Javassist 修改class字节码

import java.io.IOException; import java.lang.reflect.InvocationTargetException; import javassist.CannotCompileException; import javassist.ClassPool; import javassist.CtClass; import javassist.CtMethod; import javassist.CtNewMethod; import javassist.NotFoundException; public class JassistLearn { public static void main(String[] argv) { try { CtClass clas = ClassPool.getDefault().get("Bean"); addTiming(clas,"show"); } catch (NotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (CannotCompileException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } private static void addTiming(CtClass clas, String mname) throws NotFoundException, CannotCompileException, IOException { /* Retrieves methods with the specified name among the methods * declared in the class. Multiple methods with different parameters * may be returned.*/ CtMethod mold = clas.getDeclaredMethod(mname); //修改当前方法名称为一个临时名称 String nname = mname+"$impl"; mold.setName(nname); /* Creates a copy of a method with a new name. * This method is provided for creating * a new method based on an existing method. * This is a convenience method for calling * {@link CtMethod#CtMethod(CtMethod, CtClass, ClassMap) this constructor}. * See the description of the constructor for particular behavior of the copying. * 拷贝一个新的方法，内容跟当前方法一致，方法名与当前方法原有名称一致。后面主要修改新方法的 * 方法体的内容。 * */ CtMethod mnew = CtNewMethod.copy(mold, mname, clas, null); String type = mold.getReturnType().getName(); StringBuffer body = new StringBuffer(); //在新方法体开始处增加一个变量start标识当前执行时间点 body.append("{\nlong start = System.currentTimeMillis();\n"); if (!"void".equals(type)) { body.append(type + " result = "); } //原方法运行，格式为方法名+参数，($$)代表参数 body.append(nname + "($$);\n"); //在原方法执行后打印当前时间点。统计前后时间差可以算出原方法的执行耗时 body.append("System.out.println(\"Call to method " + mname + " took \" +\n (System.currentTimeMillis()-start) + " + "\" ms.\");\n"); if (!"void".equals(type)) { body.append("return result;\n"); } body.append("}"); //将方法体设置到新方法内 mnew.setBody(body.toString()); //将新方法添加到原方法所在类 clas.addMethod(mnew); //更改原方法所在class文件，刷新到磁盘，完成字节码修改 clas.writeFile("D:/eclipse/server/JavaSistLearn/bin"); } }