【jvm】02-手写自己的类加载器

欢迎关注b站账号/公众号【六边形战士夏宁】，一个要把各项指标拉满的男人。该文章已在github目录收录。

屏幕前的大帅比和大漂亮如果有帮助到你的话请顺手点个赞、加个收藏这对我真的很重要。别下次一定了，都不关注上哪下次一定。

1.简单手写自己的类加载器

创建一个类继承ClassLoader,然后重写findClass、loadClass这两个方法

findClass的方法

private static byte[] loadByte(String name) throws Exception {

    name = name.replaceAll("\\.", "/");

    FileInputStream fis = new FileInputStream(classPath + "/" + name

            + ".class");

    int len = fis.available();

    byte[] data = new byte[len];

    fis.read(data);

    fis.close();

    return data;

}

protected Class<?> findClass(String name) throws ClassNotFoundException {

    try {

        byte[] data = loadByte(name);

        //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节数组。

        return defineClass(name, data, 0, data.length);

    } catch (Exception e) {

        e.printStackTrace();

        throw new ClassNotFoundException();

    }

}

loadClass直接复制ClassLoader中的方法然后把错误的行数删除就行了

重点改动代码如下，让该自己的加载器加载指定包的文件，注意需要提前编译java文件为class

if (name.startsWith("com.example.demo.lesson")) {

    c = findClass(name);

} else {

    c = this.getParent().loadClass(name);

}

2.加载一个和jdk中同名的类

创建一个java.lang.Byte的类，然后用自己的类加载器去加载，然后就触发了jdk的沙箱机制，报了一个安全错误

tomcat的类加载器逻辑

如图所示，这里的核心其实就是每一个war包的代码即使含有同名的类也可以加载

做法如下，自己的classloader复制一份然后加载同一个class文件，可以看到启用的是不同的加载器，都被加载了

3.完整代码

package com.example.demo.lesson.jvm.myloader;

import java.io.FileInputStream;

/**

 * @author seal email:876651109@qq.com

 * @date 2020/9/1 7:23 PM

 * @description

 */

public class MyClassLoaderDemo {

    public static void main(String[] args) throws ClassNotFoundException {

        // Class clazz1 = new MyClassLoader().loadClass("com.example.demo.lesson.jvm.loader.A",false);

        //Class clazz1 = new MyClassLoader().loadClass("java.lang.Byte",false);

        Class clazz2 = new MyClassLoader2().loadClass("com.example.demo.lesson.jvm.loader.A",false);

        System.out.println(new MyClassLoaderDemo().getClass().getClassLoader());

        // System.out.println(clazz1.getClassLoader());

        System.out.println(clazz2.getClassLoader());

    }

    public static String classPath = "F:\\IdeaProjects\\TechingCode\\demoGrace\\src\\main\\java";

    private static byte[] loadByte(String name) throws Exception {

        name = name.replaceAll("\\.", "/");

        FileInputStream fis = new FileInputStream(classPath + "/" + name

                + ".class");

        int len = fis.available();

        byte[] data = new byte[len];

        fis.read(data);

        fis.close();

        return data;

    }

    static class MyClassLoader extends ClassLoader {

        @Override

        protected Class<?> findClass(String name) throws ClassNotFoundException {

            try {

                byte[] data = loadByte(name);

                //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节数组。

                return defineClass(name, data, 0, data.length);

            } catch (Exception e) {

                e.printStackTrace();

                throw new ClassNotFoundException();

            }

        }

        @Override

        protected Class<?> loadClass(String name, boolean resolve)

                throws ClassNotFoundException {

            synchronized (getClassLoadingLock(name)) {

                // First, check if the class has already been loaded

                Class<?> c = findLoadedClass(name);

                if (c == null) {

                    long t0 = System.nanoTime();

                    if (name.startsWith("java.lang.Byte")) {

                        c = findClass(name);

                    } else {

                        c = this.getParent().loadClass(name);

                    }

                    if (c == null) {

                        // If still not found, then invoke findClass in order

                        // to find the class.

                        long t1 = System.nanoTime();

                        c = findClass(name);

                        // this is the defining class loader; record the stats

                    }

                }

                if (resolve) {

                    resolveClass(c);

                }

                return c;

            }

        }

    }

    static class MyClassLoader2 extends ClassLoader {

        @Override

        protected Class<?> findClass(String name) throws ClassNotFoundException {

            try {

                byte[] data = loadByte(name);

                //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节数组。

                return defineClass(name, data, 0, data.length);

            } catch (Exception e) {

                e.printStackTrace();

                throw new ClassNotFoundException();

            }

        }

        @Override

        protected Class<?> loadClass(String name, boolean resolve)

                throws ClassNotFoundException {

            synchronized (getClassLoadingLock(name)) {

                // First, check if the class has already been loaded

                Class<?> c = findLoadedClass(name);

                if (c == null) {

                    long t0 = System.nanoTime();

                    if (name.startsWith("com.example.demo.lesson")) {

                        c = findClass(name);

                    } else {

                        c = this.getParent().loadClass(name);

                    }

                    if (c == null) {

                        // If still not found, then invoke findClass in order

                        // to find the class.

                        long t1 = System.nanoTime();

                        c = findClass(name);

                        // this is the defining class loader; record the stats

                    }

                }

                if (resolve) {

                    resolveClass(c);

                }

                return c;

            }

        }

    }

}

4.ConstantPool

Constant pool:

    #1 = Methodref          #2.#3         // java/lang/Object."<init>":()V

    #2 = Class              #4            // java/lang/Object

    #3 = NameAndType        #5:#6         // "<init>":()V

    #4 = Utf8               java/lang/Object

    #5 = Utf8               <init>

    #6 = Utf8               ()V

    #7 = Fieldref           #8.#9         // com/example/demo/lesson/jvm/construction/Master.master2:Lcom/example/demo/lesson/jvm/construction/Master;

    #8 = Class              #10           // com/example/demo/lesson/jvm/construction/Master

    #9 = NameAndType        #11:#12       // master2:Lcom/example/demo/lesson/jvm/construction/Master;

   #10 = Utf8               com/example/demo/lesson/jvm/construction/Master

   #11 = Utf8               master2

   #12 = Utf8               Lcom/example/demo/lesson/jvm/construction/Master;

   #13 = Fieldref           #8.#14        // com/example/demo/lesson/jvm/construction/Master.master4:Lcom/example/demo/lesson/jvm/construction/Master;

   #14 = NameAndType        #15:#12       // master4:Lcom/example/demo/lesson/jvm/construction/Master;

   #15 = Utf8               master4

   #16 = Fieldref           #8.#17        // com/example/demo/lesson/jvm/construction/Master.int2:I

   #17 = NameAndType        #18:#19       // int2:I

   #18 = Utf8               int2

   #19 = Utf8               I

   #20 = Fieldref           #8.#21        // com/example/demo/lesson/jvm/construction/Master.int4:I

   #21 = NameAndType        #22:#19       // int4:I

   #22 = Utf8               int4

   #23 = Methodref          #24.#25       // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

   #24 = Class              #26           // java/lang/Integer

   #25 = NameAndType        #27:#28       // valueOf:(I)Ljava/lang/Integer;

   #26 = Utf8               java/lang/Integer

   #27 = Utf8               valueOf

   #28 = Utf8               (I)Ljava/lang/Integer;

   #29 = Fieldref           #8.#30        // com/example/demo/lesson/jvm/construction/Master.integerMax2:Ljava/lang/Integer;

   #30 = NameAndType        #31:#32       // integerMax2:Ljava/lang/Integer;

   #31 = Utf8               integerMax2

   #32 = Utf8               Ljava/lang/Integer;

   #33 = Fieldref           #8.#34        // com/example/demo/lesson/jvm/construction/Master.integerMax4:Ljava/lang/Integer;

   #34 = NameAndType        #35:#32       // integerMax4:Ljava/lang/Integer;

   #35 = Utf8               integerMax4

   #36 = Fieldref           #8.#37        // com/example/demo/lesson/jvm/construction/Master.integerMin2:Ljava/lang/Integer;

   #37 = NameAndType        #38:#32       // integerMin2:Ljava/lang/Integer;

   #38 = Utf8               integerMin2

   #39 = Fieldref           #8.#40        // com/example/demo/lesson/jvm/construction/Master.integerMin4:Ljava/lang/Integer;

   #40 = NameAndType        #41:#32       // integerMin4:Ljava/lang/Integer;

   #41 = Utf8               integerMin4

   #42 = String             #43           // str25

   #43 = Utf8               str25

   #44 = Fieldref           #8.#45        // com/example/demo/lesson/jvm/construction/Master.str2:Ljava/lang/String;

   #45 = NameAndType        #46:#47       // str2:Ljava/lang/String;

   #46 = Utf8               str2

   #47 = Utf8               Ljava/lang/String;

   #48 = String             #49           // str27

   #49 = Utf8               str27

   #50 = Fieldref           #8.#51        // com/example/demo/lesson/jvm/construction/Master.str4:Ljava/lang/String;

   #51 = NameAndType        #52:#47       // str4:Ljava/lang/String;

   #52 = Utf8               str4

   #53 = Class              #54           // java/lang/String

   #54 = Utf8               java/lang/String

   #55 = String             #56           // str30

   #56 = Utf8               str30

   #57 = Methodref          #53.#58       // java/lang/String."<init>":(Ljava/lang/String;)V

   #58 = NameAndType        #5:#59        // "<init>":(Ljava/lang/String;)V

   #59 = Utf8               (Ljava/lang/String;)V

   #60 = Fieldref           #8.#61        // com/example/demo/lesson/jvm/construction/Master.strN2:Ljava/lang/String;

   #61 = NameAndType        #62:#47       // strN2:Ljava/lang/String;

   #62 = Utf8               strN2

   #63 = String             #64           // str32

   #64 = Utf8               str32

   #65 = Fieldref           #8.#66        // com/example/demo/lesson/jvm/construction/Master.strN4:Ljava/lang/String;

   #66 = NameAndType        #67:#47       // strN4:Ljava/lang/String;

   #67 = Utf8               strN4

   #68 = String             #69           // str35

   #69 = Utf8               str35

   #70 = Methodref          #53.#71       // java/lang/String.intern:()Ljava/lang/String;

   #71 = NameAndType        #72:#73       // intern:()Ljava/lang/String;

   #72 = Utf8               intern

   #73 = Utf8               ()Ljava/lang/String;

   #74 = Fieldref           #8.#75        // com/example/demo/lesson/jvm/construction/Master.strI2:Ljava/lang/String;

   #75 = NameAndType        #76:#47       // strI2:Ljava/lang/String;

   #76 = Utf8               strI2

   #77 = String             #78           // str37

   #78 = Utf8               str37

   #79 = Fieldref           #8.#80        // com/example/demo/lesson/jvm/construction/Master.strI4:Ljava/lang/String;

   #80 = NameAndType        #81:#47       // strI4:Ljava/lang/String;

   #81 = Utf8               strI4

   #82 = String             #83           // str54

   #83 = Utf8               str54

   #84 = String             #85           // str56

   #85 = Utf8               str56

   #86 = String             #87           // str57

   #87 = Utf8               str57

   #88 = String             #89           // str59

   #89 = Utf8               str59

   #90 = String             #91           // str60

   #91 = Utf8               str60

   #92 = Fieldref           #93.#94       // java/lang/System.out:Ljava/io/PrintStream;

   #93 = Class              #95           // java/lang/System

   #94 = NameAndType        #96:#97       // out:Ljava/io/PrintStream;

   #95 = Utf8               java/lang/System

   #96 = Utf8               out

   #97 = Utf8               Ljava/io/PrintStream;

   #98 = Fieldref           #8.#99        // com/example/demo/lesson/jvm/construction/Master.integerMin1:Ljava/lang/Integer;

   #99 = NameAndType        #100:#32      // integerMin1:Ljava/lang/Integer;

  #100 = Utf8               integerMin1

  #101 = Methodref          #102.#103     // java/io/PrintStream.println:(Z)V

  #102 = Class              #104          // java/io/PrintStream

  #103 = NameAndType        #105:#106     // println:(Z)V

  #104 = Utf8               java/io/PrintStream

  #105 = Utf8               println

  #106 = Utf8               (Z)V

  #107 = Fieldref           #8.#108       // com/example/demo/lesson/jvm/construction/Master.integerMax1:Ljava/lang/Integer;

  #108 = NameAndType        #109:#32      // integerMax1:Ljava/lang/Integer;

  #109 = Utf8               integerMax1

  #110 = Fieldref           #8.#111       // com/example/demo/lesson/jvm/construction/Master.strN1:Ljava/lang/String;

  #111 = NameAndType        #112:#47      // strN1:Ljava/lang/String;

  #112 = Utf8               strN1

  #113 = String             #114          // str29

  #114 = Utf8               str29

  #115 = String             #116          // abc

  #116 = Utf8               abc

  #117 = Fieldref           #8.#118       // com/example/demo/lesson/jvm/construction/Master.master1:Lcom/example/demo/lesson/jvm/construction/Master;

  #118 = NameAndType        #119:#12      // master1:Lcom/example/demo/lesson/jvm/construction/Master;

  #119 = Utf8               master1

  #120 = Fieldref           #8.#121       // com/example/demo/lesson/jvm/construction/Master.master3:Lcom/example/demo/lesson/jvm/construction/Master;

  #121 = NameAndType        #122:#12      // master3:Lcom/example/demo/lesson/jvm/construction/Master;

  #122 = Utf8               master3

  #123 = Fieldref           #8.#124       // com/example/demo/lesson/jvm/construction/Master.int3:I

  #124 = NameAndType        #125:#19      // int3:I

  #125 = Utf8               int3

  #126 = Fieldref           #8.#127       // com/example/demo/lesson/jvm/construction/Master.integerMax3:Ljava/lang/Integer;

  #127 = NameAndType        #128:#32      // integerMax3:Ljava/lang/Integer;

  #128 = Utf8               integerMax3

  #129 = Fieldref           #8.#130       // com/example/demo/lesson/jvm/construction/Master.integerMin3:Ljava/lang/Integer;

  #130 = NameAndType        #131:#32      // integerMin3:Ljava/lang/Integer;

  #131 = Utf8               integerMin3

  #132 = String             #133          // str26

  #133 = Utf8               str26

  #134 = Fieldref           #8.#135       // com/example/demo/lesson/jvm/construction/Master.str3:Ljava/lang/String;

  #135 = NameAndType        #136:#47      // str3:Ljava/lang/String;

  #136 = Utf8               str3

  #137 = String             #138          // str31

  #138 = Utf8               str31

  #139 = Fieldref           #8.#140       // com/example/demo/lesson/jvm/construction/Master.strN3:Ljava/lang/String;

  #140 = NameAndType        #141:#47      // strN3:Ljava/lang/String;

  #141 = Utf8               strN3

  #142 = String             #143          // str34

  #143 = Utf8               str34

  #144 = Fieldref           #8.#145       // com/example/demo/lesson/jvm/construction/Master.strI1:Ljava/lang/String;

  #145 = NameAndType        #146:#47      // strI1:Ljava/lang/String;

  #146 = Utf8               strI1

  #147 = String             #148          // str36

  #148 = Utf8               str36

  #149 = Fieldref           #8.#150       // com/example/demo/lesson/jvm/construction/Master.strI3:Ljava/lang/String;

  #150 = NameAndType        #151:#47      // strI3:Ljava/lang/String;

  #151 = Utf8               strI3

  #152 = Utf8               int1

  #153 = Utf8               ConstantValue

  #154 = Integer            9

  #155 = Integer            10

  #156 = Utf8               str1

  #157 = String             #158          // str24

  #158 = Utf8               str24

  #159 = Utf8               Code

  #160 = Utf8               LineNumberTable

  #161 = Utf8               main

  #162 = Utf8               ([Ljava/lang/String;)V

  #163 = Utf8               StackMapTable

  #164 = Class              #165          // "[Ljava/lang/String;"

  #165 = Utf8               [Ljava/lang/String;

  #166 = Utf8               <clinit>

  #167 = Utf8               SourceFile

  #168 = Utf8               Master.java

参考资料

《深入理解Java虚拟机》-周志明

【jvm】02-手写自己的类加载器的更多相关文章

JVM 类的生命周期、类加载器
类的加载.连接与初始化 • 1. 加载:查找并加载类的二进制数据 • 2. 连接 – 2.1 验证:确保被加载的类的正确性 ...
【JVM】查看JVM加载的类及类加载器的方法
查看JVM加载了哪些类 java -verbose[:class|gc|jni] 在输出设备上显示虚拟机运行信息. java -verbose:class 在程序运行的时候有多少类被加载!你可以用ve ...
JVM知识（二）：类加载器原理
我们知道我们编写的java代码,会经过编译器编译成字节码(class文件),再把字节码文件装载到JVM中,最后映射到各个内存区域中,我们的程序就可以在内存中运行了.那么问题来了,这些字节码文件是怎么装 ...
深入了解java虚拟机（JVM）第十二章类加载器
一.什么是类加载器类加载器是一个用来加载类文件的类,Java源代码通过javac编译器编译成类文件,然后JVM来执行类文件中的字节码来执行程序.需要注意的是,只有被同一个类加载器加载的类才可能会相等 ...
【JVM学习笔记】线程上下文类加载器
有许多地方能够看到线程上下文类加载的设置,比如在sun.misc.Launcher类的构造方法中,能够看到如下代码先写一个例子建立感性认识 public class Test { public st ...
写一个自定义类加载器demo
public class MyTest16 extends ClassLoader { private String classLoaderName; private String fileExten ...
JVM源码分析之自定义类加载器如何拉长YGC
概述本文重点讲述毕玄大师在其公众号上发的一个GC问题一个jstack/jmap等不能用的case,对于毕大师那篇文章,题目上没有提到GC的那个问题,不过进入到文章里可以看到,既然文章提到了jstac ...
深入JVM系列（三）之类加载、类加载器、双亲委派机制与常见问题
一．概述定义:虚拟机把描述类的数据从Class文件加载到内存,并对数据进行校验.转换解析和初始化,最终形成可以被虚拟机直接使用的java类型.类加载和连接的过程都是在运行期间完成的. 二. 类的 ...
【JVM第二篇--类加载机制】类加载器与双亲委派模型
写在前面的话:本文是在观看尚硅谷JVM教程后,整理的学习笔记.其观看地址如下:尚硅谷2020最新版宋红康JVM教程一.什么是类加载器在类加载过程中,加载阶段有一个动作是"通过一个类的全限 ...

随机推荐

解决springboot序列化 json数据到前端中文乱码问题
前言关于springboot乱码的问题,之前有文章已经介绍过了,这一篇算是作为补充,重点解决对象在序列化过程中出现的中文乱码的问题,以及后台报500的错误. 问题描述 spring Boot 中文返 ...
ORACLE dba_objects
dba_objects OWNER 对象所有者 OBJECT_NAME 对象名称 SUBOBJECT_NAME 子对象名称 OBJECT_ID 对象id DATA_OBJECT_ID 包含该对象的se ...
PhoneGap打包webApp
因为我只弄了Andriod的环境,所以在此只以Andriod为例. 使用PhoneGap搭建Android开发的项目整体步骤如下: 安装java环境. 安装ant构建工具. 安装android的开发环 ...
docker之镜像制作
#:下载镜像并初始化系统 root@ubuntu:~# docker pull centos #:创建目录 root@ubuntu:/opt# mkdir dockerfile/{web/{nginx ...
JAVA平台AOP技术研究
3.1 Java平台AOP技术概览 3.1.1 AOP技术在Java平台中的应用 AOP在实验室应用和商业应用上,Java平台始终走在前面.从最初也是目前最成熟的AOP工具--AspectJ,到目前已 ...
Taro 3.5 canary 发布：支持适配鸿蒙
一.背景鸿蒙作为华为自研开发的一款可以实现万物互联的操作系统,一经推出就受到了很大的关注,被国人寄予了厚望.而鸿蒙也没让人失望,今年 Harmony2.0 正式推出供用户进行升级之后,在短短的三个月 ...
typeScript基本概念
我一直认为学习是知识的累加,而前端技术也是进步的.所以学习的重点就是,'它有什么不同,它好在哪里'.这要求我们必须结合之前的经验和知识去学习一门新技术,而不是无情的复制粘贴机器. 首先,ts的官方定义 ...
合并项目（Project）
<Project2016 企业项目管理实践>张会斌董方好编著在熬肥的世界里,有个大难题,就是多文件合并-- 好吧,以前是大难题,现在,早就不是了,Word有主控文档,Excel有&q ...
1、学习算法和刷题的框架思维——Go版
前情提示:Go语言学习者.本文参考https://labuladong.gitee.io/algo,代码自己参考抒写,若有不妥之处,感谢指正关于golang算法文章,为了便于下载和整理,都已开源放在 ...
Blazor Webassembly多标签页实现非iframe的实现
前面用iframe实现了多标签页,这次是不用iframe实现的多标签页.参考了一下Ant Design Blazor. 基本功能已经都实现了不论是标签滚动.激活.左移.右移.关闭其他,还是在内容页跳转 ...

【jvm】02-手写自己的类加载器

【jvm】02-手写自己的类加载器

1.简单手写自己的类加载器

2.加载一个和jdk中同名的类

tomcat的类加载器逻辑

3.完整代码

4.ConstantPool

参考资料

【jvm】02-手写自己的类加载器的更多相关文章

随机推荐

热门专题