Java并发19:Lock系列-Lock接口基本方法学习实例
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本章主要通过解读Lock接口的源码，来学习Lock接口定义的方法的使用。
1.源码注释
Lock接口，定义了如下方法：
/**
* Lock接口
*
@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.
*/
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.
*/
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.
*/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.
*/
Condition newCondition();
}
2.源码解读
通过解读上面的源码，将Lock接口提供的方法总结如下：
1. void lock();
- 尝试去获得锁。
- 如果锁不可用，则为了线程调度的目的，当前线程会变得不可用，直到获得锁为止。
2.void lockInterruptibly() throws InterruptedException;
- 尝试去获得锁，除非当前线程被中断。
- 如果锁不可用，则为了线程调度的目的，当前线程会变得不可用，直到出现以下两种情况之一：
- 当前线程获取锁。
- 其他的线程中断了这个线程。
- 如果当前线程在获取锁操作中，被其他线程中断，则会抛出InterruptedException异常，并且将中断标识清除。
3.boolean tryLock();
- 只有在调用时锁是空闲的，才获取锁。
- 如果锁空闲，则获取锁，并立即返回true值。
- 如果锁不可用，则立即返回false值。
- tryLock()方法的典型使用方式如下：
Lock lock = ...; if (lock.tryLock()) {//如果锁空闲，就获取锁try { // manipulate protected state } finally { lock.unlock();//获取了锁就要记得释放 } } else {//没获取锁，那就干点别的 // perform alternative actions }
- 上述方式能够保证：如果获取了锁能够释放锁；如果没获取锁，也不会去尝试释放锁。
4.boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
- 如果在限定时间内，锁可用并且当前线程不被中断，则获取锁。
- 如果锁空闲，则获取锁，并立即返回true值。
- 如果锁不可用，则为了线程调度的目的，当前线程会变得不可用，直到出现以下三种情况之一：
- 当前线程获取锁。
- 其他的线程中断了这个线程。
- 限定时间超时。
- 针对上面三种情况，当前方法会分别作出以下操作：
- 如果获得锁，则即返回true值。
- 如果当前线程在获取锁操作中，被其他线程中断，则会抛出InterruptedException异常，并且将中断标识清除。
- 如果限定时间超时，则即返回false值。
5.void unlock();
- 解锁，或者叫释放锁。
- 通常这样释放锁：
原文：https://www.cnblogs.com/yaochunhui/p/15180346.html
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