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用Java实现JVM第一章《命令行工具》

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

jdk 1.8.0
IntelliJ IDEA Community Edition 2018.3.1 x64

配置信息

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

<!-- 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

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

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());
    }

}
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