12_类加载器与类初始化深度剖析

可以用-XX:+TraceClassLoading虚拟机参数来查看类加载的顺序。

class Parent{

    static int a = 3;

    static {
        System.out.println("Parent static Bbolck");
    }

}

class Child extends Parent{

    static int b = 4;

    static {
        System.out.println("Child static block");
    }

}

public class MyTest9 {

    static {
        System.out.println("MyTest9 static block");
    }

    public static void main(String[] args) {

        System.out.println(Child.b);

    }

}

输出：

[Loaded top.tomxwd.classloader.MyTest9 from file:/E:/IDEA-workspace/jvm-test-01/target/classes/]
MyTest9 static block
[Loaded top.tomxwd.classloader.Parent from file:/E:/IDEA-workspace/jvm-test-01/target/classes/]
[Loaded top.tomxwd.classloader.Child from file:/E:/IDEA-workspace/jvm-test-01/target/classes/]
Parent static Bbolck
Child static block
4

代码：

public class MyTest10 {

    static {
        System.out.println("MyTest10 static block");
    }

    public static void main(String[] args) {
        Parent2 parent2;
        System.out.println("------------------------------");
        parent2 = new Parent2();
        System.out.println("------------------------------");
        System.out.println(parent2.a);
        System.out.println("------------------------------");
        System.out.println(Child2.b);
    }

}

class Parent2 {
    static int a = 3;
    static {
        System.out.println("Parent2 static block");
    }
}

class Child2 extends Parent2{
    static int b = 4;
    static {
        System.out.println("Child2 static block");
    }
}

结果：

MyTest10 static block
------------------------------
Parent2 static block
------------------------------
3
------------------------------
Child2 static block
4

代码：

public class MyTest11 {

    public static void main(String[] args) {
        System.out.println(Child3.a);
        Child3.doSomething();
    }

}

class Parent3{
    static int a = 3;

    static {
        System.out.println("Parent3 static block");
    }

    static void doSomething(){
        System.out.println("do something");
    }

}

class Child3 extends Parent3 {

    static {
        System.out.println("Child3 static block");
    }

}

输出：

Parent3 static block
3
do something

谁拥有这个静态变量，就对谁主动使用，所以并没有初始化Child3。

代码：

public class MyTest12 {

    public static void main(String[] args) throws ClassNotFoundException {
        // 系统类加载器--应用类加载器
        ClassLoader loader = ClassLoader.getSystemClassLoader();
        Class<?> cl = loader.loadClass("top.tomxwd.classloader.CL");
        System.out.println(cl);
        System.out.println("----------------------------");
        cl = Class.forName("top.tomxwd.classloader.CL");
        System.out.println(cl);
    }

}

class CL{

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

}

输出：

class top.tomxwd.classloader.CL
----------------------------
Class CL
class top.tomxwd.classloader.CL

加载并不会进行初始化，只有主动使用的时候会，而上面是通过反射对CL进行主动使用。

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