Java中写双重检查加锁的单例时volatile关键字作用

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
/**
* 注意:该demo 展示了volatile变量在主内存和线程工作内存中的状态
* 1.一个线程获取回来的volatile变量会保持不变;除非重新获取一次
* 2.另外一个线程a改变了volatile变量的值,会立即同步到主内存; 当其他线程重新获取的时候,a对变量的改变可以被看到,这就是可见性.
*
*/
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button = findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//主线程获取test并打印值
Test test = Test.getTest();
Log.e("TAG", test.toString());
// 2.MyThread1获取test,sleep 2000ms后打印值
MyThread1 t1 = new MyThread1();
t1.start();
// 2.MyThread 重置Test,并打印值
MyThread t = new MyThread();
t.start();
}
});
}
static class MyThread extends Thread {
@Override
public void run() {
Test test = Test.resetTest();
System.out.println("重置volatile变量为null");
System.out.println(test);
}
}
static class MyThread1 extends Thread {
@Override
public void run() {
Test test = Test.getTest();
System.out.println("刚获取后 打印volatile变量");
System.out.println(test);
try {
Thread.sleep(2000);
System.out.println("sleep 了2000ms后 打印volatile变量");
System.out.println(test);
System.out.println("sleep 了2000ms后 打印Test中的volatile变量");
System.out.println(Test.test);
System.out.println("sleep 了2000ms后 再次获取test后打印volatile变量");
System.out.println(Test.getTest());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class Test {
private int mT;
private static volatile Test test;
private Test() {
}
public static Test getTest() {
if (test == null) {
test = new Test();
}
return test;
}
public static Test resetTest() {
test = null;
return test;
}
}
}

1
2
3
4
5
6
7
8
9
10
11
12
2020-07-07 14:05:48.530 4772-4772/com.example.myapplication E/: com.example.myapplication.MainActivity$Test@d28ed69
2020-07-07 14:05:48.531 4772-4945/com.example.myapplication I/System.out: 刚获取后 打印volatile变量
2020-07-07 14:05:48.532 4772-4945/com.example.myapplication I/System.out: com.example.myapplication.MainActivity$Test@d28ed69
2020-07-07 14:05:48.532 4772-4946/com.example.myapplication I/System.out: 重置volatile变量为null
2020-07-07 14:05:48.532 4772-4946/com.example.myapplication I/System.out: null
2020-07-07 14:05:50.532 4772-4945/com.example.myapplication I/System.out: sleep 了2000ms后 打印volatile变量
2020-07-07 14:05:50.533 4772-4945/com.example.myapplication I/System.out: com.example.myapplication.MainActivity$Test@d28ed69
2020-07-07 14:05:50.533 4772-4945/com.example.myapplication I/System.out: sleep 了2000ms后 打印Test中的volatile变量
2020-07-07 14:05:50.533 4772-4945/com.example.myapplication I/System.out: null
2020-07-07 14:05:50.533 4772-4945/com.example.myapplication I/System.out: sleep 了2000ms后 再次获取test后打印volatile变量
2020-07-07 14:05:50.533 4772-4945/com.example.myapplication I/System.out: com.example.myapplication.MainActivity$Test@6e8a08f