AQS 框架之 Lock 接口

■ 前言 - 内存锁的不足

• 不可中断：使用内部锁(指的是 synchronized) 时，不能中断正在等待获取锁的线程
• 不可超时：使用内部锁时，在请求锁失败情况下，必须无限等待，没有超时效果
• 自动释放：使用内部锁时，内部锁必须在获取它们的代码块中被自动释放（虽然对代码来说是种简化且对异常友好）
• 不可伸缩：使用内部锁时，无法细粒度控制锁(伸缩性不足)，即无法实现锁分离和锁联结，比如为每个链表节点(或部分)加锁从而允许不同的线程能够独立操作链表的不同节点(部分)，遍历或修改链表时，需先获取该节点锁并直到获取下一个节点锁时才释放当前节点锁
• 性能问题：使用内部锁时，在有竞争情况下仍会出现性能问题，尽管JDK6对内部锁进行了优化，但无论是偏向锁或是轻量级锁都是针对无竞争情况的优化，无竞争情况下与 ReentractLock 性能一致，但有竞争时Lock明显更高效

■ Lock 接口综述

• 定义： JDK1.5 引入Lock接口，其定义了一些抽象的锁操作，相比synchronized，Lock 提供了无条件、可轮询、可定时、可中断的锁获取操作，所有加锁和解锁的方法都是显式的
• 实现： Lock 的实现必须提供具有与 synchronized 相同的内存语义，但加锁的语义、调度算法、顺序保证、性能特性可以有所不同
• 使用： Lock接口的实现基本是通过聚合一个同步器 AbstractQueuedSynchronized 的子类来完成线程的访问控制
• 对比内部锁： Lock缺少隐式获取/释放锁的便捷，但却拥有了锁获取与释放的可操作性、可中断的获取锁以及超时获取锁等多种内部锁不具备的同步性，甚至还支持读写锁分离，同时允许获取和释放可以不在同一个块中
• 补充：此番为 AQS 框架之综述 (排期中) 的子番，同时也是 AQS 框架之 ReentractLock (排期中) 的预备番

　　并发包中的Lock接口实现类，其中ReadLock和WriteLock是ReentrantReadWriteLock的静态内部类，如图：

■ Lock 接口方法标准使用

//标准用法
Lock lock = new ReentrantLock();
//获取锁应在try之前，因为若获取锁时发生异常，异常抛出同时会导致锁无故释放
lock.lock();
try{
    doSometing();
}finally{
    //注意：必须在finally块中释放锁，目的是保证在获取到锁之后，最终能被释放
    lock.unlock();
}

■ Lock 接口方重点方法

  1. lock()

• lock方法应具有与内部锁加锁相同的内存语义，即无锁阻塞和支持可重入
• lock方法必须搭配unlock方法使用，同时必须在finally中显式调用unlock方法释放锁

/**
 * Acquires the lock.
 *  获取锁，调用该方法的当前线程将会获取锁，当锁获得后，从该方法返回
 * <p>If the lock is not available then the current thread becomes
 * disabled for thread scheduling purposes and lies dormant until the
 * lock has been acquired.
 *  若当前锁不可用(已被占有)，当前线程会一直休眠直到锁为可被获取状态
 * <p><b>Implementation Considerations</b>
 *  实现该方法的注意事项
 * <p>A {@code Lock} implementation may be able to detect erroneous use
 * of the lock, such as an invocation that would cause deadlock, and
 * may throw an (unchecked) exception in such circumstances.  The
 * circumstances and the exception type must be documented by that
 * {@code Lock} implementation.
 *  该方法的实现需要能发现lock被错误使用，如死锁或抛出不可查异常(即可运行期异常和Error)
 *  此时该实现必须用文档注明其可能出现的异常或需要的使用环境
 */
void lock();

  2. lockInterruptibly()

• lockInterruptibly 方法提供可中断的锁获取操作并允许在可取消的活动中使用

//样例代码
public boolean doTask throws InterruptedException(){
    lock.lockInterruptibly();
    try{
        return cancelTask();
    }finally{
        lock.unlock();
    }
}
//取消任务
private boolean cancelTask() throws InterruptedException {...}

/**
 * Acquires the lock unless the current thread is
 * {@linkplain Thread#interrupt interrupted}.
 *      可中断地获取锁，即在锁的获取中可以中断当前线程
 * <p>Acquires the lock if it is available and returns immediately.
 *  当获取锁时锁可用就立即返回
 * <p>If the lock is not available then the current thread becomes
 * disabled for thread scheduling purposes and lies dormant until
 * one of two things happens:
 * <ul>
 * <li>The lock is acquired by the current thread; or
 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
 * current thread, and interruption of lock acquisition is supported.
 * </ul>
 *  若当前锁不可用(已被占有)，当前线程会一直休眠直到以下两种情况发生：
 *      1.锁被当前线程获取
 *      2.其他线程中断当前线程，同时锁的获取允许被中断
 * <p><b>Implementation Considerations</b>
 *      实现该方法的注意事项
 * <p>The ability to interrupt a lock acquisition in some
 * implementations may not be possible, and if possible may be an
 * expensive operation.  The programmer should be aware that this
 * may be the case. An implementation should document when this is
 * the case.
 *      该方法属于拓展方法，只有需要中断服务的时候才需要实现它
 * <p>An implementation can favor responding to an interrupt over
 * normal method return.
 *      相对于返回，该方法更适合抛出一个中断响应，比如中断异常
 * @throws InterruptedException if the current thread is
 *         interrupted while acquiring the lock (and interruption
 *         of lock acquisition is supported)
 */
void lockInterruptibly() throws InterruptedException;

 3. tryLock()

• tryLock 方法提供可定时与可轮询的锁获取方式，与无条件的锁获取相比，具有更完善的错误恢复机制
• tryLock 方法能够有效的防止死锁的发生，比如使用轮询锁优雅失败规避死锁
• tryLock 方法同时提供定时锁的功能，其允许在限时活动内部使用独占锁，当线程获取锁、被中断或超时后返回
• tryLock 方法支持轮询获取锁：通过一个循环配合tryLock()来不断尝试获取锁，由于tryLock()非阻塞因此会立即返回是否成功获取锁的结果；当不能获取所有的锁时，应释放已获得的所有锁并重新尝试获取
• tryLock 方法同时支持响应中断

/**
 * Acquires the lock only if it is free at the time of invocation.
 *      尝试非阻塞的获取锁，调用该方法后立即返回是否成功获取锁true/false
 * <p>Acquires the lock if it is available and returns immediately
 * with the value {@code true}.
 * If the lock is not available then this method will return
 * immediately with the value {@code false}.
 *      当锁不可用时立即返回false
 * This usage ensures that the lock is unlocked if it was acquired, and
 * doesn't try to unlock if the lock was not acquired.
 *      该实现应确保当锁被获取时是未锁状态，当未被获取时不会尝试解锁
 * @return {@code true} if the lock was acquired and
 *         {@code false} otherwise
 */
boolean tryLock();
/**
 * Acquires the lock if it is free within the given waiting time and the
 * current thread has not been {@linkplain Thread#interrupt interrupted}.
 *      没有被中断当前线程在指定超时时间内获取锁
 * If the lock is not available then the current thread becomes disabled for
 * thread scheduling purposes and lies dormant until one of three things happens:
 * <ul>
 * <li>The lock is acquired by the current thread; or
 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
 * current thread, and interruption of lock acquisition is supported; or
 * <li>The specified waiting time elapses
 * </ul>
 * <p>If the specified waiting time elapses then the value {@code false} is returned.
 * If the time is less than or equal to zero, the method will not wait at all.
 *      当前线程在以下三种情况下会返回：
 *          1.当前线程在超时时间内获得锁
 *          2.当前线程在超时时间内被中断
 *          3.超时时间结束，返回false，线程不再被阻塞
 * @param time the maximum time to wait for the lock
 * @param unit the time unit of the {@code time} argument
 * @return {@code true} if the lock was acquired and {@code false}
 *         if the waiting time elapsed before the lock was acquired
 * @throws InterruptedException if the current thread is interrupted
 *         while acquiring the lock (and interruption of lock
 *         acquisition is supported)
 */
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;    

  4. unLock()

• 使用lock方法、lockInterruptibly方法、tryLock方法都必须显式调用unlock方法释放锁
• unlock方法必须在finally块中执行，这也是Lock使用的一个代码隐患(容易忘记执行)
• unlock方法允许与lock方法不在同一个块(即{})中执行，但业务代码必须保证在try-finally块中执行

/**
 * Releases the lock.
 *  释放锁
 */
void unlock();

 5. newCondition 方法

• Lock搭配Condition可以实现更加灵活的锁获取与释放的条件控制

/**
 * Returns a new {@link Condition} instance that is bound to this
 * {@code Lock} instance.
 *      返回一个等待通知组(条)件
 * <p>Before waiting on the condition the lock must be held by the current thread.
 * A call to {@link Condition#await()} will atomically release the lock
 * before waiting and re-acquire the lock before the wait returns.
 *      该组件与当前锁绑定，当先线程只有获得锁才能调用该组件的await方法并释放锁
 * @return A new {@link Condition} instance for this {@code Lock} instance
 * @throws UnsupportedOperationException if this {@code Lock}
 *         implementation does not support conditions
 */
Condition newCondition();