ReentrantLock 소스 코드 판독, 전재 출처 를 밝 혀 주세요!

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.Collection;

/**
 *      ，       ，  ！
 */
public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;//     ，AQS      

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {//  AQS   
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();//    lock  

        /**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {//             
            final Thread current = Thread.currentThread();//        
            int c = getState();//       
            if (c == 0) {//0       
                if (compareAndSetState(0, acquires)) {//  AQS CAS     ，       true
                    setExclusiveOwnerThread(current);//             ，       
                    return true;//  true
                }
            }
            else if (current == getExclusiveOwnerThread()) {//               ，        ，           
                int nextc = c + acquires;//       
                if (nextc < 0) // overflow        ，    
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);//        
                return true;//  true
            }
            return false;//    false
        }

        protected final boolean tryRelease(int releases) {//     
            int c = getState() - releases;//     
            if (Thread.currentThread() != getExclusiveOwnerThread())//                   
                throw new IllegalMonitorStateException();//   
            boolean free = false;//      false
            if (c == 0) {//      0，        
                free = true;//     true
                setExclusiveOwnerThread(null);//          
            }
            setState(c);//       
            return free;//     
        }

        protected final boolean isHeldExclusively() {//           
            // While we must in general read state before owner,
            // we don't need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();//    
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }//  ConditionObject    

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }//          

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }//              

        final boolean isLocked() {
            return getState() != 0;
        }//       

    }

    /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {//    
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {//  
            if (compareAndSetState(0, 1))//        cas     
                setExclusiveOwnerThread(Thread.currentThread());//         
            else
                acquire(1);//                  
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }//     
    }

    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {//   
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {//  
            acquire(1);//      
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {//     
            final Thread current = Thread.currentThread();//      
            int c = getState();//      
            if (c == 0) {//   0
                if (!hasQueuedPredecessors() &&
                        compareAndSetState(0, acquires)) {//     
                    setExclusiveOwnerThread(current);//    ，            
                    return true;//  true
                }
            }
            else if (current == getExclusiveOwnerThread()) {//                   
                int nextc = c + acquires;//    
                if (nextc < 0)//        
                    throw new Error("Maximum lock count exceeded");//    
                setState(nextc);//        
                return true;//  true
            }
            return false;//    false
        }
    }

    /**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }//    ，          

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }//              

    /**
     * Acquires the lock.
     *
     * 
Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * 
If the current thread already holds the lock then the hold
     * count is incremented by one and the method returns immediately.
     *
     * 
If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one.
     */
    public void lock() {
        sync.lock();
    }//  ，      lock  

    /**
     * Acquires the lock unless the current thread is
     * {@linkplain Thread#interrupt interrupted}.
     *
     * 
Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * 
If the current thread already holds this lock then the hold count
     * is incremented by one and the method returns immediately.
     *
     * 
If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of two things happens:
     *
     * 

     *
     * 
The lock is acquired by the current thread; or
     *
     * 
Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread.
     *
     * 
     *
     * 
If the lock is acquired by the current thread then the lock hold
     * count is set to one.
     *
     * 
If the current thread:
     *
     * 

     *
     * 
has its interrupted status set on entry to this method; or
     *
     * 
is {@linkplain Thread#interrupt interrupted} while acquiring
     * the lock,
     *
     * 
     *
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * 
In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void lockInterruptibly() throws InterruptedException {//   
        sync.acquireInterruptibly(1);//        ，         AQS    
    }

    /**
     * Acquires the lock only if it is not held by another thread at the time
     * of invocation.
     *
     * 
Acquires the lock if it is not held by another thread and
     * returns immediately with the value {@code true}, setting the
     * lock hold count to one. Even when this lock has been set to use a
     * fair ordering policy, a call to {@code tryLock()} will
     * immediately acquire the lock if it is available, whether or not
     * other threads are currently waiting for the lock.
     * This "barging" behavior can be useful in certain
     * circumstances, even though it breaks fairness. If you want to honor
     * the fairness setting for this lock, then use
     * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
     * which is almost equivalent (it also detects interruption).
     *
     * 
If the current thread already holds this lock then the hold
     * count is incremented by one and the method returns {@code true}.
     *
     * 
If the lock is held by another thread then this method will return
     * immediately with the value {@code false}.
     *
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} otherwise
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }//            


    /**
     * Attempts to release this lock.
     *
     * 
If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {
        sync.release(1);
    }//   ，     release  

    /**
     * Returns a {@link Condition} instance for use with this
     * {@link Lock} instance.
     *
     * 
The returned {@link Condition} instance supports the same
     * usages as do the {@link Object} monitor methods ({@link
     * Object#wait() wait}, {@link Object#notify notify}, and {@link
     * Object#notifyAll notifyAll}) when used with the built-in
     * monitor lock.
     *
     * 

     *
     * 
If this lock is not held when any of the {@link Condition}
     * {@linkplain Condition#await() waiting} or {@linkplain
     * Condition#signal signalling} methods are called, then an {@link
     * IllegalMonitorStateException} is thrown.
     *
     * 
When the condition {@linkplain Condition#await() waiting}
     * methods are called the lock is released and, before they
     * return, the lock is reacquired and the lock hold count restored
     * to what it was when the method was called.
     *
     * 
If a thread is {@linkplain Thread#interrupt interrupted}
     * while waiting then the wait will terminate, an {@link
     * InterruptedException} will be thrown, and the thread's
     * interrupted status will be cleared.
     *
     * 
 Waiting threads are signalled in FIFO order.
     *
     * 
The ordering of lock reacquisition for threads returning
     * from waiting methods is the same as for threads initially
     * acquiring the lock, which is in the default case not specified,
     * but for fair locks favors those threads that have been
     * waiting the longest.
     *
     * 
     *
     * @return the Condition object
     */
    public Condition newCondition() {
        return sync.newCondition();
    }//    Condition    

    /**
     * Queries the number of holds on this lock by the current thread.
     *
     * 
A thread has a hold on a lock for each lock action that is not
     * matched by an unlock action.
     *
     * 
The hold count information is typically only used for testing and
     * debugging purposes. For example, if a certain section of code should
     * not be entered with the lock already held then we can assert that
     * fact:
     *
     *  
 {@code
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *   public void m() {
     *     assert lock.getHoldCount() == 0;
     *     lock.lock();
     *     try {
     *       // ... method body
     *     } finally {
     *       lock.unlock();
     *     }
     *   }
     * }}
*
* @return the number of holds on this lock by the current thread,
*         or zero if this lock is not held by the current thread
*/
public int getHoldCount () {/ 현재 스 레 드 에서 가 져 온 상 태 를 되 돌려 줍 니 다. 그렇지 않 으 면 0 으로 돌아 갑 니 다.
return sync.getHoldCount();
}
/**
* Queries if this lock is held by the current thread.
*
* Analogous to the {@link Thread#holdsLock(Object)} method for
* built-in monitor locks, this method is typically used for
* debugging and testing. For example, a method that should only be
* called while a lock is held can assert that this is the case:
*
*   
 {@code
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert lock.isHeldByCurrentThread();
     *       // ... method body
     *   }
     * }}
*
* It can also be used to ensure that a reentrant lock is used
* in a non-reentrant manner, for example:
*
*   
 {@code
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert !lock.isHeldByCurrentThread();
     *       lock.lock();
     *       try {
     *           // ... method body
     *       } finally {
     *           lock.unlock();
     *       }
     *   }
     * }}
*
* @return {@code true} if current thread holds this lock and
*         {@code false} otherwise
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
} / / 현재 스 레 드 에 자물쇠 가 있 는 지 판단 합 니 다.
/**
* Queries if this lock is held by any thread. This method is
* designed for use in monitoring of the system state,
* not for synchronization control.
*
* @return {@code true} if any thread holds this lock and
*         {@code false} otherwise
*/
public boolean isLocked() {
return sync.isLocked();
} / / 현재 자물쇠 가 있 는 지 판단 합 니 다.
/**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync instanceof FairSync;
} / / 공평자물쇠 여 부 를 판단 합 니 다.
/**
* Returns the thread that currently owns this lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
} / / 현재 자 물 쇠 를 가지 고 있 는 스 레 드 대상 가 져 오기
/**
* Queries whether any threads are waiting to acquire this lock. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire this lock.  This method is designed primarily for use in
* monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
*         acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
} / / 대기 열 에 스 레 드 가 기다 리 고 있 는 지 판단 합 니 다. true 는 있 음 을 표시 합 니 다. 그렇지 않 으 면 false 입 니 다.
/**
* Queries whether the given thread is waiting to acquire this
* lock. Note that because cancellations may occur at any time, a
* {@code true} return does not guarantee that this thread
* will ever acquire this lock.  This method is designed primarily for use
* in monitoring of the system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
} / / 지정 한 라인 이 대기 열 에 있 는 지 판단 합 니 다. true 가 존재 하지 않 으 면 false
/**
* Returns an estimate of the number of threads waiting to
* acquire this lock.  The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures.  This method is designed for use in
* monitoring of the system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
} / / 대기 열의 길 이 를 가 져 옵 니 다.
/**
* Returns a collection containing threads that may be waiting to
* acquire this lock.  Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate.  The elements of the
* returned collection are in no particular order.  This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
protected Collection getQueuedThreads() {
return sync.getQueuedThreads();
} / / 대기 열 에 있 는 Thread 집합 을 되 돌려 줍 니 다.
/**
* Queries whether any threads are waiting on the given condition
* associated with this lock. Note that because timeouts and
* interrupts may occur at any time, a {@code true} return does
* not guarantee that a future {@code signal} will awaken any
* threads.  This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
*         not associated with this lock
* @throws NullPointerException if the condition is null
*/
public boolean hasWaiters (Condition condition) {/ 지정 한 condition 대기 열 이 지정 한 node 노드 를 깨 워 야 하 는 지 판단 합 니 다.
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this lock. Note that because
* timeouts and interrupts may occur at any time, the estimate
* serves only as an upper bound on the actual number of waiters.
* This method is designed for use in monitoring of the system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
*         not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength (Condition condition) {/ Condition 대기 열의 길 이 를 가 져 옵 니 다.
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this lock.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order.  This method is designed to
* facilitate construction of subclasses that provide more
* extensive condition monitoring facilities.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
*         not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection getWaiting Threads (Condition condition) {/ Condition 대기 열 에 있 는 모든 대기 상태 스 레 드 가 져 오기
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes either the String {@code "Unlocked"}
* or the String {@code "Locked by"} followed by the
* {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString () {/ / toString 재 작성 방법
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
}