公众号：bugstack虫洞栈 | 用Java实现JVM专题

src/itstack-demo-jvm/README.md

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+# itstack-demo-jvm 用java实现JVM
+微信公众号：bugstack虫洞栈，欢迎您的关注&获取更多案例与源码！博客栈：https://bugstack.cn 
+
+>JVM是Java Virtual Machine（Java虚拟机）的缩写，JVM是一种用于计算设备的规范，它是一个虚构出来的计算机，是通过在实际的计算机上仿真模拟各种计算机功能来实现的。
+
+
+本专题主要介绍如何通过java代码来实现JVM的基础功能（搜索解析class文件、字节码命令、运行时数据区等），从而让java程序员通过最熟知的java程序，学习JVM是如何将java程序一步步跑起来的，按照
+《java虚拟机规范》与go语言版的《自己动手写Java虚拟机》实现java版虚拟机案例如下；
+
+
+## 章节列表
+- [用Java实现JVM第一章《命令行工具》](https://bugstack.cn/?p=59)
+- [用Java实现JVM第二章《搜索class文件》](https://bugstack.cn/?p=60)
+- [用Java实现JVM第三章《解析class文件》](https://bugstack.cn/?p=63)
+- [用Java实现JVM第三章《解析class文件》附[classReader拆解]](https://bugstack.cn/?p=64)
+- [用Java实现JVM第四章《运行时数据区》](https://bugstack.cn/?p=66)
+- [用Java实现JVM第五章《指令集和解释器》](https://bugstack.cn/?p=68)
+- [用Java实现JVM第六章《类和对象》](https://bugstack.cn/?p=69)
+- [用Java实现JVM第七章《方法调用和返回》](https://bugstack.cn/?p=72)
+- [用Java实现JVM第八章《数组和字符串》](https://bugstack.cn/?p=73)
+- [用Java实现JVM第九章《本地方法调用》](https://bugstack.cn/?p=74)
+- [用Java实现JVM第十章《异常处理》](https://bugstack.cn/?p=75)
+
+------------
+
+## 微信公众号：bugstack虫洞栈，欢迎您的关注&获取源码！
+
+![微信公众号：bugstack虫洞栈，欢迎您的关注&获取源码！](https://bugstack.cn/wp-content/uploads/2019/08/qrcode清晰.png)

src/itstack-demo-jvm/itstack-demo-jvm-01/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>itstack-demo-jvm</artifactId>
+        <groupId>org.itatack.demo</groupId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>itstack-demo-jvm-01</artifactId>
+
+
+</project>

src/itstack-demo-jvm/itstack-demo-jvm-01/src/main/java/org/itstack/demo/jvm/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;
+    }
+
+}
+
+

src/itstack-demo-jvm/itstack-demo-jvm-01/src/main/java/org/itstack/demo/jvm/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());
+    }
+
+}

src/itstack-demo-jvm/itstack-demo-jvm-01/src/test/java/org/itstack/demo/test/HelloWorld.java

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+package org.itstack.demo.test;
+
+public class HelloWorld {
+
+    public static void main(String[] args) {
+        System.out.println("hello world");
+    }
+
+}

src/itstack-demo-jvm/itstack-demo-jvm-02/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>itstack-demo-jvm</artifactId>
+        <groupId>org.itatack.demo</groupId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>itstack-demo-jvm-02</artifactId>
+
+
+</project>

src/itstack-demo-jvm/itstack-demo-jvm-02/src/main/java/org/itstack/demo/jvm/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(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;
+    }
+
+}
+
+

src/itstack-demo-jvm/itstack-demo-jvm-02/src/main/java/org/itstack/demo/jvm/Main.java

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+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(Arrays.toString(classData));
+
+            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();
+        }
+    }
+
+}

src/itstack-demo-jvm/itstack-demo-jvm-02/src/main/java/org/itstack/demo/jvm/classpath/Classpath.java

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+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);
+    }
+
+}

src/itstack-demo-jvm/itstack-demo-jvm-02/src/main/java/org/itstack/demo/jvm/classpath/Entry.java

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+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);
+    }
+
+}