Synchronized 和 Lock的区别，以及使用Lock的好处

Synchronized 和 Lock 在java并发编程中广泛使用，简单的来说下这两者的区别，记录下

一，两者的构造

　　synchronized是关键字，它是属于JVM层面的

　　Lock是一个具体的类，它是属于API层面的锁  (java.util.concurrent.locks.Lock)

synchronized底层是通过monitor对象来完成

二，使用方法

　　synchronized 不需要用户手动去释放锁，当synchronized代码执行完后系统会自动让线程释放对锁的占用

　　ReentrantLock则需要用户去手动释放锁，若没有主动释放锁，可能会导致死锁现象。需要lock()和unlock()方法配合try/finally语句块来完成

三，等待是否可以中断

　　synchronized不可中断，除非抛出异常或者正常运行完成

　　ReentrantLock 可中断：

　　　　1），设置超时方法tryLock(long timeout,TimeUnit unit)

　　　　2），lockInterruptibly()放代码块中，调用interrupt()方法可中断

推荐查看此博客：https://www.cnblogs.com/dolphin0520/p/3923167.html

四，加锁是否公平

　　synchronized 非公平锁

　　ReentrantLock两者都可以，默认非公平锁，构造方法可以传入boolean值，true为公平锁，false为非公平锁 

　　

五，锁绑定多个条件Condition

　　synchronized 没有

　　ReentrantLock用来实现分组唤醒需要唤醒的线程们，可以精确唤醒，而不是像synchronized要么随机唤醒一个要么唤醒全部线程。

 Lock的好处，在第五点看来绑定多个Condition  可以精确唤醒，用一下列子来看看：

/**
 * 
 * 编写一个线程，开启三个线程，这三个线程的ID分别为A,B,C,每个线程将自己的ID在屏幕上打印10遍，要求输出结果必须按顺序显示。
 * 如：ABCABCABC... 依次递归
 *
 */
public class TestABCAlternate {
    
    public static void main(String[] args) {
        AlternateDemo ad = new AlternateDemo();
        
        new Thread(new Runnable() {
            public void run() {
                
                for (int i = 1; i <= 20; i++) {
                    ad.loopA(i);
                }
                
            }
        },"A").start();
        
        new Thread(new Runnable() {
            public void run() {
                
                for (int i = 1; i <= 20; i++) {
                    ad.loopB(i);
                }
                
            }
        },"B").start();
        
        new Thread(new Runnable() {
            public void run() {
                
                for (int i = 1; i <= 20; i++) {
                    ad.loopC(i);
                    
                    System.out.println("===========================");
                }
            }
        },"C").start();
    }
}

class AlternateDemo{
    private int number = 1;//当前正在执行线程的标记
    
    private Lock lock = new ReentrantLock();
    
    private Condition condition1 = lock.newCondition();  //多个condition 精确唤醒
    private Condition condition2 = lock.newCondition();
    private Condition condition3 = lock.newCondition();
    /**
     * 
     * @param totalLoop : 循环第几轮
     */
    public void loopA(int totalLoop) {
        lock.lock();
        try {
            //1.判断
            while(number!=1) {    //使用while  不使用if  是防止虚假唤醒
                condition1.await();
            }
            //2.打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);
            }
            //3.唤醒
            number = 2;
            condition2.signal();
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            lock.unlock();
        }
    }
    public void loopB(int totalLoop) {
        lock.lock();
        try {
            //1.判断
            while(number!=2) {
                condition2.await();
            }
            //2.打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);
            }
            //3.唤醒
            number = 3;
            condition3.signal();
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            lock.unlock();
        }
    }
    public void loopC(int totalLoop) {
        lock.lock();
        try {
            //1.判断
            while(number!=3) {
                condition3.await();
            }
            //2.打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);
            }
            //3.唤醒
            number = 1;
            condition1.signal();
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            lock.unlock();
        }
    }
}

/** *  * 编写一个线程，开启三个线程，这三个线程的ID分别为A,B,C,每个线程将自己的ID在屏幕上打印10遍，要求输出结果必须按顺序显示。 * 如：ABCABCABC... 依次递归 * */public class TestABCAlternate {public static void main(String[] args) {AlternateDemo ad = new AlternateDemo();new Thread(new Runnable() {public void run() {for (int i = 1; i <= 20; i++) {ad.loopA(i);}}},"A").start();new Thread(new Runnable() {public void run() {for (int i = 1; i <= 20; i++) {ad.loopB(i);}}},"B").start();new Thread(new Runnable() {public void run() {for (int i = 1; i <= 20; i++) {ad.loopC(i);System.out.println("===========================");}}},"C").start();}}
class AlternateDemo{private int number = 1;//当前正在执行线程的标记private Lock lock = new ReentrantLock();private Condition condition1 = lock.newCondition();private Condition condition2 = lock.newCondition();private Condition condition3 = lock.newCondition();/** *  * @param totalLoop : 循环第几轮 */public void loopA(int totalLoop) {lock.lock();try {//1.判断while(number!=1) {condition1.await();}//2.打印for (int i = 1; i <= 1; i++) {System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);}//3.唤醒number = 2;condition2.signal();} catch (Exception e) {// TODO: handle exception} finally {lock.unlock();}}public void loopB(int totalLoop) {lock.lock();try {//1.判断while(number!=2) {condition2.await();}//2.打印for (int i = 1; i <= 1; i++) {System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);}//3.唤醒number = 3;condition3.signal();} catch (Exception e) {// TODO: handle exception} finally {lock.unlock();}}public void loopC(int totalLoop) {lock.lock();try {//1.判断while(number!=3) {condition3.await();}//2.打印for (int i = 1; i <= 1; i++) {System.out.println(Thread.currentThread().getName()+"\t"+i+"\t"+totalLoop);}//3.唤醒number = 1;condition1.signal();} catch (Exception e) {// TODO: handle exception} finally {lock.unlock();}}}