Java字节码的增强技术

import java.io.File;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import org.objectweb.asm.ClassWriter;

import org.objectweb.asm.Opcodes;

public class GeneratorClass {

     public static void main(String[] args) throws IOException {

         //生成一个类只需要ClassWriter组件即可

         ClassWriter cw = new ClassWriter( 0 );

         //通过visit方法确定类的头部信息

         cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT+Opcodes.ACC_INTERFACE,

                 "com/asm3/Comparable" , null , "java/lang/Object" , new String[]{ "com/asm3/Mesurable" });

         //定义类的属性

         cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                 "LESS" , "I" , null , new Integer(- 1 )).visitEnd();

         cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                 "EQUAL" , "I" , null , new Integer( 0 )).visitEnd();

         cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                 "GREATER" , "I" , null , new Integer( 1 )).visitEnd();

         //定义类的方法

         cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo" ,

                 "(Ljava/lang/Object;)I" , null , null ).visitEnd();

         cw.visitEnd(); //使cw类已经完成

         //将cw转换成字节数组写到文件里面去

         byte [] data = cw.toByteArray();

         File file = new File( "D://Comparable.class" );

         FileOutputStream fout = new FileOutputStream(file);

         fout.write(data);

         fout.close();

     }

}

import javassist.*;

public class CreatePerson {

     public static void createPseson() throws Exception {

         ClassPool pool = ClassPool.getDefault();

         // 1. 创建一个空类

         CtClass cc = pool.makeClass( "com.test.javassist.Person" );

         // 2. 新增一个字段 private String name;

         // 字段名为name

         CtField param = new CtField(pool.get( "java.lang.String" ), "name" , cc);

         // 访问级别是 private

         param.setModifiers(Modifier.PRIVATE);

         // 初始值是 "xiaoming"

         cc.addField(param, CtField.Initializer.constant( "xiaoming" ));

         // 3. 生成 getter、setter 方法

         cc.addMethod(CtNewMethod.setter( "setName" , param));

         cc.addMethod(CtNewMethod.getter( "getName" , param));

         // 4. 添加无参的构造函数

         CtConstructor cons = new CtConstructor( new CtClass[]{}, cc);

         cons.setBody( "{name = \"xiaohong\";}" );

         cc.addConstructor(cons);

         // 5. 添加有参的构造函数

         cons = new CtConstructor( new CtClass[]{pool.get( "java.lang.String" )}, cc);

         // $0=this / $1,$2,$3... 代表方法参数

         cons.setBody( "{$0.name = $1;}" );

         cc.addConstructor(cons);

         // 6. 创建一个名为printName方法，无参数，无返回值，输出name值

         CtMethod ctMethod = new CtMethod(CtClass.voidType, "printName" , new CtClass[]{}, cc);

         ctMethod.setModifiers(Modifier.PUBLIC);

         ctMethod.setBody( "{System.out.println(name);}" );

         cc.addMethod(ctMethod);

         //这里会将这个创建的类对象编译为.class文件

         cc.writeFile( "/Users/yangyue/workspace/springboot-learn/java-agent/src/main/java/" );

     }

     public static void main(String[] args) {

         try {

             createPseson();

         } catch (Exception e) {

             e.printStackTrace();

         }

     }

}

Class<?> dynamicType = new ByteBuddy()

   .subclass(Object. class )

   .method(ElementMatchers.named( "toString" ))

   .intercept(FixedValue.value( "Hello World!" ))

   .make()

   .load(getClass().getClassLoader())

   .getLoaded();

assertThat(dynamicType.newInstance().toString(), is( "Hello World!" ));