SerialVersionUIDAdder类的特点是可以为Class文件添加一个serialVersionUID字段。
SerialVersionUIDAdder类
当SerialVersionUIDAdder类计算serialVersionUID值的时候，它有一套自己的算法。那么，serialVersionUID字段有什么作用呢？
Simply put, we use the serialVersionUID attribute to remember versions of a Serializable class to verify that a loaded class and the serialized object are compatible.
class info
SerialVersionUIDAdder类的父类是ClassVisitor类。
public class SerialVersionUIDAdder extends ClassVisitor {
}
fields
SerialVersionUIDAdder类的字段分成三组：
- 第一组，computeSvuid和hasSvuid字段，用于判断是否需要添加serialVersionUID信息。
- 第二组，access、name和interfaces字段，用于记录当前类的访问标识、类的名字和实现的接口。
- 第三组，svuidFields、svuidMethods、svuidConstructors和hasStaticInitializer字段，用于记录字段、构造方法、普通方法、是否有<clinit>()方法。
第二组和第三组部分的字段都是用于计算serialVersionUID值的，而第一组字段是则先判断是否有必要去计算serialVersionUID值。
public class SerialVersionUIDAdder extends ClassVisitor {
// 第一组，用于判断是否需要添加serialVersionUID信息
private boolean computeSvuid;
private boolean hasSvuid;
// 第二组，用于记录当前类的访问标识、类的名字和实现的接口
private int access;
private String name;
private String[] interfaces;
// 第三组，用于记录字段、构造方法、普通方法
private Collection<Item> svuidFields;
private boolean hasStaticInitializer;
private Collection<Item> svuidConstructors;
private Collection<Item> svuidMethods;
}
其中，svuidFields、svuidMethods和svuidConstructors字段都涉及到Item类。Item类的定义如下：
private static final class Item implements Comparable<Item> {
final String name;
final int access;
final String descriptor;
Item(final String name, final int access, final String descriptor) {
this.name = name;
this.access = access;
this.descriptor = descriptor;
}
}
constructors
public class SerialVersionUIDAdder extends ClassVisitor {
public SerialVersionUIDAdder(final ClassVisitor classVisitor) {
this(Opcodes.ASM9, classVisitor);
}
protected SerialVersionUIDAdder(final int api, final ClassVisitor classVisitor) {
super(api, classVisitor);
}
}
methods
public class SerialVersionUIDAdder extends ClassVisitor {
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
// Get the class name, access flags, and interfaces information (step 1, 2 and 3) for SVUID computation.
// 对于枚举类型，不计算serialVersionUID
computeSvuid = (access & Opcodes.ACC_ENUM) == 0;
if (computeSvuid) {
// 记录第二组字段的信息
this.name = name;
this.access = access;
this.interfaces = interfaces.clone();
this.svuidFields = new ArrayList<>();
this.svuidConstructors = new ArrayList<>();
this.svuidMethods = new ArrayList<>();
}
super.visit(version, access, name, signature, superName, interfaces);
}
@Override
public FieldVisitor visitField(int access, String name, String desc, String signature, Object value) {
// Get the class field information for step 4 of the algorithm. Also determine if the class already has a SVUID.
if (computeSvuid) {
if ("serialVersionUID".equals(name)) {
// Since the class already has SVUID, we won't be computing it.
computeSvuid = false;
hasSvuid = true;
}
// Collect the non private fields. Only the ACC_PUBLIC, ACC_PRIVATE, ACC_PROTECTED,
// ACC_STATIC, ACC_FINAL, ACC_VOLATILE, and ACC_TRANSIENT flags are used when computing
// serialVersionUID values.
if ((access & Opcodes.ACC_PRIVATE) == 0
|| (access & (Opcodes.ACC_STATIC | Opcodes.ACC_TRANSIENT)) == 0) {
int mods = access
& (Opcodes.ACC_PUBLIC
| Opcodes.ACC_PRIVATE
| Opcodes.ACC_PROTECTED
| Opcodes.ACC_STATIC
| Opcodes.ACC_FINAL
| Opcodes.ACC_VOLATILE
| Opcodes.ACC_TRANSIENT);
svuidFields.add(new Item(name, mods, desc));
}
}
return super.visitField(access, name, desc, signature, value);
}
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
// Get constructor and method information (step 5 and 7). Also determine if there is a class initializer (step 6).
if (computeSvuid) {
// 这里是对静态初始方法进行判断
if (CLINIT.equals(name)) {
hasStaticInitializer = true;
}
// Collect the non private constructors and methods. Only the ACC_PUBLIC, ACC_PRIVATE,
// ACC_PROTECTED, ACC_STATIC, ACC_FINAL, ACC_SYNCHRONIZED, ACC_NATIVE, ACC_ABSTRACT and
// ACC_STRICT flags are used.
int mods = access
& (Opcodes.ACC_PUBLIC
| Opcodes.ACC_PRIVATE
| Opcodes.ACC_PROTECTED
| Opcodes.ACC_STATIC
| Opcodes.ACC_FINAL
| Opcodes.ACC_SYNCHRONIZED
| Opcodes.ACC_NATIVE
| Opcodes.ACC_ABSTRACT
| Opcodes.ACC_STRICT);
if ((access & Opcodes.ACC_PRIVATE) == 0) {
if ("<init>".equals(name)) {
// 这里是对构造方法进行判断
svuidConstructors.add(new Item(name, mods, descriptor));
} else if (!CLINIT.equals(name)) {
// 这里是对普通方法进行判断
svuidMethods.add(new Item(name, mods, descriptor));
}
}
}
return super.visitMethod(access, name, descriptor, signature, exceptions);
}
@Override
public void visitEnd() {
// Add the SVUID field to the class if it doesn't have one.
if (computeSvuid && !hasSvuid) {
try {
addSVUID(computeSVUID());
} catch (IOException e) {
throw new IllegalStateException("Error while computing SVUID for " + name, e);
}
}
super.visitEnd();
}
protected void addSVUID(final long svuid) {
FieldVisitor fieldVisitor =
super.visitField(Opcodes.ACC_FINAL + Opcodes.ACC_STATIC, "serialVersionUID", "J", null, svuid);
if (fieldVisitor != null) {
fieldVisitor.visitEnd();
}
}
protected long computeSVUID() throws IOException {
// ...
}
}
示例
预期目标
假如有一个HelloWorld类，代码如下：
import java.io.Serializable;
public class HelloWorld implements Serializable {
public String name;
public int age;
public HelloWorld(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public String toString() {
return String.format("HelloWorld { name='%s', age=%d }", name, age);
}
}
我们想实现的预期目标：为HelloWorld类添加serialVersionUID字段。
进行转换
import lsieun.utils.FileUtils;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.commons.SerialVersionUIDAdder;
public class SerialVersionUIDAdderExample01 {
public static void main(String[] args) {
String relative_path = "sample/HelloWorld.class";
String filepath = FileUtils.getFilePath(relative_path);
byte[] bytes1 = FileUtils.readBytes(filepath);
//（1）构建ClassReader
ClassReader cr = new ClassReader(bytes1);
//（2）构建ClassWriter
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
//（3）串连ClassVisitor
int api = Opcodes.ASM9;
ClassVisitor cv = new SerialVersionUIDAdder(cw);
//（4）结合ClassReader和ClassVisitor
int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES;
cr.accept(cv, parsingOptions);
//（5）生成byte[]
byte[] bytes2 = cw.toByteArray();
FileUtils.writeBytes(filepath, bytes2);
}
}
验证结果
import lsieun.utils.FileUtils;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
public class HelloWorldRun {
private static final String FILE_DATA = "obj.data";
public static void main(String[] args) throws Exception {
HelloWorld obj = new HelloWorld("Tomcat", 10);
writeObject(obj);
readObject();
}
public static void readObject() throws Exception {
String filepath = FileUtils.getFilePath(FILE_DATA);
byte[] bytes = FileUtils.readBytes(filepath);
ByteArrayInputStream bai = new ByteArrayInputStream(bytes);
ObjectInputStream in = new ObjectInputStream(bai);
Object obj = in.readObject();
System.out.println(obj);
}
public static void writeObject(Object obj) throws Exception {
ByteArrayOutputStream bao = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bao);
out.writeObject(obj);
out.flush();
out.close();
byte[] bytes = bao.toByteArray();
String filepath = FileUtils.getFilePath(FILE_DATA);
FileUtils.writeBytes(filepath, bytes);
}
}
总结
本文对SerialVersionUIDAdder类进行介绍，内容总结如下：
- 第一点，SerialVersionUIDAdder类的特点是可以为Class文件添加一个serialVersionUID字段。
- 第二点，了解SerialVersionUIDAdder类的各个部分的信息，以便理解它的工作原理。
- 第三点，如何使用SerialVersionUIDAdder类添加serialVersionUID字段。
