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Java(TM) 2 Platform, Standard Edition, v1.2.2 API Specification: Class Thread - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

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java.lang
Class Thread

java.lang.Object
  |
  +--java.lang.Thread
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:

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()

Field Summary
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
Thread()
          Allocates a new Thread object.
Thread(Runnable target)
          Allocates a new Thread object.
Thread(Runnable target, String name)
          Allocates a new Thread object.
Thread(String name)
          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, String name)
          Allocates a new Thread object.
 
Method Summary
static int activeCount()
          Returns the current number of active threads in this thread's thread group.
 void checkAccess()
          Determines if the currently running thread has permission to modify this thread.
 int countStackFrames()
          Deprecated. The definition of this call depends on suspend(), which is deprecated. Further, the results of this call were never well-defined.
static Thread currentThread()
          Returns a reference to the currently executing thread object.
 void destroy()
          Destroys this thread, without any cleanup.
static void dumpStack()
          Prints a stack trace of the current thread.
static int enumerate(Thread[] tarray)
          Copies into the specified array every active thread in this thread's thread group and its subgroups.
 ClassLoader getContextClassLoader()
          Returns the context ClassLoader for this Thread.
 String getName()
          Returns this thread's name.
 int getPriority()
          Returns this thread's priority.
 ThreadGroup getThreadGroup()
          Returns the thread group to which this thread belongs.
 void interrupt()
          Interrupts this thread.
static boolean interrupted()
          Tests whether the current thread has been interrupted.
 boolean isAlive()
          Tests if this thread is alive.
 boolean isDaemon()
          Tests if this thread is a daemon thread.
 boolean isInterrupted()
          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()
          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?.
 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 setContextClassLoader(ClassLoader cl)
          Sets the context ClassLoader for this Thread.
 void setDaemon(boolean on)
          Marks this thread as either a daemon thread or a user thread.
 void setName(String name)
          Changes the name of this thread to be equal to the argument name.
 void setPriority(int newPriority)
          Changes the priority of this thread.
static void sleep(long millis)
          Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds.
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.
 void start()
          Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
 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?.
 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?.
 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?.
 String toString()
          Returns a string representation of this thread, including the thread's name, priority, and thread group.
static void yield()
          Causes the currently executing thread object to temporarily pause and allow other threads to execute.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Field Detail

MIN_PRIORITY

public static final int MIN_PRIORITY
The minimum priority that a thread can have.

NORM_PRIORITY

public static final int NORM_PRIORITY
The default priority that is assigned to a thread.

MAX_PRIORITY

public static final int MAX_PRIORITY
The maximum priority that a thread can have.
Constructor Detail

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.

Threads created this way must have overridden their run() method to actually do anything. An example illustrating this method being used follows: 

     import java.lang.*;

     class plain01 implements Runnable {
         String name; 
         plain01() {
             name = null;
         }
         plain01(String s) {
             name = s;
         }
         public void run() {
             if (name == null)
                 System.out.println("A new thread created");
             else
                 System.out.println("A new thread with name " + name +
                                    " created");
         }
     }
     class threadtest01 {
         public static void main(String args[] ) {
             int failed = 0 ;

             Thread t1 = new Thread();  
             if (t1 != null)
                 System.out.println("new Thread() succeed");
             else {
                 System.out.println("new Thread() failed"); 
                 failed++; 
             }
         }
     }
See Also:
Thread(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String)

Thread

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

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

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

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)

Thread

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

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, 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)
Method Detail

currentThread

public static Thread currentThread()
Returns a reference to the currently executing thread object.
Returns:
the currently executing thread.

yield

public static void yield()
Causes the currently executing thread object to temporarily pause and allow other threads to execute.

sleep

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()

sleep

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()

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()

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()

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)

stop

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)

interrupt

public void interrupt()
Interrupts this thread.

First the checkAccess method of this thread is called with no arguments. This may result in throwing a SecurityException.

Throws:
SecurityException - if the current thread cannot modify this thread.

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()

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()

destroy

public void destroy()
Destroys this thread, without any cleanup. Any monitors it has locked remain locked. (This method is not implemented.)

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.

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()

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()

setPriority

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.

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:
checkAccess(), getPriority(), getThreadGroup(), MAX_PRIORITY, MIN_PRIORITY, ThreadGroup.getMaxPriority()

getPriority

public final int getPriority()
Returns this thread's priority.
Returns:
this thread's name.
See Also:
setPriority(int)

setName

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:
checkAccess(), getName()

getName

public final String getName()
Returns this thread's name.
Returns:
this thread's name.
See Also:
setName(java.lang.String)

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.

activeCount

public static int activeCount()
Returns the current number of active threads in this thread's thread group.
Returns:
the current number of threads in this thread's thread group.

enumerate

public static int enumerate(Thread[] tarray)
Copies into the specified array every active thread in this thread's thread group and its subgroups. This method simply calls the enumerate method of this 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.

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)

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.

join

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.

join

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.

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.

dumpStack

public static void dumpStack()
Prints a stack trace of the current thread. This method is used only for debugging.
See Also:
Throwable.printStackTrace()

setDaemon

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()

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)

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 JDK 1.2.

Throws:
SecurityException - if the current thread is not allowed to access this thread.
See Also:
SecurityManager.checkAccess(java.lang.Thread)

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.

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:
JDK1.2
See Also:
SecurityManager.checkPermission(java.security.Permission), RuntimePermission

setContextClassLoader

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:
JDK1.2
See Also:
SecurityManager.checkPermission(java.security.Permission), RuntimePermission
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