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()