A classic JVM class loaded interview question class singleton{static singleton instance = new singleton(); private singleton() {}

One 、 subject

Almost the same code , There are two cases ：

Situation 1 ：new SingleTon() rearwards , And what you get is ：count1 = 1 and count2 = 1

Situation two ：new SingleTon() in front , And what you get is ：count1 = 1 and count2 = 0

 【 ask 】 Please give us a brief introduction , Why the output results of these two cases are different ？

Two 、 analysis

The main knowledge involved here is ,JVM Class loading mechanism of .

1、 The life cycle of a class ： load ->  verification ->  Get ready ->  analysis ->  initialization ->  Use ->  uninstall , And verification , Preparation and parsing can be classified as the connection phase .

2、 When calling a static method in a class getSingleTon() when , Will trigger class initialization .

（1） For case one ：

1） Connection phase , Will assign initial values to static variables , At this time count1=0,count2=0;

2） Then the initialization phase , Code execution from top to bottom , Assignment operations and static code blocks , At this time count1=0,count2=0;

Create objects SingleTon after , Yes count1 and count2 Conduct ++, Last count1=1,count2=1.

（2） For case two ：

1） Connection phase , It is also a static variable that gives an initial value , It's the same time count1=0,count2=0;

2） Initialization phase , Execute code from top to bottom , Create the object first , Yes count1 and count2 Conduct ++, At this time count1=1,count2=1; then  public static int  count1 Yes count1 There is no assignment , At this time count1 Or is it equal to 1, however count2 Assigned a value to 0, At this time count2=0, So the final output count1=1,count2=0

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【 Code 】

class SingleTon{
    private static SingleTon singleTon = new SingleTon();
    public static int  count1;
    public static int  count2 = 0;

    private SingleTon(){
        count1++;
        count2++;
    }
    public static  SingleTon getSingleTon(){
        return singleTon;
    }
}

public class ClassLoaderTest {
    public static void main(String[] args) {
        SingleTon.getSingleTon();
        System.out.println("count1 = "+ SingleTon.count1);
        System.out.println("count2 = "+ SingleTon.count2);
    }
}