Skip to content

2019 05 02 用Java实现JVM第二章《搜索class文件》

Jump to bottom
fuzhengwei edited this page May 16, 2020 · 1 revision

作者：小傅哥
博客：https://bugstack.cn - 原创系列专题

沉淀、分享、成长，让自己和他人都能有所收获！

案例简述

本章节主要了解Java虚拟机从哪里寻找class文件并且读取class内字节码

环境准备

  1. jdk 1.8.0
  2. IntelliJ IDEA Community Edition 2018.3.1 x64
  3. Notepad++ （插件安装HEX-Editor，用于查看class字节）

配置信息

  1. 调试配置
    1. 配置位置：Run/Debug Configurations -> program arguments
    2. 配置内容：-Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-02\target\test-classes\org\itstack\demo\test\HelloWorld

代码示例

itstack-demo-jvm-02
├── pom.xml
└── src
    └── main
    │    └── java
    │        └── org.itstack.demo.jvm
    │             ├── classpath
    │             │   ├── impl
    │             │   │   ├── CompositeEntry.java
    │             │   │   ├── DirEntry.java 
    │             │   │   ├── WildcardEntry.java 
    │             │   │   └── ZipEntry.java    
    │             │   ├── Classpath.java
    │             │   └── Entry.java    
    │             ├── Cmd.java
    │             └── Main.java
    └── test
         └── java
             └── org.itstack.demo.test
                 └── HelloWorld.java

pom.xml

<!-- 命令行参数解析器 -->
<dependency>
  <groupId>com.beust</groupId>
  <artifactId>jcommander</artifactId>
  <version>1.72</version>
</dependency>

CompositeEntry.java

package org.itstack.demo.jvm.classpath.impl;

import org.itstack.demo.jvm.classpath.Entry;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 */
public class CompositeEntry implements Entry {

    private final List<Entry> entryList = new ArrayList<>();

    public CompositeEntry(String pathList) {
        String[] paths = pathList.split(File.pathSeparator);
        for (String path : paths) {
            entryList.add(Entry.create(path));
        }
    }

    @Override
    public byte[] readClass(String className) throws IOException {
        for (Entry entry : entryList) {
            try {
                return entry.readClass(className);
            } catch (Exception ignored) {
                //ignored
            }
        }
        throw new IOException("class not found " + className);
    }


    @Override
    public String toString() {
        String[] strs = new String[entryList.size()];
        for (int i = 0; i < entryList.size(); i++) {
            strs[i] = entryList.get(i).toString();
        }
        return String.join(File.pathSeparator, strs);
    }
    
}

DirEntry.java

package org.itstack.demo.jvm.classpath.impl;

import org.itstack.demo.jvm.classpath.Entry;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * 目录形式的类路径
 */
public class DirEntry implements Entry {

    private Path absolutePath;

    public DirEntry(String path){
        //获取绝对路径
        this.absolutePath = Paths.get(path).toAbsolutePath();
    }

    @Override
    public byte[] readClass(String className) throws IOException {
        return Files.readAllBytes(absolutePath.resolve(className));
    }

    @Override
    public String toString() {
        return this.absolutePath.toString();
    }
}

WildcardEntry.java

package org.itstack.demo.jvm.classpath.impl;

import org.itstack.demo.jvm.classpath.Entry;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Collectors;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * 通配符类路径，继承CompositeEntry
 */
public class WildcardEntry extends CompositeEntry {

    public WildcardEntry(String path) {
        super(toPathList(path));
    }

    private static String toPathList(String wildcardPath) {
        String baseDir = wildcardPath.replace("*", ""); // remove *
        try {
            return Files.walk(Paths.get(baseDir))
                    .filter(Files::isRegularFile)
                    .map(Path::toString)
                    .filter(p -> p.endsWith(".jar") || p.endsWith(".JAR"))
                    .collect(Collectors.joining(File.pathSeparator));
        } catch (IOException e) {
            return "";
        }
    }

}

ZipEntry.java

package org.itstack.demo.jvm.classpath.impl;

import org.itstack.demo.jvm.classpath.Entry;

import java.io.IOException;
import java.nio.file.*;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * zip/zar文件形式类路径
 */
public class ZipEntry implements Entry {

    private Path absolutePath;

    public ZipEntry(String path) {
        //获取绝对路径
        this.absolutePath = Paths.get(path).toAbsolutePath();
    }

    @Override
    public byte[] readClass(String className) throws IOException {
        try (FileSystem zipFs = FileSystems.newFileSystem(absolutePath, null)) {
            return Files.readAllBytes(zipFs.getPath(className));
        }
    }

    @Override
    public String toString() {
        return this.absolutePath.toString();
    }

}

Classpath.java

package org.itstack.demo.jvm.classpath;

import org.itstack.demo.jvm.classpath.impl.WildcardEntry;

import java.io.File;
import java.nio.file.Files;
import java.nio.file.Paths;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * 类路径
 */
public class Classpath {

    private Entry bootstrapClasspath;  //启动类路径
    private Entry extensionClasspath;  //扩展类路径
    private Entry userClasspath;       //用户类路径

    public Classpath(String jreOption, String cpOption) {
        //启动类&扩展类 "C:\Program Files\Java\jdk1.8.0_161\jre"
        bootstrapAndExtensionClasspath(jreOption);
        //用户类 E:\..\org\itstack\demo\test\HelloWorld
        parseUserClasspath(cpOption);
    }

    private void bootstrapAndExtensionClasspath(String jreOption) {
        
        String jreDir = getJreDir(jreOption);

        //..jre/lib/*
        String jreLibPath = Paths.get(jreDir, "lib") + File.separator + "*";
        bootstrapClasspath = new WildcardEntry(jreLibPath);

        //..jre/lib/ext/*
        String jreExtPath = Paths.get(jreDir, "lib", "ext") + File.separator + "*";
        extensionClasspath = new WildcardEntry(jreExtPath);

    }

    private static String getJreDir(String jreOption) {
        if (jreOption != null && Files.exists(Paths.get(jreOption))) {
            return jreOption;
        }
        if (Files.exists(Paths.get("./jre"))) {
            return "./jre";
        }
        String jh = System.getenv("JAVA_HOME");
        if (jh != null) {
            return Paths.get(jh, "jre").toString();
        }
        throw new RuntimeException("Can not find JRE folder!");
    }

    private void parseUserClasspath(String cpOption) {
        if (cpOption == null) {
            cpOption = ".";
        }
        userClasspath = Entry.create(cpOption);
    }

    public byte[] readClass(String className) throws Exception {
        className = className + ".class";

        //[readClass]启动类路径
        try {
            return bootstrapClasspath.readClass(className);
        } catch (Exception ignored) {
            //ignored
        }

        //[readClass]扩展类路径
        try {
            return extensionClasspath.readClass(className);
        } catch (Exception ignored) {
            //ignored
        }

        //[readClass]用户类路径
        return userClasspath.readClass(className);
    }

}

Entry.java

package org.itstack.demo.jvm.classpath;

import org.itstack.demo.jvm.classpath.impl.CompositeEntry;
import org.itstack.demo.jvm.classpath.impl.DirEntry;
import org.itstack.demo.jvm.classpath.impl.WildcardEntry;
import org.itstack.demo.jvm.classpath.impl.ZipEntry;

import java.io.File;
import java.io.IOException;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * 类路径接口
 */
public interface Entry {

    byte[] readClass(String className) throws IOException;

    static Entry create(String path) {

        //File.pathSeparator；路径分隔符(win\linux)
        if (path.contains(File.pathSeparator)) {
            return new CompositeEntry(path);
        }

        if (path.endsWith("*")) {
            return new WildcardEntry(path);
        }

        if (path.endsWith(".jar") || path.endsWith(".JAR") ||
                path.endsWith(".zip") || path.endsWith(".ZIP")) {
            return new ZipEntry(path);
        }

        return new DirEntry(path);
    }

}

Cmd.java

package org.itstack.demo.jvm;

import com.beust.jcommander.JCommander;
import com.beust.jcommander.Parameter;

import java.util.List;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 */
public class Cmd {

    @Parameter(names = {"-?", "-help"}, description = "print help message", order = 3, help = true)
    boolean helpFlag = false;

    @Parameter(names = "-version", description = "print version and exit", order = 2)
    boolean versionFlag = false;

    @Parameter(names = {"-cp", "-classpath"}, description = "classpath", order = 1)
    String classpath;

    @Parameter(names = "-Xjre", description = "path to jre", order = 4)
    String jre;

    @Parameter(description = "main class and args")
    List<String> mainClassAndArgs;

    boolean ok;

    String getMainClass() {
        return mainClassAndArgs != null && !mainClassAndArgs.isEmpty()
                ? mainClassAndArgs.get(0)
                : null;
    }

    List<String> getAppArgs() {
        return mainClassAndArgs != null && mainClassAndArgs.size() > 1
                ? mainClassAndArgs.subList(1, mainClassAndArgs.size())
                : null;
    }

    static Cmd parse(String[] argv) {
        Cmd args = new Cmd();
        JCommander cmd = JCommander.newBuilder().addObject(args).build();
        cmd.parse(argv);
        args.ok = true;
        return args;
    }
    
}

Main.java

package org.itstack.demo.jvm;

import org.itstack.demo.jvm.classpath.Classpath;

import java.util.Arrays;

/**
 * http://www.itstack.org
 * create by fuzhengwei on 2019/4/24
 * program arguments：-Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-02\target\test-classes\org\itstack\demo\test\HelloWorld
 */
public class Main {

    public static void main(String[] args) {
        Cmd cmd = Cmd.parse(args);
        if (!cmd.ok || cmd.helpFlag) {
            System.out.println("Usage: <main class> [-options] class [args...]");
            return;
        }
        if (cmd.versionFlag) {
            //注意案例测试都是基于1.8，另外jdk1.9以后使用模块化没有rt.jar
            System.out.println("java version \"1.8.0\"");
            return;
        }
        startJVM(cmd);
    }

    private static void startJVM(Cmd cmd) {
        Classpath cp = new Classpath(cmd.jre, cmd.classpath);
        System.out.printf("classpath：%s class：%s args：%s\n", cp, cmd.getMainClass(), cmd.getAppArgs());
        //获取className
        String className = cmd.getMainClass().replace(".", "/");
        try {
            byte[] classData = cp.readClass(className);
            System.out.println("classData：");
            for (byte b : classData) {
                //16进制输出
                System.out.print(String.format("%02x", b & 0xff) + " ");
            }
        } catch (Exception e) {
            System.out.println("Could not find or load main class " + cmd.getMainClass());
            e.printStackTrace();
        }
    }

}

测试结果

classpath：org.itstack.demo.jvm.classpath.Classpath@4bf558aa class：E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-02\target\test-classes\org\itstack\demo\test\HelloWorld args：null
classData：
ca fe ba be 00 00 00 34 00 22 0a 00 06 00 14 09 
00 15 00 16 08 00 17 0a 00 18 00 19 07 00 1a 07 
00 1b 01 00 06 3c 69 6e 69 74 3e 01 00 03 28 29 
56 01 00 04 43 6f 64 65 01 00 0f 4c 69 6e 65 4e 
75 6d 62 65 72 54 61 62 6c 65 01 00 12 4c 6f 63 
61 6c 56 61 72 69 61 62 6c 65 54 61 62 6c 65 01 
00 04 74 68 69 73 01 00 22 4c 6f 72 67 2f 69 74 
73 74 61 63 6b 2f 64 65 6d 6f 2f 74 65 73 74 2f 
48 65 6c 6c 6f 57 6f 72 6c 64 3b 01 00 04 6d 61 
69 6e 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 
67 2f 53 74 72 69 6e 67 3b 29 56 01 00 04 61 72 
67 73 01 00 13 5b 4c 6a 61 76 61 2f 6c 61 6e 67 
2f 53 74 72 69 6e 67 3b 01 00 0a 53 6f 75 72 63 
65 46 69 6c 65 01 00 0f 48 65 6c 6c 6f 57 6f 72 
6c 64 2e 6a 61 76 61 0c 00 07 00 08 07 00 1c 0c 
00 1d 00 1e 01 00 0d 48 65 6c 6c 6f 2c 20 77 6f 
72 6c 64 21 07 00 1f 0c 00 20 00 21 01 00 20 6f 
72 67 2f 69 74 73 74 61 63 6b 2f 64 65 6d 6f 2f 
74 65 73 74 2f 48 65 6c 6c 6f 57 6f 72 6c 64 01 
00 10 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 
63 74 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 
79 73 74 65 6d 01 00 03 6f 75 74 01 00 15 4c 6a 
61 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 
61 6d 3b 01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 
69 6e 74 53 74 72 65 61 6d 01 00 07 70 72 69 6e 
74 6c 6e 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e 
67 2f 53 74 72 69 6e 67 3b 29 56 00 21 00 05 00 
06 00 00 00 00 00 02 00 01 00 07 00 08 00 01 00 
09 00 00 00 2f 00 01 00 01 00 00 00 05 2a b7 00 
01 b1 00 00 00 02 00 0a 00 00 00 06 00 01 00 00 
00 03 00 0b 00 00 00 0c 00 01 00 00 00 05 00 0c 
00 0d 00 00 00 09 00 0e 00 0f 00 01 00 09 00 00 
00 37 00 02 00 01 00 00 00 09 b2 00 02 12 03 b6 
00 04 b1 00 00 00 02 00 0a 00 00 00 0a 00 02 00 
00 00 06 00 08 00 07 00 0b 00 00 00 0c 00 01 00 
00 00 09 00 10 00 11 00 00 00 01 00 12 00 00 00 
02 00 13

结果验证 Notepad++ 打开HelloWorld.class，在'插件'工具中选HEX-Editor设置为View in Hex 默认为8-bit

上一篇：用Java实现JVM第一章《命令行工具》

下一篇：用Java实现JVM第三章《解析class文件》

微信搜索「bugstack虫洞栈」公众号，关注后回复「用Java实现jvm源码」获取本文源码&更多原创专题案例！

小傅哥(微信：fustack)，公众号：bugstack虫洞栈 | bugstack.cn - 沉淀、分享、成长，让自己和他人都能有所收获！

📝 首页

🌏 知识星球：码农会锁

实战项目：「DDD+RPC分布式抽奖系统」、专属小册、问题解答、简历指导、架构图稿、视频课程

🐲 头条

⛳ 目录

  1. 源码 - :octocat: 公众号：bugstack虫洞栈 文章所涉及到的全部开源代码
  2. Java
  3. Spring
  4. 面向对象
  5. 中间件
  6. Netty 4.x
  7. 字节码编程
  8. 💯实战项目
  9. 部署 Dev-Ops
  10. 📚PDF 下载
  11. 关于

💋 精选

🐾 友链

转载分享

建立本开源项目的初衷是基于个人学习与工作中对 Java 相关技术栈的总结记录，在这里也希望能帮助一些在学习 Java 过程中遇到问题的小伙伴，如果您需要转载本仓库的一些文章到自己的博客，请按照以下格式注明出处，谢谢合作。

作者：小傅哥
链接：https://bugstack.cn
来源：bugstack虫洞栈

参与贡献

2021年10月24日，小傅哥 的文章全部开源到代码库 CodeGuide 中，与同好同行，一起进步，共同维护。

这里我提供 3 种方式：

  1. 提出 Issue ：在 Issue 中指出你觉得需要改进/完善的地方(能够独立解决的话，可以在提出 Issue 后再提交 PR )。
  2. 处理 Issue : 帮忙处理一些待处理的 Issue。
  3. 提交 PR： 对于错别字/笔误这类问题可以直接提交PR，无需提交Issue 确认。

详细参考：CodeGuide 贡献指南 - 非常感谢你的支持，这里会留下你的足迹

与我联系 💌

  • 加群交流 本群的宗旨是给大家提供一个良好的技术学习交流平台，所以杜绝一切广告！由于微信群人满 100 之后无法加入，请扫描下方二维码先添加作者 “小傅哥” 微信(fustack)，备注：加群。
微信：fustack

  • 公众号(bugstack虫洞栈) - 沉淀、分享、成长，专注于原创专题案例，以最易学习编程的方式分享知识，让自己和他人都能有所收获。
公众号：bugstack虫洞栈

致谢

感谢以下人员对本仓库做出的贡献或者对小傅哥的赞赏，当然不仅仅只有这些贡献者，这里就不一一列举了。如果你希望被添加到这个名单中，并且提交过 Issue 或者 PR，请与我联系。

Clone this wiki locally