Module java.base
Package java.lang

Class ThreadGroup

java.lang.Object
java.lang.ThreadGroup
All Implemented Interfaces:
Thread.UncaughtExceptionHandler
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.

Since:
1.0

  • Constructor Summary

    Constructors
    Constructor
    Description
    ThreadGroup​(String name)
    Constructs a new thread group.
    ThreadGroup​(ThreadGroup parent, String name)
    Creates a new thread group.

  • Method Summary

    Modifier and Type
    Method
    Description
    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.
    boolean
    allowThreadSuspension​(boolean b)
    Deprecated, for removal: This API element is subject to removal in a future version.
    The definition of this call depends on suspend(), which is deprecated.
    void
    checkAccess()
    Determines if the currently running thread has permission to modify this thread group.
    void
    destroy()
    Deprecated, for removal: This API element is subject to removal in a future version.
    The API and mechanism for destroying a ThreadGroup is inherently flawed.
    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.
    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
    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()
    Deprecated, for removal: This API element is subject to removal in a future version.
    The API and mechanism for destroying a ThreadGroup is inherently flawed.
    boolean
    isDestroyed()
    Deprecated, for removal: This API element is subject to removal in a future version.
    The API and mechanism for destroying a ThreadGroup is inherently flawed.
    void
    list()
    Prints information about this thread group to the standard output.
    boolean
    parentOf​(ThreadGroup g)
    Tests if this thread group is either the thread group argument or one of its ancestor thread groups.
    void
    resume()
    Deprecated, for removal: This API element is subject to removal in a future version.
    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.
    void
    setDaemon​(boolean daemon)
    Deprecated, for removal: This API element is subject to removal in a future version.
    The API and mechanism for destroying a ThreadGroup is inherently flawed.
    void
    setMaxPriority​(int pri)
    Sets the maximum priority of the group.
    void
    stop()
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method is inherently unsafe.
    void
    suspend()
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method is inherently deadlock-prone.
    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.

    Methods declared in class java.lang.Object

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

  • Constructor Details

    • 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:
      1.0
      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:
      1.0
      See Also:
      SecurityException, checkAccess()

  • Method Details

    • getName

      public final String getName()
      Returns the name of this thread group.
      Returns:
      the name of this thread group.
      Since:
      1.0

    • 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:
      1.0
      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:
      1.0
      See Also:
      setMaxPriority(int)

    • isDaemon

      @Deprecated(since="16", forRemoval=true) public final boolean isDaemon()
      Deprecated, for removal: This API element is subject to removal in a future version.
      The API and mechanism for destroying a ThreadGroup is inherently flawed. The ability to explicitly or automatically destroy a thread group, and the concept of daemon thread group, will be removed in a future release.
      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:
      1.0

    • isDestroyed

      @Deprecated(since="16", forRemoval=true) public boolean isDestroyed()
      Deprecated, for removal: This API element is subject to removal in a future version.
      The API and mechanism for destroying a ThreadGroup is inherently flawed. The ability to explicitly or automatically destroy a thread group will be removed in a future release.
      Tests if this thread group has been destroyed.
      Returns:
      true if this object is destroyed
      Since:
      1.1

    • setDaemon

      @Deprecated(since="16", forRemoval=true) public final void setDaemon​(boolean daemon)
      Deprecated, for removal: This API element is subject to removal in a future version.
      The API and mechanism for destroying a ThreadGroup is inherently flawed. The ability to explicitly or automatically destroy a thread group, and the concept of daemon thread group, will be removed in a future release.
      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:
      1.0
      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:
      1.0
      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:
      1.0

    • 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:
      1.0
      See Also:
      SecurityManager.checkAccess(java.lang.ThreadGroup)

    • activeCount

      public int activeCount()
      Returns an estimate of the number of active threads in this thread group 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
      Since:
      1.0

    • enumerate

      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

      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 checkAccess determines that the current thread cannot access this thread group
      Since:
      1.0

    • enumerate

      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 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 checkAccess determines that the current thread cannot access this thread group
      Since:
      1.0

    • activeGroupCount

      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
      Since:
      1.0

    • enumerate

      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

      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 checkAccess determines that the current thread cannot access this thread group
      Since:
      1.0

    • enumerate

      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 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 checkAccess determines that the current thread cannot access this thread group
      Since:
      1.0

    • stop

      @Deprecated(since="1.2", forRemoval=true) public final void stop()
      Deprecated, for removal: This API element is subject to removal in a future version.
      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:
      1.0
      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

      @Deprecated(since="1.2", forRemoval=true) public final void suspend()
      Deprecated, for removal: This API element is subject to removal in a future version.
      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:
      1.0
      See Also:
      Thread.suspend(), SecurityException, checkAccess()

    • resume

      @Deprecated(since="1.2", forRemoval=true) public final void resume()
      Deprecated, for removal: This API element is subject to removal in a future version.
      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:
      1.0
      See Also:
      SecurityException, Thread.resume(), checkAccess()

    • destroy

      @Deprecated(since="16", forRemoval=true) public final void destroy()
      Deprecated, for removal: This API element is subject to removal in a future version.
      The API and mechanism for destroying a ThreadGroup is inherently flawed. The ability to explicitly or automatically destroy a thread group will be removed in a future release.
      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:
      1.0
      See Also:
      checkAccess()

    • list

      public void list()
      Prints information about this thread group to the standard output. This method is useful only for debugging.
      Since:
      1.0

    • 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, 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.

      Specified by:
      uncaughtException in interface Thread.UncaughtExceptionHandler
      Parameters:
      t - the thread that is about to exit.
      e - the uncaught exception.
      Since:
      1.0

    • allowThreadSuspension

      @Deprecated(since="1.2", forRemoval=true) public boolean allowThreadSuspension​(boolean b)
      Deprecated, for removal: This API element is subject to removal in a future version.
      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:
      1.1

    • 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:
      1.0