Java动态编程之javassist

灰太狼 2021-09-15 12:20 392阅读 0赞

## 概述 ##

Javassist是一款字节码编辑工具，可以直接编辑和生成Java生成的字节码，以达到对.class文件进行动态修改的效果。熟练使用这套工具，可以让Java编程更接近与动态语言编程。

## 教程 ##

### maven依赖 ###

pom.xml

<dependency>
        <groupId>org.javassist</groupId>
        <artifactId>javassist</artifactId>
        <version>3.20.0-GA</version>
    </dependency>

### API讲解 ###

### 1、创建一个新的Class ###

ClassPool pool = ClassPool.getDefault();
    
    //定义类
    CtClass stuClass = pool.makeClass("com.ricky.Student");

当然，如果某个类已经存在，可以直接加载它，如下：

CtClass cc = pool.get("java.lang.String");

2、构造类成员变量

//id属性
    CtField idField = new CtField(CtClass.longType, "id", stuClass);
    stuClass.addField(idField);

3、构造类方法

CtMethod getMethod = CtNewMethod.make("public int getAge() { return this.age;}", stuClass);
    
    CtMethod setMethod = CtNewMethod.make("public void setAge(int age) { this.age = age;}", stuClass);
    
    stuClass.addMethod(getMethod);
    stuClass.addMethod(setMethod);

### 应用示例 ###

1、动态构造一个Class

package com.ricky.codelab.javassist;
    
    import java.io.IOException;
    import java.lang.reflect.Field;
    import java.lang.reflect.Method;
    import java.util.Arrays;
    import javassist.CannotCompileException;
    import javassist.ClassPool;
    import javassist.CtClass;
    import javassist.CtField;
    import javassist.CtMethod;
    import javassist.CtNewMethod;
    import javassist.NotFoundException;
    
    /**
     * 动态构造Class
     * @author Ricky
     *
     */
    public class JavassistDemo {
    
        public static void main(String[] args) throws CannotCompileException, IOException, NotFoundException {
    
            ClassPool pool = ClassPool.getDefault();
    
            //定义类
            CtClass stuClass = pool.makeClass("com.ricky.Student");
    
            //加载类
    //      CtClass cc =  pool.get(classname);
    
            //id属性
            CtField idField = new CtField(CtClass.longType, "id", stuClass);
            stuClass.addField(idField);
    
            //name属性
            CtField nameField = new CtField(pool.get("java.lang.String"), "name", stuClass);
            stuClass.addField(nameField);
    
            //age属性
            CtField ageField = new CtField(CtClass.intType, "age", stuClass);
            stuClass.addField(ageField);
    
            CtMethod getMethod = CtNewMethod.make("public int getAge() { return this.age;}", stuClass);
            CtMethod setMethod = CtNewMethod.make("public void setAge(int age) { this.age = age;}", stuClass);
    
            stuClass.addMethod(getMethod);
            stuClass.addMethod(setMethod);
    
    //        stuClass.writeFile("F:\\Practice_Demo");
    
            Class<?> clazz = stuClass.toClass();
            System.out.println("class:"+clazz.getName());
    
            System.out.println("------------属性列表------------");
            Field[] fields = clazz.getDeclaredFields();
            for (Field field : fields) {
                System.out.println(field.getType()+"\t"+field.getName());
            }
    
            System.out.println("------------方法列表------------");
            //方法
            Method[] methods = clazz.getDeclaredMethods();
            for (Method method: methods){
                System.out.println(method.getReturnType()+"\t"+method.getName()+"\t"+Arrays.toString(method.getParameterTypes()));
            }
    
        }
    
    }

2、指定父类

3、动态注入代码

package com.ricky.codelab.javassist;
    
    import java.io.IOException;
    
    import javassist.CannotCompileException;
    import javassist.ClassPool;
    import javassist.CtClass;
    import javassist.CtMethod;
    import javassist.CtNewMethod;
    import javassist.NotFoundException;
    
    /**
     * 动态注入代码
     * 
     * @author Ricky
     *
     */
    public class JavassistInsertDemo {
    
        public static void main(String[] args) throws CannotCompileException, IOException, NotFoundException, InstantiationException, IllegalAccessException {
    
            ClassPool pool = ClassPool.getDefault();
    
            // 定义类
            CtClass ctClass = pool.get("com.ricky.codelab.javassist.Calculator");
    
            // 需要修改的方法名称
            String mname = "getSum";
            CtMethod mold = ctClass.getDeclaredMethod(mname);
            // 修改原有的方法名称
            String nname = mname + "$impl";
            mold.setName(nname);
    
            //创建新的方法，复制原来的方法
            CtMethod mnew = CtNewMethod.copy(mold, mname, ctClass, null);
            // 主要的注入代码
            StringBuffer body = new StringBuffer();
            body.append("{\nlong start = System.currentTimeMillis();\n");
            // 调用原有代码，类似于method();($$)表示所有的参数
    		body.append(nname + "($$);\n");
            body.append("System.out.println(\"Call to method " + mname
                    + " took \" +\n (System.currentTimeMillis()-start) + " + "\" ms.\");\n");
    
            body.append("}");
            // 替换新方法
            mnew.setBody(body.toString());
            // 增加新方法
            ctClass.addMethod(mnew);
    
            Calculator calculator =(Calculator)ctClass.toClass().newInstance(); 
    
            calculator.getSum(10000);
        }
    
    }
    
    class Calculator {
    
        public void getSum(long n) {
            long sum = 0;
            for (int i = 0; i < n; i++) {
                sum += i;
            }
            System.out.println("n="+n+",sum="+sum);
        }
    }