在Java多线程中，可以使用synchronized关键字实现线程之间的同步互斥，在jdk1.5后新增的ReentrantLock类同样可达到此效果，且在使用上比synchronized更加灵活。
观察ReentrantLock类可以发现其实现了Lock接口
public class ReentrantLock implements Lock,java.io.Serializable
1、使用ReentrantLock实现同步
lock()方法:上锁
unlock()方法：释放锁
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/*
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* 使用ReentrantLock类实现同步
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* */
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class MyReenrantLock implements Runnable{
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//向上转型
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private Lock lock = new ReentrantLock();
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public void run() {
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//上锁
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lock.lock();
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for(int i = 0; i < 5; i++) {
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System.out.println("当前线程名： "+ Thread.currentThread().getName()+" ,i = "+i);
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}
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//释放锁
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lock.unlock();
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}
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}
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public class MyLock {
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public static void main(String[] args) {
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MyReenrantLock myReenrantLock = new MyReenrantLock();
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Thread thread1 = new Thread(myReenrantLock);
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Thread thread2 = new Thread(myReenrantLock);
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Thread thread3 = new Thread(myReenrantLock);
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thread1.start();
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thread2.start();
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thread3.start();
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}
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}
由此我们可以看出，只有当当前线程打印完毕后，其他的线程才可继续打印，线程打印的数据是分组打印，因为当前线程持有锁，但线程之间的打印顺序是随机的。
即调用lock.lock()代码的线程就持有了“对象监视器”，其他线程只有等待锁被释放再次争抢。
2、使用Condition实现等待/通知
synchronized关键字结合wait()和notify()及notifyAll()方法的使用可以实现线程的等待与通知模式。在使用notify()、notifyAll()方法进行通知时，被通知的线程是JVM随机选择的。
类ReentrantLock类同样可以实现该功能，需要借助Condition对象，可实现“选择性通知”。Condition类是jdk1.5提供的，且在一个Lock对象中可以创建多个Condition(对象监视器)实例。
Condition类的await():是当前执行任务的线程处于等待状态
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/*
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* 错误的使用Condition实现等待、通知
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* */
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class MyCondition implements Runnable{
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private Lock lock = new ReentrantLock();
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public Condition condition = lock.newCondition();
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public void run() {
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try {
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System.out.println("当前线程名:"+Thread.currentThread().getName()+" 开始等待时间："+System.currentTimeMillis());
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//线程等待
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condition.await();
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System.out.println("我陷入了等待...");
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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MyCondition myCondition = new MyCondition();
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Thread thread1 = new Thread(myCondition,"线程1");
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thread1.start();
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}
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}
观察运行结果可以发现，报出监视器出错的异常，解决的办法是我们必须在condition.await()方法调用前用lock.lock()代码获得同步监视器。对上述代码做出如下修改：
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/*
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* 使用Condition实现等待
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* */
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class MyCondition implements Runnable{
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private Lock lock = new ReentrantLock();
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public Condition condition = lock.newCondition();
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public void run() {
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try {
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//上锁
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lock.lock();
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System.out.println("当前线程名:"+Thread.currentThread().getName()+" 开始等待时间："+System.currentTimeMillis());
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//线程等待
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condition.await();
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System.out.println("我陷入了等待...");
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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//释放锁
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lock.unlock();
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System.out.println("锁释放了!");
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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MyCondition myCondition = new MyCondition();
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Thread thread1 = new Thread(myCondition,"线程1");
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thread1.start();
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}
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}
在控制台只打印出一句，原因是调用了Condition对象的await()方法，是的当前执行任务的线程进入等待状态。
Condition类的signal():是当前执行任务的线程处于等待状态
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/*
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* 使用Condition实现等待、通知
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* */
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class MyCondition implements Runnable{
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private Lock lock = new ReentrantLock();
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public Condition condition = lock.newCondition();
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public void run() {
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try {
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//上锁
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lock.lock();
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System.out.println(" 开始等待时间："+System.currentTimeMillis());
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System.out.println("我陷入了等待...");
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//线程等待
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condition.await();
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//释放锁
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lock.unlock();
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System.out.println("锁释放了!");
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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//通知方法
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public void signal(){
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try {
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lock.lock();
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System.out.println("结束等待时间："+System.currentTimeMillis());
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//通知等待线程
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condition.signal();
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} finally {
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lock.unlock();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) throws InterruptedException {
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MyCondition myCondition = new MyCondition();
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Thread thread1 = new Thread(myCondition,"线程1");
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thread1.start();
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Thread.sleep(3000);
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myCondition.signal();
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}
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}
观察结果我们成功地实现了等待通知。
可以得知：Object类中的wait()方法等同于Condition类中的await()方法。
Object类中的wait(long timeout)方法等同于Condition类中的await(long time,TimeUnit unit)方法。
Object类中的notify()方法等同于Condition类中的singal()方法。
Object类中的notifyAll()方法等同于Condition类中的singalAll()方法。
3、生产者消费者模式
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/*
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* 生产者、消费者模式
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* 一对一交替打印
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* */
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class MyServer{
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private ReentrantLock lock = new ReentrantLock();
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public Condition condition = lock.newCondition();
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public Boolean flag = false;
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public void set() {
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try {
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lock.lock();
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while(flag == true) {
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condition.await();
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}
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System.out.println("当前线程名："+Thread.currentThread().getName()+" hello");
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flag = true;
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condition.signal();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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lock.unlock();
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}
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}
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public void get() {
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try {
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lock.lock();
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while(flag == false) {
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condition.await();
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}
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System.out.println("当前线程名："+Thread.currentThread().getName()+" lemon");
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flag = false;
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condition.signal();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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lock.unlock();
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}
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}
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}
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class MyCondition1 extends Thread{
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private MyServer myServer;
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public MyCondition1(MyServer myServer) {
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super();
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this.myServer = myServer;
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}
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public void run() {
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for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
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myServer.set();
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}
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}
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}
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class MyCondition2 extends Thread{
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private MyServer myServer;
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public MyCondition2(MyServer myServer) {
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super();
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this.myServer = myServer;
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}
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public void run() {
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for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
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myServer.get();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) throws InterruptedException {
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MyServer myServer = new MyServer();
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MyCondition1 myCondition1 = new MyCondition1(myServer);
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MyCondition2 myCondition2 = new MyCondition2(myServer);
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myCondition1.start();
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myCondition2.start();
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}
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}
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/*
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* 生产者、消费者模式
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* 多对多交替打印
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* */
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class MyServer{
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private ReentrantLock lock = new ReentrantLock();
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public Condition condition = lock.newCondition();
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public Boolean flag = false;
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public void set() {
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try {
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lock.lock();
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while(flag == true) {
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System.out.println("可能会有连续的hello进行打印");
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condition.await();
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}
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System.out.println("当前线程名："+Thread.currentThread().getName()+" hello");
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flag = true;
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condition.signal();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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lock.unlock();
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}
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}
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public void get() {
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try {
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lock.lock();
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while(flag == false) {
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System.out.println("可能会有连续的lemon进行打印");
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condition.await();
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}
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System.out.println("当前线程名："+Thread.currentThread().getName()+" lemon");
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flag = false;
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condition.signal();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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lock.unlock();
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}
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}
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}
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class MyCondition1 extends Thread{
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private MyServer myServer;
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public MyCondition1(MyServer myServer) {
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super();
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this.myServer = myServer;
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}
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public void run() {
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for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
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myServer.set();
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}
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}
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}
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class MyCondition2 extends Thread{
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private MyServer myServer;
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public MyCondition2(MyServer myServer) {
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super();
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this.myServer = myServer;
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}
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public void run() {
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for(int i = 0 ;i < Integer.MAX_VALUE;i++) {
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myServer.get();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) throws InterruptedException {
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MyServer myServer = new MyServer();
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MyCondition1[] myCondition1 = new MyCondition1[10];
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MyCondition2[] myCondition2 = new MyCondition2[10];
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for(int i = 0; i < 10; i++) {
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myCondition1[i] = new MyCondition1(myServer);
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myCondition2[i] = new MyCondition2(myServer);
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myCondition1[i].start();
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myCondition2[i].start();
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}
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}
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}
4、公平锁与非公平锁
锁Lock分为“公平锁”和“非公平锁”。
公平锁：表示线程获取锁的顺序是按照线程加锁的顺序来的进行分配的，即先来先得FIFO先进先出顺序。
非公平锁：一种获取锁的抢占机制，是随机拿到锁的，和公平锁不一样的是先来的不一定先拿到锁，这个方式可能造成某些线程一直拿不到锁，结果就是不公平的·。
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/*
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* 公平锁
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* */
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class MyService{
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private ReentrantLock lock;
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public MyService(boolean isFair) {
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super();
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lock = new ReentrantLock(isFair);
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}
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public void serviceMethod() {
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try {
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lock.lock();
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System.out.println("线程名："+Thread.currentThread().getName()+"获得锁定");
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} finally {
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lock.unlock();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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//设置当前为true公平锁
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final MyService myService = new MyService(true);
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Runnable runnable = new Runnable() {
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public void run() {
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System.out.println("线程名："+Thread.currentThread().getName()+"运行了");
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myService.serviceMethod();
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}
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};
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Thread[] threads = new Thread[10];
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for(int i = 0;i < 10; i++) {
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threads[i] = new Thread(runnable);
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}
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for(int i = 0;i < 10; i++) {
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threads[i].start();
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}
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}
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}
由打印结果可以看出，基本呈现有序的状态，这就是公平锁的特点。
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/*
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* 非公平锁
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* */
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class MyService{
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private ReentrantLock lock;
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public MyService(boolean isFair) {
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super();
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lock = new ReentrantLock(isFair);
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}
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public void serviceMethod() {
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try {
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lock.lock();
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System.out.println("线程名："+Thread.currentThread().getName()+"获得锁定");
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} finally {
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lock.unlock();
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}
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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//设置当前为true公平锁
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final MyService myService = new MyService(false);
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Runnable runnable = new Runnable() {
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public void run() {
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System.out.println("线程名："+Thread.currentThread().getName()+"运行了");
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myService.serviceMethod();
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}
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};
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Thread[] threads = new Thread[10];
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for(int i = 0;i < 10; i++) {
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threads[i] = new Thread(runnable);
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}
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for(int i = 0;i < 10; i++) {
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threads[i].start();
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}
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}
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}
非公平锁的运行结果基本都是无须的，则可以表明先start()启动的线程并不一定先获得锁。
5、使用ReentrantReadWriteLock类
类ReentrantLock具有完全互斥排他的效果，即同一时间只有一个线程在执行ReentrantLock.lock()方法后的任务。这样虽然保证了实例变量的线程安全性，但是效率低下。所以在Java中提供有读写锁ReentrantReadWriteLock类，使其效率可以加快。在某些不需要操作实例变量的方法中，完全可以使用ReentrantReadWriteLock来提升该方法代码运行速度。
读写锁表示两个锁：
读操作相关的锁，也成为共享锁。
写操作相关的锁，也叫排他锁。
多个读锁之间不互斥，读锁与写锁互斥，多个写锁互斥。
在没有线程Thread进行写入操作时，进行读操作的多个Thread可以获取读锁，但是进行写入操作时的Thread只有获取写锁后才能进行写入操作。
（1）多个读锁共享
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/*
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* 多个读锁共享
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* */
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class MyService{
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private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
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public void read() {
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try {
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//读锁
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lock.readLock().lock();
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System.out.println("线程名: "+Thread.currentThread().getName()+"获取读锁" );
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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//释放读锁
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lock.readLock().unlock();
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}
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}
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}
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//线程1
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class Thread1 extends Thread{
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private MyService myService;
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public Thread1(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.read();
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}
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}
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//线程2
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class Thread2 extends Thread{
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private MyService myService;
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public Thread2(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.read();
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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MyService myService = new MyService();
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Thread1 thread1 = new Thread1(myService);
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Thread2 thread2 = new Thread2(myService);
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thread1.start();
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thread2.start();
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}
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}
从打印结果可以看出，两个线程几乎同时进入lock()方法后面的代码。
说明在此时使用lock.readLock()读锁可以提高程序运行效率，允许多个线程同时执行lock()方法后的代码。
（2）多个写锁互斥
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/*
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* 多个写锁互斥
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* */
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class MyService{
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private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
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public void write() {
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try {
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//写锁
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lock.writeLock().lock();
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System.out.println("线程名: "+Thread.currentThread().getName()+"获取写锁，获得时间："+System.currentTimeMillis() );
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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//释放写锁
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lock.writeLock().unlock();
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}
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}
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}
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//线程1
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class Thread1 extends Thread{
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private MyService myService;
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public Thread1(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.write();
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}
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}
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//线程2
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class Thread2 extends Thread{
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private MyService myService;
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public Thread2(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.write();
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}
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}
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public class MyLock{
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public static void main(String[] args) {
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MyService myService = new MyService();
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Thread1 thread1 = new Thread1(myService);
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Thread2 thread2 = new Thread2(myService);
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thread1.start();
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thread2.start();
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}
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}
使用写锁代码writeLock.lock()的效果就是同一时间只允许一个线程执行lock()方法后的代码。
（3）读写/写读互斥
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/*
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* 读写/写读互斥,
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* */
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class MyService{
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private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
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public void read() {
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try {
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//读锁
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lock.readLock().lock();
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System.out.println("线程名: "+Thread.currentThread().getName()+"获取读锁，获得时间:"+System.currentTimeMillis() );
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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//释放读锁
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lock.readLock().unlock();
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}
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}
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public void write() {
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try {
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//写锁
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lock.writeLock().lock();
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System.out.println("线程名: "+Thread.currentThread().getName()+"获取写锁，获得时间："+System.currentTimeMillis() );
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally {
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//释放写锁
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lock.writeLock().unlock();
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}
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}
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}
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//线程1
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class Thread1 extends Thread{
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private MyService myService;
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public Thread1(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.read();
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}
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}
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//线程2
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class Thread2 extends Thread{
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private MyService myService;
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public Thread2(MyService myService) {
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super();
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this.myService = myService;
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}
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public void run() {
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myService.write();
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}
-
}
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public class MyLock{
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public static void main(String[] args) {
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MyService myService = new MyService();
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Thread1 thread1 = new Thread1(myService);
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Thread2 thread2 = new Thread2(myService);
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thread1.start();
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thread2.start();
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}
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}
此运行结果说明“读写/写读”操作是互斥的。
由此可表明：只要出现“写”操作，就是互斥的。