synchronized和volatile使用

synchronized和volatile

volatile ：保证内存可见性，但是不保证原子性；

synchronized：同步锁，既能保证内存可见性，又能保证原子性；

synchronized实现可重入锁 (1.持有同一锁自动获取   2.继承锁)

锁定的对象有两种:1.类的实例（对象锁） 2.类对象(类锁)

对象锁（synchronized修饰普通方法或代码块）    对象锁已被其他调用者占用，则需要等待此锁被释放 

/**
* 对象锁的两种方式
*/
//方式一

private int count =10;
public synchronized void test01() {
count--;
System.out.println(Thread.currentThread().getName()+"count="+count);
}

//方式二
public void test02() {
synchronized(this) {
count--;
System.out.println(Thread.currentThread().getName()+"count="+count);
}
}

类锁（synchronized修饰静态方法）   所有类实例化对象互斥拥有一把类锁

private static int count =10;
    /**
     * 类锁两种表现方式
     */
    public static synchronized void test01() {
        count -- ;
        System.out.println(Thread.currentThread().getName()+"count="+count);
    }
    
    
    public static  void test02() {
        synchronized(CurrentDemo01.class) {
            count -- ;
            System.out.println(Thread.currentThread().getName()+"count="+count);
        }
    }

同一字符串常量代表同一把锁对象

//t1执行结束，t2再执行，t1和t2持有同一把锁
public class CurrentDemo03 {

    String s1 = "yew";
    String s2 = "yew";

    public void test01(){
        synchronized (s1){
            System.out.println(Thread.currentThread().getName()+"---start");
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                System.out.println("interrupt");
            }
            System.out.println(Thread.currentThread().getName()+"---end");
        }
    }

    public void test02(){
          synchronized (s2){
              System.out.println(Thread.currentThread().getName()+"---start");
              try {
                  TimeUnit.SECONDS.sleep(1);
              } catch (InterruptedException e) {
                  System.out.println("interrupt");
              }
              System.out.println(Thread.currentThread().getName()+"---end");
          }
    }

    public static void main(String[] args) {
        CurrentDemo03 demo3 = new CurrentDemo03();
        new Thread(demo3::test01,"t1").start();
        new Thread(demo3::test02,"t2").start();
    }
}

synchronized同步代码块粒度越小，执行效率越高

public class CurrentDemo04 {

 //总数
 private static int count = 0;
 public synchronized  void test01() throws InterruptedException {
     System.out.println(Thread.currentThread().getName()+"---start");
     TimeUnit.SECONDS.sleep(2);
     for (int i = 0; i <10000000 ; i++) {
         count++ ;
     }
     TimeUnit.SECONDS.sleep(2);
     System.out.println(Thread.currentThread().getName()+"---end");
 }
 
 public void test02() throws InterruptedException {
     System.out.println(Thread.currentThread().getName()+"---start");
     TimeUnit.SECONDS.sleep(2);
     synchronized (this){
         for (int i = 0; i <10000000 ; i++) {
             count++ ;
         }
     }
     TimeUnit.SECONDS.sleep(2);
     System.out.println(Thread.currentThread().getName()+"---end");
 }

    //demo04.test01();8043ms
    // demo04.test02();4116ms
    public static void main(String[] args){
        CurrentDemo04 demo04 = new CurrentDemo04();
        long start = System.currentTimeMillis();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    demo04.test02();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    demo04.test02();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
        System.out.println(Thread.activeCount());
        while(Thread.activeCount()>2){
            Thread.yield();
        }
        long end = System.currentTimeMillis();
        System.out.println("耗时:"+(end-start)+"ms");
        System.out.println(count);
    }
}

synchronized方法正常返回或者抛异常而终止，jvm会自动释放对象锁(捕获异常则不会自动释放)

public class CurrentDemo08 {
    
    //总数值
    private int count = 0;

    public synchronized void test01() throws Exception {
        System.out.println(Thread.currentThread().getName()+"---start");

        while (true){
            count++ ;
            TimeUnit.SECONDS.sleep(1);
            if(count <5){
                System.out.println(count);
            }else{
                try {
                    int m =  count/0;
                }catch (Exception e){
                    System.out.println("ERROR :"+e.getMessage());
                   // return;
                }
            }
        }
    }

    public synchronized void test02() throws InterruptedException {
        System.out.println(Thread.currentThread().getName()+"----start");

        TimeUnit.SECONDS.sleep(1);
        System.out.println(Thread.currentThread().getName()+"---end");
    }

    public static void main(String[] args) {

        CurrentDemo08 demo08 = new CurrentDemo08();

        new Thread(()-> {
            try {
                demo08.test01();
            } catch (Exception e) {
                System.out.println("test01中断");
            }
        },"t1").start();

        new Thread(()-> {
            try {
                demo08.test02();
            } catch (InterruptedException e) {
                System.out.println("test02中断");
            }
        },"t2").start();
    }
}

/**
 * @author yew
 * @date on 2019/11/18 - 11:49
 * 一道面试题：实现一个容器，提供两个方法，add,size
 * 写两个线程，线程1添加10个元素到容器中，线程2实现监控元素的个数，
 * 当个数到5个时，线程2给出提示并结束线程2
 * 1.无volatile,线程间不可见，线程t2不会结束
 * 2.volatile可以保证原子的可见性,存在不确定性  while(true) 占用CPU资源
 */
public class CurrentDemo11 {
   //List