1. Java class文件解析

为了能够更加深入的学习class结构，本章节将写一个ClassByteCodeParser类（有极小部分数据结构较复杂没解析）来实现简单的class文件解析。
首先我们创建一个用于测试的TestHelloWorld.java文件，源码如下：

package com.anbai.sec.bytecode;
import java.io.Serializable;
/**
 * Creator: yz
 * Date: 2019/12/17
 */
@Deprecated
public class TestHelloWorld implements Serializable {
  private static final long serialVersionUID = -7366591802115333975L;
  private long id = 1l;
  private String username;
  private String password;
  public String hello(String content) {
    String str = "Hello:";
    return str + content;
  }
  public static void main(String[] args) {
    TestHelloWorld test = new TestHelloWorld();
    String         str  = test.hello("Hello World~");
    System.out.println(str);
  }
  public long getId() {
    return id;
  }
  public void setId(long id) {
    this.id = id;
  }
  public String getUsername() {
    return username;
  }
  public void setUsername(String username) {
    this.username = username;
  }
  public String getPassword() {
    return password;
  }
  public void setPassword(String password) {
    this.password = password;
  }
  @Override
  public String toString() {
    return "TestHelloWorld{" +
        "id=" + id +
        ", username='" + username + '\'' +
        ", password='" + password + '\'' +
        '}';
  }
}

然后使用javac将TestHelloWorld.java编译成TestHelloWorld.class文件，或者使用maven构建javaweb-sec/javaweb-sec-source/javase/项目，构建成功后在javaweb-sec/javaweb-sec-source/javase/target/classes/com/anbai/sec/bytecode/目录下可以找到TestHelloWorld.class文件。

最后编写一个ClassByteCodeParser类，严格按照JVM规范中的类文件格式文档规定，依次解析class文件的各种数据类型就可以实现字节码解析了。
ClassByteCodeParser代码片段（省略了getter/setter和解析逻辑）：

package com.anbai.sec.bytecode;
/**
 * Java类字节码解析，参考：https://docs.oracle.com/javase/specs/jvms/se15/jvms15.pdf和https://github.com/ingokegel/jclasslib
 */
public class ClassByteCodeParser {
  /**
   * 转换为数据输入流
   */
  private DataInputStream dis;
  /**
   * Class文件魔数
   */
  private int magic;
  /**
   * Class小版本号
   */
  private int minor;
  /**
   * Class大版本号
   */
  private int major;
  /**
   * 常量池中的对象数量
   */
  private int poolCount;
  /**
   * 创建常量池Map
   */
  private final Map<Integer, Map<String, Object>> constantPoolMap = new LinkedHashMap<>();
  /**
   * 类访问修饰符
   */
  private int accessFlags;
  /**
   * thisClass
   */
  private String thisClass;
  /**
   * superClass
   */
  private String superClass;
  /**
   * 接口数
   */
  private int interfacesCount;
  /**
   * 接口Index数组
   */
  private String[] interfaces;
  /**
   * 成员变量数量
   */
  private int fieldsCount;
  /**
   * 成员变量数组
   */
  private final Set<Map<String, Object>> fieldList = new HashSet<>();
  /**
   * 方法数
   */
  private int methodsCount;
  /**
   * 方法数组
   */
  private final Set<Map<String, Object>> methodList = new HashSet<>();
  /**
   * 属性数
   */
  private int attributesCount;
  /**
   * 属性
   */
  private Map<String, Object> attributes;
  /**
   * 解析Class字节码
   *
   * @param in 类字节码输入流
   * @throws IOException 解析IO异常
   */
  private void parseByteCode(InputStream in) throws IOException {
    // 将输入流转换成DataInputStream
    this.dis = new DataInputStream(in);
    // 解析字节码逻辑代码
  }
  public static void main(String[] args) throws IOException {
    // 解析单个class文件
    File                classFile  = new File(System.getProperty("user.dir"), "javaweb-sec-source/javase/target/classes/com/anbai/sec/bytecode/TestHelloWorld.class");
    ClassByteCodeParser codeParser = new ClassByteCodeParser();
    codeParser.parseByteCode(new FileInputStream(classFile));
    System.out.println(JSON.toJSONString(codeParser));
  }
}

解析完TestHelloWorld.class后将会生成一个json字符串，省略掉复杂的constantPoolMap、fieldList、methodList、attributes属性后格式如下：

{
    "accessFlags": 33, 
    "attributes": {}, 
    "attributesCount": 3, 
    "constantPoolMap": {}, 
    "fieldList": [], 
    "fieldsCount": 4, 
    "interfaces": [
        "java/io/Serializable"
    ], 
    "interfacesCount": 1, 
    "magic": -889275714, 
    "major": 51, 
    "methodList": [], 
    "methodsCount": 10, 
    "minor": 0, 
    "poolCount": 95, 
    "superClass": "java/lang/Object", 
    "thisClass": "com/anbai/sec/bytecode/TestHelloWorld"
}