public class

ThreadGroup

extends Object
implements Thread.UncaughtExceptionHandler
java.lang.Object
   ↳ java.lang.ThreadGroup
Known Direct Subclasses

Class Overview

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.

Summary

Public Constructors
ThreadGroup(String name)
Constructs a new thread group.
ThreadGroup(ThreadGroup parent, String name)
Creates a new thread group.
Public Methods
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.
boolean allowThreadSuspension(boolean b)
This method is deprecated. The definition of this call depends on suspend(), which is deprecated. Further, the behavior of this call was never specified.
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(Thread[] list)
Copies into the specified array every active thread 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, boolean recurse)
Copies into the specified array every active thread in this thread group.
int enumerate(ThreadGroup[] list)
Copies into the specified array references to every active subgroup 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 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 stop() for details.
final void suspend()
This method is deprecated. This method is inherently deadlock-prone. See 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.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.lang.Thread.UncaughtExceptionHandler

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

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

Public Methods

public int activeCount ()

Returns an estimate of the number of active threads in this thread group. The result might not reflect concurrent activity, and might be affected by the presence of certain system threads.

Due to the inherently imprecise nature of the result, it is recommended that this method only be used for informational 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. The result might not reflect concurrent activity.

Due to the inherently imprecise nature of the result, it is recommended that this method only be used for informational purposes.

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

public boolean allowThreadSuspension (boolean b)

This method is 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

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

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

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 might use the 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 and its subgroups, 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 informational purposes.

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.
See Also

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 might use the 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 informational purposes.

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.
See Also

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 might use the 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 informational purposes.

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.
See Also

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 might use the 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 informational purposes.

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.
See Also

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

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

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

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 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

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

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 MIN_PRIORITY or greater than 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.
See Also

public final void stop ()

This method is deprecated.
This method is inherently unsafe. See 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

public final void suspend ()

This method is deprecated.
This method is inherently deadlock-prone. See 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

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 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 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.