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之前笔者解析了AQS的源码，在JUC中有很多锁是基于AQS实现的，今天想写个简单的ReentrantLock实现，代码也基本是在看了ReentrantLock源码后写出来的，做个笔记。
总结一下AQS的原理，就是使用一个int类型来表示可申请的锁资源，提供了一系列的原子操作，以及用于放置申请锁的线程的等待队列。实际上定义了一整套完整的多线程访问共享资源的同步框架，具体的解析可以看我的另一篇文章
ReentrantLock非公平锁的实现
首先来看ReentrantLock的非公平锁实现，它的类定义：
public class UnfairReentrantLockImpl extends AbstractQueuedSynchronizer implements Lock, Serializable
非公平锁实现了Lock接口，因此需要实现以下的方法：
基本上只要了解接口的语义，基于AQS提供的接口，可以快速实现Lock的接口功能，代码如下，可以看到AQS提供的是线程同步部分的实现：
public void lock() {//cas的方式修改state值，如果成功，即获得锁if (compareAndSetState(0, 1)) {setExclusiveOwnerThread(Thread.currentThread());} else {//否则加入等待队列，自旋方式申请锁acquire(1);}}@Overridepublic void unlock() {//释放锁release(1);}@Overridepublic boolean tryLock(long time, TimeUnit unit) throws InterruptedException {//阻塞方式申请锁，指定时间后无论是否申请成功都返回return tryAcquireNanos(1, unit.toNanos(time));}@Overridepublic boolean tryLock() {//非阻塞方式申请锁return nofairTryAcquire(1);}@Overridepublic void lockInterruptibly() throws InterruptedException {//可中断式的申请锁acquireInterruptibly(1);}@Overridepublic Condition newCondition() {return new ConditionObject();}//尝试获取锁private final boolean nofairTryAcquire(int acquires) {final Thread current = Thread.currentThread();int c = getState();if (c == 0) {if (compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current);return true;}} else if (current == getExclusiveOwnerThread()) {//如果当前线程已经持有锁，只修改一下state的值即返回，//体现了可重入锁的特点int nextC = c + acquires;if (nextC < 0) {throw new Error("Maximum lock lcount exceeded");}setState(nextC);return true;}return false;}
ReentrantLock锁继承AQS，必须要实现AQS的两个方法：tryAcquire和tryRelease，这两个方法在AQS只提供了一个抛出异常的实现。tryAcquire的语义是非阻塞式的申请锁，而tryRelease的语义是释放锁，并恢复state的值。在AQS内部，会使用这两个方法构造整个同步器的实现。
//AbstractQueuedSynchronizer定义的方法，该方法在AbstractQueuedSynchronizer中只有一个抛出异常的默认实现@Overrideprotected boolean tryAcquire(int arg) {final Thread current = Thread.currentThread();int c = getState();if (c == 0) {if (compareAndSetState(0, arg)) {setExclusiveOwnerThread(current);return true;}} else if (current == getExclusiveOwnerThread()) {int nextC = c + arg;if (nextC < 0) {throw new Error("Maximum lock lcount exceeded");}setState(nextC);return true;}return false;}//AbstractQueuedSynchronizer定义的方法，该方法在AbstractQueuedSynchronizer中只有一个抛出异常的默认实现@Overrideprotected boolean tryRelease(int arg) {int c = getState() - arg;if (Thread.currentThread() != getExclusiveOwnerThread())throw new IllegalMonitorStateException();boolean free = false;if (c == 0) {free = true;setExclusiveOwnerThread(null);}setState(c);return free;}
ReentrantLock公平锁的实现
ReentrantLock公平锁强调根据线程等待时间长短来分配锁，等待时间最长的获取锁，在实现上，会让等待队列头部的线程获得锁。
公平锁的代码实现与非公平锁基本一致，主要区别在以下两个方法上，公平锁的lock方法会将申请锁的线程直接加入等待队列，根据等待时间排序来决定获取锁的线程。公平锁的tryAcquire方法在判断一个线程能否获得锁时，会比非公平锁多加入一个请求线程是否为头线程的判断，只有头线程能获得线程。读者可以与非公平锁的这两个方法实现进行对比，即可清楚比较出两者的区别
@Overridepublic void lock() {//与非公平锁的区别：直接将当前申请锁请求加入队列acquire(1);}@Overrideprotected boolean tryAcquire(int arg) {final Thread current = Thread.currentThread();int c = getState();if (c == 0) {//只有在当前线程为头结点时，才能去获得锁if (!hasQueuedPredecessors() && compareAndSetState(0, arg)) {setExclusiveOwnerThread(current);return true;}} else if (current == getExclusiveOwnerThread()) {int nextC = c + arg;if (nextC < 0) {throw new Error("Maximum lock count exceeded");}setState(nextC);return true;}return false;}
ReentrantLock的测试
最后，给出一个简单的测试类，这个测试类创建了10个银行账户，初始金额都为1000元，然后有30个管理线程，会随机挑选两个账户，转账一个10以内的随机金额，每个线程独立转账10次。完成所有的转账操作后，计算所有账户总金额，如果总金额与原来一致，证明整个转账过程没有同步错误。
public class LockTest {private static Account[] accounts = new Account[10];private static AccountManager[] threads = new AccountManager[30];public static void main(String[] args) throws Exception {for (int i = 0; i < 10; i++) {accounts[i] = new Account(1000);}double sum = 0;for (int i = 0; i < 10; i++) {sum += accounts[i].getMoney();}System.out.println("src sum：" + sum);for (int i = 0; i < 30; i++) {threads[i] = new AccountManager();threads[i].start();}for (int i = 0; i < 30; i++) {threads[i].join();}sum = 0;for (int i = 0; i < 10; i++) {sum += accounts[i].getMoney();}System.out.println("after operation sum：" + sum);for (int i = 0; i < accounts.length; i++) {System.out.println("acount-" + i + " res money: " + accounts[i].getMoney());}}private static class AccountManager extends Thread {@Overridepublic void run() {int index = 10;int fromAccount = 0, toAccount = 0;while (index > 0) {fromAccount = (int) (Math.random() * 10);toAccount = (int) (Math.random() * 10);if (toAccount == fromAccount) {toAccount = (toAccount + 1) % 10;}try {AccountMgr.transfer(accounts[fromAccount], accounts[toAccount], Math.random() * 10);} catch (Exception e) {e.printStackTrace();}index--;System.out.println(Thread.currentThread() + " complete " + (10 - index) + " transformation from " + fromAccount + " to " + toAccount);}}}
}public class Account {private Lock lock = new FairReentrantLockImpl();private volatile double money;public Account(final double money) {this.money = money;}public void add(double money) {lock.lock();try {this.money += money;} finally {lock.unlock();}}public void reduce(double money) {lock.lock();try {this.money -= money;} finally {lock.unlock();}}public double getMoney() {return money;}void lock() {lock.lock();}void unLock() {lock.unlock();}boolean tryLock() {return lock.tryLock();}
}
public class AccountMgr {public static boolean tryTransfer(Account from, Account to, Double money) throws NoEnoughMoneyException {if (from.tryLock()) {try {if (to.tryLock()) {try {if (from.getMoney() >= money) {from.reduce(money);to.add(money);} else {System.out.println("operation failed ");
// throw new NoEnoughMoneyException();}return true;} finally {to.unLock();}}} finally {from.unLock();}}return false;}public static void transfer(Account from, Account to, Double money) throws NoEnoughMoneyException {boolean success = false;do {success = tryTransfer(from, to, money);if (!success)Thread.yield();} while (!success);}public static class NoEnoughMoneyException extends Exception {}
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