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public class Thread extends Object implements Runnable
A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.
Every thread has a priority. Threads with higher priority are
executed in preference to threads with lower priority. Each thread
may or may not also be marked as a daemon. When code running in
some thread creates a new Thread
object, the new
thread has its priority initially set equal to the priority of the
creating thread, and is a daemon thread if and only if the
creating thread is a daemon.
When a Java Virtual Machine starts up, there is usually a single
non-daemon thread (which typically calls the method named
main
of some designated class). The Java Virtual
Machine continues to execute threads until either of the following
occurs:
exit
method of class Runtime
has been
called and the security manager has permitted the exit operation
to take place.
run
method or by
throwing an exception that propagates beyond the run
method.
There are two ways to create a new thread of execution. One is to
declare a class to be a subclass of Thread
. This
subclass should override the run
method of class
Thread
. An instance of the subclass can then be
allocated and started. For example, a thread that computes primes
larger than a stated value could be written as follows:
class PrimeThread extends Thread { long minPrime; PrimeThread(long minPrime) { this.minPrime = minPrime; } public void run() { // compute primes larger than minPrime . . . } }
The following code would then create a thread and start it running:
PrimeThread p = new PrimeThread(143); p.start();
The other way to create a thread is to declare a class that
implements the Runnable
interface. That class then
implements the run
method. An instance of the class can
then be allocated, passed as an argument when creating
Thread
, and started. The same example in this other
style looks like the following:
class PrimeRun implements Runnable { long minPrime; PrimeRun(long minPrime) { this.minPrime = minPrime; } public void run() { // compute primes larger than minPrime . . . } }
The following code would then create a thread and start it running:
PrimeRun p = new PrimeRun(143); new Thread(p).start();
Every thread has a name for identification purposes. More than one thread may have the same name. If a name is not specified when a thread is created, a new name is generated for it.
Nested Class Summary | |
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static enum |
A thread state. |
static interface |
Interface for handlers invoked when a Thread abruptly terminates due to an uncaught exception. |
Field Summary | |
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static int |
MAX_PRIORITY
The maximum priority that a thread can have. |
static int |
MIN_PRIORITY
The minimum priority that a thread can have. |
static int |
NORM_PRIORITY
The default priority that is assigned to a thread. |
Constructor Summary | |
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Thread() Allocates a new Thread object.
|
|
Allocates a new Thread object.
|
|
Allocates a new Thread object.
|
|
Allocates a new Thread object.
|
|
Thread(ThreadGroup group, Runnable target) Allocates a new Thread object.
|
|
Thread(ThreadGroup group, Runnable target, String name) Allocates a new Thread object so that it has
target as its run object, has the specified
name as its name, and belongs to the thread group
referred to by group .
|
|
Thread(ThreadGroup group, Runnable target, String name, long stackSize) Allocates a new Thread object so that it has
target as its run object, has the specified
name as its name, belongs to the thread group referred to
by group , and has the specified stack size.
|
|
Thread(ThreadGroup group, String name) Allocates a new Thread object.
|
Method Summary | |
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static int |
Returns the number of active threads in the current thread's thread group. |
void |
Determines if the currently running thread has permission to modify this thread. |
native int |
Counts the number of stack frames in this thread. |
static native Thread |
Returns a reference to the currently executing thread object. |
void |
destroy() Throws java.lang.NoSuchMethodError. |
static void |
Prints a stack trace of the current thread to the standard error stream. |
static int |
Copies into the specified array every active thread in the current thread's thread group and its subgroups. |
static Map |
Returns a map of stack traces for all live threads. |
ClassLoader |
Returns the context ClassLoader for this Thread. |
static Thread.UncaughtExceptionHandler |
Returns the default handler invoked when a thread abruptly terminates due to an uncaught exception. |
long |
getId() Returns the identifier of this Thread. |
String |
getName() Returns this thread's name. |
int |
Returns this thread's priority. |
StackTraceElement[] |
Returns an array of stack trace elements representing the stack dump of this thread. |
Thread.State |
getState() Returns the state of this thread. |
ThreadGroup |
Returns the thread group to which this thread belongs. |
Thread.UncaughtExceptionHandler |
Returns the handler invoked when this thread abruptly terminates due to an uncaught exception. |
static native boolean |
Returns true if and only if the current thread holds the monitor lock on the specified object. |
void |
Interrupts this thread. |
static boolean |
Tests whether the current thread has been interrupted. |
native boolean |
isAlive() Tests if this thread is alive. |
boolean |
isDaemon() Tests if this thread is a daemon thread. |
boolean |
Tests whether this thread has been interrupted. |
void |
join() Waits for this thread to die. |
void |
join(long millis) Waits at most millis milliseconds for this thread to
die.
|
void |
join(long millis, int nanos) Waits at most millis milliseconds plus
nanos nanoseconds for this thread to die.
|
void |
resume() Resumes a suspended thread. |
void |
run() If this thread was constructed using a separate Runnable run object, then that
Runnable object's run method is called;
otherwise, this method does nothing and returns.
|
void |
Sets the context ClassLoader for this Thread. |
void |
setDaemon(boolean on) Marks this thread as either a daemon thread or a user thread. |
static void |
Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread. |
void |
Changes the name of this thread to be equal to the argument name .
|
void |
setPriority(int newPriority) Changes the priority of this thread. |
void |
Set the handler invoked when this thread abruptly terminates due to an uncaught exception. |
static native void |
sleep(long millis) Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers. |
static void |
sleep(long millis, int nanos) Causes the currently executing thread to sleep (cease execution) for the specified number of milliseconds plus the specified number of nanoseconds, subject to the precision and accuracy of system timers and schedulers. |
void |
start() Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
|
void |
stop() Forces the thread to stop executing. |
void |
Forces the thread to stop executing. |
void |
suspend() Suspends this thread. |
String |
toString() Returns a string representation of this thread, including the thread's name, priority, and thread group. |
static native void |
yield() Causes the currently executing thread object to temporarily pause and allow other threads to execute. |
Methods inherited from class java.lang.Object |
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clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Field Detail |
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public static final int MAX_PRIORITY
public static final int MIN_PRIORITY
public static final int NORM_PRIORITY
Constructor Detail |
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public Thread()
Thread
object. This constructor has
the same effect as Thread(null, null,
gname)
, where gname is
a newly generated name. Automatically generated names are of the
form "Thread-"+
n, where n is an integer.
public Thread(Runnable target)
Thread
object. This constructor has
the same effect as Thread(null, target,
gname)
, where gname is
a newly generated name. Automatically generated names are of the
form "Thread-"+
n, where n is an integer.
target
- the object whose run
method is called.public Thread(Runnable target, String name)
Thread
object. This constructor has
the same effect as Thread(null, target, name)
.
target
- the object whose run
method is called.name
- the name of the new thread.public Thread(String name)
Thread
object. This constructor has
the same effect as Thread(null, null, name)
.
name
- the name of the new thread.public Thread(ThreadGroup group, Runnable target)
Thread
object. This constructor has
the same effect as Thread(group, target,
gname)
, where gname is
a newly generated name. Automatically generated names are of the
form "Thread-"+
n, where n is an integer.
group
- the thread group.target
- the object whose run
method is called.public Thread(ThreadGroup group, Runnable target, String name)
Thread
object so that it has
target
as its run object, has the specified
name
as its name, and belongs to the thread group
referred to by group
.
If group
is null
and there is a
security manager, the group is determined by the security manager's
getThreadGroup
method. If group
is
null
and there is not a security manager, or the
security manager's getThreadGroup
method returns
null
, the group is set to be the same ThreadGroup
as the thread that is creating the new thread.
If there is a security manager, its checkAccess
method is called with the ThreadGroup as its argument.
In addition, its checkPermission
method is called with the
RuntimePermission("enableContextClassLoaderOverride")
permission when invoked directly or indirectly by the constructor
of a subclass which overrides the getContextClassLoader
or setContextClassLoader
methods.
This may result in a SecurityException.
If the target
argument is not null
, the
run
method of the target
is called when
this thread is started. If the target argument is
null
, this thread's run
method is called
when this thread is started.
The priority of the newly created thread is set equal to the
priority of the thread creating it, that is, the currently running
thread. The method setPriority
may be used to
change the priority to a new value.
The newly created thread is initially marked as being a daemon
thread if and only if the thread creating it is currently marked
as a daemon thread. The method setDaemon
may be used
to change whether or not a thread is a daemon.
group
- the thread group.target
- the object whose run
method is called.name
- the name of the new thread.public Thread(ThreadGroup group, Runnable target, String name, long stackSize)
Thread
object so that it has
target
as its run object, has the specified
name
as its name, belongs to the thread group referred to
by group
, and has the specified stack size.
This constructor is identical to {@link #Thread(ThreadGroup,Runnable,String)} with the exception of the fact that it allows the thread stack size to be specified. The stack size is the approximate number of bytes of address space that the virtual machine is to allocate for this thread's stack. The effect of the stackSize parameter, if any, is highly platform dependent.
On some platforms, specifying a higher value for the stackSize parameter may allow a thread to achieve greater recursion depth before throwing a {@link StackOverflowError}. Similarly, specifying a lower value may allow a greater number of threads to exist concurrently without throwing an {@link OutOfMemoryError} (or other internal error). The details of the relationship between the value of the stackSize parameter and the maximum recursion depth and concurrency level are platform-dependent. On some platforms, the value of the stackSize parameter may have no effect whatsoever.
The virtual machine is free to treat the stackSize parameter as a suggestion. If the specified value is unreasonably low for the platform, the virtual machine may instead use some platform-specific minimum value; if the specified value is unreasonably high, the virtual machine may instead use some platform-specific maximum. Likewise, the virtual machine is free to round the specified value up or down as it sees fit (or to ignore it completely).
Specifying a value of zero for the stackSize parameter will cause this constructor to behave exactly like the Thread(ThreadGroup, Runnable, String) constructor.
Due to the platform-dependent nature of the behavior of this constructor, extreme care should be exercised in its use. The thread stack size necessary to perform a given computation will likely vary from one JRE implementation to another. In light of this variation, careful tuning of the stack size parameter may be required, and the tuning may need to be repeated for each JRE implementation on which an application is to run.
Implementation note: Java platform implementers are encouraged to document their implementation's behavior with respect to the stackSize parameter.
group
- the thread group.target
- the object whose run
method is called.name
- the name of the new thread.stackSize
- the desired stack size for the new thread, or
zero to indicate that this parameter is to be ignored.public Thread(ThreadGroup group, String name)
Thread
object. This constructor has
the same effect as Thread(group, null, name)
group
- the thread group.name
- the name of the new thread.Method Detail |
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public static int activeCount()
public final void checkAccess()
If there is a security manager, its checkAccess
method
is called with this thread as its argument. This may result in
throwing a SecurityException
.
public native int countStackFrames()
public static native Thread currentThread()
public void destroy()
public static void dumpStack()
public static int enumerate(Thread[] tarray)
enumerate
method of the current thread's thread
group with the array argument.
First, if there is a security manager, that enumerate
method calls the security
manager's checkAccess
method
with the thread group as its argument. This may result
in throwing a SecurityException
.
tarray
- an array of Thread objects to copy topublic static Map getAllStackTraces()
The threads may be executing while this method is called. The stack trace of each thread only represents a snapshot and each stack trace may be obtained at different time. A zero-length array will be returned in the map value if the virtual machine has no stack trace information about a thread.
If there is a security manager, then the security manager's checkPermission method is called with a RuntimePermission("getStackTrace") permission as well as RuntimePermission("modifyThreadGroup") permission to see if it is ok to get the stack trace of all threads.
public ClassLoader getContextClassLoader()
First, if there is a security manager, and the caller's class
loader is not null and the caller's class loader is not the same as or
an ancestor of the context class loader for the thread whose
context class loader is being requested, then the security manager's
checkPermission
method is called with a
RuntimePermission("getClassLoader")
permission
to see if it's ok to get the context ClassLoader..
public static Thread.UncaughtExceptionHandler getDefaultUncaughtExceptionHandler()
public long getId()
public final String getName()
public final int getPriority()
public StackTraceElement[] getStackTrace()
If there is a security manager, and this thread is not the current thread, then the security manager's checkPermission method is called with a RuntimePermission("getStackTrace") permission to see if it's ok to get the stack trace.
Some virtual machines may, under some circumstances, omit one or more stack frames from the stack trace. In the extreme case, a virtual machine that has no stack trace information concerning this thread is permitted to return a zero-length array from this method.
public Thread.State getState()
public final ThreadGroup getThreadGroup()
public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
public static native boolean holdsLock(Object obj)
This method is designed to allow a program to assert that the current thread already holds a specified lock:
assert Thread.holdsLock(obj);
obj
- the object on which to test lock ownershippublic void interrupt()
Unless the current thread is interrupting itself, which is always permitted, the {@link #checkAccess() checkAccess} method of this thread is invoked, which may cause a {@link SecurityException} to be thrown.
If this thread is blocked in an invocation of the {@link Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link Object#wait(long, int) wait(long, int)} methods of the {@link Object} class, or of the {@link #join()}, {@link #join(long)}, {@link #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)}, methods of this class, then its interrupt status will be cleared and it will receive an {@link InterruptedException}.
If this thread is blocked in an I/O operation upon an {@link
java.nio.channels.InterruptibleChannel interruptible
channel If this thread is blocked in a {@link java.nio.channels.Selector}
then the thread's interrupt status will be set and it will return
immediately from the selection operation, possibly with a non-zero
value, just as if the selector's {@link
java.nio.channels.Selector#wakeup wakeup} method were invoked.
If none of the previous conditions hold then this thread's interrupt
status will be set. Interrupting a thread that is not alive need not have any effect.
} then the channel will be closed, the thread's interrupt
status will be set, and the thread will receive a {@link
java.nio.channels.ClosedByInterruptException}.
public static boolean interrupted()
A thread interruption ignored because a thread was not alive at the time of the interrupt will be reflected by this method returning false.
true
if the current thread has been interrupted;
false
otherwise.public final native boolean isAlive()
true
if this thread is alive;
false
otherwise.public final boolean isDaemon()
true
if this thread is a daemon thread;
false
otherwise.public boolean isInterrupted()
A thread interruption ignored because a thread was not alive at the time of the interrupt will be reflected by this method returning false.
true
if this thread has been interrupted;
false
otherwise.public final void join() throws InterruptedException
InterruptedException
- if any thread has interrupted
the current thread. The interrupted status of the
current thread is cleared when this exception is thrown.public final synchronized void join(long millis) throws InterruptedException
millis
milliseconds for this thread to
die. A timeout of 0
means to wait forever.
millis
- the time to wait in milliseconds.InterruptedException
- if any thread has interrupted
the current thread. The interrupted status of the
current thread is cleared when this exception is thrown.public final synchronized void join(long millis, int nanos) throws InterruptedException
millis
milliseconds plus
nanos
nanoseconds for this thread to die.
millis
- the time to wait in milliseconds.nanos
- 0-999999 additional nanoseconds to wait.InterruptedException
- if any thread has interrupted
the current thread. The interrupted status of the
current thread is cleared when this exception is thrown.public final void resume()
First, the checkAccess
method of this thread is called
with no arguments. This may result in throwing a
SecurityException
(in the current thread).
If the thread is alive but suspended, it is resumed and is permitted to make progress in its execution.
public void run()
Runnable
run object, then that
Runnable
object's run
method is called;
otherwise, this method does nothing and returns.
Subclasses of Thread
should override this method.
public void setContextClassLoader(ClassLoader cl)
First, if there is a security manager, its checkPermission
method is called with a
RuntimePermission("setContextClassLoader")
permission
to see if it's ok to set the context ClassLoader..
cl
- the context ClassLoader for this Threadpublic final void setDaemon(boolean on)
This method must be called before the thread is started.
This method first calls the checkAccess
method
of this thread
with no arguments. This may result in throwing a
SecurityException
(in the current thread).
on
- if true
, marks this thread as a
daemon thread.public static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh)
Uncaught exception handling is controlled first by the thread, then by the thread's {@link ThreadGroup} object and finally by the default uncaught exception handler. If the thread does not have an explicit uncaught exception handler set, and the thread's thread group (including parent thread groups) does not specialize its uncaughtException method, then the default handler's uncaughtException method will be invoked.
By setting the default uncaught exception handler, an application can change the way in which uncaught exceptions are handled (such as logging to a specific device, or file) for those threads that would already accept whatever "default" behavior the system provided.
Note that the default uncaught exception handler should not usually defer to the thread's ThreadGroup object, as that could cause infinite recursion.
eh
- the object to use as the default uncaught exception handler.
If null then there is no default handler.public final void setName(String name)
name
.
First the checkAccess
method of this thread is called
with no arguments. This may result in throwing a
SecurityException
.
name
- the new name for this thread.public final void setPriority(int newPriority)
First the checkAccess
method of this thread is called
with no arguments. This may result in throwing a
SecurityException
.
Otherwise, the priority of this thread is set to the smaller of
the specified newPriority
and the maximum permitted
priority of the thread's thread group.
newPriority
- priority to set this thread topublic void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh)
A thread can take full control of how it responds to uncaught exceptions by having its uncaught exception handler explicitly set. If no such handler is set then the thread's ThreadGroup object acts as its handler.
eh
- the object to use as this thread's uncaught exception
handler. If null then this thread has no explicit handler.public static native void sleep(long millis) throws InterruptedException
millis
- the length of time to sleep in milliseconds.InterruptedException
- if any thread has interrupted
the current thread. The interrupted status of the
current thread is cleared when this exception is thrown.public static void sleep(long millis, int nanos) throws InterruptedException
millis
- the length of time to sleep in milliseconds.nanos
- 0-999999 additional nanoseconds to sleep.InterruptedException
- if any thread has interrupted
the current thread. The interrupted status of the
current thread is cleared when this exception is thrown.public synchronized void start()
run
method of this thread.
The result is that two threads are running concurrently: the
current thread (which returns from the call to the
start
method) and the other thread (which executes its
run
method).
It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.
public final void stop()
If there is a security manager installed, its checkAccess
method is called with this
as its argument. This may result in a
SecurityException
being raised (in the current thread).
If this thread is different from the current thread (that is, the current
thread is trying to stop a thread other than itself), the
security manager's checkPermission
method (with a
RuntimePermission("stopThread")
argument) is called in
addition.
Again, this may result in throwing a
SecurityException
(in the current thread).
The thread represented by this thread is forced to stop whatever
it is doing abnormally and to throw a newly created
ThreadDeath
object as an exception.
It is permitted to stop a thread that has not yet been started. If the thread is eventually started, it immediately terminates.
An application should not normally try to catch
ThreadDeath
unless it must do some extraordinary
cleanup operation (note that the throwing of
ThreadDeath
causes finally
clauses of
try
statements to be executed before the thread
officially dies). If a catch
clause catches a
ThreadDeath
object, it is important to rethrow the
object so that the thread actually dies.
The top-level error handler that reacts to otherwise uncaught
exceptions does not print out a message or otherwise notify the
application if the uncaught exception is an instance of
ThreadDeath
.
public final synchronized void stop(Throwable obj)
If there is a security manager installed, the checkAccess
method of this thread is called, which may result in a
SecurityException
being raised (in the current thread).
If this thread is different from the current thread (that is, the current
thread is trying to stop a thread other than itself) or
obj
is not an instance of ThreadDeath
, the
security manager's checkPermission
method (with the
RuntimePermission("stopThread")
argument) is called in
addition.
Again, this may result in throwing a
SecurityException
(in the current thread).
If the argument obj
is null, a
NullPointerException
is thrown (in the current thread).
The thread represented by this thread is forced to stop
whatever it is doing abnormally and to throw the
Throwable
object obj
as an exception. This
is an unusual action to take; normally, the stop
method
that takes no arguments should be used.
It is permitted to stop a thread that has not yet been started. If the thread is eventually started, it immediately terminates.
obj
- the Throwable object to be thrown.public final void suspend()
First, the checkAccess
method of this thread is called
with no arguments. This may result in throwing a
SecurityException
(in the current thread).
If the thread is alive, it is suspended and makes no further progress unless and until it is resumed.
public String toString()
toString
in class Object
public static native void yield()
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