannot create a thread in the specified thread group.

Since:

JDK1.0

See Also:

SecurityException, checkAccess()

Method Detail

getName

public final String getName()

Returns the name of this thread group.

Returns:

the name of this thread group.

Since:

JDK1.0

getParent

public final ThreadGroup getParent()

Returns the parent of this thread group.

First, if the parent is not null, the checkAccess method of the parent thread group is called with no arguments; this may result in a security exception.

Returns:

the parent of this thread group. The top-level thread group is the only thread group whose parent is null.

Throws:

SecurityException - if the current thread cannot modify this thread group.

Since:

JDK1.0

See Also:

checkAccess(), SecurityException, RuntimePermission

getMaxPriority

public final int getMaxPriority()

Returns the maximum priority of this thread group. Threads that are part of this group cannot have a higher priority than the maximum priority.

Returns:

the maximum priority that a thread in this thread group can have.

Since:

JDK1.0

See Also:

setMaxPriority(int)

isDaemon

public final boolean isDaemon()

Tests if this thread group is a daemon thread group. A daemon thread group is automatically destroyed when its last thread is stopped or its last thread group is destroyed.

Returns:

true if this thread group is a daemon thread group; false otherwise.

Since:

JDK1.0

isDestroyed

public boolean isDestroyed()

Tests if this thread group has been destroyed.

Returns:

true if this object is destroyed

Since:

JDK1.1

setDaemon

public final void setDaemon(boolean daemon)

Changes the daemon status of this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

A daemon thread group is automatically destroyed when its last thread is stopped or its last thread group is destroyed.

Parameters:

daemon - if true, marks this thread group as a daemon thread group; otherwise, marks this thread group as normal.

Throws:

SecurityException - if the current thread cannot modify this thread group.

Since:

JDK1.0

See Also:

SecurityException, checkAccess()

setMaxPriority

public final void setMaxPriority(int pri)

Sets the maximum priority of the group. Threads in the thread group that already have a higher priority are not affected.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

If the pri argument is less than Thread.MIN_PRIORITY or greater than Thread.MAX_PRIORITY, the maximum priority of the group remains unchanged.

Otherwise, the priority of this ThreadGroup object is set to the smaller of the specified pri and the maximum permitted priority of the parent of this thread group. (If this thread group is the system thread group, which has no parent, then its maximum priority is simply set to pri.) Then this method is called recursively, with pri as its argument, for every thread group that belongs to this thread group.

Parameters:

pri - the new priority of the thread group.

Throws:

SecurityException - if the current thread cannot modify this thread group.

Since:

JDK1.0

See Also:

getMaxPriority(), SecurityException, checkAccess()

parentOf

public final boolean parentOf(ThreadGroup g)

Tests if this thread group is either the thread group argument or one of its ancestor thread groups.

Parameters:

g - a thread group.

Returns:

true if this thread group is the thread group argument or one of its ancestor thread groups; false otherwise.

Since:

JDK1.0

checkAccess

public final void checkAccess()

Determines if the currently running thread has permission to modify this thread group.

If there is a security manager, its checkAccess method is called with this thread group as its argument. This may result in throwing a SecurityException.

Throws:

SecurityException - if the current thread is not allowed to access this thread group.

Since:

JDK1.0

See Also:

SecurityManager.checkAccess(java.lang.ThreadGroup)

activeCount

public int activeCount()

Returns an estimate of the number of active threads in this thread group.

Returns:

the number of active threads in this thread group and in any other thread group that has this thread group as an ancestor.

Since:

JDK1.0

enumerate

public int enumerate(Thread[] list)

Copies into the specified array every active thread in this thread group and its subgroups.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

An application should use the activeCount method to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored.

Parameters:

list - an array into which to place the list of threads.

Returns:

the number of threads put into the array.

Throws:

SecurityException - if the current thread does not have permission to enumerate this thread group.

Since:

JDK1.0

See Also:

activeCount(), checkAccess()

enumerate

public int enumerate(Thread[] list,
                     boolean recurse)

Copies into the specified array every active thread in this thread group. If the recurse flag is true, references to every active thread in this thread's subgroups are also included. If the array is too short to hold all the threads, the extra threads are silently ignored.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

An application should use the activeCount method to get an estimate of how big the array should be.

Parameters:

list - an array into which to place the list of threads.

recurse - a flag indicating whether also to include threads in thread groups that are subgroups of this thread group.

Returns:

the number of threads placed into the array.

Throws:

SecurityException - if the current thread does not have permission to enumerate this thread group.

Since:

JDK1.0

See Also:

activeCount(), checkAccess()

enumerate

private int enumerate(Thread[] list,
                      int n,
                      boolean recurse)

activeGroupCount

public int activeGroupCount()

Returns an estimate of the number of active groups in this thread group.

Returns:

the number of active thread groups with this thread group as an ancestor.

Since:

JDK1.0

enumerate

public int enumerate(ThreadGroup[] list)

Copies into the specified array references to every active subgroup in this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

An application should use the activeGroupCount method to get an estimate of how big the array should be. If the array is too short to hold all the thread groups, the extra thread groups are silently ignored.

Parameters:

list - an array into which to place the list of thread groups.

Returns:

the number of thread groups put into the array.

Throws:

SecurityException - if the current thread does not have permission to enumerate this thread group.

Since:

JDK1.0

See Also:

activeGroupCount(), checkAccess()

enumerate

public int enumerate(ThreadGroup[] list,
                     boolean recurse)

Copies into the specified array references to every active subgroup in this thread group. If the recurse flag is true, references to all active subgroups of the subgroups and so forth are also included.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

An application should use the activeGroupCount method to get an estimate of how big the array should be.

Parameters:

list - an array into which to place the list of threads.

recurse - a flag indicating whether to recursively enumerate all included thread groups.

Returns:

the number of thread groups put into the array.

Throws:

SecurityException - if the current thread does not have permission to enumerate this thread group.

Since:

JDK1.0

See Also:

activeGroupCount(), checkAccess()

enumerate

private int enumerate(ThreadGroup[] list,
                      int n,
                      boolean recurse)

stop

public final void stop()

Deprecated. This method is inherently unsafe. See Thread.stop() for details.

Stops all threads in this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

This method then calls the stop method on all the threads in this thread group and in all of its subgroups.

Throws:

SecurityException - if the current thread is not allowed to access this thread group or any of the threads in the thread group.

Since:

JDK1.0

See Also:

SecurityException, Thread.stop(), checkAccess()

interrupt

public final void interrupt()

Interrupts all threads in this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

This method then calls the interrupt method on all the threads in this thread group and in all of its subgroups.

Throws:

SecurityException - if the current thread is not allowed to access this thread group or any of the threads in the thread group.

Since:

1.2

See Also:

Thread.interrupt(), SecurityException, checkAccess()

suspend

public final void suspend()

Deprecated. This method is inherently deadlock-prone. See Thread.suspend() for details.

Suspends all threads in this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

This method then calls the suspend method on all the threads in this thread group and in all of its subgroups.

Throws:

SecurityException - if the current thread is not allowed to access this thread group or any of the threads in the thread group.

Since:

JDK1.0

See Also:

Thread.suspend(), SecurityException, checkAccess()

stopOrSuspend

private boolean stopOrSuspend(boolean suspend)

Helper method: recursively stops or suspends (as directed by the boolean argument) all of the threads in this thread group and its subgroups, except the current thread. This method returns true if (and only if) the current thread is found to be in this thread group or one of its subgroups.

resume

public final void resume()

Deprecated. This method is used solely in conjunction with Thread.suspend and ThreadGroup.suspend, both of which have been deprecated, as they are inherently deadlock-prone. See Thread.suspend() for details.

Resumes all threads in this thread group.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

This method then calls the resume method on all the threads in this thread group and in all of its sub groups.

Throws:

SecurityException - if the current thread is not allowed to access this thread group or any of the threads in the thread group.

Since:

JDK1.0

See Also:

SecurityException, Thread.resume(), checkAccess()

destroy

public final void destroy()

Destroys this thread group and all of its subgroups. This thread group must be empty, indicating that all threads that had been in this thread group have since stopped.

First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

Throws:

IllegalThreadStateException - if the thread group is not empty or if the thread group has already been destroyed.

SecurityException - if the current thread cannot modify this thread group.

Since:

JDK1.0

See Also:

checkAccess()

add

private final void add(ThreadGroup g)

Adds the specified Thread group to this group.

Parameters:

g - the specified Thread group to be added

Throws:

IllegalThreadStateException - If the Thread group has been destroyed.

remove

private void remove(ThreadGroup g)

Removes the specified Thread group from this group.

Parameters:

g - the Thread group to be removed

Returns:

if this Thread has already been destroyed.

add

void add(Thread t)

Adds the specified Thread to this group.

Parameters:

t - the Thread to be added

Throws:

IllegalThreadStateException - If the Thread group has been destroyed.

remove

void remove(Thread t)

Removes the specified Thread from this group.

Parameters:

t - the Thread to be removed

Returns:

if the Thread has already been destroyed.

list

public void list()

Prints information about this thread group to the standard output. This method is useful only for debugging.

Since:

JDK1.0

list

void list(PrintStream out,
          int indent)

uncaughtException

public void uncaughtException(Thread t,
                              Throwable e)

Called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception.

The uncaughtException method of ThreadGroup does the following:

• If this thread group has a parent thread group, the uncaughtException method of that parent is called with the same two arguments.
• Otherwise, this method determines if the Throwable argument is an instance of ThreadDeath. If so, nothing special is done. Otherwise, the Throwable's printStackTrace method is called to print a stack backtrace to the standard error stream.

Applications can override this method in subclasses of ThreadGroup to provide alternative handling of uncaught exceptions.

Parameters:

t - the thread that is about to exit.

e - the uncaught exception.

Since:

JDK1.0

See Also:

System.err, ThreadDeath, Throwable.printStackTrace(java.io.PrintStream)

allowThreadSuspension

public boolean allowThreadSuspension(boolean b)

Deprecated. The definition of this call depends on suspend(), which is deprecated. Further, the behavior of this call was never specified.

Used by VM to control lowmem implicit suspension.

Parameters:

b - boolean to allow or disallow suspension

Returns:

true on success

Since:

JDK1.1

toString

public String toString()

Returns a string representation of this Thread group.

Overrides:

toString in class Object

Returns:

a string representation of this thread group.

Since:

JDK1.0