ITPub博客

首页 > Linux操作系统 > Linux操作系统 > [转载]Class文件详解

[转载]Class文件详解

原创 Linux操作系统 作者:dinner1007 时间:2019-02-04 09:57:06 0 删除 编辑
Class文件详解
我们都知道,Java编译器负责将.java文件编译成.class文件,class文件存储的是java字节码,与.java文件无关(只要你愿意写一个编译器,也可以将别的语言写的源代码编译成.class文件),本文准备详细解剖class文件的内部结构,并且把class文件结构读取并显示出来。
  
  Class文件的格式由JVM规范规定,一共有以下部分:
  
  1. magic number,必须是0xCAFEBABE,用于快速识别是否是一个class文件。
  
  2. version,包括major和minor,如果版本号超过了JVM的识别范围,JVM将拒绝执行。
  
  3. constant pool,常量池,存放所有用到的常量。
  
  4. access flag,定义类的访问权限。
  
  5. this class和super class,指示如何找到this class和super class。
  
  6. interfaces,存放所有interfaces。
  
  7. fields,存放所有fields。
  
  8. methods,存放所有methods。
  
  9. attributes,存放所有attributes。
  
  先写一个Test.java:
  
  package example.test;
  
  public final class TestClass {
  public int id = 1234567;
  public void test() {}
  }
  
  然后编译,放在C:example estTest.class。
  
  我们用Java来读取和分析class,ClassAnalyzer的功能便是读取Test.class,分析结构,然后显示出来:
  
  package classfile.format;
  
  import java.io.*;
  
  public class ClassAnalyzer {
  
  public static void main(String[] args) {
  DataInputStream input = null;
  try {
  input = new DataInputStream(new BufferedInputStream(new FileInputStream(
  "C:\example\test\TestClass.class"
  )));
  analyze(input);
  }
  catch(Exception e) {
  System.out.println("Analyze failed!");
  }
  finally {
  try { input.close(); } catch(Exception e) {}
  }
  }
  
  public static void analyze(DataInputStream input) throws IOException {
  // read magic number:
  int magic = input.readInt();
  if(magic==0xCAFEBABE)
  System.out.println("magic number = 0xCAFEBABE");
  else
  throw new RuntimeException("Invalid magic number!");
  // read minor version and major version:
  short minor_ver = input.readShort();
  short major_ver = input.readShort();
  System.out.println("Version = " + major_ver + "." + minor_ver);
  // read constant pool:
  short const_pool_count = input.readShort();
  System.out.println("constant pool size = " + const_pool_count);
  // read each constant:
  for(int i=1; i  analyzeConstant(input, i);
  }
  }
  
  public static void analyzeConstant(DataInputStream input, int index) throws IOException {
  byte flag = input.readByte();
  // for read:
  byte n8;
  short n16;
  int n32;
  long n64;
  float f;
  double d;
  byte[] buffer;
  System.out.println(" const index = " + index + ", flag = " + (int)flag);
  switch(flag) {
  case 1: // utf-8 string
  System.out.println(" const type = Utf8");
  n16 = input.readShort();
  System.out.println("   length = " + n16);
  buffer = new byte[n16];
  input.readFully(buffer);
  System.out.println("   value = " + new String(buffer));
  break;
  case 3: // integer
  System.out.println(" const type = Integer");
  n32 = input.readInt();
  System.out.println("   value = " + n32);
  break;
  case 4: // float
  System.out.println(" const type = Float");
  f = input.readFloat();
  System.out.println("   value = " + f);
  break;
  case 5: // long
  System.out.println(" const type = Long");
  n64 = input.readLong();
  System.out.println("   value = " + n64);
  break;
  case 6: // double
  System.out.println(" const type = Double");
  d = input.readDouble();
  System.out.println("   value = " + d);
  break;
  case 7: // class or interface reference
  System.out.println(" const type = Class");
  n16 = input.readShort();
  System.out.println("   index = " + n16 + " (where to find the class name)");
  break;
  case 8: // string
  System.out.println(" const type = String");
  n16 = input.readShort();
  System.out.println("   index = " + n16);
  break;
  case 9: // field reference
  System.out.println(" const type = Fieldref");
  n16 = input.readShort();
  System.out.println("class index = " + n16 + " (where to find the class)");
  n16 = input.readShort();
  System.out.println("nameAndType = " + n16 + " (where to find the NameAndType)");
  break;
  case 10: // method reference
  System.out.println(" const type = Methodref");
  n16 = input.readShort();
  System.out.println("class index = " + n16 + " (where to find the class)");
  n16 = input.readShort();
  System.out.println("nameAndType = " + n16 + " (where to find the NameAndType)");
  break;
  case 11: // interface method reference
  System.out.println(" const type = InterfaceMethodref");
  n16 = input.readShort();
  System.out.println("class index = " + n16 + " (where to find the interface)");
  n16 = input.readShort();
  System.out.println("nameAndType = " + n16 + " (where to find the NameAndType)");
  break;
  case 12: // name and type reference
  System.out.println(" const type = NameAndType");
  n16 = input.readShort();
  System.out.println(" name index = " + n16 + " (where to find the name)");
  n16 = input.readShort();
  System.out.println(" descripter = " + n16 + " (where to find the descriptor)");
  break;
  default:
  throw new RuntimeException("Invalid constant pool flag: " + flag);
  }
  }
  }
  
  输出结果为:
  
  magic number = 0xCAFEBABE
  Version = 48.0
  constant pool size = 22
  
  const index = 1, flag = 1
  const type = Utf8
  length = 22
  value = example/test/TestClass
  
  const index = 2, flag = 7
  const type = Class
  index = 1 (where to find the class name)
  
  const index = 3, flag = 1
  const type = Utf8
  length = 16
  value = java/lang/Object
  
  const index = 4, flag = 7
  const type = Class
  index = 3 (where to find the class name)
  
  const index = 5, flag = 1
  const type = Utf8
  length = 2
  value = id
  
  const index = 6, flag = 1
  const type = Utf8
  length = 1
  value = I
  
  const index = 7, flag = 1
  const type = Utf8
  length = 6
  value =
  
  const index = 8, flag = 1
  const type = Utf8
  length = 3
  value = ()V
  
  const index = 9, flag = 1
  const type = Utf8
  length = 4
  value = Code
  
  const index = 10, flag = 12
  const type = NameAndType
  name index = 7 (where to find the name)
  descripter = 8 (where to find the descriptor)
  
  const index = 11, flag = 10
  const type = Methodref
  class index = 4 (where to find the class)
  nameAndType = 10 (where to find the NameAndType)
  
  const index = 12, flag = 3
  const type = Integer
  value = 1234567
  
  const index = 13, flag = 12
  const type = NameAndType
  name index = 5 (where to find the name)
  descripter = 6 (where to find the descriptor)
  
  const index = 14, flag = 9
  const type = Fieldref
  class index = 2 (where to find the class)
  nameAndType = 13 (where to find the NameAndType)
  
  const index = 15, flag = 1
  const type = Utf8
  length = 15
  value = LineNumberTable
  
  const index = 16, flag = 1
  const type = Utf8
  length = 18
  value = LocalVariableTable
  
  const index = 17, flag = 1
  const type = Utf8
  length = 4
  value = this
  
  const index = 18, flag = 1
  const type = Utf8
  length = 24
  value = Lexample/test/TestClass;
  
  const index = 19, flag = 1
  const type = Utf8
  length = 4
  value = test
  
  const index = 20, flag = 1
  const type = Utf8
  length = 10
  value = SourceFile
  
  const index = 21, flag = 1
  const type = Utf8
  length = 14
  value = TestClass.java
  
  我们暂时只读取到constant pool,后面的信息下次再继续 :)
  继续分析Class文件的结构。
  
  上次读取了constant pool,紧接着是这个类或接口的Access flags,JVM定义的关于class或interface的Access flags有:
  
  private static short ACC_PUBLIC  = 0x0001;
  private static short ACC_FINAL   = 0x0010;
  private static short ACC_SUPER   = 0x0020;
  private static short ACC_INTERFACE = 0x0200;
  private static short ACC_ABSTRACT = 0x0400;
  
  // read access flags:
  short access_flags = input.readShort();
  System.out.print("Access flags:");
  if((access_flags & ACC_PUBLIC) == ACC_PUBLIC)
  System.out.print(" public");
  if((access_flags & ACC_FINAL) == ACC_FINAL)
  System.out.print(" final");
  if((access_flags & ACC_SUPER) == ACC_SUPER)
  System.out.print(" super");
  if((access_flags & ACC_INTERFACE) == ACC_INTERFACE)
  System.out.print(" interface");
  if((access_flags & ACC_ABSTRACT) == ACC_ABSTRACT)
  System.out.print(" abstract");
  System.out.println();
  
  然后是this class和super class:
  
  // read this class and super class:
  short this_class_index = input.readShort();
  short super_class_index = input.readShort();
  System.out.println("This class = " + this_class_index);
  System.out.println("Super class = " + super_class_index);
  
  根据this class的index可以从constant pool中得到这个class的信息,super class也一样。每个class都有super class,只有Object的super class index=0。
  
  接下来是这个class继承了多少个interface和每个interface在constant pool中的index:
  
  // read interfaces count:
  short interfaces_count = input.readShort();
  System.out.println("Interfaces count = " + interfaces_count);
  // read each interface:
  for(int i=1; i<=interfaces_count; i++) {
  short interface_index = input.readShort();
  System.out.println("No. " + i + " interface index = " + interface_index);
  }
  
  结果如下:
  
  Access flags: public final super
  This class = 2
  Super class = 4
  Interfaces count = 0

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/374079/viewspace-130896/,如需转载,请注明出处,否则将追究法律责任。

请登录后发表评论 登录
全部评论

注册时间:2018-08-23

  • 博文量
    177
  • 访问量
    86502