Java中ClassLoader與Class.forName的區別

package com.demo.classLoader;

public class Main {

    public static void main(String[] args) {
        getClassLoaderParent();

        forName1();
        forName2();
        forName3();
    }

    /**
     * @Title: getClassLoaderParent
     * @Date: 2018-12-07 9:31
     * @Description: 類載入時的載入器，AppClassLoader->ExtClassLoader->null(BootstrapClass)
     */
 

    private static void getClassLoaderParent() {
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println("classLoader:\t" + systemClassLoader);

        ClassLoader parent1 = systemClassLoader.getParent();
        System.out.println("parent1:\t" + parent1)
 
;

        ClassLoader parent2 = parent1.getParent();
        System.out.println("parent2:\t" + parent2);
    }

    /**
     * @Title: forName1
     * @Date: 2018-12-07 9:41
     * @Description: 執行時，將類載入到JVM中，同時執行類中的static程式碼塊，對類的例項物件做初始化
     */
    private static void forName1() {
        try {
            System.
 
out.println("before forName1");
            Class<?> aClass = Class.forName("com.demo.classLoader.DemoClass");
            System.out.println("after forName1");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    /**
     * @Title: forName2
     * @Date: 2018-12-07 9:43
     * @Description: 在同一個JVM中，一個類的例項物件只有一個，也就是說該類中的static程式碼塊在類載入時就執行了，而且只執行一次
     */
    private static void forName2() {
        try {
            System.out.println("before forName2");
            Class<?> aClass1 = Class.forName("com.demo.classLoader.DemoClass", true, Main.class.getClassLoader());
            System.out.println("after forName2");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    /**
     * @Title: forName3
     * @Date: 2018-12-07 9:44
     * @Description: 類的載入方式是通過獲取當前類的ClassLoader的父類，進行載入（雙親委派）
     */
    private static void forName3() {
        try {
            System.out.println("before forName3");
            Class<?> aClass2 = ClassLoader.getSystemClassLoader().loadClass("com.demo.classLoader.DemoClass");
            System.out.println("after forName3");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

}

class DemoClass {

    private DemoClass(String s) {
        System.out.println("Constructor " + s);
    }

    static {
        System.out.println("static DemoClass");
    }

}