Java马士兵高并发编程视频学习笔记（一）

1.同一个资源，同步和非同步的方法可以同时调用

package com.dingyu;

public class Y {
    public synchronized void m1() {
        System.out.println(Thread.currentThread().getName() + " m1 begin---------");
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " m1 end---------");
    }

    public void m2() {
        System.out.println(Thread.currentThread().getName() + " m2 begin---------");
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " m2 end---------");
    }
    public static void main(String[] args) {
        Y t = new Y();
//        new Thread(()->t.m1(),"t1").start();
//        new Thread(()->t.m2(),"t2").start();
        new Thread(new Runnable() {

            @Override
            public void run() {
                t.m1();

            }
        },"t1").start();;
    new Thread(new Runnable() {

            @Override
            public void run() {
                t.m2();

            }
        },"t2").start();;
    }
}

可以看到t1先执行，如果不能同时调用那么t2是不能执行的，必须等t1结束，释放锁后才能调用，但这里t2确先执行了，所以是可以同时调用的。

2.对业务写代码进行加锁，对读代码不进行加锁，会产生脏读

　　

package com.dingyu;

public class U {
    private String name;
    private double banlance;

    public synchronized void set(String name, double balance) {
        this.name = name;
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.banlance = balance;
    }

    public double getBalance() {
        return banlance;
    }

    public static void main(String[] args) {
        U u = new U();
        new Thread(() -> u.set("zhangsan", 500)).start();
        System.out.println(u.getBalance());
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(u.getBalance());
    }
}

3.同线程内一个同步方法可以去调用另一个同步方法（重入锁 还有一种重入锁就是子类调用父类的同步方法）

package com.dingyu;

public class I {
    public synchronized void m1() {
        System.out.println("m1 start");
        m2();
        System.out.println("m1 end");
    }

    private synchronized void m2() {
        System.out.println("m2 start");
        System.out.println("m2 end");
    }

    public static void main(String[] args) {
        I i = new I();
        new Thread(() -> i.m1()).start();
    }

}

4.模拟一个简单的死锁

package com.dingyu;

public class DeadLock {
    private Object o1 = new Object();
    private Object o2 = new Object();

    public void m1() {
        synchronized (o1) {
            try {
                Thread.sleep(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            synchronized (o2) {
                System.out.println("如果出现这句话表示没有死锁");
            }
        }

    }

    public void m2() {
        synchronized(o2) {

            synchronized (o1) {
                System.out.println("如果出现这句话表示没有死锁");
            }

        }

    }
    public static void main(String[] args) {
        DeadLock deadLock=new DeadLock();
        new Thread(()->deadLock.m1()).start();
        new Thread(()->deadLock.m2()).start();
    }
}

5.如果执行同步方法中出现异常，那么就会自动释放锁，如果不想释放锁，加上try catch

package com.dingyu;

public class ReleaseLock {
    private int count = 0;
    private int i = 0;

    public synchronized void m1() {
        while (true) {
            System.out.println(Thread.currentThread().getName() + " " + count++);
            if (count % 10 == 0)
                i = 1 / 0;
        }
    }

    public static void main(String[] args) {
        ReleaseLock releaseLock = new ReleaseLock();
        new Thread(() -> releaseLock.m1(), "t1").start();
        new Thread(() -> releaseLock.m1(), "t2").start();
    }
}

6.volatile关键字（无锁同步）

volatile关键字 每个线程都有自己的一小块内存，执行的时候会把变量copy过来，修改了后在写回对象，

执行m1方法的线程把 running读到内存里，与此同时主线程也把running读到内存，并进行修改，写回对象为false

但是执行m1的线程里的内存一直都是true啊（因为太忙了没空去刷新）所以会形成死循环，

volatile就是当running改了之后 *立马去通知其他线程，你们记得去主存刷新一下，一刷新，running为false，退出while循环。

package com.dingyu;
/**
 * volatile关键字   每个线程都有自己的一小块内存，执行的时候会把变量copy过来，修改了后在写回对象，
 * 执行m1方法的线程把 running读到内存里，与此同时主线程也把running读到内存，并进行修改，写回对象为false
 * 但是执行m1的线程里的内存一直都是true啊（因为太忙了没空去刷新）所以会形成死循环，volatile就是当running改了之后
 * 立马去通知其他线程，你们记得去主存刷新一下，一刷新，running为false，退出while循环。
 * @author dingyu
 *
 */
public class VolatileDemo {
    private volatile boolean running = true;

    public void m1() {
        System.out.println("m1 start");
        while (running) {

        }
        System.out.println("m1 end");
    }

    public static void main(String[] args) {
        VolatileDemo volatileDemo = new VolatileDemo();
        new Thread(() -> volatileDemo.m1()).start();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        volatileDemo.running = false;

    }
}

7.voliatile 不能保证原子性 不能替换synchronized

package com.dingyu;

/**
 * voliatile 不能保证原子性 不能替换synchronized
 *
 * @author dingyu
 *
 */
public class VolatileDemo02 {
    public volatile int count = 0;

    public void m1() {
        for (int i = 0; i <= 10000; i++)
            count++;
    }

    public static void main(String[] args) {
        VolatileDemo02 volatileDemo02 = new VolatileDemo02();
        for (int i = 0; i < 3; i++) {
            new Thread(() -> volatileDemo02.m1()).start();
        }
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(volatileDemo02.count);
    }
}

8.多个原子类的方法之间不具备原子性

package com.dingyu;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * 原子类 具有原子性，但两个原子类的方法之间不具备原子性
 *
 * @author dingyu
 *
 */
public class AtomicDemo {
    private AtomicInteger count = new AtomicInteger();

    public void m1() {
        for (int i = 0; i < 100; i++) {
            count.incrementAndGet();
            //两个原子类的方法之间不具备原子性
            count.incrementAndGet();

        }
    }
}

9.原子类的不具备可见性

package com.dingyu;

import java.util.concurrent.atomic.AtomicBoolean;

public class AtomicDemo02 {
    public AtomicBoolean running = new AtomicBoolean(true);

    public void m1() {
        while (running.get()) {

        }
    }

    public static void main(String[] args) {
        AtomicDemo02 demo02 = new AtomicDemo02();
        new Thread(()->demo02.m1()).start();
        demo02.running.set(false);
    }
}

10.锁是锁在堆内存的那个对象上，而不是引用

package com.dingyu;

/**
 * 锁是锁在堆内存的那个对象上，而不是引用
 *
 * @author dingyu
 *
 */
public class ChangeReference {
    public Object o = new Object();

    public void m1() {
        //锁o
        synchronized (o) {
            while (true) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
            }
        }
    }

    public static void main(String[] args) {
        ChangeReference changeReference = new ChangeReference();
        new Thread(() -> changeReference.m1(), "t1").start();//启动一个线程 叫t1
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        changeReference.o = new Object();//引用变了
        new Thread(() -> changeReference.m1(),"t2").start();//启动线程 t2
    }
}

11.不要锁字符串常量

package com.dingyu;
/**
 * 不要锁字符串常量
 * @author dingyu
 *
 */
public class SynchronizedString {
    private String s1 = "hello";
    private String s2 = "hello";

    public void m1() {
        synchronized (s1) {
            while(true) {}
        }
    }

    public void m2() {
        synchronized (s2) {
            System.out.println("m2 start");
        }
    }

    public static void main(String[] args) {
        SynchronizedString synchronizedString = new SynchronizedString();
        new Thread(()->synchronizedString.m1()).start();
        new Thread(()->synchronizedString.m2()).start();
    }
}

12.wait 让线程暂停，释放锁， notify 唤醒线程，不释放锁

package com.dingyu2;

/**
 * wait 让线程暂停，释放锁， notify 唤醒线程，不释放锁
 *
 * @author dingyu
 *
 */
public class WaitAndNotyifyDemo {
    private volatile int count = 0;
    private Object lock = new Object();

    public void m1() {
        synchronized (lock) {
            System.out.println("m1 start");
            for (int i = 0; i < 10; i++) {
                count++;
                System.out.println(count);
                if (count == 5) {
                    lock.notify();
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }

        }
    }

    public void m2() {
        synchronized (lock) {
            System.out.println("m2 start");
            if (count != 5) {
                try {
                    System.out.println("m2 在等着 但把锁释放了");
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("m2 end");
            lock.notify();
        }
    }

    public static void main(String[] args) {
        WaitAndNotyifyDemo waitAndNotyifyDemo = new WaitAndNotyifyDemo();
        new Thread(() -> waitAndNotyifyDemo.m2()).start();
        new Thread(() -> waitAndNotyifyDemo.m1()).start();
    }
}