多个线程多个锁

1. 多个线程多个锁概念

多个线程多个锁：多个线程，每个线程都可以拿到自己指定的锁，分别获得锁之后，执行synchronized方法体的内容。

2. 示例【com.bjsxt.base.sync002】MultiThread

public class MultiThread {
    private int num = 0;
    public synchronized void printNum(String tag){

            try {
                if(tag.equals("a")){
                    num = 100;
                    System.out.println("tag a, set num over!");
                    Thread.sleep(1000);
                }else{
                    num = 200;
                    System.out.println("tag b, set num over!");
                }
                System.out.println("tag " + tag + ", num = " + num);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
    }

    public static void main(String[] args){
        MultiThread multiThread1 = new MultiThread();
        MultiThread multiThread2 = new MultiThread();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                multiThread1.printNum("a");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                multiThread2.printNum("b");
            }
        });

        t1.start();
        t2.start();
    }
}

执行结果

public class MultiThread {
    private static int num = 0;
    public static synchronized void printNum(String tag){

            try {
                if(tag.equals("a")){
                    num = 100;
                    System.out.println("tag a, set num over!");
                    Thread.sleep(1000);
                }else{
                    num = 200;
                    System.out.println("tag b, set num over!");
                }
                System.out.println("tag " + tag + ", num = " + num);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
    }

    public static void main(String[] args){
        MultiThread multiThread1 = new MultiThread();
        MultiThread multiThread2 = new MultiThread();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                multiThread1.printNum("a");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                multiThread2.printNum("b");
            }
        });

        t1.start();
        t2.start();
    }
}

执行结果

3. 示例总结

关键字synchronized取得的锁都是对象锁，而不是把一段代码（方法）当做锁，所以示例代码中那个那个线程先执行synchronized关键字的方法，那个线程就持有该方法所属对象的锁（Lock），两个对象，线程获得的就是两个不同的锁，他们互不影响。
有一种情况则是相同的锁，即在静态方法上synchronized关键字，表示锁定.class类，类一级别的锁（独占.class类）