JVM 初始化阶段的重要意义分析

1、创建一个Mytest6类和Singleton类

public class MyTest6 {

    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();
        System.out.println("counter1:" +Singleton.counter1);
        System.out.println("counter2:" +Singleton.counter2);
    }
}

class  Singleton{
    public static  int counter1 ;

    public static int counter2 = 0;

    private static  Singleton singleton = new Singleton();

    private Singleton(){
        counter1 ++;
        counter2 ++;
    }

    public static  Singleton getInstance(){
        return  singleton;
    }

}

　　输出结果

counter1:1
counter2:1

　　

2、将counter2成员变量的位置移动到构造函数后面

public class MyTest6 {

    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();
        System.out.println("counter1:" +Singleton.counter1);
        System.out.println("counter2:" +Singleton.counter2);
    }
}

class  Singleton{
    public static  int counter1 ;

    private static  Singleton singleton = new Singleton();

    private Singleton(){
        counter1 ++;
        counter2 ++;
        System.out.println("Singleton counter1:" +counter1);
        System.out.println("Singleton counter2:" +counter2);
    }

    public static int counter2 = 0;

    public static  Singleton getInstance(){
        return  singleton;
    }

}

　　输出结果如下：

Singleton counter1:1
Singleton counter2:1
counter1:1
counter2:0

　　首先Singleton singleton = Singleton.getInstance(); 是调用Singleton类的getInstance()，属于主动调用。Singleton在准备阶段，按照声明的顺序，赋予所有成员变量默认值。在初始化阶段，构造函数里couonter1和counter2的值变为1，但是后面counter2的值又被赋值为0。 所以打印了上面的结果。

上面代码中的构造函数里counter2 ++;

准备阶段的意义：如果没有准备阶段，counter2是没有值的，更不会有++操作