最近接触了下javassist,试着进行字节码操纵，javassist的上手还是比较简单，官方文档说的很详细，而且例子也给的蛮多。
传送门：Javassist官方文档地址

下面是自己写的一个方法，其中生成了一个经典的Spring的controller类。具体写法如下，代码已经贴上了注释。

public void makeclass(String className,String methodName, CONSTANTS.INVOKETYPE invoketype,String interfaceCode) throws NotFoundException, CannotCompileException, IOException {
        ClassPool pool = ClassPool.getDefault();
        CtClass clazz = pool.makeClass(className);
        ClassFile ccFile = clazz.getClassFile();
        ConstPool constpool = ccFile.getConstPool();

        CtClass executor = pool.get("com.javassist.test.Executor");
        CtClass requst = pool.get("javax.servlet.http.HttpServletRequest");
        CtClass response = pool.get("javax.servlet.http.HttpServletResponse");

        String fieldName = invoketype.getValue());
        // 增加字段
        CtField field = new CtField(executor,fieldName,clazz);
        field.setModifiers(Modifier.PUBLIC);
        FieldInfo fieldInfo = field.getFieldInfo();

        // 属性附上注解
        AnnotationsAttribute fieldAttr = new AnnotationsAttribute(constpool, AnnotationsAttribute.visibleTag);
        Annotation autowired = new Annotation("org.springframework.beans.factory.annotation.Autowired",constpool);
        fieldAttr.addAnnotation(autowired);
        fieldInfo.addAttribute(fieldAttr);
        clazz.addField(field);

        // 增加方法，javassist可以直接将字符串set到方法体中，所以使用时非常方便
        CtMethod method = new CtMethod(new CtClassType(CtClass.javaLangObject,pool),methodName,new CtClass[]{requst,response},clazz);
        method.setModifiers(java.lang.reflect.Modifier.PUBLIC);
        StringBuffer methodBody = new StringBuffer();
        methodBody.append("{return "+fieldName+".execute(\""+interfaceCode+"\",(com.javassist.test.RequestVo)$1.getAttribute(\"request\"));}");
        method.setBody(methodBody.toString());



        // 类附上注解
        AnnotationsAttribute classAttr = new AnnotationsAttribute(constpool, AnnotationsAttribute.visibleTag);
        Annotation controller = new Annotation("org.springframework.stereotype.Controller",constpool);
        Annotation requestMapping = new Annotation("org.springframework.web.bind.annotation.RequestMapping.RequestMapping",constpool);
        String visitPath = "/api/department";
        requestMapping.addMemberValue("value",new StringMemberValue(visitPath,constpool));
        classAttr.addAnnotation(controller);
        classAttr.addAnnotation(requestMapping);
        ccFile.addAttribute(classAttr);

        //方法附上注解
        AnnotationsAttribute methodAttr = new AnnotationsAttribute(constpool, AnnotationsAttribute.visibleTag);
        //Annotation annotation3 = new Annotation("org.springframework.web.bind.annotation.RequestMapping.RequestMapping",constpool);
        requestMapping.addMemberValue("value",new StringMemberValue("/register",constpool));

        Annotation responseBody = new Annotation("org.springframework.web.bind.annotation.RequestMapping.ResponseBody",constpool);
        methodAttr.addAnnotation(requestMapping);
        methodAttr.addAnnotation(responseBody);
        MethodInfo info = method.getMethodInfo();
        info.addAttribute(methodAttr);
        clazz.addMethod(method);
        clazz.writeFile();

    }

需要注意的是，
1，运行在web容器或者Jboss下可能会出现找不到类，很有可能是类搜索路径的问题，可以通过增加搜索路径在试着解决问题，pool.insertClassPath(new ClassClassPath(this.getClass()));。
2，运行在web容器或者Jboss下可能会出现类加载器问题，报ClassCastException，可以不使用默认的类加载器，也可以自定义类加载器进行加载。