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 has a name, maximum priority, and a daemon status. The name is specified when creating the group and cannot be changed. The group's maximum priority is the maximum priority for threads created in the group. It is initially inherited from the parent thread group but may be changed using the setMaxPriority(int) method.

The daemon status determines if the group is weakly or strongly reachable from its parent group. A newly created thread group is a daemon thread group. It is weakly reachable from its parent group so that it is eligible for garbage collection when there are no live threads in the group and is otherwise unreachable. The setDaemon(boolean) method can be used to change a group to be a non-daemon thread group. A non-daemon thread group is strongly reachable from its parent. The need to keep a group strongly reachable from its parent may be important for cases where a parent group is enumerated to find a subgroup by name.

Unless otherwise specified, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown.

API Note:
The concept of daemon thread group is not related to the concept of daemon threads. There may be both daemon and non-daemon threads in a thread group, independent of whether the group is a daemon or non-daemon thread group.
Since:
1.0
  • Constructor Summary

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

    Modifier and Type
    Method
    Description
    int
    Returns an estimate of the number of active (meaning alive) threads in this thread group and its subgroups.
    int
    Returns an estimate of the number of 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.
    This method was originally intended for controlling suspension in low memory conditions.
    void
    Determines if the currently running thread has permission to modify this thread group.
    void
    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)
    Copies into the specified array every active (meaning alive) thread in this thread group and its subgroups.
    int
    enumerate​(Thread[] list, boolean recurse)
    Copies into the specified array every active (meaning alive) thread in this thread group.
    int
    Copies into the specified array references to every subgroup in this thread group and its subgroups.
    int
    enumerate​(ThreadGroup[] list, boolean recurse)
    Copies into the specified array references to every subgroup in this thread group.
    int
    Returns the maximum priority of this thread group.
    Returns the name of this thread group.
    Returns the parent of this thread group.
    void
    Interrupts all active (meaning alive) in this thread group and its subgroups.
    boolean
    Tests if this thread group is a daemon thread group.
    boolean
    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
    Prints information about this thread group to the standard output.
    boolean
    Tests if this thread group is either the thread group argument or one of its ancestor thread groups.
    void
    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)
    Changes the daemon status of this thread group.
    void
    setMaxPriority​(int pri)
    Sets the maximum priority of the group.
    void
    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
    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.
    Returns a string representation of this Thread group.
    void
    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.
      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:
      SecurityException - if the current thread cannot create a thread in the specified thread group.
      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, may be null
    • 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.
      See Also:
      checkAccess(), SecurityException, RuntimePermission
    • getMaxPriority

      public final int getMaxPriority()
      Returns the maximum priority of this thread group. This is the maximum priority for new threads created in the thread group.
      Returns:
      the maximum priority for new threads created in the thread group
      See Also:
      setMaxPriority(int)
    • isDaemon

      public final boolean isDaemon()
      Tests if this thread group is a daemon thread group. A daemon thread group is weakly reachable from its parent so that it is eligible for garbage collection when there are no live threads in the group and is otherwise unreachable. A non-daemon thread is strongly reachable from its parent.
      Returns:
      true if this thread group is a daemon thread group; false otherwise.
    • 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

      public final void setDaemon(boolean daemon)
      Changes the daemon status of this thread group. A daemon thread group is weakly reachable from its parent so that it is eligible for garbage collection when there are no live threads in the group and is otherwise unreachable. A non-daemon thread is strongly reachable from its parent. This method does not change the daemon status of subgroups.

      First, the checkAccess method of this thread group is called with no arguments; this may result in a security exception.

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

      public int activeCount()
      Returns an estimate of the number of active (meaning alive) 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
    • enumerate

      public int enumerate(Thread[] list)
      Copies into the specified array every active (meaning alive) 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
    • enumerate

      public int enumerate(Thread[] list, boolean recurse)
      Copies into the specified array every active (meaning alive) 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.

      API Note:
      ThreadMXBean supports monitoring and management of active threads in the Java virtual machine and may be a more suitable API some many 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
    • activeGroupCount

      public int activeGroupCount()
      Returns an estimate of the number of 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 thread groups with this thread group as an ancestor
    • enumerate

      public int enumerate(ThreadGroup[] list)
      Copies into the specified array references to every 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
    • enumerate

      public int enumerate(ThreadGroup[] list, boolean recurse)
      Copies into the specified array references to every subgroup in this thread group. If recurse is true, this method recursively enumerates all subgroups of this thread group and references to every 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 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
    • 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.
    • interrupt

      public final void interrupt()
      Interrupts all active (meaning alive) in this thread group and 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.
    • 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.
    • 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.
    • list

      public void list()
      Prints information about this thread group to the standard output. This method is useful only for debugging.
    • 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.
    • 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.
      Does nothing.
      Parameters:
      b - ignored
      Returns:
      false
      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.