查看文件权限 linux Class文件详解 (1)

Class文件详解 (1)  

　　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 　　　　 magic number 必须是 xCAFEBABE 用于快速识别是否是一个class文件 　　　　 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 　　　　 constant pool 常量池 存放所有用到的常量 　　　　 access flag 定义类的访问权限 　　　　 this class和super class 指示如何找到this class和super class 　　　　 interfaces 存放所有interfaces 　　　　 fields 存放所有fields 　　　　 methods 存放所有methods 　　　　 attributes 存放所有attributes 　　　　先写一个Test java 　　　　package example test;　　　　public final class TestClass {　　public int id = ;　　public void test() {}　　}　　　　然后编译 放在C:exampletestTest 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== xCAFEBABE)　　System out println( magic number = xCAFEBABE );　　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= ; 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("nconst 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 lishixinzhi/Article/program/Java/JSP/201311/19269 ;>