javassist学习三

这里举个简单的例子，来记录下如何用CtClass创建一个类，并且往这个类里面新加方法，

下面是代码，可以直接运行：

 package seeeyou.app.test;

 import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;

 import javassist.CannotCompileException;
 import javassist.ClassPool;
 import javassist.CtClass;
 import javassist.CtMethod;
 import javassist.CtNewMethod;
 import javassist.NotFoundException;

 /**
  *
  * @author seeeyou
  *
  */
 public class TestHelloWorld2 {
     public static void main(String[] args) throws NotFoundException,
             IOException, CannotCompileException, InstantiationException,
             IllegalAccessException, SecurityException, NoSuchMethodException,
             IllegalArgumentException, InvocationTargetException {
         // 用于取得字节码类，必须在当前的classpath中，使用全称
         ClassPool pool = ClassPool.getDefault();
         /**
          * makeClass() cannot create a new interface; makeInterface() in
          * ClassPool can do. Member methods in an interface can be created with
          * abstractMethod() in CtNewMethod. Note that an interface method is an
          * abstract method.
          */
         CtClass ccClass = pool.makeClass("Point");
         String bodyString = "{System.out.println(\"Call to method \");}";
         //为新创建的类新加一个方法execute，无任何参数
         CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,
                 bodyString, ccClass);
         ccClass.addMethod(n1);
         /**
          * 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：
         *Caused by: java.lang.LinkageError: loader (instance of  sun/misc/Launcher$AppClassLoader):
         *attempted  duplicate class definition for name: "Point"
         **/
         Object oo = ccClass.toClass().newInstance();
         Method mms = oo.getClass().getMethod("execute", null);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, null));
         System.out.println("---------------------------------------------");
         //这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：
         //Exception in thread "main" java.lang.RuntimeException: Point class is frozen
         ccClass.freeze();
         try{
             ccClass.setName("Point2");
         }catch (Exception e) {
             System.out.println(e);
         }
         //对已经冻结的class解冻之后还可以继续编辑修改
          ccClass.defrost();
         "color: #ff0000;"> System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");
          //第二个方法
          bodyString = "public int getNumber(Integer num){System.out.println(\"Point2 Call to method \");return 10+num;}";
          CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值
          ccClass.addMethod(n2);
          Class[] params = new Class[1];
          Integer num = new Integer(15);
          params[0] = num.getClass();
          mms = oo.getClass().getMethod("getNumber", params);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, 100));
         System.out.println("---------------------------------------------");
     }
 }

 上面的结果是：new class name is : Point
 Call to method
 new class's method is : null
 ---------------------------------------------
 java.lang.RuntimeException: Point class is frozen
 ---------------------------------------------
 Exception in thread "main" java.lang.NoSuchMethodException: Point.getNumber(java.lang.Integer)
  at java.lang.Class.getMethod(Class.java:1605)
  at seeeyou.app.test.TestHelloWorld2.main(TestHelloWorld2.java:66)

 

错误的原因其实和简单，因为我第二次新加一个方法后，没有再次实例化一个对象，所以oo还是原来的对象，他的成员函数肯定没有新加的方法。

那我可以再次实例化下试试，代码和结果如下：

 package seeeyou.app.test;

 import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;

 import javassist.CannotCompileException;
 import javassist.ClassPool;
 import javassist.CtClass;
 import javassist.CtMethod;
 import javassist.CtNewMethod;
 import javassist.NotFoundException;

 /**
  *
  * @author seeeyou
  *
  */
 public class TestHelloWorld2 {
     public static void main(String[] args) throws NotFoundException,
             IOException, CannotCompileException, InstantiationException,
             IllegalAccessException, SecurityException, NoSuchMethodException,
             IllegalArgumentException, InvocationTargetException {
         // 用于取得字节码类，必须在当前的classpath中，使用全称
         ClassPool pool = ClassPool.getDefault();
         /**
          * makeClass() cannot create a new interface; makeInterface() in
          * ClassPool can do. Member methods in an interface can be created with
          * abstractMethod() in CtNewMethod. Note that an interface method is an
          * abstract method.
          */
         CtClass ccClass = pool.makeClass("Point");
         String bodyString = "{System.out.println(\"Call to method \");}";
         //为新创建的类新加一个方法execute，无任何参数
         CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,
                 bodyString, ccClass);
         ccClass.addMethod(n1);
         /**
          * 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：
         *Caused by: java.lang.LinkageError: loader (instance of  sun/misc/Launcher$AppClassLoader):
         *attempted  duplicate class definition for name: "Point"
         **/
         Object oo = ccClass.toClass().newInstance();
         Method mms = oo.getClass().getMethod("execute", null);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, null));
         System.out.println("---------------------------------------------");
         //这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：
         //Exception in thread "main" java.lang.RuntimeException: Point class is frozen
         ccClass.freeze();
         try{
             ccClass.setName("Point2");
         }catch (Exception e) {
             System.out.println(e);
         }
         //对已经冻结的class解冻之后还可以继续编辑修改
          ccClass.defrost();
          System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");
          //第二个方法
          bodyString = "public int getNumber(Integer num){System.out.println(\"Point2 Call to method \");return 10+num;}";
          CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值
          ccClass.addMethod(n2);
          Class[] params = new Class[1];
          Integer num = new Integer(15);
          params[0] = num.getClass();//就多了下面这个实例化，但是这样会导致一个错误
          oo = ccClass.toClass().newInstance();
          mms = oo.getClass().getMethod("getNumber", params);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, 100));
         System.out.println("---------------------------------------------");
     }
 }

 这也会导致一个错误：new class name is : Point
 Call to method
 new class's method is : null
 ---------------------------------------------
 java.lang.RuntimeException: Point class is frozen
 ------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------
 Exception in thread "main" javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of  sun/misc/Launcher$AppClassLoader): attempted  duplicate class definition for name: "Point"
  at javassist.ClassPool.toClass(ClassPool.java:1051)
  at javassist.ClassPool.toClass(ClassPool.java:994)
  at javassist.ClassPool.toClass(ClassPool.java:952)
  at javassist.CtClass.toClass(CtClass.java:1079)
  at seeeyou.app.test.TestHelloWorld2.main(TestHelloWorld2.java:66)
 Caused by: java.lang.LinkageError: loader (instance of  sun/misc/Launcher$AppClassLoader): attempted  duplicate class definition for name: "Point"
  at java.lang.ClassLoader.defineClass1(Native Method)
  at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
  at java.lang.ClassLoader.defineClass(ClassLoader.java:466)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  at java.lang.reflect.Method.invoke(Method.java:597)
  at javassist.ClassPool.toClass2(ClassPool.java:1063)
  at javassist.ClassPool.toClass(ClassPool.java:1045)
  ... 4 more

 原因也很简单，一个classloader里面怎么有两个重复的对象呢，除非是两个不同的classloader。。所以爆了个重复加载类的错误

 

对的方式是只实例化一次：如下：

 package seeeyou.app.test;

 import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;

 import javassist.CannotCompileException;
 import javassist.ClassPool;
 import javassist.CtClass;
 import javassist.CtMethod;
 import javassist.CtNewMethod;
 import javassist.NotFoundException;

 /**
  *
  * @author seeeyou
  *
  */
 public class TestHelloWorld3 {

     public static void main(String[] args) throws NotFoundException,
             IOException, CannotCompileException, InstantiationException,
             IllegalAccessException, SecurityException, NoSuchMethodException,
             IllegalArgumentException, InvocationTargetException {
         // 用于取得字节码类，必须在当前的classpath中，使用全称
         ClassPool pool = ClassPool.getDefault();
         /**
          * makeClass() cannot create a new interface; makeInterface() in
          * ClassPool can do. Member methods in an interface can be created with
          * abstractMethod() in CtNewMethod. Note that an interface method is an
          * abstract method.
          */
         CtClass ccClass = pool.makeClass("Point");
         String bodyString = "{System.out.println(\"Call to method \");}";
         //为新创建的类新加一个方法execute，无任何参数
         CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,
                 bodyString, ccClass);
         ccClass.addMethod(n1);
         //新加第二个方法
          bodyString = "public Integer getNumber(Integer num);";
          CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值
          n2.setBody("{System.out.println(\"Point Call to method \");return $1;}");
          ccClass.addMethod(n2);

         /**
          * 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：
         *Caused by: java.lang.LinkageError: loader (instance of  sun/misc/Launcher$AppClassLoader):
         *attempted  duplicate class definition for name: "Point"
         **/
         Object oo = ccClass.toClass().newInstance();
         Method mms = oo.getClass().getMethod("execute", null);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, null));
         System.out.println("---------------------------------------------");
         //这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：
         //Exception in thread "main" java.lang.RuntimeException: Point class is frozen
         ccClass.freeze();
         try{
             ccClass.setName("Point2");
         }catch (Exception e) {
             System.out.println(e);
         }
         //对已经冻结的class解冻之后还可以继续编辑修改
          ccClass.defrost();
          System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");
          Class[] params = new Class[1];
          Integer num = new Integer(0);
          params[0] = num.getClass();
          mms = oo.getClass().getMethod("getNumber",params);
         System.out.println("new class name is : " + oo.getClass().getName());
         System.out.println("new class's method is : " + mms.invoke(oo, 100));
         System.out.println("---------------------------------------------");
     }
 }

 结果如下：new class name is : Point
 Call to method
 new class's method is : null
 ---------------------------------------------
 java.lang.RuntimeException: Point class is frozen
 ------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------
 new class name is : Point
 Point2 Call to method
 new class's method is : 100
 ---------------------------------------------