ThreadGroup

AI-generated Key Takeaways

• In Java, a ThreadGroup organizes and manages threads hierarchically, allowing for grouping and control over multiple threads.

• ThreadGroup provides methods to monitor (e.g., activeCount()), manage (e.g., interrupt()), and manipulate (e.g., setMaxPriority()) threads and thread groups within its structure.

• Each ThreadGroup, except the initial one, belongs to a parent ThreadGroup, forming a tree-like structure, but threads can only access their own group's details.

• ThreadGroup offers uncaught exception handling through uncaughtException(), allowing custom actions when a thread within the group encounters an unhandled exception.

• Some methods, like stop(), suspend(), and resume(), are deprecated due to safety and deadlock concerns and should be avoided.

public class ThreadGroup extends Object
implements Thread.UncaughtExceptionHandler

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.

Public Constructor Summary

	ThreadGroup(String name) Constructs a new thread group.
	ThreadGroup(ThreadGroup parent, String name) Creates a new thread group.

Public Method Summary

int	activeCount() Returns an estimate of the number of active threads in this thread group and its subgroups.
int	activeGroupCount() Returns an estimate of the number of active groups in this thread group and its subgroups.
final void	checkAccess() Determines if the currently running thread has permission to modify this thread group.
final void	destroy() Destroys this thread group and all of its subgroups.
int	enumerate(ThreadGroup[] list) Copies into the specified array references to every active subgroup in this thread group and its subgroups.
int	enumerate(ThreadGroup[] list, boolean recurse) Copies into the specified array references to every active subgroup in this thread group.
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.
final int	getMaxPriority() Returns the maximum priority of this thread group.
final String	getName() Returns the name of this thread group.
final ThreadGroup	getParent() Returns the parent of this thread group.
final void	interrupt() Interrupts all threads in this thread group.
final boolean	isDaemon() Tests if this thread group is a daemon thread group.
synchronized boolean	isDestroyed() Tests if this thread group has been destroyed.
void	list() Prints information about this thread group to the standard output.
final boolean	parentOf(ThreadGroup g) Tests if this thread group is either the thread group argument or one of its ancestor thread groups.
final void	resume() This method is 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.
final void	setDaemon(boolean daemon) Changes the daemon status of this thread group.
final void	setMaxPriority(int pri) Sets the maximum priority of the group.
final void	stop() This method is deprecated. This method is inherently unsafe. See Thread.stop() for details.
final void	suspend() This method is 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, and the thread does not have a specific Thread.UncaughtExceptionHandler installed.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.lang.Thread.UncaughtExceptionHandler

abstract void

uncaughtException(Thread arg0, Throwable arg1)

Public Constructors

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.

See Also

• checkAccess()

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.

See Also

• SecurityException
• checkAccess()

Public Methods

public int activeCount ()

Returns an estimate of the number of active threads in this thread group and its subgroups. Recursively iterates over all subgroups in this thread group.

The value returned is only an estimate because the number of threads may change dynamically while this method traverses internal data structures, and might be affected by the presence of certain system threads. This method is intended primarily for debugging and monitoring purposes.

Returns

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

public int activeGroupCount ()

Returns an estimate of the number of active groups in this thread group and its subgroups. Recursively iterates over all subgroups in this thread group.

The value returned is only an estimate because the number of thread groups may change dynamically while this method traverses internal data structures. This method is intended primarily for debugging and monitoring purposes.

Returns

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

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.

See Also

• SecurityManager.checkAccess(java.lang.ThreadGroup)

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.

See Also

• checkAccess()

public int enumerate (ThreadGroup[] list)

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

An invocation of this method behaves in exactly the same way as the invocation

{@linkplain #enumerate(ThreadGroup[], boolean) enumerate}(list, true)

Parameters

list	an array into which to put the list of thread groups

Returns

• the number of thread groups put into the array

Throws

SecurityException	if {@linkplain #checkAccess checkAccess} determines that the current thread cannot access this thread group

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

Copies into the specified array references to every active subgroup in this thread group. If recurse is true, this method recursively enumerates all subgroups of this thread group and references to every active thread group in these subgroups are also included.

An application might use the {@linkplain #activeGroupCount activeGroupCount} method to get an estimate of how big the array should be, however if the array is too short to hold all the thread groups, the extra thread groups are silently ignored. If it is critical to obtain every active subgroup in this thread group, the caller should verify that the returned int value is strictly less than the length of list.

Due to the inherent race condition in this method, it is recommended that the method only be used for debugging and monitoring purposes.

Parameters

list	an array into which to put the list of thread groups
recurse	if true, recursively enumerate all subgroups

Returns

• the number of thread groups put into the array

Throws

SecurityException	if {@linkplain #checkAccess checkAccess} determines that the current thread cannot access this thread group

public int enumerate (Thread[] list)

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

An invocation of this method behaves in exactly the same way as the invocation

{@linkplain #enumerate(Thread[], boolean) enumerate}(list, true)

Parameters

list	an array into which to put the list of threads

Returns

• the number of threads put into the array

Throws

SecurityException	if {@linkplain #checkAccess checkAccess} determines that the current thread cannot access this thread group

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

Copies into the specified array every active thread in this thread group. If recurse is true, this method recursively enumerates all subgroups of this thread group and references to every active thread in these subgroups are also included. If the array is too short to hold all the threads, the extra threads are silently ignored.

An application might use the {@linkplain #activeCount activeCount} method to get an estimate of how big the array should be, however if the array is too short to hold all the threads, the extra threads are silently ignored. If it is critical to obtain every active thread in this thread group, the caller should verify that the returned int value is strictly less than the length of list.

Due to the inherent race condition in this method, it is recommended that the method only be used for debugging and monitoring purposes.

Parameters

list	an array into which to put the list of threads
recurse	if true, recursively enumerate all subgroups of this thread group

Returns

• the number of threads put into the array

Throws

SecurityException	if {@linkplain #checkAccess checkAccess} determines that the current thread cannot access this thread group

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.

See Also

• setMaxPriority(int)

public final String getName ()

Returns the name of this thread group.

Returns

• the name of this thread group.

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.

See Also

• checkAccess()
• SecurityException
• RuntimePermission

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.

See Also

• Thread.interrupt()
• SecurityException
• checkAccess()

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.

public synchronized boolean isDestroyed ()

Tests if this thread group has been destroyed.

Returns

• true if this object is destroyed

public void list ()

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

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.

public final void resume ()

This method is 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.

See Also

• SecurityException
• Thread.resume()
• checkAccess()

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.

See Also

• SecurityException
• checkAccess()

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, it is clamped to those values.

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.

See Also

• getMaxPriority()
• SecurityException
• checkAccess()

public final void stop ()

This method is 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.

See Also

• SecurityException
• Thread.stop()
• checkAccess()

public final void suspend ()

This method is 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.

See Also

• Thread.suspend()
• SecurityException
• checkAccess()

public String toString ()

Returns a string representation of this Thread group.

Returns

• a string representation of this thread group.

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, and the thread does not have a specific Thread.UncaughtExceptionHandler installed.

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 checks to see if there is a {@linkplain Thread#getDefaultUncaughtExceptionHandler default uncaught exception handler} installed, and if so, its uncaughtException method 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, a message containing the thread's name, as returned from the thread's getName method, and a stack backtrace, using the Throwable's printStackTrace method, is printed to the {@linkplain System#err 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.