13.显式锁
/**
* {@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
* "hand-over-hand" or "chain locking": 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/se8/html/jls-17.html#jls-17.4">
* Chapter 17 of
* <cite>The Java™ Language Specification</cite></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
*/
public interface Lock {
/**
* 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();
/**
* 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;
/**
* 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();
/**
* 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;
/**
* 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();
/**
* 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();
}
与内置加锁机制不同的是，Lock提供了一种无条件的、可轮询的、定时的以及可中断的锁获取操作，所有加锁和解锁的方式都是显式的。在Lock的实现中必须提供与内置锁相同的内存可见性语义，但在加锁语义、调度算法、顺序保证以及性能特性等方面可以有所不同。
在大多数情况下，内置锁都能很好地工作，但在功能上存在一些局限性，例如，无法中断一个正在等待获取锁的线程，或者无法在请求获取一个锁时无限地等待下去。内置锁必须在获取该锁的代码块中释放，这就简化了编码工作，并且与异常处理操作实现了很好的交互，但却无法实现非阻塞结构的加锁规则。
Lock lock = new ReentrantLock();
...
lock.lock();
try {
// 更新对象状态
// 捕获异常，并在必要时恢复不变性条件
} finally {
lock.unlock();
}
必须在finally块中释放锁，否则，如果在被保护的代码中抛出了异常，那么这个锁永远都无法释放。
可定时的与可轮询的锁获取模式是由tryLock方法实现的，与无条件的锁获取模式相比，它具有更完善的错误恢复机制。在内置锁中，死锁是一个严重的问题，恢复程序的唯一方法是重新启动程序，而防止死锁的唯一方法就是在构造程序时避免出现不一致的锁顺序。可定时的与可轮询的锁提供了另一种选择：避免死锁的发生。
在实现具有时间限制的操作时，定时锁同样非常有用。当在带有时间限制的操作中调用了一个阻塞方法时，它能根据剩余时间来提供一个时限。如果操作不能在指定时间内给出结果，那么就会使程序提前结束。当使用内置锁时，在开始请求锁后，这个操作将无法取消，因此内置锁很难实现带有时间限制的操作。
正如定时的锁获取操作能在带有时间限制的操作中使用独占锁，可中断的锁获取操作同样能在可取消的操作中使用加锁。lockInterruptibly方法能够在获得锁的同时保持对中断的响应。
在ReentrantLock的构造函数中提供了两种公平性选择：创建一个非公平的锁（默认）或者一个公平的锁。在公平的锁上，线程将按照它们发出请求的顺序来获得锁，但在非公平的锁上，则允许插队：当一个线程请求非公平的锁时，如果在发出请求的同时该锁的状态变为可用，那么这个线程将跳过队列中所欲的等待线程并获得这个锁。非公平的ReentrantLock并不提倡插队行为，但无法防止某个线程在合适的时候进行插队。在公平的锁中，如果有另一个线程持有这个锁或者有其他线程在队列中等待这个锁，那么新发出请求的线程将被放入队列中。在非公平的锁中，只有当锁被某个线程持有时，新发出请求的线程才会被放入队列中。
当持有锁的时间相对较长，或者请求锁的平均时间间隔较长，那么应该使用公平锁。在这种情况下，插队带来的吞吐量提升（当锁处于可用状态时，线程却还处于被唤醒的过程中）则可能不会出现。
与默认的ReentrantLock一样，内置锁并不会提供确定的公平性保证，但在大多数情况下，在锁实现上实现统计上的公平性保证已经足够了。Java语言规范并没有要求JVM以公平的方式实现内置锁，而在各种JVM中也没有这样做。
ReentrantLock在加锁和内存上提供的语义域内置锁相同，此外它还提供了一些其他功能，包括定时的锁等待、可中断的锁等待、公平性，以及实现非块结构的加锁。ReentrantLock在性能上似乎优于内置锁，其中在Java6中略有胜出，在Java5中则远远胜出。ReentrantLock的危险性比同步机制要高，如果忘记在finally块中调用unlock，那么虽然代码表面上能正常运行，但实际上已经埋下了一颗定时炸弹，并很有可能伤及其他代码。仅当内置锁不能满足需求时，才可以考虑使用ReentrantLock。
在一些内置锁无法满足需求的情况下，ReentrantLock可以作为一种高级工具。当需要一些高级功能时才应该使用ReentrantLock，这些功能包括：可定时的、可轮询的与可中断的锁获取操作，公平队列，以及非块结构的锁。否则，还是应该优先使用synchronized。
ReentrantLock实现了一种标准的互斥锁：每次最多只有一个线程能持有ReentrantLock。互斥是一种保守的加锁策略，虽然可以避免“写/写”冲突和“读/写”冲突，但同样也避免了“读/读”冲突。
读写锁：一个资源可以被多个读操作访问，或者被一个写操作访问，但两者不能同时进行。
ReentrantReadWriteLock为这两种锁都提供了可重入的加锁语义。与ReentrantLock类似，ReentrantReadWriteLock在构造时也可以选择是一个非公平的锁（默认）还是一个公平的锁。在公平的锁中，等待时间最长的线程将优先获取锁。如果这个锁由读线程持有，而另一个线程请求写入锁，那么其他读线程都不能获得读取锁，直到写线程使用完并且释放了写入锁。在非公平的锁中，线程获得访问许可的顺序是不确定的。写线程降级为读线程是可以的，但从读线程升级为写线程则是不可以的（这样做会导致死锁）。
读写锁允许多个读线程并发地访问被保护的对象，当访问以读取操作为主的数据结构时，它能提高程序的可伸缩性。