synchronized

package springcloud.controller;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.locks.ReentrantLock;

public class Jmm07 {

    private static int total = 0;
    private static Object object = new Object();

    private static ReentrantLock lock = new ReentrantLock();//显示锁

    public static void main(String[] args) throws Exception {

        final CountDownLatch countDownLatch = new CountDownLatch(1);
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                try {
                    countDownLatch.await();//等到其他线程执行完毕
                    for (int j = 0; j < 1000; j++) {
                        synchronized (object) {//内置锁、隐式锁
                            total++;
                        }
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }).start();
            Thread.sleep(50);
            countDownLatch.countDown();
            Thread.sleep(200);
        }
        System.out.println(total);
    }
}

1. 在1.6之后，对象锁偏向锁（针对单线程）
2. 对象锁轻量级锁（针对多线程，但线程竞争不激烈（锁的时间占用端，可以多线程可以交替执行）），内部循环执行，会一直占着cpu（spin）
3. 占着cpu执行的时间比重新唤醒线程的效率要高
4. 但是循环的次数是有限制的，比如while设置为100，则锁进行升级（说明竞争激烈，要释放锁）
5. synchronized 锁的升级过程是不可逆的
6. ReentrantLock 和synchronized 区别不大

以下是synchronized 锁加的地方

package springcloud.controller;

import java.util.concurrent.locks.ReentrantLock;

public class Jmm08 {
//修饰静态方法
    private static int total = 0;
    private static Object object = new Object();

    private static ReentrantLock lock = new ReentrantLock();//显示锁

    public static void main(String[] args) {
        Jmm08.decrStock();
    }

    static int stock;

    public static synchronized void decrStock() {
        //加锁加在Jmm08这个对象上
        System.out.println(stock);
    }
}

package springcloud.controller;

import java.util.concurrent.locks.ReentrantLock;

public class Jmm09 {
//修饰非静态方法
    private static ReentrantLock lock = new ReentrantLock();//显示锁

    public static void main(String[] args) {
        new Jmm09().decrStock();
    }
    static int stock;
    public synchronized void decrStock() {
        //加锁加在this对象上
        System.out.println(stock);
    }
}

package springcloud.controller;

public class Jmm10 {
    private static Object object = new Object();
//加在方法内部
    public static void main(String[] args) {
        new Jmm10().decrStock();
    }

    static int stock;

    public synchronized void decrStock() {
        //方法内部加同步块
        synchronized (object) {
            if (stock <= 0) {
                System.out.println("库存为0");
            } else {
                System.out.println(stock);
            }
        }

    }
}