ReadWriteLock是继承ReantrantReadWriteLock的类，这就声明了一个重入读写锁，有点类似于ReentrantLock.
你可以通过以下的方法来初始化ReentrantReadWriteLock的实例。
ReentrantReadWriteLock():创建一个ReentrantReadWriteLock的实例。这个构造器类似于ReenReadWriteLock(false)。 ReentrantReadWritelock(booleanfair):创建一个ReentrantReadWriteLock的实例，并且明确指定的公平策略。当锁需要的序地执行时，需要设置为true。
Note 对于公平的有序策略，当前持有的锁被释放，或长时间等待的独有写的线程会被注册写的锁；或一组读的线程等待的时间比写的线程来的长，那个这个组将会注册读的锁。
当一个写的锁被持有或这里已经有等待写的锁，那么一个试图请求公平的读的锁（不是重入的）会处于阻塞状态。这个线程将不会请求读的锁，直到最老的等待写的线程释放写的锁。如果写的线程无边界地在等待，将一个或多个读的线程作为长时间等待的线程在队列中，这些读的线程将会被注册为读的锁。
一个线程尝试去公平的写的锁将会处于阻塞状态，除非读和写的锁都是自由的。（非阻塞的tryLock()方法不会遵守这个公平的设置，如果有可能的话它会更快速的请求锁，而不管等待线程情况如何。）
你可以用下面的方法来实例化这个类：
ReentrantReadWriteLock.ReadLockreadLock()：返回读的锁应用。 ReentrantReadWriteLock.WriteLockwriteLock():返回写的锁的应用。每一个嵌套ReadLock和WriteLock的类都继承了Lock的接口和声明自己的方法。然而，ReentrantReadWriteLock声明了中添加了如下的方法： int getReadHoldCount()：重入读请求线程而持有的锁的数量将会返回，当请求线程中并没有持有锁将返回0。一个读的线程会持有每一个锁的操作，而不是非锁的操作。 int getWriteHoldCount()：重入写请求线程，持有的锁的数量将会返回，当请求线程中并没有持有锁将返回0。一个写的线程会持有每一个锁的操作，而不是非锁的操作。 为了验证ReadWriteLock和ReentrantReadWriteLock，例子7-3实现了一个应用程序，写的线程会填充单词的定义条目，读的线程会不断的访问并随机输出。
package com.owen.thread.chapter7;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class Dictionary
{
public static void main(String[] args)
{
final String[] words = { "hypocalcemia", "prolixity", "assiduous",
"indefatigable", "castellan" };
final String[] definitions = { "a deficiency of calcium in the blood",
"unduly prolonged or drawn out",
"showing great care, attention, and effort",
"able to work or continue for a lengthy time without tiring",
"the govenor or warden of a castle or fort" };
final Map dictionary = new HashMap();
ReadWriteLock rwl = new ReentrantReadWriteLock(true);
final Lock rlock = rwl.readLock();
final Lock wlock = rwl.writeLock();
Runnable writer = () -> {
for (int i = 0; i < words.length; i++)
{
wlock.lock();
try
{
dictionary.put(words[i], definitions[i]);
System.out.println("writer storing " + words[i] + " entry");
} finally
{
wlock.unlock();
}
try
{
Thread.sleep(1);
} catch (InterruptedException ie)
{
System.err.println("writer " + "interrupted");
}
}
};
ExecutorService es = Executors.newFixedThreadPool(1);
es.submit(writer);
Runnable reader = () -> {
while (true)
{
rlock.lock();
try
{
int i = (int) (Math.random() * words.length);
System.out.println("reader accessing " + words[i] + ": "
+ dictionary.get(words[i]) + " entry");
} finally
{
rlock.unlock();
}
}
};
es = Executors.newFixedThreadPool(1);
es.submit(reader);
}
}
例子7-3主线程创建单词和解释的数组，它们都声明为final，因为它们将会用于内部类。之后创建一个Map储存单词和解释，它含有重入的读和写的锁。
一个可运行写的线程被创建。它的run()方法遍历单词数组。每一个写的锁将会被遍历。当这个方法返回时，写的线程就会执行写的锁和更新map。执行这些操作主要是通过map的put()方法。当添加信息的添加单词之后，写的线程就会释放，并且休眠一毫秒。执行器包含线程池和用于执行写线程的运用。
一个写的线程随后被创建。它的run()方法重复地去获取读的锁，通过随机的进入map，输出条目，和解锁读锁。执行器包含线程池和用于执行写线程的运用。
执行上面的代码，你可能会得到以下的结果：
reader accessing indefatigable: able to work or continue for a lengthy time without tiring entry reader accessing prolixity: unduly prolonged or drawn out entry reader accessing indefatigable: able to work or continue for a lengthy time without tiring entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing castellan: the govenor or warden of a castle or fort entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing prolixity: unduly prolonged or drawn out entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing prolixity: unduly prolonged or drawn out entry reader accessing castellan: the govenor or warden of a castle or fort entry reader accessing indefatigable: able to work or continue for a lengthy time without tiring entry reader accessing indefatigable: able to work or continue for a lengthy time without tiring entry reader accessing castellan: the govenor or warden of a castle or fort entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing castellan: the govenor or warden of a castle or fort entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing hypocalcemia: a deficiency of calcium in the blood entry reader accessing assiduous: showing great care, attention, and effort entry reader accessing assiduous: showing great care, attention, and effort entry
