Java 锁、线程执行顺序

Java 锁、线程执行顺序

• 锁是什么 ？
• 锁的什么？
• 怎么加锁 ？
• 什么时候加锁 ？
• 怎么实现高效率加锁 ？

锁是什么？

• java中的锁是一种线程同步机制，控制对共享资源的访问，来达到数据安全的一种工具。
• 本质是依赖于底层的 *** 作系统的 Mutex Lock（互斥锁）来实现。

怎么加锁 ？

• synchronized 关键字、Lock 锁

什么时候加锁 ？

• 线程是一个单独的资源类
• 多线程则是，多个线程 争抢同一共享资源
• 在资源竞争中，就会存在线程安全的问题，需要加锁控制

加锁线程排队-》基本表现
同步前-》同步后-》

public class SaleTicketDemo {

    public static void main(String[] args) {
        Ticket ticket = new Ticket();
        new Thread(()->{
            for (int i = 0; i < 30; i++) {
                ticket.sale();
            }
        },"A").start();
        new Thread(()->{
            for (int i = 0; i < 30; i++) {
                ticket.sale();
            }
        },"B").start();
        new Thread(()->{
            for (int i = 0; i < 30; i++) {
                ticket.sale();
            }
        },"C").start();
    }

}

//资源类
class Ticket{

    private int number=30;

    //synchronized 排队，锁 - 对象，class
    public synchronized void sale(){
        if (number>0){
            System.out.println(Thread.currentThread().getName()+"卖出了"+number--+"票，剩余"+number);
        }
    }

}

锁的是什么？

• 普通同步方法，锁的是方法的调用者，对象的实例,该资源类对象 P p1 = new P(); P p2 = new P(); p1与p2 方法调用时，锁的就不一样。
• public synchronized void m1(){}
• 静态同步方法，锁的是 资源类 对象.Class 类模板
• public static synchronized void m1(){}

Lock 加锁

	private Lock lock = new ReentrantLock();

    public  void m1(){
        //加锁
        lock.lock();
        try {
            //业务代码 ...
        }catch (Exception e){
            e.printStackTrace();
        }finally {
        	//解锁
            lock.unlock();
        }
    }

• Synchronized 适合少量代码同步，Lock适合锁大量的同步代码
• Synchronized 内置的java关键字，Lock是一个java 类
• Synchronized 无法判断获取锁的状态，Lock 可以判断是否获取到了锁
• Synchronized 会自动释放锁，lock 必须要手动释放锁! 如果不释放，可能出现死锁
• Synchronized 没有获取锁超时，会一直等待，Lock可以
• Synchronized 可重入锁，不可中断，非公平；Lock 可重入锁，可以判断锁，非公平（可设置）

线程执行顺序控制

• 1.Condition 精确控制通知唤醒
• 2.join() 方法线程插队

public class C {

    public static void main(String[] args) {
        Data3 data = new Data3();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printA();
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printB();
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printC();
            }
        },"C").start();
    }
}

class Data3{

    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  3C

    public void printA(){
        lock.lock();
        try {
            //业务， 判断 -》 执行 -》通知
            while (number!=1){
                //等待
                condition1.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>AAAAAAA");
            //唤醒指定线程, B
            //唤醒B
            number =2;
            condition2.signal();
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }

    public void printB(){
        lock.lock();
        try {
            while (number!=2){
                condition2.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>BBBBBBBBB");
            number =3;
            condition3.signal();
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }

    public void printC(){
        lock.lock();
        try {
            while (number!=3){
                condition3.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>CCCCCCCCC");
            number =1;
            condition1.signal();
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
}

		Thread thread = new Thread(threadOrder::methodA1);
        thread.start();
        thread.join();
        Thread thread1 = new Thread(threadOrder::methodB1);
        thread1.start();
        thread1.join();

线程近乎同时执行

• CountDownLatch、CyclicBarrier
• 并行流.parallelStream()

public class CountDownLatchTest implements Runnable{

    private CountDownLatch countDownLatch;

    public CountDownLatchTest(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            countDownLatch.countDown();
            System.out.println(Thread.currentThread().getName() + "启动" + System.currentTimeMillis());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        int count = Runtime.getRuntime().availableProcessors();
        for (int i = 0; i < count ; i++) {
            new Thread(new CountDownLatchTest(count)).start();
        }
    }
}

public class CountDownLatchTest implements Runnable{

    private CyclicBarrier cyclicBarrier;

    public CountDownLatchTest(CyclicBarrier cyclicBarrier) {
        this.cyclicBarrier = cyclicBarrier;
    }

    @Override
    public void run() {
        try {
            cyclicBarrier.await();
            System.out.println(Thread.currentThread().getName() + "启动" + System.currentTimeMillis());
        } catch (InterruptedException | BrokenBarrierException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        int count = Runtime.getRuntime().availableProcessors();
        CyclicBarrier cyclicBarrier = new CyclicBarrier(count);
        for (int i = 0; i < count; i++) {
            new Thread(new CountDownLatchTest(cyclicBarrier)).start();
        }
    }
}

public static void main(String[] args) {
        List threads = new ArrayList<>();
        for (int i = 0; i < 50; i++) {
            int finalI = i;
            threads.add(new Thread(()->{System.out.println("线程"+ finalI +"执行: "+ LocalDateTime.now());}));
        }
        threads.parallelStream().forEach(e->{
            new Thread(e).start();
        });
    }