Java初始化

Java虚拟机在首次主动使用类型时初始化它们。下面几种情况下会主动使用:

1.创建类的新实例

2.调用类中声明的静态方法

3.操作类或接口中声明的非常量静态字段

4.调用Java API中特定反射方法

5.初始化一个类的子类

6.指定一个类作为Java虚拟机启动时的初始化类


而被动使用(对于子类、子接口和实现了接口的类引用父亲的非常量静态字段)不会触发初始化。

来看下面的一段代码

=====父类

public class NewParent {

 static int hoursOfSleep=(int)(Math.random()*3.0);
 static{
  System.out.println("New parent was initialized...");
 }

 public NewParent() {

  System.out.println("NewParent.NewParent()");
  // TODO Auto-generated constructor stub
 }
}

========子类

public class NewBornBody extends NewParent{

 static int houseOfCrying=6+(int)(Math.random()*2.0);

 static{
  System.out.println("NewBronBody was initiazed.");
 }
}

========JVM启动类

public class Example2 {

 public static void main(String[] args){
  int hours=NewBornBody.hoursOfSleep;
  System.out.println(hours);


 }

 static{

  System.out.println("Example2 was initialized!");
 }
}

这段代码的执行结果为:

Example2 was initialized!
New parent was initialized...
0


===================================

如果一个既是static又是final的，使用时就不是对这个类的主动使用。示例代码如下：

public interface Angry {

 String greeting="Grrr1";

}


public class Dog {

 static final String greeting="wolf,wolf,world!";
 static{
  System.out.println("Dog was initialized");

 }


 static int getAngerLevel(){
  System.out.println("Angel was initialized!");
  return 1;
 }


}


public class Example3 {

 public static void main(String[] args){

  System.out.println(Angry.greeting);
  System.out.println(Dog.greeting);

 }

 static{
  System.out.println("Example3 was initialized!");
 }
}

========代码的执行结果为:

Example3 was initialized!
Grrr1
wolf,wolf,world!

//可见Dog并没有被初始化