Class ThreadGroup
- All Implemented Interfaces:
Thread.UncaughtExceptionHandler
A thread group has a name and maximum priority. 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
method.
A thread group 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 the thread group is otherwise unreachable.
Unless otherwise specified, passing a null
argument to a constructor
or method in this class will cause a NullPointerException
to be thrown.
Thread groups and virtual threads
The Java runtime creates a special thread group for virtual threads. This group is returned by theThread.getThreadGroup
method
when invoked on a virtual thread. The thread group differs to other thread
groups in that its maximum priority is fixed and cannot be changed with the
setMaxPriority
method.
Virtual threads are not included in the estimated thread count returned by the
activeCount
method, are not enumerated by the enumerate
method, and are not interrupted by the interrupt
method.- API Note:
- Thread groups provided a way in early Java releases to group threads and provide a form of job control for threads. Thread groups supported the isolation of applets and defined methods intended for diagnostic purposes. It should be rare for new applications to create ThreadGroups and interact with this API.
- Since:
- 1.0
-
Constructor Summary
ConstructorDescriptionThreadGroup
(String name) Constructs a new thread group.ThreadGroup
(ThreadGroup parent, String name) Creates a new thread group. -
Method Summary
Modifier and TypeMethodDescriptionint
Returns an estimate of the number of live platform threads in this thread group and its subgroups.int
Returns an estimate of the number of groups in this thread group and its subgroups.final void
Deprecated, for removal: This API element is subject to removal in a future version.This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release.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.int
Copies into the specified array every live platform thread in this thread group and its subgroups.int
Copies into the specified array every live platform thread in this thread group.int
enumerate
(ThreadGroup[] list) 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.final int
Returns the maximum priority of this thread group.final String
getName()
Returns the name of this thread group.final ThreadGroup
Returns the parent of this thread group.final void
Interrupts all live platform threads in this thread group and its subgroups.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 is a daemon thread group that is automatically destroyed when its last thread terminates.boolean
Deprecated, for removal: This API element is subject to removal in a future version.This method originally indicated if the thread group is destroyed.void
list()
Prints information about this thread group to the standard output.final boolean
Tests if this thread group is either the thread group argument or one of its ancestor thread groups.final void
setDaemon
(boolean daemon) Deprecated, for removal: This API element is subject to removal in a future version.This method originally configured whether the thread group is a daemon thread group that is automatically destroyed when its last thread terminates.final void
setMaxPriority
(int pri) Sets the maximum priority of the group.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 specificThread.UncaughtExceptionHandler
installed.
-
Constructor Details
-
ThreadGroup
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 benull
- Throws:
SecurityException
- if the current thread cannot create a thread in the specified thread group.- See Also:
-
ThreadGroup
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 benull
- Throws:
SecurityException
- if the current thread cannot create a thread in the specified thread group.- See Also:
-
-
Method Details
-
getName
Returns the name of this thread group.- Returns:
- the name of this thread group, may be
null
-
getParent
Returns the parent of this thread group.First, if the parent is not
null
, thecheckAccess
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:
-
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:
-
isDaemon
Deprecated, for removal: This API element is subject to removal in a future version.This method originally indicated if the thread group is a daemon thread group that is automatically destroyed when its last thread terminates. The concept of daemon thread group no longer exists. A thread group is eligible to be GC'ed when there are no live threads in the group and it is otherwise unreachable.Returns the daemon status of this thread group. The daemon status is not used for anything.- Returns:
- the daemon status of this thread group
-
isDestroyed
Deprecated, for removal: This API element is subject to removal in a future version.This method originally indicated if the thread group is destroyed. The ability to destroy a thread group and the concept of a destroyed thread group no longer exists. A thread group is eligible to be GC'ed when there are no live threads in the group and it is otherwise unreachable.Returns false.- Returns:
- false
- Since:
- 1.1
-
setDaemon
Deprecated, for removal: This API element is subject to removal in a future version.This method originally configured whether the thread group is a daemon thread group that is automatically destroyed when its last thread terminates. The concept of daemon thread group no longer exists. A thread group is eligible to be GC'ed when there are no live threads in the group and it is otherwise unreachable.Sets the daemon status of this thread group. The daemon status is not used for anything.First, the
checkAccess
method of this thread group is called with no arguments; this may result in a security exception.- Parameters:
daemon
- the daemon status- Throws:
SecurityException
- if the current thread cannot modify this thread group.- See Also:
-
setMaxPriority
public final void setMaxPriority(int pri) Sets the maximum priority of the group. The maximum priority of the ThreadGroup for virtual threads is not changed by this method (the new priority is ignored). Threads in the thread group (or subgroups) that already have a higher priority are not affected by this method.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 thanThread.MIN_PRIORITY
or greater thanThread.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 topri
.) Then this method is called recursively, withpri
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:
-
parentOf
Tests if this thread group is either the thread group argument or one of its ancestor thread groups.- Parameters:
g
- a thread group, can benull
- Returns:
true
if this thread group is the thread group argument or one of its ancestor thread groups;false
otherwise.
-
checkAccess
Deprecated, for removal: This API element is subject to removal in a future version.This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. Consequently, this method is also deprecated and subject to removal. There is no replacement for the Security Manager or this method.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 aSecurityException
.- Throws:
SecurityException
- if the current thread is not allowed to access this thread group.- See Also:
-
activeCount
public int activeCount()Returns an estimate of the number of live platform threads in this thread group and its subgroups. Virtual threads are not included in the estimate. This method 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 live threads in this thread group and in any other thread group that has this thread group as an ancestor
-
enumerate
Copies into the specified array every live platform thread in this thread group and its subgroups. Virtual threads are not enumerated by this method.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
Copies into the specified array every live platform thread in this thread group. Virtual threads are not enumerated by this method. Ifrecurse
istrue
, this method recursively enumerates all subgroups of this thread group and references to every live platform 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 live 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 threadsrecurse
- iftrue
, 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
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
Copies into the specified array references to every subgroup in this thread group. Ifrecurse
istrue
, 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 groupsrecurse
- iftrue
, 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
-
interrupt
public final void interrupt()Interrupts all live platform threads 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:
-
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 explicitly destroy a thread group no longer exists. A thread group is eligible to be GC'ed when there are no live threads in the group and it is otherwise unreachable.Does nothing. -
list
public void list()Prints information about this thread group to the standard output. This method is useful only for debugging. -
uncaughtException
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 specificThread.UncaughtExceptionHandler
installed.The
uncaughtException
method ofThreadGroup
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, a message containing the thread's name, as returned
from the thread's
getName
method, and a stack backtrace, using theThrowable
'sprintStackTrace
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 interfaceThread.UncaughtExceptionHandler
- Parameters:
t
- the thread that is about to exit.e
- the uncaught exception.
- If this thread group has a parent thread group, the
-
toString
-