ASM在运行时通过修改字节码动态给接口添加Annotation

1. 对接口预先添加一个自定义注解：@ServiceType

 

2. 系统启动时，通过Spring扫描有包含注解：@ServiceType的接口，然后通过ASM类库修改接口字节码

 

3. 用到一些辅助工具：java命令，asm-util.jar，asm.jar

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具体操作：

IIndividualTax接口代码如下（import相关代码省略）：

@ServiceType

public interface IIndividualTax {

BusinessResponse request(BusinessRequest request) throws GatewayException;

}

 

目的是要在IIndividualTax 接口上添加注解，并且删掉ServiceType注解，以及在方法request上添加注解，最终想要运行时的代码如下（import相关代码省略）：

@Path("it")

@Consumes({"application/json", "text/xml"})

@Produces({"application/json; charset=UTF-8", "text/xml; charset=UTF-8"})

public abstract interface IIndividualTax

{

  @POST

  @Path("request")

  BusinessResponse request(BusinessRequest paramBusinessRequest) throws GatewayException;

}

 

首先：java -classpath D:\;D:\asm-util-5.1.jar;D:\asm-5.1.jar;. org.objectweb.asm.util.ASMifier cn.com.*.*.gateway.service.IIndividualTax

运行这个命令，需要注意的是classpath参数以及，在当前cmd位置下要有IIndividualTax这个类的全包路径

借助ASMifier类，可以看到这个接口IIndividualTax修改前完整的字节码，如下：

package asm.*****************************.service;

import java.util.*;

import org.objectweb.asm.*;

public class IIndividualTaxDump implements Opcodes {

 

public static byte[] dump () throws Exception {

 

ClassWriter cw = new ClassWriter(0);

FieldVisitor fv;

MethodVisitor mv;

AnnotationVisitor av0;

 

cw.visit(V1_7, ACC_PUBLIC + ACC_ABSTRACT + ACC_INTERFACE, "cn/com/*/*/gateway/service/IIndividualTax", null, "java/lang/Object", null);

 

{

av0 = cw.visitAnnotation("Lcn/com/*/*/gateway/service/ServiceType;", false);

av0.visitEnd();

}

{

mv = cw.visitMethod(ACC_PUBLIC + ACC_ABSTRACT, "request", "(Lcn/com/*/*/gateway/dto/BusinessRequest;)Lcn/com/*/*/gateway/dto/BusinessResponse;", null, new String[] { "cn/com/*/*/gateway/exception/GatewayException" });

mv.visitEnd();

}

cw.visitEnd();

 

return cw.toByteArray();

}

}

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在次运行：java -classpath D:\;D:\asm-util-5.1.jar;D:\asm-5.1.jar;. org.objectweb.asm.util.ASMifier cn.com.*.*.gateway.service.IIndividualTax，查看想要修改后接口完整的字节码，如下：

package asm.*************************************service;

import java.util.*;

import org.objectweb.asm.*;

public class IIndividualTaxDump implements Opcodes {

 

public static byte[] dump () throws Exception {

 

ClassWriter cw = new ClassWriter(0);

FieldVisitor fv;

MethodVisitor mv;

AnnotationVisitor av0;

 

cw.visit(V1_7, ACC_PUBLIC + ACC_ABSTRACT + ACC_INTERFACE, "cn/com/*/*/gateway/service/IIndividualTax", null, "java/lang/Object", null);

 

{

av0 = cw.visitAnnotation("Ljavax/ws/rs/Path;", true);

av0.visit("value", "it");

av0.visitEnd();

}

{

av0 = cw.visitAnnotation("Ljavax/ws/rs/Consumes;", true);

{

AnnotationVisitor av1 = av0.visitArray("value");

av1.visit(null, "application/json");

av1.visit(null, "text/xml");

av1.visitEnd();

}

av0.visitEnd();

}

{

av0 = cw.visitAnnotation("Ljavax/ws/rs/Produces;", true);

{

AnnotationVisitor av1 = av0.visitArray("value");

av1.visit(null, "application/json; charset=UTF-8");

av1.visit(null, "text/xml; charset=UTF-8");

av1.visitEnd();

}

av0.visitEnd();

}

{

av0 = cw.visitAnnotation("Lcn/com/*/*/gateway/service/ServiceType;", false);

av0.visitEnd();

}

{

mv = cw.visitMethod(ACC_PUBLIC + ACC_ABSTRACT, "request", "(Lcn/com/*/*/gateway/dto/BusinessRequest;)Lcn/com/*/*/gateway/dto/BusinessResponse;", null, new String[] { "cn/com/*/*/gateway/exception/GatewayException" });

{

av0 = mv.visitAnnotation("Ljavax/ws/rs/POST;", true);

av0.visitEnd();

}

{

av0 = mv.visitAnnotation("Ljavax/ws/rs/Path;", true);

av0.visit("value", "request");

av0.visitEnd();

}

mv.visitEnd();

}

cw.visitEnd();

 

return cw.toByteArray();

}

}

 

 

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对于新手玩ASM不熟悉的情况，通过对比上面的二段字节码存在的差异，用ASM提供的API进行码代码，生成运行时的关键代码如下：

 

public class TaxVisitor extends ClassVisitor implements Opcodes{

 

public TaxVisitor(int api, ClassVisitor cv) {

super(api, cv);

}

 

@Override

public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {

MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);

if (name.equals("request")) {

AnnotationVisitor  av1 = mv.visitAnnotation("Ljavax/ws/rs/POST;", true);//在request上添加@POST注解，对比看上面红色粗体类源码

AnnotationVisitor  av2 = mv.visitAnnotation("Ljavax/ws/rs/Path;", true);//同上

av2.visit("value", "request");

av2.visitEnd();//这些api用法，查看asm相关说明

av1.visitEnd();

}

 

return mv;

}

 

@Override

public AnnotationVisitor visitAnnotation(String name, boolean arg1) {

if (name.equals("Lcn/com/*/*/gateway/service/ServiceType;")) {

return null;//返回null说明，要删掉接口上@ServiceType注解

}

 

return super.visitAnnotation(name, arg1);

}

}

 

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public class AsmTest extends ClassLoader{

 

public static void main(String[] args) throws Exception {

 

ClassReader cr=new ClassReader(IIndividualTax.class.getName());

ClassWriter cw=new ClassWriter(ClassWriter.COMPUTE_MAXS);

 

AnnotationVisitor av0 = cw.visitAnnotation("Ljavax/ws/rs/Path;", true);//在接口上添加注解@Path

av0.visit("value", "it");

av0.visitEnd();

 

AnnotationVisitor av1 = cw.visitAnnotation("Ljavax/ws/rs/Consumes;", true);//在接口上添加注解@Consumes

AnnotationVisitor av2 = av1.visitArray("value");

av2.visit(null, "application/json");

av2.visit(null, "text/xml");

av2.visitEnd();

av1.visitEnd();

 

AnnotationVisitor av3 = cw.visitAnnotation("Ljavax/ws/rs/Produces;", true);//在接口上添加注解@Produces

AnnotationVisitor av4 = av3.visitArray("value");

av4.visit(null, "application/json; charset=UTF-8");

av4.visit(null, "text/xml; charset=UTF-8");

av4.visitEnd();

av3.visitEnd();

 

TaxVisitor myv=new TaxVisitor(Opcodes.ASM4,cw);

cr.accept(myv, 0);

 

byte[] code=cw.toByteArray();//最终想要类的字节码

 

AsmTest loader=new AsmTest();

Class<?> appClass=loader.defineClass(null, code, 0,code.length);//字节码加载到jvm

System.out.println(AnnotationUtils.findAnnotation(appClass, Path.class));

FileOutputStream fos = new FileOutputStream(appClass.getResource("").getPath()+"/IIndividualTax.class");//覆盖当前class文件

fos.write(code);

fos.close();

 

}

}

 

注意：对类或接口的字节码的修改必须用ClassWriter的实例进行api相关操作，对类的方法，属性字节码操作需要通过对应的Visitor操作，最终调用visitEnd()