用Java实现JVM第二章《搜索class文件》

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
2.1、配置位置:Run/Debug Configurations -> program arguments
2.2、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

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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

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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>

CompositeEntry.java

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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

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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

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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

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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

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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);
}

}

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);
}

}

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;
}

}

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("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

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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
HelloWorld.class.png