【Java开发】JUC进阶 01：Lock锁详解

1 Lock锁介绍

已经在【JUC基础】04简单介绍过了，本文做进一步的拓展，比如公平锁和非公平锁、

📌 明白锁的核心

• 四个对象：线程，共享资源，锁，锁操作

• 包括线程如何操作资源，使用锁锁哪个资源，锁让谁等待，谁唤醒，这是我们在加锁时需要考虑的

📌 synchronized 与Lock区别

• synchronized是一个关键字，lock是一个类

• synchronized自动释放锁，lock需要手动释放

• synchronized线程1获得了锁进入阻塞，线程2会死等。lock不会

• synchronized非公平，lock可以使用公平锁

• synchronized适合锁少量代码同步问题，lock适合锁大量同步代码

📌 Lock重点

Lock接口的实现类均需要主动加锁和解锁：

主要有三个实现类，ReentrantLock最常用：

2 公平锁和非公平锁

📌 要点

• 公平锁：必须先来后到，十分公平

• 非公平锁：允许插队（默认-为了效率）

以下是ReentrantLock的构造方式👇（synchronized默认也是非公平锁）

📌 代码举例

首先在主线程中启动一个T线程，给他上锁，休眠2秒（在它释放锁之前，启动一个T1线程，T1线程中创建10个B线程，因为T已经上锁了，那么后边的A线程就必须等T1线程启动后才能获取锁，但是10个B线程已经排在了10个B线程后边），在主线程中再启动10个A线程获取锁，此时可观察公平锁和非公平锁的具体情况。

• 公平锁：B线程一直再A线程后边

• 非公平锁：B线程可能会插队到A线程前边

public class testLock {public static void main(String[] args) throws InterruptedException {test1(false);//非公平锁
//        test1(true);//公平锁}public static void test1(boolean fair) throws InterruptedException {ReentrantLock lock = new ReentrantLock(fair);new Thread(() -> {lock.lock();try {System.out.println("start");try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {e.printStackTrace();}new Thread(() -> {create10T(lock, "B");},"T1").start();System.out.println("end");} finally {lock.unlock();}},"T").start();create10T(lock, "A");}public static void create10T(ReentrantLock lock, String threadPre) {for (int i = 0; i < 10; i++) {Thread thread = new Thread(() -> {lock.lock();try {System.out.println(Thread.currentThread().getName()+"获取到锁!");} finally {lock.unlock();}});thread.setName(threadPre + "-" + i);thread.start();}}
}

非公平锁：

公平锁：

3 Lock版的生产者消费者问题

📌 Condition 通知和唤醒线程

• Condition是个接口，基本的方法就是await()和signal()方法；

• Conditon中的await()对应Object的wait()；

• Condition中的signal()对应Object的notify()；

• Condition中的signalAll()对应Object的notifyAll()。

核心：锁的condition执行await，就是让该线程携带对应的condition进入等待队列，当condition执行signal就是让携带该condition的线程唤醒，使用于lock与unlock之间。

3.1 Lock处理生产者消费者问题

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class LockPC {public static void main(String[] args) {Data data = new Data();new Thread(()->{for (int i = 0; i < 5; i++) {try {data.plus();} catch (InterruptedException e) {e.printStackTrace();}}},"A").start();new Thread(()->{for (int i = 0; i < 5; i++) {try {data.minus();} catch (InterruptedException e) {e.printStackTrace();}}},"B").start();new Thread(()->{for (int i = 0; i < 5; i++) {try {data.plus();} catch (InterruptedException e) {e.printStackTrace();}}},"C").start();new Thread(()->{for (int i = 0; i < 5; i++) {try {data.minus();} catch (InterruptedException e) {e.printStackTrace();}}},"D").start();}
}class Data{//资源类private int number = 0;Lock lock = new ReentrantLock();Condition condition = lock.newCondition();//+1public void plus() throws InterruptedException {lock.lock();try {while (number!=0){condition.await();//等待}number++;System.out.println(Thread.currentThread().getName()+"=>"+number);condition.signalAll();//通知其他线程，+1结束} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}//-1public void minus() throws InterruptedException {lock.lock();try {while (number==0){condition.await();//等待}number--;System.out.println(Thread.currentThread().getName()+"=>"+number);condition.signalAll();//通知其他线程，-1结束} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}
}

控制台输出：

3.2 Condition 精准通知和唤醒线程

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;//A执行完调用B，B执行完调用C，C执行完调用A
public class LockCondition {public static void main(String[] args) {Data2 data = new Data2();new Thread(()->{for (int i = 0; i < 5; i++) {data.pringtA();}},"A").start();new Thread(()->{for (int i = 0; i < 5; i++) {data.pringtB();}},"B").start();new Thread(()->{for (int i = 0; i < 5; i++) {data.pringtC();}},"C").start();}
}class Data2{private Lock lock= new ReentrantLock();private Condition condition1 = lock.newCondition();private Condition condition2 = lock.newCondition();private Condition condition3 = lock.newCondition();private int number = 1; //1A、2B、3Cpublic void pringtA(){lock.lock();try { //业务，判断->执行->通知while (number!=1){condition1.await();}System.out.println(Thread.currentThread().getName()+"=>"+number);//唤醒指定的Bnumber = 2;condition2.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}public void pringtB(){lock.lock();try {while (number!=2){condition2.await();}System.out.println(Thread.currentThread().getName()+"=>"+number);//唤醒指定的Bnumber = 3;condition3.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}public void pringtC(){lock.lock();try {while (number!=3){condition3.await();}System.out.println(Thread.currentThread().getName()+"=>"+number);//唤醒指定的Bnumber = 1;condition1.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}
}

控制台输出：