java.lang
Class Thread

java.lang.Object
  java.lang.Thread

All Implemented Interfaces:

Runnable

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:

• The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
• All threads that are not daemon threads have died, either by returning from the call to the 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.

Since:

JDK1.0

See Also:

Runnable, Runtime.exit(int), run(), stop()

Method Summary
static int	activeCount() public static int activeCount() Returns the number of active threads in the current thread's thread group. Returns: the number of active threads in the current thread's thread group.
void	checkAccess() public final void checkAccess() Determines if the currently running thread has permission to modify this thread. If there is a security manager, its checkAccess method is called with this thread as its argument. This may result in throwing a SecurityException. Note: This method was mistakenly non-final in JDK 1.1. It has been made final in the Java 2 Platform. Throws: SecurityException - if the current thread is not allowed to access this thread. See Also: SecurityManager.checkAccess(java.lang.Thread)
int	countStackFrames() public int countStackFrames() Deprecated. The definition of this call depends on suspend(), which is deprecated. Further, the results of this call were never well-defined. Counts the number of stack frames in this thread. The thread must be suspended. Returns: the number of stack frames in this thread. Throws: IllegalThreadStateException - if this thread is not suspended.
static Thread	currentThread() public static Thread currentThread() Returns a reference to the currently executing thread object. Returns: the currently executing thread.
void	destroy() public void destroy() Destroys this thread, without any cleanup. Any monitors it has locked remain locked. (This method is not implemented.)
static void	dumpStack() public static void dumpStack() Prints a stack trace of the current thread. This method is used only for debugging. See Also: Throwable.printStackTrace()
static int	enumerate(Thread[] tarray) public static int enumerate(Thread[] tarray) Copies into the specified array every active thread in the current thread's thread group and its subgroups. This method simply calls the 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. Parameters: tarray - an array of Thread objects to copy to Returns: the number of threads put into the array Throws: SecurityException - if a security manager exists and its checkAccess method doesn't allow the operation. See Also: ThreadGroup.enumerate(java.lang.Thread[]), SecurityManager.checkAccess(java.lang.ThreadGroup)
ClassLoader	getContextClassLoader() public ClassLoader getContextClassLoader() Returns the context ClassLoader for this Thread. The context ClassLoader is provided by the creator of the thread for use by code running in this thread when loading classes and resources. If not set, the default is the ClassLoader context of the parent Thread. The context ClassLoader of the primordial thread is typically set to the class loader used to load the application. 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.. Returns: the context ClassLoader for this Thread Throws: SecurityException - if a security manager exists and its checkPermission method doesn't allow getting the context ClassLoader. Since: 1.2 See Also: setContextClassLoader(java.lang.ClassLoader), SecurityManager.checkPermission(java.security.Permission), RuntimePermission
String	getName() public final String getName() Returns this thread's name. Returns: this thread's name. See Also: setName(java.lang.String), setName(java.lang.String)
int	getPriority() public final int getPriority() Returns this thread's priority. Returns: this thread's priority. See Also: setPriority(int), setPriority(int)
ThreadGroup	getThreadGroup() public final ThreadGroup getThreadGroup() Returns the thread group to which this thread belongs. This method returns null if this thread has died (been stopped). Returns: this thread's thread group.
static boolean	holdsLock(Object obj) public static boolean holdsLock(Object obj) Returns true if and only if the current thread holds the monitor lock on the specified object. This method is designed to allow a program to assert that the current thread already holds a specified lock: assert Thread.holdsLock(obj); Parameters: obj - the object on which to test lock ownership Returns: true if the current thread holds the monitor lock on the specified object. Throws: NullPointerException - if obj is null Since: 1.4
void	interrupt() public void interrupt() Interrupts this thread. First the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown. If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException. If this thread is blocked in an I/O operation upon an interruptible channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException. If this thread is blocked in a 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 wakeup method were invoked. If none of the previous conditions hold then this thread's interrupt status will be set. Throws: SecurityException - if the current thread cannot modify this thread
static boolean	interrupted() public static boolean interrupted() Tests whether the current thread has been interrupted. The interrupted status of the thread is cleared by this method. In other words, if this method were to be called twice in succession, the second call would return false (unless the current thread were interrupted again, after the first call had cleared its interrupted status and before the second call had examined it). Returns: true if the current thread has been interrupted; false otherwise. See Also: isInterrupted()
boolean	isAlive() public final boolean isAlive() Tests if this thread is alive. A thread is alive if it has been started and has not yet died. Returns: true if this thread is alive; false otherwise.
boolean	isDaemon() public final boolean isDaemon() Tests if this thread is a daemon thread. Returns: true if this thread is a daemon thread; false otherwise. See Also: setDaemon(boolean)
boolean	isInterrupted() public boolean isInterrupted() Tests whether this thread has been interrupted. The interrupted status of the thread is unaffected by this method. Returns: true if this thread has been interrupted; false otherwise. See Also: interrupted()
void	join() public final void join() throws InterruptedException Waits for this thread to die. Throws: InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
void	join(long millis) public final void join(long millis) throws InterruptedException Waits at most millis milliseconds for this thread to die. A timeout of 0 means to wait forever. Parameters: millis - the time to wait in milliseconds. Throws: InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
void	join(long millis, int nanos) public final void join(long millis, int nanos) throws InterruptedException Waits at most millis milliseconds plus nanos nanoseconds for this thread to die. Parameters: millis - the time to wait in milliseconds. nanos - 0-999999 additional nanoseconds to wait. Throws: IllegalArgumentException - if the value of millis is negative the value of nanos is not in the range 0-999999. InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
void	resume() public final void resume() Deprecated. This method exists solely for use with suspend(), which has been deprecated because it is deadlock-prone. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?. Resumes a suspended thread. 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. Throws: SecurityException - if the current thread cannot modify this thread. See Also: checkAccess(), suspend()
void	run() public 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. Subclasses of Thread should override this method. Specified by: run in interface Runnable See Also: start(), stop(), Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String), Runnable.run()
void	setContextClassLoader(ClassLoader cl) public void setContextClassLoader(ClassLoader cl) Sets the context ClassLoader for this Thread. The context ClassLoader can be set when a thread is created, and allows the creator of the thread to provide the appropriate class loader to code running in the thread when loading classes and resources. 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.. Parameters: cl - the context ClassLoader for this Thread Throws: SecurityException - if the current thread cannot set the context ClassLoader. Since: 1.2 See Also: getContextClassLoader(), SecurityManager.checkPermission(java.security.Permission), RuntimePermission
void	setDaemon(boolean on) public final void setDaemon(boolean on) Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads. 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). Parameters: on - if true, marks this thread as a daemon thread. Throws: IllegalThreadStateException - if this thread is active. SecurityException - if the current thread cannot modify this thread. See Also: isDaemon(), checkAccess()
void	setName(String name) public final void setName(String name) Changes the name of this thread to be equal to the argument name. First the checkAccess method of this thread is called with no arguments. This may result in throwing a SecurityException. Parameters: name - the new name for this thread. Throws: SecurityException - if the current thread cannot modify this thread. See Also: getName(), checkAccess(), getName()
void	setPriority(int newPriority) public final void setPriority(int newPriority) Changes the priority of this thread. 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. Parameters: newPriority - priority to set this thread to Throws: IllegalArgumentException - If the priority is not in the range MIN_PRIORITY to MAX_PRIORITY. SecurityException - if the current thread cannot modify this thread. See Also: getPriority(), checkAccess(), getPriority(), getThreadGroup(), MAX_PRIORITY, MIN_PRIORITY, ThreadGroup.getMaxPriority()
static void	sleep(long millis) public static void sleep(long millis) throws InterruptedException Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds. The thread does not lose ownership of any monitors. Parameters: millis - the length of time to sleep in milliseconds. Throws: InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown. See Also: Object.notify()
static void	sleep(long millis, int nanos) public static void sleep(long millis, int nanos) throws InterruptedException Causes the currently executing thread to sleep (cease execution) for the specified number of milliseconds plus the specified number of nanoseconds. The thread does not lose ownership of any monitors. Parameters: millis - the length of time to sleep in milliseconds. nanos - 0-999999 additional nanoseconds to sleep. Throws: IllegalArgumentException - if the value of millis is negative or the value of nanos is not in the range 0-999999. InterruptedException - if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown. See Also: Object.notify()
void	start() public void start() Causes this thread to begin execution; the Java Virtual Machine calls the 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). Throws: IllegalThreadStateException - if the thread was already started. See Also: run(), stop()
void	stop() public final void stop() Deprecated. This method is inherently unsafe. Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked (as a natural consequence of the unchecked ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects become visible to other threads, potentially resulting in arbitrary behavior. Many uses of stop should be replaced by code that simply modifies some variable to indicate that the target thread should stop running. The target thread should check this variable regularly, and return from its run method in an orderly fashion if the variable indicates that it is to stop running. If the target thread waits for long periods (on a condition variable, for example), the interrupt method should be used to interrupt the wait. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?. Forces the thread to stop executing. 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. Throws: SecurityException - if the current thread cannot modify this thread. See Also: interrupt(), checkAccess(), run(), start(), ThreadDeath, ThreadGroup.uncaughtException(java.lang.Thread, java.lang.Throwable), SecurityManager.checkAccess(Thread), SecurityManager.checkPermission(java.security.Permission)
void	stop(Throwable obj) public final void stop(Throwable obj) Deprecated. This method is inherently unsafe. See stop() (with no arguments) for details. An additional danger of this method is that it may be used to generate exceptions that the target thread is unprepared to handle (including checked exceptions that the thread could not possibly throw, were it not for this method). For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?. Forces the thread to stop executing. 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 complete 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. Parameters: obj - the Throwable object to be thrown. Throws: SecurityException - if the current thread cannot modify this thread. See Also: interrupt(), checkAccess(), run(), start(), stop(), SecurityManager.checkAccess(Thread), SecurityManager.checkPermission(java.security.Permission)
void	suspend() public final void suspend() Deprecated. This method has been deprecated, as it is inherently deadlock-prone. If the target thread holds a lock on the monitor protecting a critical system resource when it is suspended, no thread can access this resource until the target thread is resumed. If the thread that would resume the target thread attempts to lock this monitor prior to calling resume, deadlock results. Such deadlocks typically manifest themselves as "frozen" processes. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?. Suspends this thread. 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. Throws: SecurityException - if the current thread cannot modify this thread. See Also: checkAccess()
String	toString() public String toString() Returns a string representation of this thread, including the thread's name, priority, and thread group. Overrides: toString in class Object Returns: a string representation of this thread.
static void	yield() public static void yield() Causes the currently executing thread object to temporarily pause and allow other threads to execute.

 

Constructor Summary

Thread() public Thread() Allocates a new 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. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread(Runnable target) public Thread(Runnable target) Allocates a new 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. Parameters: target - the object whose run method is called. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread(Runnable target, String name) public Thread(Runnable target, String name) Allocates a new Thread object. This constructor has the same effect as Thread(null, target, name). Parameters: target - the object whose run method is called. name - the name of the new thread. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread(String name) public Thread(String name) Allocates a new Thread object. This constructor has the same effect as Thread(null, null, name). Parameters: name - the name of the new thread. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread(ThreadGroup group, Runnable target) public Thread(ThreadGroup group, Runnable target) Allocates a new 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. Parameters: group - the thread group. target - the object whose run method is called. Throws: SecurityException - if the current thread cannot create a thread in the specified thread group. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread(ThreadGroup group, Runnable target, String name) public 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. 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. 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. Parameters: group - the thread group. target - the object whose run method is called. name - the name of the new thread. Throws: SecurityException - if the current thread cannot create a thread in the specified thread group. See Also: Runnable.run(), run(), setDaemon(boolean), setPriority(int), ThreadGroup.checkAccess(), SecurityManager.checkAccess(java.lang.Thread)

Thread(ThreadGroup group, Runnable target, String name, long stackSize) public 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. This constructor is identical to 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 StackOverflowError. Similarly, specifying a lower value may allow a greater number of threads to exist concurrently without throwing an an 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. Parameters: 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. Throws: SecurityException - if the current thread cannot create a thread in the specified thread group.

Thread(ThreadGroup group, String name) public Thread(ThreadGroup group, String name) Allocates a new Thread object. This constructor has the same effect as Thread(group, null, name) Parameters: group - the thread group. name - the name of the new thread. Throws: SecurityException - if the current thread cannot create a thread in the specified thread group. See Also: Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

 

Field Summary
static int	MAX_PRIORITY The maximum priority that a thread can have. See Also: Constant Field Values
static int	MIN_PRIORITY The minimum priority that a thread can have. See Also: Constant Field Values
static int	NORM_PRIORITY The default priority that is assigned to a thread. See Also: Constant Field Values