用Java实现JVM第一章《命令行工具》

背景描述

为了更好的学习jvm阅读过《Java虚拟机规范》、《自己动手写Java虚拟机》，尤其是《自动动手写java虚拟机》可以更加清晰的看到全貌。对于程序开发者来说学习一个新东西最好事必躬亲，亲力亲为的做出一些demo，只有输出了结果心里才踏实。

案例简述

本章节主要是通过编写java代码，从main方法入口进行获取指令。例如；-version

环境准备

1. jdk 1.8.0
2. IntelliJ IDEA Community Edition 2018.3.1 x64

配置信息

1. 调试配置
   1. 配置位置：Run/Debug Configurations -> program arguments
   2. 配置内容：-version

代码示例

itstack-demo-jvm-01├── pom.xml└── src    └── main    │    └── java    │        └── org.itstack.demo.jvm    │		      ├── Cmd.java    │ 		      └── Main.java    └── test		 └── java			 └── org.itstack.demo.test				 └── HelloWorld.java			  

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pom.xml

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

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Cmd.java

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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(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;    }    }

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Main.java

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package org.itstack.demo.jvm;/** * http://www.itstack.org * create by fuzhengwei on 2019/4/24 * program arguments：-version */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) {            System.out.println("java version /"1.8.0/"");            return;        }        startJVM(cmd);    }    private static void startJVM(Cmd cmd) {        System.out.printf("classpath:%s class:%s args:%s/n", cmd.classpath, cmd.getMainClass(), cmd.getAppArgs());    }}

测试结果

java version "1.8.0"

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