用Java实现JVM第一章《命令列工具》
背景描述
为了更好的学习jvm阅读过《Java虚拟机器规范》、《自己动手写Java虚拟机器》，尤其是《自动动手写java虚拟机器》可以更加清晰的看到全貌。对于程式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。
Jvm架构图
案例简述
本章节主要是通过编写java程式码，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
配置资讯
1、除错配置
2.1、配置位置：Run/Debug Configurations -> program arguments
2.2、配置内容：-version
程式码示例
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
pom.xml
com.beust
jcommander
1.72
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 mainClassAndArgs;
boolean ok;
String getMainClass() {
return mainClassAndArgs != null && !mainClassAndArgs.isEmpty()
? mainClassAndArgs.get(0)
: null;
}
List 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;
/**
* 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: [-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 ", cmd.classpath, cmd.getMainClass(), cmd.getAppArgs());
}
}
测试结果
java version "1.8.0"
