ThreadGroup

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## java.lang
Class ThreadGroup

    java.lang.Object
      java.lang.ThreadGroup

    public class ThreadGroup

extends Object

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:
  JDK1.0
Version:
  1.55 07/27/01
Author:
  unascribed

Constructor Summary
`ThreadGroup(String name)` Constructs a new thread group.
`ThreadGroup(ThreadGroup parent, String name)` Creates a new thread group.

Method Summary
`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.
`void` | `checkAccess()` Determines if the currently running thread has permission to modify this thread group.
`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(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.
`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()` Tests if this thread group is a daemon thread group.
`boolean` | `isDestroyed()` Tests if this thread group has been destroyed.
`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` | `setDaemon(boolean daemon)` Changes the daemon status of this thread group.
`void` | `setMaxPriority(int pri)` Sets the maximum priority of the 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.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Constructor Detail

### 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:
    JDK1.0
  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:
    JDK1.0
  See Also:
    `SecurityException`, `checkAccess()`

Method Detail

### getName

    public final String getName()

  Returns the name of this thread group.

  Returns:
    the name of this thread group.
  Since:
    JDK1.0

### 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:
    JDK1.0
  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:
    JDK1.0
  See Also:
    `setMaxPriority(int)`

### isDaemon

    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.
  Since:
    JDK1.0

### isDestroyed

    public boolean isDestroyed()

  Tests if this thread group has been destroyed.

  Returns:
    true if this object is destroyed
  Since:
    JDK1.1

### setDaemon

    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.
  Since:
    JDK1.0
  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:
    JDK1.0
  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:
    JDK1.0

### 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:
    JDK1.0
  See Also:
    `SecurityManager.checkAccess(java.lang.ThreadGroup)`

### activeCount

    public int activeCount()

  Returns an estimate of the number of active threads in this thread group.

  Returns:
    the number of active threads in this thread group and in any other thread group that has this thread group as an ancestor.
  Since:
    JDK1.0

### enumerate

    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 should use the `activeCount` method to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored.

  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.
  Since:
    JDK1.0
  See Also:
    `activeCount()`, `checkAccess()`

### enumerate

    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 should use the `activeCount` method to get an estimate of how big the array should be.

  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.
  Since:
    JDK1.0
  See Also:
    `activeCount()`, `checkAccess()`

### activeGroupCount

    public int activeGroupCount()

  Returns an estimate of the number of active groups in this thread group.

  Returns:
    the number of active thread groups with this thread group as an ancestor.
  Since:
    JDK1.0

### enumerate

    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 should use the `activeGroupCount` method to get an estimate of how big the array should be. If the array is too short to hold all the thread groups, the extra thread groups are silently ignored.

  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.
  Since:
    JDK1.0
  See Also:
    `activeGroupCount()`, `checkAccess()`

### enumerate

    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 should use the `activeGroupCount` method to get an estimate of how big the array should be.

  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.
  Since:
    JDK1.0
  See Also:
    `activeGroupCount()`, `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()`

### destroy

    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.
  Since:
    JDK1.0
  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:
    JDK1.0

### 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.

  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 determines if the `Throwable` argument is an instance of `ThreadDeath`. If so, nothing special is done. Otherwise, the `Throwable`'s `printStackTrace` method is called to print a stack backtrace 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.
  Since:
    JDK1.0
  See Also:
    `System.err`, `ThreadDeath`, `Throwable.printStackTrace(java.io.PrintStream)`

### 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:
    JDK1.0

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