Class: java.lang.ThreadGroup

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.

Authors:
@author unascribed
Since:
@since JDK1.0

Inheritance

Superclass tree: Implements:

Methods

  • ThreadGrouptop

    private ThreadGroup()
    Creates an empty Thread group that is not in any Thread group. This method is used to create the system Thread group.
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  • ThreadGrouptop

    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:
    @param name the name of the new thread group.
    Exceptions:
    @exception SecurityException if the current thread cannot create a thread in the specified thread group.
    See:
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • ThreadGrouptop

    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:
    @param parent the parent thread group.
    @param name the name of the new thread group.
    Exceptions:
    @exception NullPointerException if the thread group argument is null.
    @exception SecurityException if the current thread cannot create a thread in the specified thread group.
    See:
    @see SecurityException
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • activeCounttop

    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.

    Return:
    @return 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.
    Since:
    @since JDK1.0
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  • activeGroupCounttop

    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.

    Return:
    @return the number of active thread groups with this thread group as an ancestor.
    Since:
    @since JDK1.0
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  • addtop

    void add(Thread t)
    Adds the specified Thread to this group.
    Parameters:
    @param t the Thread to be added
    Exceptions:
    @exception IllegalThreadStateException If the Thread group has been destroyed.
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  • addtop

    final private void add(ThreadGroup g)
    Adds the specified Thread group to this group.
    Parameters:
    @param g the specified Thread group to be added
    Exceptions:
    @exception IllegalThreadStateException If the Thread group has been destroyed.
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  • addUnstartedtop

    void addUnstarted()
    Increments the count of unstarted threads in the thread group. Unstarted threads are not added to the thread group so that they can be collected if they are never started, but they must be counted so that daemon thread groups with unstarted threads in them are not destroyed.
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  • allowThreadSuspensiontop

    public boolean allowThreadSuspension(boolean b)
    Used by VM to control lowmem implicit suspension.
    Parameters:
    @param b boolean to allow or disallow suspension
    Return:
    @return true on success
    Deprecated:
    @deprecated The definition of this call depends on ThreadGroup.suspend() , which is deprecated. Further, the behavior of this call was never specified.
    Since:
    @since JDK1.1
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  • checkAccesstop

    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.

    Exceptions:
    @exception SecurityException if the current thread is not allowed to access this thread group.
    See:
    @see SecurityManager.checkAccess(java.lang.ThreadGroup)
    Since:
    @since JDK1.0
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  • destroytop

    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.

    Exceptions:
    @exception IllegalThreadStateException if the thread group is not empty or if the thread group has already been destroyed.
    @exception SecurityException if the current thread cannot modify this thread group.
    See:
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • enumeratetop

    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:
    @param list an array into which to place the list of threads.
    Return:
    @return the number of threads put into the array.
    Exceptions:
    @exception SecurityException if the current thread does not have permission to enumerate this thread group.
    See:
    @see ThreadGroup.activeCount()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • enumeratetop

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

    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:
    @param list an array into which to place the list of threads.
    @param recurse a flag indicating whether also to include threads in thread groups that are subgroups of this thread group.
    Return:
    @return the number of threads placed into the array.
    Exceptions:
    @exception SecurityException if the current thread does not have permission to enumerate this thread group.
    See:
    @see ThreadGroup.activeCount()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • enumeratetop

    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:
    @param list an array into which to place the list of thread groups.
    Return:
    @return the number of thread groups put into the array.
    Exceptions:
    @exception SecurityException if the current thread does not have permission to enumerate this thread group.
    See:
    @see ThreadGroup.activeGroupCount()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • enumeratetop

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

    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:
    @param list an array into which to place the list of threads.
    @param recurse a flag indicating whether to recursively enumerate all included thread groups.
    Return:
    @return the number of thread groups put into the array.
    Exceptions:
    @exception SecurityException if the current thread does not have permission to enumerate this thread group.
    See:
    @see ThreadGroup.activeGroupCount()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • getMaxPrioritytop

    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.
    Return:
    @return the maximum priority that a thread in this thread group can have.
    See:
    @see ThreadGroup.setMaxPriority(int)
    Since:
    @since JDK1.0
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  • getNametop

    public final String getName()
    Returns the name of this thread group.
    Return:
    @return the name of this thread group.
    Since:
    @since JDK1.0
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  • getParenttop

    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.

    Return:
    @return the parent of this thread group. The top-level thread group is the only thread group whose parent is null.
    Exceptions:
    @exception SecurityException if the current thread cannot modify this thread group.
    See:
    @see ThreadGroup.checkAccess()
    @see SecurityException
    @see RuntimePermission
    Since:
    @since JDK1.0
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  • interrupttop

    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.

    Exceptions:
    @exception SecurityException if the current thread is not allowed to access this thread group or any of the threads in the thread group.
    See:
    @see Thread.interrupt()
    @see SecurityException
    @see ThreadGroup.checkAccess()
    Since:
    @since 1.2
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  • isDaemontop

    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.
    Return:
    @return true if this thread group is a daemon thread group; false otherwise.
    Since:
    @since JDK1.0
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  • isDestroyedtop

    public synchronized boolean isDestroyed()
    Tests if this thread group has been destroyed.
    Return:
    @return true if this object is destroyed
    Since:
    @since JDK1.1
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  • listtop

    public void list()
    Prints information about this thread group to the standard output. This method is useful only for debugging.
    Since:
    @since JDK1.0
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  • listtop

    void list(PrintStream out, int indent)
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  • parentOftop

    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:
    @param g a thread group.
    Return:
    @return true if this thread group is the thread group argument or one of its ancestor thread groups; false otherwise.
    Since:
    @since JDK1.0
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  • removetop

    void remove(Thread t)
    Removes the specified Thread from this group.
    Parameters:
    @param t the Thread to be removed
    Return:
    @return if the Thread has already been destroyed.
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  • removetop

    private void remove(ThreadGroup g)
    Removes the specified Thread group from this group.
    Parameters:
    @param g the Thread group to be removed
    Return:
    @return if this Thread has already been destroyed.
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  • resumetop

    public final void resume()
    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.

    Exceptions:
    @exception SecurityException if the current thread is not allowed to access this thread group or any of the threads in the thread group.
    Deprecated:
    @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.
    See:
    @see SecurityException
    @see Thread.resume()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • setDaemontop

    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:
    @param daemon if true, marks this thread group as a daemon thread group; otherwise, marks this thread group as normal.
    Exceptions:
    @exception SecurityException if the current thread cannot modify this thread group.
    See:
    @see SecurityException
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • setMaxPrioritytop

    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:
    @param pri the new priority of the thread group.
    Exceptions:
    @exception SecurityException if the current thread cannot modify this thread group.
    See:
    @see ThreadGroup.getMaxPriority()
    @see SecurityException
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • stoptop

    public final void stop()
    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.

    Exceptions:
    @exception SecurityException if the current thread is not allowed to access this thread group or any of the threads in the thread group.
    Deprecated:
    @deprecated This method is inherently unsafe. See Thread.stop() for details.
    See:
    @see SecurityException
    @see Thread.stop()
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • stopOrSuspendtop

    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.
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  • suspendtop

    public final void suspend()
    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.

    Exceptions:
    @exception SecurityException if the current thread is not allowed to access this thread group or any of the threads in the thread group.
    Deprecated:
    @deprecated This method is inherently deadlock-prone. See Thread.suspend() for details.
    See:
    @see Thread.suspend()
    @see SecurityException
    @see ThreadGroup.checkAccess()
    Since:
    @since JDK1.0
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  • toStringtop

    public String toString()
    Returns a string representation of this Thread group.
    Return:
    @return a string representation of this thread group.
    Since:
    @since JDK1.0
    Override hierarchy:
    toString from Object
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  • uncaughtExceptiontop

    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:
    @param t the thread that is about to exit.
    @param e the uncaught exception.
    Since:
    @since JDK1.0
    Specified by:
    uncaughtException from Thread.UncaughtExceptionHandler
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Fields

  • daemon

    boolean daemon

  • destroyed

    boolean destroyed

  • groups

    ThreadGroup[] groups

  • maxPriority

    int maxPriority

  • nUnstartedThreads

    int nUnstartedThreads

  • name

    String name

  • ngroups

    int ngroups

  • nthreads

    int nthreads

  • parent

    ThreadGroup parent

  • threads

    Thread[] threads

  • vmAllowSuspension

    boolean vmAllowSuspension