案例介绍
本案例主要介绍通过java代码从class文件中解析；class文件、常量池、属性表；
作为类（或者接口）信息的载体，每个class文件都完整地定义了一个类。为了使java程序可以“编写一次，处处运行”，Java虚拟机规范对class文件格式进行了严格的规定。但是另外一方面，对于从哪里加载class文件，给了足够多的自由。Java虚拟机实现可以从文件系统读取和从JAR（或ZIP）压缩包中提取clss文件。除此之外，也可以通过网络下载、从数据库加载，甚至是在运行中直接生成class文件。Java虚拟机规范中所指的class文件，并非特指位于磁盘中的.class文件，而是泛指任何格式符号规范的class数据。
环境准备
- jdk 1.8.0
- IntelliJ IDEA Community Edition 2018.3.1 x64
配置信息
- 调试配置
- 配置位置：Run/Debug Configurations -> program arguments
- 配置内容：-Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" java.lang.String
代码示例
itstack-demo-jvm-03
├── pom.xml
└── src
└── main
│ └── java
│ └── org.itstack.demo.jvm
│ ├── classfile
│ │ ├── attributes {BootstrapMethods/Code/ConstantValue...}
│ │ ├── constantpool {CONSTANT_TAG_CLASS/CONSTANT_TAG_FIELDREF/CONSTANT_TAG_METHODREF...}
│ │ ├── ClassFile.java
│ │ ├── ClassReader.java
│ │ └── MemberInfo.java
│ ├── classpath
│ │ ├── impl
│ │ │ ├── CompositeEntry.java
│ │ │ ├── DirEntry.java
│ │ │ ├── WildcardEntry.java
│ │ │ └── ZipEntry.java
│ │ ├── Classpath.java
│ │ └── Entry.java
│ ├── Cmd.java
│ └── Main.java
└── test
└── java
└── org.itstack.demo.test
└── HelloWorld.java
代码篇幅较长，不一一列举，需要源码可以关注公众号：bugstack虫洞栈
AttributeInfo.java
package org.itstack.demo.jvm.classfile.attributes;
import org.itstack.demo.jvm.classfile.ClassReader;
import org.itstack.demo.jvm.classfile.attributes.impl.*;
import org.itstack.demo.jvm.classfile.constantpool.ConstantPool;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/4/26
*/
public interface AttributeInfo {
void readInfo(ClassReader reader);
static AttributeInfo[] readAttributes(ClassReader reader, ConstantPool constantPool) {
int attributesCount = reader.readU2ToInt();
AttributeInfo[] attributes = new AttributeInfo[attributesCount];
for (int i = 0; i < attributesCount; i++) {
attributes[i] = readAttribute(reader, constantPool);
}
return attributes;
}
static AttributeInfo readAttribute(ClassReader reader, ConstantPool constantPool) {
int attrNameIdx = reader.readU2ToInt();
String attrName = constantPool.getUTF8(attrNameIdx);
int attrLen = reader.readU4ToInt();
AttributeInfo attrInfo = newAttributeInfo(attrName, attrLen, constantPool);
attrInfo.readInfo(reader);
return attrInfo;
}
static AttributeInfo newAttributeInfo(String attrName, int attrLen, ConstantPool constantPool) {
switch (attrName) {
case "BootstrapMethods":
return new BootstrapMethodsAttribute();
case "Code":
return new CodeAttribute(constantPool);
case "ConstantValue":
return new ConstantValueAttribute();
case "Deprecated":
return new DeprecatedAttribute();
case "EnclosingMethod":
return new EnclosingMethodAttribute(constantPool);
case "Exceptions":
return new ExceptionsAttribute();
case "InnerClasses":
return new InnerClassesAttribute();
case "LineNumberTable":
return new LineNumberTableAttribute();
case "LocalVariableTable":
return new LocalVariableTableAttribute();
case "LocalVariableTypeTable":
return new LocalVariableTypeTableAttribute();
// case "MethodParameters":
// case "RuntimeInvisibleAnnotations":
// case "RuntimeInvisibleParameterAnnotations":
// case "RuntimeInvisibleTypeAnnotations":
// case "RuntimeVisibleAnnotations":
// case "RuntimeVisibleParameterAnnotations":
// case "RuntimeVisibleTypeAnnotations":
case "Signature":
return new SignatureAttribute(constantPool);
case "SourceFile":
return new SourceFileAttribute(constantPool);
// case "SourceDebugExtension":
// case "StackMapTable":
case "Synthetic":
return new SyntheticAttribute();
default:
return new UnparsedAttribute(attrName, attrLen);
}
}
}
ConstantInfo.java
package org.itstack.demo.jvm.classfile.constantpool;
import org.itstack.demo.jvm.classfile.ClassReader;
import org.itstack.demo.jvm.classfile.constantpool.impl.*;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/4/26
*/
public interface ConstantInfo {
int CONSTANT_TAG_CLASS = 7;
int CONSTANT_TAG_FIELDREF = 9;
int CONSTANT_TAG_METHODREF = 10;
int CONSTANT_TAG_INTERFACEMETHODREF = 11;
int CONSTANT_TAG_STRING = 8;
int CONSTANT_TAG_INTEGER = 3;
int CONSTANT_TAG_FLOAT = 4;
int CONSTANT_TAG_LONG = 5;
int CONSTANT_TAG_DOUBLE = 6;
int CONSTANT_TAG_NAMEANDTYPE = 12;
int CONSTANT_TAG_UTF8 = 1;
int CONSTANT_TAG_METHODHANDLE = 15;
int CONSTANT_TAG_METHODTYPE = 16;
int CONSTANT_TAG_INVOKEDYNAMIC = 18;
void readInfo(ClassReader reader);
int tag();
static ConstantInfo readConstantInfo(ClassReader reader, ConstantPool constantPool) {
int tag = reader.readU1ToInt();
ConstantInfo constantInfo = newConstantInfo(tag, constantPool);
constantInfo.readInfo(reader);
return constantInfo;
}
static ConstantInfo newConstantInfo(int tag, ConstantPool constantPool) {
switch (tag) {
case CONSTANT_TAG_INTEGER:
return new ConstantIntegerInfo();
case CONSTANT_TAG_FLOAT:
return new ConstantFloatInfo();
case CONSTANT_TAG_LONG:
return new ConstantLongInfo();
case CONSTANT_TAG_DOUBLE:
return new ConstantDoubleInfo();
case CONSTANT_TAG_UTF8:
return new ConstantUtf8Info();
case CONSTANT_TAG_STRING:
return new ConstantStringInfo(constantPool);
case CONSTANT_TAG_CLASS:
return new ConstantClassInfo(constantPool);
case CONSTANT_TAG_FIELDREF:
return new ConstantFieldRefInfo(constantPool);
case CONSTANT_TAG_METHODREF:
return new ConstantMethodRefInfo(constantPool);
case CONSTANT_TAG_INTERFACEMETHODREF:
return new ConstantInterfaceMethodRefInfo(constantPool);
case CONSTANT_TAG_NAMEANDTYPE:
return new ConstantNameAndTypeInfo();
case CONSTANT_TAG_METHODTYPE:
return new ConstantMethodTypeInfo();
case CONSTANT_TAG_METHODHANDLE:
return new ConstantMethodHandleInfo();
case CONSTANT_TAG_INVOKEDYNAMIC:
return new ConstantInvokeDynamicInfo();
default:
throw new ClassFormatError("constant pool tag");
}
}
}
ClassFile.java
package org.itstack.demo.jvm.classfile;
import org.itstack.demo.jvm.classfile.attributes.AttributeInfo;
import org.itstack.demo.jvm.classfile.constantpool.ConstantPool;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/4/26
*/
public class ClassFile {
private int minorVersion;
private int majorVersion;
private ConstantPool constantPool;
private int accessFlags;
private int thisClassIdx;
private int supperClassIdx;
private int[] interfaces;
private MemberInfo[] fields;
private MemberInfo[] methods;
private AttributeInfo[] attributes;
public ClassFile(byte[] classData) {
ClassReader reader = new ClassReader(classData);
this.readAndCheckMagic(reader);
this.readAndCheckVersion(reader);
this.constantPool = this.readConstantPool(reader);
this.accessFlags = reader.readU2ToInt();
this.thisClassIdx = reader.readU2ToInt();
this.supperClassIdx = reader.readU2ToInt();
this.interfaces = reader.readUInt16s();
this.fields = MemberInfo.readMembers(reader, constantPool);
this.methods = MemberInfo.readMembers(reader, constantPool);
this.attributes = AttributeInfo.readAttributes(reader, constantPool);
}
private void readAndCheckMagic(ClassReader reader) {
String magic = reader.readU4ToHexStr();
if (!"cafebabe".equals(magic)) {
throw new ClassFormatError("magic!");
}
}
private void readAndCheckVersion(ClassReader reader) {
this.minorVersion = reader.readU2ToInt();
this.majorVersion = reader.readU2ToInt();
switch (this.majorVersion) {
case 45:
return;
case 46:
case 47:
case 48:
case 49:
case 50:
case 51:
case 52:
if (this.minorVersion == 0)
return;
}
throw new UnsupportedClassVersionError();
}
private ConstantPool readConstantPool(ClassReader reader) {
return new ConstantPool(reader);
}
public int minorVersion(){
return this.minorVersion;
}
public int majorVersion(){
return this.majorVersion;
}
public ConstantPool constantPool(){
return this.constantPool;
}
public int accessFlags() {
return this.accessFlags;
}
public MemberInfo[] fields() {
return this.fields;
}
public MemberInfo[] methods() {
return this.methods;
}
public String className() {
return this.constantPool.getClassName(this.thisClassIdx);
}
public String superClassName() {
if (this.supperClassIdx <= 0) return "";
return this.constantPool.getClassName(this.supperClassIdx);
}
public String[] interfaceNames() {
String[] interfaceNames = new String[this.interfaces.length];
for (int i = 0; i < this.interfaces.length; i++) {
interfaceNames[i] = this.constantPool.getClassName(interfaces[i]);
}
return interfaceNames;
}
}
ClassReader.java
package org.itstack.demo.jvm.classfile;
import java.math.BigInteger;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/5/13
* <p>
* java虚拟机定义了u1、u2、u4三种数据类型来表示；1字节、2字节、4字节，无符号整数。
* 在如下实现中，用增位方式表示无符号类型：
* u1、u2可以用int类型存储，因为int类型是4字节
* u4 需要用long类型存储，因为long类型是8字节
*/
public class ClassReader {
private byte[] data;
public ClassReader(byte[] data) {
this.data = data;
}
//u1
public int readUint8() {
byte[] val = readByte(1);
return byte2int(val);
}
//u2
public int readUint16() {
byte[] val = readByte(2);
return byte2int(val);
}
//u4
public long readUint32() {
byte[] val = readByte(4);
String str_hex = new BigInteger(1, val).toString(16);
return Long.parseLong(str_hex, 16);
}
public float readUint64TFloat() {
byte[] val = readByte(8);
return new BigInteger(1, val).floatValue();
}
public long readUint64TLong() {
byte[] val = readByte(8);
return new BigInteger(1, val).longValue();
}
public double readUint64TDouble() {
byte[] val = readByte(8);
return new BigInteger(1, val).doubleValue();
}
public int[] readUint16s() {
int n = this.readUint16();
int[] s = new int[n];
for (int i = 0; i < n; i++) {
s[i] = this.readUint16();
}
return s;
}
public byte[] readBytes(int n) {
return readByte(n);
}
private byte[] readByte(int length) {
byte[] copy = new byte[length];
System.arraycopy(data, 0, copy, 0, length);
System.arraycopy(data, length, data, 0, data.length - length);
return copy;
}
private int byte2int(byte[] val) {
String str_hex = new BigInteger(1, val).toString(16);
return Integer.parseInt(str_hex, 16);
}
}
MemberInfo.java
package org.itstack.demo.jvm.classfile;
import org.itstack.demo.jvm.classfile.attributes.AttributeInfo;
import org.itstack.demo.jvm.classfile.attributes.impl.CodeAttribute;
import org.itstack.demo.jvm.classfile.attributes.impl.ConstantValueAttribute;
import org.itstack.demo.jvm.classfile.constantpool.ConstantPool;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/4/26
*/
public class MemberInfo {
private ConstantPool constantPool;
private int accessFlags;
private int nameIdx;
private int descriptorIdx;
private AttributeInfo[] attributes;
public MemberInfo(ClassReader reader, ConstantPool constantPool) {
this.constantPool = constantPool;
this.accessFlags = reader.readU2ToInt();
this.nameIdx = reader.readU2ToInt();
this.descriptorIdx = reader.readU2ToInt();
this.attributes = AttributeInfo.readAttributes(reader, constantPool);
}
public static MemberInfo[] readMembers(ClassReader reader, ConstantPool constantPool) {
int fieldCount = reader.readU2ToInt();
MemberInfo[] fields = new MemberInfo[fieldCount];
for (int i = 0; i < fieldCount; i++) {
fields[i] = new MemberInfo(reader, constantPool);
}
return fields;
}
public int accessFlags() {
return this.accessFlags;
}
public String name() {
return this.constantPool.getUTF8(this.nameIdx);
}
public String descriptor() {
return this.constantPool.getUTF8(this.descriptorIdx);
}
public CodeAttribute codeAttribute() {
for (AttributeInfo attrInfo : attributes) {
if (attrInfo instanceof CodeAttribute) return (CodeAttribute) attrInfo;
}
return null;
}
public ConstantValueAttribute ConstantValueAttribute() {
for (AttributeInfo attrInfo : attributes) {
if (attrInfo instanceof ConstantValueAttribute) return (ConstantValueAttribute) attrInfo;
}
return null;
}
}
Main.java
package org.itstack.demo.jvm;
import org.itstack.demo.jvm.classfile.ClassFile;
import org.itstack.demo.jvm.classfile.MemberInfo;
import org.itstack.demo.jvm.classpath.Classpath;
import java.util.Arrays;
/**
* http://www.itstack.org
* create by fuzhengwei on 2019/4/24
*/
public class Main {
public static void main(String[] args) {
Cmd cmd = Cmd.parse(args);
if (!cmd.ok || cmd.helpFlag) {
System.out.println("Usage: <main class> [-options] class [args...]");
return;
}
if (cmd.versionFlag) {
//注意案例测试都是基于1.8，另外jdk1.9以后使用模块化没有rt.jar
System.out.println("java version \"1.8.0\"");
return;
}
startJVM(cmd);
}
private static void startJVM(Cmd cmd) {
Classpath classpath = new Classpath(cmd.jre, cmd.classpath);
System.out.printf("classpath：%s class：%s args：%s\n",
classpath, cmd.getMainClass(), cmd.getAppArgs());
//获取className
String className = cmd.getMainClass().replace(".", "/");
ClassFile classFile = loadClass(className, classpath);
assert classFile != null;
printClassInfo(classFile);
}
private static ClassFile loadClass(String className, Classpath classpath) {
try {
byte[] classData = classpath.readClass(className);
return new ClassFile(classData);
} catch (Exception e) {
System.out.println("Could not find or load main class " + className);
return null;
}
}
private static void printClassInfo(ClassFile cf) {
System.out.println("version: " + cf.majorVersion() + "." + cf.minorVersion());
System.out.println("constants count：" + cf.constantPool().getSiz());
System.out.format("access flags：0x%x\n", cf.accessFlags());
System.out.println("this class：" + cf.className());
System.out.println("super class：" + cf.superClassName());
System.out.println("interfaces：" + Arrays.toString(cf.interfaceNames()));
System.out.println("fields count：" + cf.fields().length);
for (MemberInfo memberInfo : cf.fields()) {
System.out.format(" %s\n", memberInfo.name());
}
System.out.println("methods count: " + cf.methods().length);
for (MemberInfo memberInfo : cf.methods()) {
System.out.format(" %s\n", memberInfo.name());
}
}
}
测试结果
"C:\Program Files\Java\jdk1.8.0_161\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2018.3.1\lib\idea_rt.jar=61458:D:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2018.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_161\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\rt.jar;E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-03\target\classes;D:\Program Files (x86)\apache-maven-2.2.1\repository\com\beust\jcommander\1.72\jcommander-1.72.jar;D:\Program Files (x86)\apache-maven-2.2.1\repository\org\projectlombok\lombok\1.18.0\lombok-1.18.0.jar;D:\Program Files (x86)\apache-maven-2.2.1\repository\com\alibaba\fastjson\1.2.40\fastjson-1.2.40.jar" org.itstack.demo.jvm.Main -Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" java.lang.String
classpath：org.itstack.demo.jvm.classpath.Classpath@4bf558aa class：java.lang.String args：null
version: 52.0
constants count：540
access flags：0x31
this class：java/lang/String
super class：java/lang/Object
interfaces：[java/io/Serializable, java/lang/Comparable, java/lang/CharSequence]
fields count：5
value [C
hash I
serialVersionUID J
serialPersistentFields [Ljava/io/ObjectStreamField;
CASE_INSENSITIVE_ORDER Ljava/util/Comparator;
methods count: 94
<init> ()V
<init> (Ljava/lang/String;)V
<init> ([C)V
<init> ([CII)V
<init> ([III)V
<init> ([BIII)V
<init> ([BI)V
checkBounds ([BII)V
<init> ([BIILjava/lang/String;)V
<init> ([BIILjava/nio/charset/Charset;)V
<init> ([BLjava/lang/String;)V
<init> ([BLjava/nio/charset/Charset;)V
<init> ([BII)V
<init> ([B)V
<init> (Ljava/lang/StringBuffer;)V
<init> (Ljava/lang/StringBuilder;)V
<init> ([CZ)V
length ()I
isEmpty ()Z
charAt (I)C
codePointAt (I)I
codePointBefore (I)I
codePointCount (II)I
offsetByCodePoints (II)I
getChars ([CI)V
getChars (II[CI)V
getBytes (II[BI)V
getBytes (Ljava/lang/String;)[B
getBytes (Ljava/nio/charset/Charset;)[B
getBytes ()[B
equals (Ljava/lang/Object;)Z
contentEquals (Ljava/lang/StringBuffer;)Z
nonSyncContentEquals (Ljava/lang/AbstractStringBuilder;)Z
contentEquals (Ljava/lang/CharSequence;)Z
equalsIgnoreCase (Ljava/lang/String;)Z
compareTo (Ljava/lang/String;)I
compareToIgnoreCase (Ljava/lang/String;)I
regionMatches (ILjava/lang/String;II)Z
regionMatches (ZILjava/lang/String;II)Z
startsWith (Ljava/lang/String;I)Z
startsWith (Ljava/lang/String;)Z
endsWith (Ljava/lang/String;)Z
hashCode ()I
indexOf (I)I
indexOf (II)I
indexOfSupplementary (II)I
lastIndexOf (I)I
lastIndexOf (II)I
lastIndexOfSupplementary (II)I
indexOf (Ljava/lang/String;)I
indexOf (Ljava/lang/String;I)I
indexOf ([CIILjava/lang/String;I)I
indexOf ([CII[CIII)I
lastIndexOf (Ljava/lang/String;)I
lastIndexOf (Ljava/lang/String;I)I
lastIndexOf ([CIILjava/lang/String;I)I
lastIndexOf ([CII[CIII)I
substring (I)Ljava/lang/String;
substring (II)Ljava/lang/String;
subSequence (II)Ljava/lang/CharSequence;
concat (Ljava/lang/String;)Ljava/lang/String;
replace (CC)Ljava/lang/String;
matches (Ljava/lang/String;)Z
contains (Ljava/lang/CharSequence;)Z
replaceFirst (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
replaceAll (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
replace (Ljava/lang/CharSequence;Ljava/lang/CharSequence;)Ljava/lang/String;
split (Ljava/lang/String;I)[Ljava/lang/String;
split (Ljava/lang/String;)[Ljava/lang/String;
join (Ljava/lang/CharSequence;[Ljava/lang/CharSequence;)Ljava/lang/String;
join (Ljava/lang/CharSequence;Ljava/lang/Iterable;)Ljava/lang/String;
toLowerCase (Ljava/util/Locale;)Ljava/lang/String;
toLowerCase ()Ljava/lang/String;
toUpperCase (Ljava/util/Locale;)Ljava/lang/String;
toUpperCase ()Ljava/lang/String;
trim ()Ljava/lang/String;
toString ()Ljava/lang/String;
toCharArray ()[C
format (Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;
format (Ljava/util/Locale;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;
valueOf (Ljava/lang/Object;)Ljava/lang/String;
valueOf ([C)Ljava/lang/String;
valueOf ([CII)Ljava/lang/String;
copyValueOf ([CII)Ljava/lang/String;
copyValueOf ([C)Ljava/lang/String;
valueOf (Z)Ljava/lang/String;
valueOf (C)Ljava/lang/String;
valueOf (I)Ljava/lang/String;
valueOf (J)Ljava/lang/String;
valueOf (F)Ljava/lang/String;
valueOf (D)Ljava/lang/String;
intern ()Ljava/lang/String;
compareTo (Ljava/lang/Object;)I
<clinit> ()V
Process finished with exit code 0
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