ReentrantLock源码解读，转载请注明出处！
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/*
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* 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;
/**
* 赵泉伟原创，转载请注明出处，谢谢！
*/
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;//内部抽象类，AQS同步器的子类
/**
* 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 {//集成AQS同步器
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();//定义抽象lock方法
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {//以非公平锁的方式尝试获取锁
final Thread current = Thread.currentThread();//获取当前线程对象
int c = getState();//获取当前锁状态
if (c == 0) {//0代表能获取到锁
if (compareAndSetState(0, acquires)) {//调用AQS以CAS方式获取锁，若获取成功返回true
setExclusiveOwnerThread(current);//设置当前线程为获取锁的线程，主要为了可重入
return true;//返回true
}
}
else if (current == getExclusiveOwnerThread()) {//如果当前线程为已经获取锁的线程，则可以继续进入锁，因为这是可重入锁的实现
int nextc = c + acquires;//计数器进行累加
if (nextc < 0) // overflow代表超过了最大值，抛出异常
throw new Error("Maximum lock count exceeded");
setState(nextc);//设置最新的状态值
return true;//返回true
}
return false;//否则返回false
}
protected final boolean tryRelease(int releases) {//尝试释放锁
int c = getState() - releases;//状态值递减
if (Thread.currentThread() != getExclusiveOwnerThread())//如果当前线程不是已经获取锁的线程则报错
throw new IllegalMonitorStateException();//抛异常
boolean free = false;//标志位默认为false
if (c == 0) {//如果状态值为0，则代表锁释放成功
free = true;//标志位变为true
setExclusiveOwnerThread(null);//清空当前持有锁的线程
}
setState(c);//设定最新状态值
return free;//返回标志位
}
protected final boolean isHeldExclusively() {//判断当前线程是否持有锁
// 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();//返回结果
}
final ConditionObject newCondition() {
return new ConditionObject();
}//创建ConditionObject等待队列
// Methods relayed from outer class
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}//获取当前持有锁的对象
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}//获取当前持有锁的线程的状态值
final boolean isLocked() {
return getState() != 0;
}//判断是否存在锁
}
/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {//非公平锁
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {//上锁
if (compareAndSetState(0, 1))//调用父类方法获以cas方式获取锁
setExclusiveOwnerThread(Thread.currentThread());//设置当前线程持有锁
else
acquire(1);//否则以调用父类默认获取锁的方式获取锁
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}//尝试获取锁
}
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {//公平锁
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {//加锁
acquire(1);//调用加锁方法
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
protected final boolean tryAcquire(int acquires) {//尝试获取锁
final Thread current = Thread.currentThread();//获取当前线程
int c = getState();//获取当前状态
if (c == 0) {//如果为0
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {//尝试获取锁
setExclusiveOwnerThread(current);//获取成功，将当前线程设为持有锁状态
return true;//返回true
}
}
else if (current == getExclusiveOwnerThread()) {//否则判断当前线程是否已经是获取锁的线程
int nextc = c + acquires;//状态累加
if (nextc < 0)//超过最大值则报错
throw new Error("Maximum lock count exceeded");//抛出异常
setState(nextc);//设置最新的状态值
return true;//返回true
}
return false;//否则返回false
}
}
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}//构造方法，默认构造的是非公平锁
/**
* 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();
}//根据传入的布尔值设定锁的方式
/**
* 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();
}//加锁，调用内部类的lock方法
/**
* 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 {//中断锁
sync.acquireInterruptibly(1);//调用内部类的方法，最终调用的是父类的AQS中的方法
}
/**
* 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);
}//尝试获取锁以非公平锁方式
/**
* 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);
}//释放锁，调用子类的release方法
/**
* 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();
}//返回一个Condition等待队列
/**
* 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() {//返回当前线程已获取的状态，否则返回0
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();
}//判断当前线程是否持有锁
/**
* 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();
}//判断当前是否存在锁
/**
* 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;
}//判断是否为公平锁
/**
* 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();
}//获取当前持有锁的线程对象
/**
* 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();
}//判断队列中是否有线程在等待，true表示有，否则为false
/**
* 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);
}//判断指定线程是否在等待队列中,true存在否则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();
}//获取等待队列的长度
/**
* 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();
}//返回等待队列中的Thread集合
/**
* 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) {//判断指定condition队列是否有待唤醒指定的node节点
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) {//获取Condition等待队列的长度
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 getWaitingThreads(Condition condition) {//获取Condition等待队列中所有处于等待状态的线程
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() {//重写toString方法
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
}