synchronized對普通同步方法和對靜態方法的區別

synchronized是一個重量級鎖,我們都知道該關鍵字鎖住的是對象而不是代碼本身,那么對於靜態方法和同步方法有什么不同呢,通過如下代碼進行測試

public class SynchronizedTest {
 
    private static int num;
 
    private synchronized void test(String param){
        if(StringUtils.equals(param,"a")){
            num = 100;
            System.out.println("set " + param + " num over");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else{
            num = 200;
            System.out.println("set " + param + " num over");
        }
        System.out.println("i am : " + param + "; num = " + num);
    }
 
    public static void main(String[] args){
        SynchronizedTest s1 = new SynchronizedTest();
        SynchronizedTest s2 = new SynchronizedTest();
 
        new Thread(new Runnable() {
            @Override
            public void run() {
                s1.test("a");
            }
        }).start();
 
        new Thread(new Runnable() {
            @Override
            public void run() {
                s2.test("b");
            }
        }).start();
    }
}

 
set a num over
set b num over
i am : b; num = 200
i am : a; num = 200
 
Process finished with exit code 0

我們可以看出兩個不同的對象s1和s2並沒有互斥,因為這里synchronized是分別持有兩個對象的鎖。如果要想m1,m2兩個對象競爭同一個鎖，則需要在method01()上加上static修飾,如下:

public class SynchronizedTest {
 
    private static int num;
 
    private synchronized static void test(String param){
        if(StringUtils.equals(param,"a")){
            num = 100;
            System.out.println("set " + param + " num over");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else{
            num = 200;
            System.out.println("set " + param + " num over");
        }
        System.out.println("i am : " + param + "; num = " + num);
    }
 
    public static void main(String[] args){
        SynchronizedTest s1 = new SynchronizedTest();
        SynchronizedTest s2 = new SynchronizedTest();
 
        new Thread(new Runnable() {
            @Override
            public void run() {
                s1.test("a");
            }
        }).start();
 
        new Thread(new Runnable() {
            @Override
            public void run() {
                s2.test("b");
            }
        }).start();
    }
}

運行結果：

set a num over
i am : a; num = 100
set b num over
i am : b; num = 200
 
Process finished with exit code 0

synchronized修飾不加static的方法，鎖是加在單個對象上，不同的對象沒有競爭關系；修飾加了static的方法，鎖是加載類上，這個類所有的對象競爭一把鎖.

版權聲明：本文為CSDN博主「珍惜每一個遇見」的原創文章，遵循 CC 4.0 BY-SA 版權協議，轉載請附上原文出處鏈接及本聲明。
原文鏈接：https://blog.csdn.net/LQM1991/article/details/86143278