JVM 初始化階段的重要意義分析

 一、建立一個Mytest6類和Singleton類

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;
    }


}

　　輸出結果

counter1:1
counter2:1

　　

 

二、將counter2成員變量的位置移動到構造函數後面

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;
    }


}

　　輸出結果以下：

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

　　首先Singleton singleton = Singleton.getInstance(); 是調用Singleton類的getInstance()，屬於主動調用。Singleton在準備階段，按照聲明的順序，賦予全部成員變量默認值。在初始化階段，構造函數裏couonter1和counter2的值變爲1，可是後面counter2的值又被賦值爲0。 因此打印了上面的結果。

 

上面代碼中的構造函數裏counter2 ++;

準備階段的意義：若是沒有準備階段，counter2是沒有值的，更不會有++操做