Interface - java.util.concurrent.locks.Lock

Created by : Mr Dk.

2019 / 12 / 23 21:22

Nanjing, Jiangsu, China

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Definition

锁 是用于控制多个线程访问共享资源的工具。synchroinzed 关键字提供对每个对象的 monitor 的隐式访问。所有的锁应当按照获得的顺序被释放 (防止死锁)。

public interface Lock {

}

该接口的实现类能够提供更灵活的锁获取和锁释放机制。锁的使用应当遵循：

Lock l = ...;
l.lock();
try {
    // access the resource protected by this lock
} finally {
    l.unlock();
}

保证获得锁的代码被 try-catch 块保护，以便锁被释放。此外，本接口还提供：

• 非阻塞尝试获得锁的 tryLock() 函数
• 可以被中断的尝试获得锁函数 lockInterruptibly()
• 可以超时的尝试获得锁函数

由于 Lock 本身也是对象，因此也可以通过 synchronized 获得 monitor 锁。但为防止混淆，最好不要这样做。

/**
 * {@code Lock} implementations provide more extensive locking
 * operations than can be obtained using {@code synchronized} methods
 * and statements.  They allow more flexible structuring, may have
 * quite different properties, and may support multiple associated
 * {@link Condition} objects.
 *
 * <p>A lock is a tool for controlling access to a shared resource by
 * multiple threads. Commonly, a lock provides exclusive access to a
 * shared resource: only one thread at a time can acquire the lock and
 * all access to the shared resource requires that the lock be
 * acquired first. However, some locks may allow concurrent access to
 * a shared resource, such as the read lock of a {@link ReadWriteLock}.
 *
 * <p>The use of {@code synchronized} methods or statements provides
 * access to the implicit monitor lock associated with every object, but
 * forces all lock acquisition and release to occur in a block-structured way:
 * when multiple locks are acquired they must be released in the opposite
 * order, and all locks must be released in the same lexical scope in which
 * they were acquired.
 *
 * <p>While the scoping mechanism for {@code synchronized} methods
 * and statements makes it much easier to program with monitor locks,
 * and helps avoid many common programming errors involving locks,
 * there are occasions where you need to work with locks in a more
 * flexible way. For example, some algorithms for traversing
 * concurrently accessed data structures require the use of
 * &quot;hand-over-hand&quot; or &quot;chain locking&quot;: you
 * acquire the lock of node A, then node B, then release A and acquire
 * C, then release B and acquire D and so on.  Implementations of the
 * {@code Lock} interface enable the use of such techniques by
 * allowing a lock to be acquired and released in different scopes,
 * and allowing multiple locks to be acquired and released in any
 * order.
 *
 * <p>With this increased flexibility comes additional
 * responsibility. The absence of block-structured locking removes the
 * automatic release of locks that occurs with {@code synchronized}
 * methods and statements. In most cases, the following idiom
 * should be used:
 *
 *  <pre> {@code
 * Lock l = ...;
 * l.lock();
 * try {
 *   // access the resource protected by this lock
 * } finally {
 *   l.unlock();
 * }}</pre>
 *
 * When locking and unlocking occur in different scopes, care must be
 * taken to ensure that all code that is executed while the lock is
 * held is protected by try-finally or try-catch to ensure that the
 * lock is released when necessary.
 *
 * <p>{@code Lock} implementations provide additional functionality
 * over the use of {@code synchronized} methods and statements by
 * providing a non-blocking attempt to acquire a lock ({@link
 * #tryLock()}), an attempt to acquire the lock that can be
 * interrupted ({@link #lockInterruptibly}, and an attempt to acquire
 * the lock that can timeout ({@link #tryLock(long, TimeUnit)}).
 *
 * <p>A {@code Lock} class can also provide behavior and semantics
 * that is quite different from that of the implicit monitor lock,
 * such as guaranteed ordering, non-reentrant usage, or deadlock
 * detection. If an implementation provides such specialized semantics
 * then the implementation must document those semantics.
 *
 * <p>Note that {@code Lock} instances are just normal objects and can
 * themselves be used as the target in a {@code synchronized} statement.
 * Acquiring the
 * monitor lock of a {@code Lock} instance has no specified relationship
 * with invoking any of the {@link #lock} methods of that instance.
 * It is recommended that to avoid confusion you never use {@code Lock}
 * instances in this way, except within their own implementation.
 *
 * <p>Except where noted, passing a {@code null} value for any
 * parameter will result in a {@link NullPointerException} being
 * thrown.
 *
 * <h3>Memory Synchronization</h3>
 *
 * <p>All {@code Lock} implementations <em>must</em> enforce the same
 * memory synchronization semantics as provided by the built-in monitor
 * lock, as described in
 * <a href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4">
 * The Java Language Specification (17.4 Memory Model)</a>:
 * <ul>
 * <li>A successful {@code lock} operation has the same memory
 * synchronization effects as a successful <em>Lock</em> action.
 * <li>A successful {@code unlock} operation has the same
 * memory synchronization effects as a successful <em>Unlock</em> action.
 * </ul>
 *
 * Unsuccessful locking and unlocking operations, and reentrant
 * locking/unlocking operations, do not require any memory
 * synchronization effects.
 *
 * <h3>Implementation Considerations</h3>
 *
 * <p>The three forms of lock acquisition (interruptible,
 * non-interruptible, and timed) may differ in their performance
 * characteristics, ordering guarantees, or other implementation
 * qualities.  Further, the ability to interrupt the <em>ongoing</em>
 * acquisition of a lock may not be available in a given {@code Lock}
 * class.  Consequently, an implementation is not required to define
 * exactly the same guarantees or semantics for all three forms of
 * lock acquisition, nor is it required to support interruption of an
 * ongoing lock acquisition.  An implementation is required to clearly
 * document the semantics and guarantees provided by each of the
 * locking methods. It must also obey the interruption semantics as
 * defined in this interface, to the extent that interruption of lock
 * acquisition is supported: which is either totally, or only on
 * method entry.
 *
 * <p>As interruption generally implies cancellation, and checks for
 * interruption are often infrequent, an implementation can favor responding
 * to an interrupt over normal method return. This is true even if it can be
 * shown that the interrupt occurred after another action may have unblocked
 * the thread. An implementation should document this behavior.
 *
 * @see ReentrantLock
 * @see Condition
 * @see ReadWriteLock
 *
 * @since 1.5
 * @author Doug Lea
 */

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Interfaces

Lock

获得锁。如果当前的锁不可获得，那么该进程进入 BLOCKED 状态并被重新调度，直到锁可被获得。在实现中，应当具有检测错误获得锁的能力 (死锁等)。

/**
 * Acquires the lock.
 *
 * <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.
 *
 * <p><b>Implementation Considerations</b>
 *
 * <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.
 */
void lock();

Interruptible Lock

获得锁，除非当前线程被中断。lock() 是不可中断等待锁，lockInterruptibly() 是可中断等待锁。如果锁可用，则立刻返回；如果锁不可用，那么当前线程进行重新竞争，直到以下两种事件发生:

1. 锁可以被获得
2. 其它线程中断了当前线程 - 如果当前线程在等待锁时被中断，则抛出 InterruptedException 异常

/**
 * Acquires the lock unless the current thread is
 * {@linkplain Thread#interrupt interrupted}.
 *
 * <p>Acquires the lock if it is available and returns immediately.
 *
 * <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>
 *
 * <p>If the current thread:
 * <ul>
 * <li>has its interrupted status set on entry to this method; or
 * <li>is {@linkplain Thread#interrupt interrupted} while acquiring the
 * lock, and interruption of lock acquisition is supported,
 * </ul>
 * then {@link InterruptedException} is thrown and the current thread's
 * interrupted status is cleared.
 *
 * <p><b>Implementation Considerations</b>
 *
 * <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.
 *
 * <p>An implementation can favor responding to an interrupt over
 * normal method return.
 *
 * <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.
 *
 * @throws InterruptedException if the current thread is
 *         interrupted while acquiring the lock (and interruption
 *         of lock acquisition is supported)
 */
void lockInterruptibly() throws InterruptedException;

Try Lock

如果锁可用，就获得锁并返回 true，如果锁不可用，则立刻返回 false。

/**
 * Acquires the lock only if it is free at the time of invocation.
 *
 * <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}.
 *
 * <p>A typical usage idiom for this method would be:
 *  <pre> {@code
 * Lock lock = ...;
 * if (lock.tryLock()) {
 *   try {
 *     // manipulate protected state
 *   } finally {
 *     lock.unlock();
 *   }
 * } else {
 *   // perform alternative actions
 * }}</pre>
 *
 * This usage ensures that the lock is unlocked if it was acquired, and
 * doesn't try to unlock if the lock was not acquired.
 *
 * @return {@code true} if the lock was acquired and
 *         {@code false} otherwise
 */
boolean tryLock();

示例用法:

Lock lock = ...;
if (lock.tryLock()) {
    try {
        // manipulate protected state
    } finally {
        lock.unlock();
    }
} else {
    // perform alternative actions
}

Try Lock Timeout

在给定的时间内获得锁。如果锁可以被获得，那么立刻获得锁，并返回 true；如果锁暂时不可获得，那么线程进入休眠，直到以下事件之一发生:

1. 锁被当前线程获得
2. 其它线程中断当前线程 - InterruptedException 抛出
3. 给定的等待时间超时 - 返回 false

/**
 * Acquires the lock if it is free within the given waiting time and the
 * current thread has not been {@linkplain Thread#interrupt interrupted}.
 *
 * <p>If the lock is available this method returns immediately
 * with the value {@code true}.
 * 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 lock is acquired then the value {@code true} is returned.
 *
 * <p>If the current thread:
 * <ul>
 * <li>has its interrupted status set on entry to this method; or
 * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
 * the lock, and interruption of lock acquisition is supported,
 * </ul>
 * then {@link InterruptedException} is thrown and the current thread's
 * interrupted status is cleared.
 *
 * <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.
 *
 * <p><b>Implementation Considerations</b>
 *
 * <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.
 *
 * <p>An implementation can favor responding to an interrupt over normal
 * method return, or reporting a timeout.
 *
 * <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.
 *
 * @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;

Release

释放锁。

/**
 * Releases the lock.
 *
 * <p><b>Implementation Considerations</b>
 *
 * <p>A {@code Lock} implementation will usually impose
 * restrictions on which thread can release a lock (typically only the
 * holder of the lock can release it) and may throw
 * an (unchecked) exception if the restriction is violated.
 * Any restrictions and the exception
 * type must be documented by that {@code Lock} implementation.
 */
void unlock();

Condition

创建一个 condition 条件变量，用于实现 await() / notify()。

/**
 * Returns a new {@link Condition} instance that is bound to this
 * {@code Lock} instance.
 *
 * <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.
 *
 * <p><b>Implementation Considerations</b>
 *
 * <p>The exact operation of the {@link Condition} instance depends on
 * the {@code Lock} implementation and must be documented by that
 * implementation.
 *
 * @return A new {@link Condition} instance for this {@code Lock} instance
 * @throws UnsupportedOperationException if this {@code Lock}
 *         implementation does not support conditions
 */
Condition newCondition();

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