JavaClass字节码文件结构详解

1、Class字节码中有两种数据类型：1. 字节数据直接量：这是基本的数据类型。共细分为u1、u2、u4、u8四种，分别代表连续的1个字节、2个字节、4个字节、8个字节组成的整体数据。2. 表：表是由多个基本数据或其他表，按照既定顺序组成的大的数据集合。表是有结构的，它的结构体现在，组成表的成分所在的位置和顺序都是已经严格定义好的。Class字节码总体结构如下：具体详解请参考我在这里要说明几个细节问题：1. 为什么说常量表的数量是constant_pool_count-1，且索引从1开始而不是0。其实根本原因在于，索引为0也是一个常量（保留常量），只不过它不存在常量表，这个常量就对应null值。因

2、此加上这个系统保留常量，常量个数共为constant_pool_count个，但是常量表数量要减1。2. 在常量池中，如果存在long型或double型字面量，它们会占用两个连续索引。比如：假设一个类中只有一个int型字面量1和一个double型字面量1（当然这种假设是不可能的，因为总会有类名字面量等），则常量池个数为3，而不是2。这正是因为double字面量占用了两个连续的索引。接下来，贴出一个小demo来展示如何读取字节码：ClassParser负责把握Class字节码整体结构的解析。packagecom.lixin;importjava.io.IOException;importjava

3、.io.InputStream;publicclassClassParserprivateInputStreamin;publicClassParser(InputStreamin)this.in=in;publicvoidparse()throwsIOException/魔数magicNumber();/主次版本号version();/常量池constantPool();/类或接口修饰符accessFlag();/继承关系（当前类、父类、父接口）inheritence();/字段集合fieldList();/方法集合methodList();/属性集合attributeList();priv

4、atevoidattributeList()throwsIOExceptionline();intattrLength=StreamUtils.read2(in);System.out.println(共有+attrLength+个属性);for(inti=0;iattrLength;i+)line();attribute();privatevoidattribute()throwsIOExceptionintnameIndex=StreamUtils.read2(in);intlength=StreamUtils.read4(in);byteinfo=StreamUtils.read(in,

5、length);System.out.println(nameIndex:+nameIndex);System.out.println(length:+length);System.out.println(info:+info);privatevoidmethodList()throwsIOExceptionintlength=StreamUtils.read2(in);System.out.println(共有+length+个方法);for(inti=0;ilength;i+)method();privatevoidmethod()throwsIOExceptionSystem.out.p

6、rintln(-);intaccessFlag=StreamUtils.read2(in);intnameIndex=StreamUtils.read2(in);intdescriptorIndex=StreamUtils.read2(in);System.out.println(accessFlag:+accessFlag);System.out.println(nameIndex:+nameIndex);System.out.println(descriptorIndex:+descriptorIndex);attributeList();privatevoidfieldList()thr

7、owsIOExceptionline();intlength=StreamUtils.read2(in);System.out.println(共有+length+个字段);for(inti=0;ilength;i+)System.out.println(-);intaccessFlag=StreamUtils.read2(in);intnameIndex=StreamUtils.read2(in);intdescriptorIndex=StreamUtils.read2(in);System.out.println(accessFlag:+accessFlag);System.out.pri

8、ntln(nameIndex:+nameIndex);System.out.println(descriptorIndex:+descriptorIndex);attributeList();privatevoidinheritence()throwsIOExceptionline();intthisClassRef=StreamUtils.read2(in);intsuperClassRef=StreamUtils.read2(in);System.out.println(thisClassRef:+thisClassRef);System.out.println(superClassRef

9、:+superClassRef);intinterfaceLen=StreamUtils.read2(in);System.out.println(接口数量：+interfaceLen);for(inti=0;iinterfaceLen;i+)intinterfaceRef=StreamUtils.read2(in);System.out.println(interfaceRef:+interfaceRef);privatevoidaccessFlag()throwsIOExceptionline();intaccessFlag=StreamUtils.read2(in);System.out

10、.println(accessFlag:0x+Integer.toHexString(accessFlag)+(+accessFlag+);privatevoidconstantPool()throwsIOExceptionnewConstantPoolParser(in).constPool();privatevoidversion()throwsIOExceptionline();intminorVersion=StreamUtils.read2(in);intmajorVersion=StreamUtils.read2(in);System.out.println(版本：+majorVe

11、rsion+.+minorVersion);privatevoidmagicNumber()throwsIOExceptionline();intmagic=StreamUtils.read4(in);System.out.println(魔数：+Integer.toHexString(magic).toUpperCase();privatevoidline()System.out.println(-);ConstPoolParser负责常量池的解析（因为常量池表较多，且数据量也较大，因此单独拉出来解析）packagecom.lixin;importjava.io.IOException;im

12、portjava.io.InputStream;publicclassConstPoolParserpublicstaticfinalintUtf8_info=1;publicstaticfinalintInteger_info=3;publicstaticfinalintFloat_info=4;publicstaticfinalintLong_info=5;publicstaticfinalintDouble_info=6;publicstaticfinalintClass_info=7;publicstaticfinalintString_info=8;publicstaticfinal

13、intFieldref_info=9;publicstaticfinalintMethodref_info=10;publicstaticfinalintInterfaceMethodref_info=11;publicstaticfinalintNameAndType_info=12;publicstaticfinalintMethodHandle_info=15;publicstaticfinalintMethodType_info=16;publicstaticfinalintInvokeDynamic_info=18;privateInputStreamin;publicConstPo

14、olParser(InputStreamin)this.in=in;publicvoidconstPool()throwsIOExceptionline();intlength=StreamUtils.read2(in);System.out.println(共有+length+个常量);booleandoubleBytes=false;for(inti=1;ilength;i+)if(doubleBytes)doubleBytes=false;continue;line();System.out.println(常量索引：+i);intflag=StreamUtils.read1(in);/

15、System.out.println(标志:+flag);switch(flag)caseUtf8_info:utf8Info();continue;caseInteger_info:integerInfo();continue;caseFloat_info:floatInfo();continue;caseLong_info:doubleBytes=true;longInfo();continue;caseDouble_info:doubleBytes=true;doubleInfo();continue;caseClass_info:classInfo();continue;caseStr

16、ing_info:stringInfo();continue;caseFieldref_info:fieldrefInfo();continue;caseMethodref_info:methodrefInfo();continue;caseInterfaceMethodref_info:interfaceMethodrefInfo();continue;caseNameAndType_info:nameAndTypeInfo();continue;caseMethodHandle_info:methodHandleInfo();continue;caseMethodType_info:met

17、hodTypeInfo();continue;caseInvokeDynamic_info:invokeDynamicInfo();continue;default:System.err.println(flag);thrownewRuntimeException(unknown);privatevoidline()System.out.println(-);privatevoidutf8Info()throwsIOExceptionintlength=StreamUtils.read2(in);bytebuf=StreamUtils.read(in,length);Strings=newSt

18、ring(buf,0,buf.length);System.out.println(utf8Info表：);System.out.println(值：+s);privatevoidintegerInfo()throwsIOExceptionSystem.out.println(integerInfo表：);intvalue=StreamUtils.read4(in);System.out.println(值：+value);privatevoidfloatInfo()throwsIOExceptionSystem.out.println(floatInfo表：);intvalue=Stream

19、Utils.read4(in);floatf=FBitsToFloat(value);System.out.println(值：+f);privatevoidlongInfo()throwsIOExceptionSystem.out.println(longInfo表：);longvalue=StreamUtils.read8(in);System.out.println(值：+value);privatevoiddoubleInfo()throwsIOExceptionSystem.out.println(doubleInfo表：);longvalue=StreamUtils

20、.read8(in);doubled=Double.longBitsToDouble(value);System.out.println(值：+d);privatevoidclassInfo()throwsIOExceptionSystem.out.println(classInfo表:);intindex=StreamUtils.read2(in);System.out.println(index:+index);privatevoidstringInfo()throwsIOExceptionSystem.out.println(stringInfo表:);intindex=StreamUt

21、ils.read2(in);System.out.println(index:+index);privatevoidfieldrefInfo()throwsIOExceptionintclassIndex=StreamUtils.read2(in);intnameAndTypeIndex=StreamUtils.read2(in);System.out.println(fieldrefInfo表:);System.out.println(classIndex:+classIndex);System.out.println(nameAndTypeIndex:+nameAndTypeIndex);

22、privatevoidmethodrefInfo()throwsIOExceptionintclassIndex=StreamUtils.read2(in);intnameAndTypeIndex=StreamUtils.read2(in);System.out.println(methodrefInfo表:);System.out.println(classIndex:+classIndex);System.out.println(nameAndTypeIndex:+nameAndTypeIndex);privatevoidinterfaceMethodrefInfo()throwsIOEx

23、ceptionintclassIndex=StreamUtils.read2(in);intnameAndTypeIndex=StreamUtils.read2(in);System.out.println(interfaceMethodrefInfo表:);System.out.println(classIndex:+classIndex);System.out.println(nameAndTypeIndex:+nameAndTypeIndex);privatevoidnameAndTypeInfo()throwsIOExceptionintnameIndex=StreamUtils.re

24、ad2(in);inttypeIndex=StreamUtils.read2(in);System.out.println(nameAndTypeInfo表:);System.out.println(nameIndex:+nameIndex);System.out.println(typeIndex:+typeIndex);privatevoidmethodHandleInfo()throwsIOExceptionintreferenceKind=StreamUtils.read1(in);intreferenceIndex=StreamUtils.read2(in);System.out.println(methodHandleInfo表：);System.out.println(referenceKind:+referenceKind);System.out.println(referenceIndex:+referenceIndex);privatevoidmethodTypeInfo()throwsIOExceptionSystem.out.println(methodTypeInfo表：);intdescriptorIndex=