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 group is considered active if there are any alive threads in the group or any of its subgroups.

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()
    Deprecated.
    ThreadMXBean provides a more suitable interface for monitoring threads.
    int activeGroupCount()
    Deprecated.
    ThreadMXBean provides a more suitable interface for monitoring threads.
    boolean allowThreadSuspension​(boolean b)
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method was originally intended for controlling suspension in low memory conditions.
    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.
    This method was originally specified to destroy an empty thread group.
    int enumerate​(Thread[] list)
    Deprecated.
    This method is inherently racy.
    int enumerate​(Thread[] list, boolean recurse)
    Deprecated.
    This method is inherently racy.
    int enumerate​(ThreadGroup[] list)
    Deprecated.
    This method is inherently racy.
    int enumerate​(ThreadGroup[] list, boolean recurse)
    Deprecated.
    This method is inherently racy.
    int getMaxPriority()
    Returns the effective 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.
    This method originally indicated if the thread group was automatically destroyed when the last thread in the group terminated or its last thread group was destroyed.
    boolean isDestroyed()
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method originally indicated if the thread group was destroyed.
    void list()
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method was originally intended for debugging purposes.
    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 was originally specified to resume all threads in the thread group.
    void setDaemon​(boolean daemon)
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method originally changed the daemon status of the thread group.
    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 was originally specified to stop all threads in the thread group.
    void suspend()
    Deprecated, for removal: This API element is subject to removal in a future version.
    This method was originally specified to suspend all threads in the thread group.
    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, can be null
      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, can be null
      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 effective maximum priority of this thread group. This is the maximum priority for new threads created in the group.

      The effective maximum priority of the group is the smaller of this group's maximum priority and the effective maximum priority of the parent of this thread group. If this thread group is the system thread group, which has no parent, then the effective maximum priority is the group's maximum priority.

      Returns:
      the effective maximum priority of this thread group, the maximum priority for new threads created in the group
      Since:
      1.0
      See Also:
      setMaxPriority(int)
    • isDaemon

      @Deprecated(since="99", forRemoval=true) public final boolean isDaemon()
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method originally indicated if the thread group was automatically destroyed when the last thread in the group terminated or its last thread group was destroyed. The concept of daemon thread group no longer exists.
      Returns false.
      Returns:
      false
      Since:
      1.0
    • isDestroyed

      @Deprecated(since="99", forRemoval=true) public boolean isDestroyed()
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method originally indicated if the thread group was destroyed. The ability to destroy a thread group no longer exists.
      Returns false.
      Returns:
      false
      Since:
      1.1
    • setDaemon

      @Deprecated(since="99", forRemoval=true) public final void setDaemon​(boolean daemon)
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method originally changed the daemon status of the thread group. The concept of daemon thread group no longer exists.
      Does nothing.
      Parameters:
      daemon - ignored
      Since:
      1.0
    • 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 maximum priority of the group is set to pri. The effective maximum priority of the group will be the smaller of pri and the effective maximum priority of the parent of this thread group. If this thread group is the system thread group, which has no parent, then the effective maximum priority will be pri.

      Parameters:
      pri - the new maximum 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

      @Deprecated(since="99") public int activeCount()
      Deprecated.
      ThreadMXBean provides a more suitable interface for monitoring threads.
      Returns an estimate of the number of active threads in this thread group and its subgroups.

      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

      @Deprecated(since="99") public int enumerate​(Thread[] list)
      Deprecated.
      This method is inherently racy. ThreadMXBean provides a more suitable interface for monitoring threads.
      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

      @Deprecated(since="99") public int enumerate​(Thread[] list, boolean recurse)
      Deprecated.
      This method is inherently racy. ThreadMXBean provides a more suitable interface for monitoring threads.
      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

      @Deprecated(since="99") public int activeGroupCount()
      Deprecated.
      ThreadMXBean provides a more suitable interface for monitoring threads.
      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

      @Deprecated(since="99") public int enumerate​(ThreadGroup[] list)
      Deprecated.
      This method is inherently racy. ThreadMXBean provides a more suitable interface for monitoring threads.
      Copies into the specified array references to every active subgroup in this thread group and its subgroups. A thread group is considered active if there are any alive threads in the group or any of 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

      @Deprecated(since="99") public int enumerate​(ThreadGroup[] list, boolean recurse)
      Deprecated.
      This method is inherently racy. ThreadMXBean provides a more suitable interface for monitoring threads.
      Copies into the specified array references to every active subgroup in this thread group. A thread group is considered active if there are any alive threads in the group or any of its subgroups. 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 was originally specified to stop all threads in the thread group. It was inherently unsafe.
      Throws UnsupportedOperationException.
      Since:
      1.0
    • 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 was originally specified to suspend all threads in the thread group.
      Throws UnsupportedOperationException.
      Since:
      1.0
    • 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 was originally specified to resume all threads in the thread group.
      Throws UnsupportedOperationException.
      Since:
      1.0
    • destroy

      @Deprecated(since="99", forRemoval=true) public final void destroy()
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method was originally specified to destroy an empty thread group. The ability to destroy a thread group no longer exists.
      Does nothing.
      Since:
      1.0
    • list

      @Deprecated(since="99", forRemoval=true) public void list()
      Deprecated, for removal: This API element is subject to removal in a future version.
      This method was originally intended for debugging purposes.
      Does nothing.
      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.
      This method was originally intended for controlling suspension in low memory conditions. It was never specified.
      Throws UnsupportedOperationException.
      Parameters:
      b - ignore
      Returns:
      nothing
      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