java反射初探

java反射

反射是java的重要特性之一，java.lang.reflect 是jdk支持反射的重要包，我下面可能会对构造器Constructor,属性Filed,方法Method会用到。反射其实很常见，在java源码、spring就有很多运用，Spring AOP就应用到了反射。设计模式中代理模式就可以用java反射实现。

反射的例子

创建一个Bean

package com.java.javabase.reflect;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;
import lombok.extern.slf4j.Slf4j;

/**
 * @author
 */
@Slf4j
@Data
//@AllArgsConstructor
@ToString
@NoArgsConstructor
public class Book {
    private final String tag="tag";
    private String auther;
    private String name;

    private Book(String auther, String name) {
        this.auther = auther;
        this.name = name;
    }
    private void printAlarm(String args){
        log.info("alarm occured! "+args);
    }
}

创建测试程序
从获取Class、创建Book的对象Obect、构造器、私有属性（Field）、私有方法（Method）四个维度测试

package com.java.javabase.reflect;

import lombok.extern.slf4j.Slf4j;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * @author
 */
@Slf4j
public class BookReflectTest {
    //创建对象测试
    public static void reflectNewInstance() {
        try {
            Class bookClass = Class.forName("com.java.javabase.reflect.Book");
            //需要一个无参构造器，否则会报错：Caused by: java.lang.NoSuchMethodException: com.java.javabase.reflect.Book.<init>()
            Book bookObject = (Book) bookClass.newInstance();
            bookObject.setName("English");
            bookObject.setAuther("jack");
            log.info("NewInstance test :{}", bookObject.toString());

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

    }

    public static void reflcectConstructor() {
        Class bookClass = null;
        try {
            bookClass = Class.forName("com.java.javabase.reflect.Book");
            //共有的构造器用getConstructor和getDeclaredConstructor不需要constructor.setAccessible(true);
            //私有的构造器constructor.setAccessible(true);
            //Constructor constructor = bookClass.getConstructor(String.class, String.class);
            Constructor constructor = bookClass.getDeclaredConstructor(String.class, String.class);
            constructor.setAccessible(true);
            Book bookObject = (Book) constructor.newInstance("jack2", "English 2");
            log.info("Constructor test :{}", bookObject.toString());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }

    }

    public static void reflectfield() {
        Class bookClass = null;
        try {
            bookClass = Class.forName("com.java.javabase.reflect.Book");

//            Book bookObject = null;
//            bookObject = (Book) bookClass.newInstance();
            Book bookObject = new Book();

            Field field = bookClass.getDeclaredField("tag");
            Field nameField = bookClass.getDeclaredField("name");
            field.setAccessible(true);
            nameField.setAccessible(true);

            bookObject.setName("English3");
            bookObject.setAuther("jack3");
            //public Object get(Object obj)
            Object object = field.get(bookObject);
            Object nameObject = nameField.get(bookObject);
            log.info("reflectfield tag :{}", object);
            log.info("reflectfield name :{}", nameObject);
            log.info("reflectfield test :{}", bookObject.toString());

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
//        catch (InstantiationException e) {
//            e.printStackTrace();
//        }
    }

    public static void reflectMethod() {
        Class bookClass = Book.class;
        //            Book bookObject = null;
        //            bookObject = (Book) bookClass.newInstance();
        Book bookObject = null;
        try {
            bookObject = (Book) bookClass.newInstance();
            //public Method getDeclaredMethod(String name, Class<?>... parameterTypes)
            Method method = bookClass.getDeclaredMethod("printAlarm",String.class);
            method.setAccessible(true);
            try {
                //public Object invoke(Object obj, Object... args)
                method.invoke(bookObject,"end");
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }

        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        reflectNewInstance();
        reflcectConstructor();
        reflectfield();
        reflectMethod();
    }
}

结果

2019-08-26 18:54:34,573   [main] INFO  BookReflectTest  - NewInstance test :Book(tag=tag, auther=jack, name=English)
2019-08-26 18:54:34,588   [main] INFO  BookReflectTest  - Constructor test :Book(tag=tag, auther=jack2, name=English 2)
2019-08-26 18:54:34,588   [main] INFO  BookReflectTest  - reflectfield tag :tag
2019-08-26 18:54:34,588   [main] INFO  BookReflectTest  - reflectfield name :English3
2019-08-26 18:54:34,588   [main] INFO  BookReflectTest  - reflectfield test :Book(tag=tag, auther=jack3, name=English3)
2019-08-26 18:54:34,588   [main] INFO  Book  - alarm occured! end