Java-JVM 类的初始化

public class Test {
    protected static final Logger logger = LoggerFactory.getLogger(Test.class);

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

class Singleton {
    private static Singleton singleton = new Singleton();
    public static int counter1;
    public static int counter2 = 0;

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

    public static Singleton getInstance() {
        return singleton;
    }
}

Singleton singleton = Singleton.getInstance();这行代码首先给Singleton中的变量赋予默认的初始值：

即：private static Singleton singleton的singleton赋予默认初始值null、

public static int counter1 的counter1赋予默认初始值0、

public static int counter2 的counter2赋予默认初始值0（不是显式的0）、

然后会显式地给变量赋值：

private static Singleton singleton的singleton赋予new Singleton()，此时会执行Singleton类的构造函数：

private Singleton(){

counter1++;

counter2++;

}

counter1 = 1；

counter2 = 1；

接着给counet1、counter2显式赋值

public static int counter1 ;

public static int counter2 = 0;

由于counter1没有显式值

所以赋值完成后counter1 = 1、counter2 = 0；

最后输出为counter1 = 1、counter2 = 0；

如果把 private static Singleton singleton = new Singleton();放在 public static int counter2 = 0;的后面，则会输出counter1 = 1、counter2 = 1；