javassist初接触

直接上代码：

pom增加依赖：

    <dependency>
    <groupId>org.javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.18.2-GA</version>
</dependency>

写个Hello类：

package JavassistTest.JavassistTest;

public class Hello {

    public void sayHello(){
        System.out.println("Hello world");
    }
}

主类：

package JavassistTest.JavassistTest;

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

/**
 * Hello world!
 *
 */
public class App {
    public static void main(String[] args) throws NotFoundException,
            CannotCompileException, InstantiationException, IllegalAccessException, ClassNotFoundException {

        //~加上报错，和类加载有关系，这里new的话会把Hello类加载，之后再加载的话就会报错
        //Hello hell=new Hello();
        //hell.sayHello();
        //~

        CtClass ctClass = ClassPool.getDefault().getCtClass("JavassistTest.JavassistTest.Hello");

        CtMethod ctMethod = ctClass.getDeclaredMethod("sayHello");
        ctMethod.setBody("System.out.println(\"hi\");");    

        Class ch=ctClass.toClass();

        Hello h=(Hello) ch.newInstance();
        h.sayHello();        

        Hello hel=new Hello();
        hel.sayHello();

    }
}

输出结果：

hi
hi

结果一样，说明是class被修改了。