1 准备工作
获取class文件byte[]
public static byte[] getFileBytes(File file) {
try (FileInputStream fileInputStream = new FileInputStream(file)) {
int available = fileInputStream.available();
byte[] data=new byte[available];
fileInputStream.read(data,0,available-1);
return data;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
这里使用jdk的ByteBuffer包装bytes
ByteBuffer data = ByteBuffer.wrap(getFileBytes(file));
因为ByteBuffer没有无符号的读取方法,所以自己实现一下,也可以直接用netty的Bytebuf,里面方法齐全
// 图方便直接返回int
private int readUnsignedByte(ByteBuffer data) {
return data.get() & 0xff;
}
// 图方便直接返回int
private int readUnsignedShort(ByteBuffer data) {
return data.getShort() & 0xffff;
}
private long readUnsignedInt(ByteBuffer data) {
return data.getInt() & 0xffffffffL;
}
定义class文件结构
参考: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html
private static class ClassFileStructure {
long magic;
int minorVersion;
int majorVersion;
int constantPoolCount;
ConstantPool[] constantPool;
int accessFlags;
int thisClass;
int superClass;
int interfacesCount;
int[] interfaces;
int fieldsCount;
FieldInfo[] fields;
int methodsCount;
MethodInfo[] methods;
int attributesCount;
AttributeInfo[] attributes;
}
2 开始解析
2.1 magic
private void magic(ClassFileStructure structure, ByteBuffer data) {
structure.magic = readUnsignedInt(data);
}
2.2 minorVersion
private void minorVersion(ClassFileStructure structure, ByteBuffer data) {
structure.minorVersion = readUnsignedShort(data);
}
2.3 majorVersion
private void majorVersion(ClassFileStructure structure, ByteBuffer data) {
structure.majorVersion = readUnsignedShort(data);
}
2.4 constantPoolCount
private void constantPoolCount(ClassFileStructure structure, ByteBuffer data) {
structure.constantPoolCount = readUnsignedShort(data);
}
2.5 ConstantPool[]
ConstantPool不同tag解析方式不同,定义抽象类ConstantPool,子类按规则解析
private abstract static class ConstantPool {
int tag;
public ConstantPool(int tag) {
this.tag = tag;
}
abstract void parse(ByteBuffer data);
}
子类实现ConstantPool
ConstantUtf8:
private class ConstantUtf8 extends ConstantPool {
int length;
byte[] bytes;
public ConstantUtf8(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.length = readUnsignedShort(data);
bytes = new byte[this.length];
for (int i = 0; i < this.length; i++) {
bytes[i] = (byte) readUnsignedByte(data);
}
}
}
ConstantMethodHandle:
private class ConstantMethodHandle extends ConstantPool {
short referenceKind;
int referenceIndex;
public ConstantMethodHandle(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.referenceKind = (short) readUnsignedByte(data);
this.referenceIndex = readUnsignedShort(data);
}
}
ConstantMethodType:
private class ConstantMethodType extends ConstantPool {
int descriptorIndex;
public ConstantMethodType(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.descriptorIndex = readUnsignedShort(data);
}
}
ConstantClass:
private class ConstantClass extends ConstantPool {
int nameIndex;
public ConstantClass(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.nameIndex = readUnsignedShort(data);
}
}
ConstantClass:
private class ConstantClass extends ConstantPool {
int nameIndex;
public ConstantClass(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.nameIndex = readUnsignedShort(data);
}
}
ConstantFieldref:
private class ConstantFieldref extends ConstantPool {
int classIndex;
int nameAndTypeIndex;
public ConstantFieldref(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.classIndex = readUnsignedShort(data);
this.nameAndTypeIndex = readUnsignedShort(data);
}
}
ConstantMethodref:
private class ConstantMethodref extends ConstantPool {
int classIndex;
int nameAndTypeIndex;
public ConstantMethodref(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.classIndex = readUnsignedShort(data);
this.nameAndTypeIndex = readUnsignedShort(data);
}
}
ConstantInterfaceMethodref:
private class ConstantInterfaceMethodref extends ConstantPool {
int classIndex;
int nameAndTypeIndex;
public ConstantInterfaceMethodref(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.classIndex = readUnsignedShort(data);
this.nameAndTypeIndex = readUnsignedShort(data);
}
}
ConstantString:
private class ConstantString extends ConstantPool {
int stringIndex;
public ConstantString(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.stringIndex = readUnsignedShort(data);
}
}
ConstantInteger:
private class ConstantInteger extends ConstantPool {
long bytes;
public ConstantInteger(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.bytes = readUnsignedInt(data);
}
}
ConstantFloat:
private class ConstantFloat extends ConstantPool {
long bytes;
public ConstantFloat(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.bytes = readUnsignedInt(data);
}
}
ConstantLong:
private class ConstantLong extends ConstantPool {
long highBytes;
long lowBytes;
public ConstantLong(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.highBytes = readUnsignedInt(data);
this.lowBytes = readUnsignedInt(data);
}
}
ConstantDouble:
private class ConstantDouble extends ConstantPool {
long highBytes;
long lowBytes;
public ConstantDouble(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.highBytes = readUnsignedInt(data);
this.lowBytes = readUnsignedInt(data);
}
}
ConstantNameAndType:
private class ConstantNameAndType extends ConstantPool {
int nameIndex;
int descriptorIndex;
public ConstantNameAndType(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.nameIndex = readUnsignedShort(data);
this.descriptorIndex = readUnsignedShort(data);
}
}
ConstantInvokeDynamic:
private class ConstantInvokeDynamic extends ConstantPool {
int bootstrapMethodAttrIndex;
int nameAndTypeIndex;
public ConstantInvokeDynamic(int tag) {
super(tag);
}
@Override
void parse(ByteBuffer data) {
this.bootstrapMethodAttrIndex = readUnsignedShort(data);
this.nameAndTypeIndex = readUnsignedShort(data);
}
}
以上是所有 constantPool 子类型,现在开始解析
坑：“所有 8 字节常量都占用文件constant_pool表中的两个条目class。 如果
CONSTANT_Long_infoorCONSTANT_Double_info结构是constant_pool表中索引n处的项目，则池中的下一个可用项目位于索引n +2 处。
constant_pool 索引n +1 必须有效但被视为不可用 。 回想起来，让 8 字节常量占用两个常量池条目是一个糟糕的选择。”
private void constantPool(ClassFileStructure structure, ByteBuffer data) {
structure.constantPool = new ConstantPool[structure.constantPoolCount - 1];
for (int i = 0; i < structure.constantPool.length; i++) {
int type = readUnsignedByte(data);
int index = i;
switch (type) {
case 1: //
structure.constantPool[i] = new ConstantUtf8(type);
break;
case 3: //
structure.constantPool[i] = new ConstantInteger(type);
break;
case 4:
structure.constantPool[i] = new ConstantFloat(type);
break;
case 5:
structure.constantPool[i] = new ConstantLong(type);
i++;// 占2位
break;
case 6:
structure.constantPool[i] = new ConstantDouble(type);
i++;// 占2位
break;
case 7: //
structure.constantPool[i] = new ConstantClass(type);
break;
case 8: //
structure.constantPool[i] = new ConstantString(type);
break;
case 9: //
structure.constantPool[i] = new ConstantFieldref(type);
break;
case 10: //
structure.constantPool[i] = new ConstantMethodref(type);
break;
case 11: //
structure.constantPool[i] = new ConstantInterfaceMethodref(type);
break;
case 12: //
structure.constantPool[i] = new ConstantNameAndType(type);
break;
case 15: //
structure.constantPool[i] = new ConstantMethodHandle(type);
break;
case 16: //
structure.constantPool[i] = new ConstantMethodType(type);
break;
case 18: //
structure.constantPool[i] = new ConstantInvokeDynamic(type);
break;
default:
throw new ParserException("class file parser exception");
}
structure.constantPool[index].parse(data);
}
}
2.6 accessFlags
private void accessFlags(ClassFileStructure structure, ByteBuffer data) {
structure.accessFlags = readUnsignedShort(data);
}
2.7 thisClass
private void thisClass(ClassFileStructure structure, ByteBuffer data) {
structure.thisClass = readUnsignedShort(data);
}
2.8 superClass
private void superClass(ClassFileStructure structure, ByteBuffer data) {
structure.superClass = readUnsignedShort(data);
}
2.9 interfacesCount
private void interfacesCount(ClassFileStructure structure, ByteBuffer data) {
structure.interfacesCount = readUnsignedShort(data);
}
2.10 int[] interfaces
private void interfaces(ClassFileStructure structure, ByteBuffer data) {
structure.interfaces = new int[structure.interfacesCount];
for (int i = 0; i < structure.interfacesCount; i++) {
structure.interfaces[i] = readUnsignedShort(data);
}
}
2.11 fieldsCount
private ClassFile fieldsCount(ClassFileStructure structure, ByteBuffer data) {
structure.fieldsCount = readUnsignedShort(data);
return this;
}
2.12 FieldInfo[] fields
FieldInfo：
private class FieldInfo {
int accessFlags;
int nameIndex;
int descriptorIndex;
int attributesCount;
AttributeInfo[] attributes;
public FieldInfo parse(ByteBuffer data) {
this.accessFlags = readUnsignedShort(data);
this.nameIndex = readUnsignedShort(data);
this.descriptorIndex = readUnsignedShort(data);
this.attributesCount = readUnsignedShort(data);
this.attributes = new AttributeInfo[attributesCount];
for (int i = 0; i < this.attributesCount; i++) {
this.attributes[i] = new AttributeInfo().parse(data);
}
return this;
}
}
AttributeInfo:
private class AttributeInfo {
int attributeNameIndex;
long attributeLength;
short[] info;
public AttributeInfo parse(ByteBuffer data) {
this.attributeNameIndex = readUnsignedShort(data);
this.attributeLength = readUnsignedInt(data);
this.info = new short[(int) attributeLength];
for (int i = 0; i < this.attributeLength; i++) {
this.info[i] = (short) readUnsignedByte(data);
}
return this;
}
}
private void fields(ClassFileStructure structure, ByteBuffer data) {
structure.fields = new FieldInfo[structure.fieldsCount];
for (int i = 0; i < structure.fieldsCount; i++) {
structure.fields[i] = new FieldInfo().parse(data);
}
}
2.13 methodsCount
private ClassFile methodsCount(ClassFileStructure structure, ByteBuffer data) {
structure.methodsCount = readUnsignedShort(data);
return this;
}
2.14 MethodInfo[]
MethodInfo:
private class MethodInfo {
int accessFlags;
int nameIndex;
int descriptorIndex;
int attributesCount;
AttributeInfo[] attributes;
public MethodInfo parse(ByteBuffer data) {
this.accessFlags = readUnsignedShort(data);
this.nameIndex = readUnsignedShort(data);
this.descriptorIndex = readUnsignedShort(data);
this.attributesCount = readUnsignedShort(data);
this.attributes = new AttributeInfo[attributesCount];
for (int i = 0; i < this.attributesCount; i++) {
this.attributes[i] = new AttributeInfo().parse(data);
}
return this;
}
}
private void methods(ClassFileStructure structure, ByteBuffer data) {
structure.methods = new MethodInfo[structure.methodsCount];
for (int i = 0; i < structure.methodsCount; i++) {
structure.methods[i] = new MethodInfo().parse(data);
}
}
2.15 attributesCount
private void attributesCount(ClassFileStructure structure, ByteBuffer data) {
structure.attributesCount = readUnsignedShort(data);
}
2.16 AttributeInfo[]
private void attributes(ClassFileStructure structure, ByteBuffer data) {
structure.attributes = new AttributeInfo[structure.attributesCount];
for (int i = 0; i < structure.attributesCount; i++) {
structure.attributes[i] = new AttributeInfo().parse(data);
}
}