多线程-synchronized锁在类级别和方法级执行结果的区别

synchronized 方法级别代码：

public class Thread1 {
    private int num = 0;
    public synchronized void printNum(String arg){

        if (arg.equals("a")){
            num = 100;
            System.out.println("num value is "+ num);
            try {
                new Thread().sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else {
            num = 200;
            System.out.println("num value is "+ num);
        }
        System.out.println("tag = " + arg + " num = " + num);
    }

    public static void main(String[] args) {
        final Thread1 t1 = new Thread1();
        final Thread1 t2 = new Thread1();

        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                t1.printNum("a");
            }
        });

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                t2.printNum("b");
            }
        });

        thread1.start();
        thread2.start();
    }
}

执行结果：

num value is 200
tag = b num = 200
num value is 100
tag = a num = 100

synchronized 类级别代码：

public class Thread1 {
    private static int num = 0;
    public static synchronized void printNum(String arg){

        if (arg.equals("a")){
            num = 100;
            System.out.println("num value is "+ num);
            try {
                new Thread().sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else {
            num = 200;
            System.out.println("num value is "+ num);
        }
        System.out.println("tag = " + arg + " num = " + num);
    }

    public static void main(String[] args) {
        final Thread1 t1 = new Thread1();
        final Thread1 t2 = new Thread1();

        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                t1.printNum("a");
            }
        });

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                t2.printNum("b");
            }
        });

        thread1.start();
        thread2.start();
    }
}

执行结果：

num value is 100
tag = a num = 100
num value is 200
tag = b num = 200

总结：

关键字synchronized取得的锁都试对象锁，而不是把一段代码（方法）当做锁，所以十里代码中哪个线程限制性synchronized关键字的方法，哪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通的锁，他们互补影响。
有一种情况则是相同的锁，即在静态方法上加synchronized关键字，表示锁定.class类，类一级别的锁（独占.class类）。