JAVA - 深入JAVA 虚拟机 3

Posted 2020-09-27 dawangandy

类的初始化时机

package practise;

class Parent{
    
    static int a =3;
    static{
        
        System.out.println("Parent static block");
    }
}
class Child extends Parent
{
    static int b = 4;
    static{
        
        System.out.println("Child static block");
    }
}
public class Test4 {
    
    static{
        
        System.out.println("Test4 static block");
    }
    public static void main(String[] args) {
        
        System.out.println(Child.b);
        
    }
}

Result:
Test4 static block
Parent static block
Child static block
4

package practise;

class Parent5{
    
    static int a =3;
    static{
        
        System.out.println("Parent5 static block");
    }
}
class Child5 extends Parent5
{
    static int b = 4;
    static{
        
        System.out.println("Child5 static block");
    }
}
public class Test5 {
    
    static{
        
        System.out.println("Test5 static block");
    }
    public static void main(String[] args) {
        
        Parent5 parent = new Parent5();
        
        System.out.println(Parent5.a);
        System.out.println(Child5.b);
        
    }
}

Result:
Test5 static block
Parent5 static block
3
Child5 static block
4

程序中对子类的“主动使用”会导致父类被初始化；但对父类的“主动”使用并不会导致子类
初始化。

调用 ClassLoader类的loadClass方法加载一个类，并不是对类的主动使用，不会导致类的初始化。

package practise;

class C{
    
    static{
        
        System.out.println("Class C");
    }
}

public class Test6 {
    public static void main(String[] args) {
        
        ClassLoader loader = ClassLoader.getSystemClassLoader();
        
        try {
            Class<?> clz = loader.loadClass("practise.C");
            System.out.println("----------------");
            clz = Class.forName("practise.C");
            
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Result:
----------------
Class C