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我们都知道，Spring框架的IOC是基于Java反射机制实现的，下面我们先回顾一下java反射。

1、回顾Java反射

Java反射机制是在运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法；对于任意一个对象，都能够调用它的任意方法和属性；这种动态获取信息以及动态调用对象方法的功能称为Java语言的反射机制。简单来说，反射机制指的是程序在运行时能够获取自身的信息。

要想解剖一个类，必须先要获取到该类的Class对象。而剖析一个类或用反射解决具体的问题就是使用相关API**（1）java.lang.Class（2）java.lang.reflect**，所以，Class对象是反射的根源。

自定义类

package com.atguigu.reflect;public class Car {//属性private String name;private int age;private String color;//无参数构造public Car() {}//有参数构造public Car(String name, int age, String color) {this.name = name;this.age = age;this.color = color;}//普通方法private void run() {System.out.println("私有方法-run.....");}//get和set方法public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}@Overridepublic String toString() {return "Car{" +"name='" + name + '\'' +", age=" + age +", color='" + color + '\'' +'}';}
}

编写测试类

package com.atguigu.reflect;import org.junit.jupiter.api.Test;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;public class TestCar {//1、获取Class对象多种方式@Testpublic void test01() throws Exception {//1 类名.classClass clazz1 = Car.class;//2 对象.getClass()Class clazz2 = new Car().getClass();//3 Class.forName("全路径")Class clazz3 = Class.forName("com.atguigu.reflect.Car");//实例化Car car = (Car)clazz3.getConstructor().newInstance();System.out.println(car);}//2、获取构造方法@Testpublic void test02() throws Exception {Class clazz = Car.class;//获取所有构造// getConstructors()获取所有public的构造方法
//        Constructor[] constructors = clazz.getConstructors();// getDeclaredConstructors()获取所有的构造方法public  privateConstructor[] constructors = clazz.getDeclaredConstructors();for (Constructor c:constructors) {System.out.println("方法名称："+c.getName()+" 参数个数："+c.getParameterCount());}//指定有参数构造创建对象//1 构造public
//        Constructor c1 = clazz.getConstructor(String.class, int.class, String.class);
//        Car car1 = (Car)c1.newInstance("夏利", 10, "红色");
//        System.out.println(car1);//2 构造privateConstructor c2 = clazz.getDeclaredConstructor(String.class, int.class, String.class);c2.setAccessible(true);Car car2 = (Car)c2.newInstance("捷达", 15, "白色");System.out.println(car2);}//3、获取属性@Testpublic void test03() throws Exception {Class clazz = Car.class;Car car = (Car)clazz.getDeclaredConstructor().newInstance();//获取所有public属性//Field[] fields = clazz.getFields();//获取所有属性（包含私有属性）Field[] fields = clazz.getDeclaredFields();for (Field field:fields) {if(field.getName().equals("name")) {//设置允许访问field.setAccessible(true);field.set(car,"五菱宏光");System.out.println(car);}System.out.println(field.getName());}}//4、获取方法@Testpublic void test04() throws Exception {Car car = new Car("奔驰",10,"黑色");Class clazz = car.getClass();//1 public方法Method[] methods = clazz.getMethods();for (Method m1:methods) {//System.out.println(m1.getName());//执行方法 toStringif(m1.getName().equals("toString")) {String invoke = (String)m1.invoke(car);//System.out.println("toString执行了："+invoke);}}//2 private方法Method[] methodsAll = clazz.getDeclaredMethods();for (Method m:methodsAll) {//执行方法 runif(m.getName().equals("run")) {m.setAccessible(true);m.invoke(car);}}}
}

2、实现Spring的IoC

我们知道，IoC（控制反转）和DI（依赖注入）是Spring里面核心的东西，那么，我们如何自己手写出这样的代码呢？下面我们就一步一步写出Spring框架最核心的部分。

①搭建子模块

搭建模块：guigu-spring，搭建方式如其他spring子模块

②准备测试需要的bean

添加依赖

<dependencies><!--junit5测试--><dependency><groupId>org.junit.jupiter</groupId><artifactId>junit-jupiter-api</artifactId><version>5.3.1</version></dependency>
</dependencies>

创建UserDao接口

package com.atguigu.spring6.test.dao;public interface UserDao {public void print();
}

创建UserDaoImpl实现

package com.atguigu.spring6.test.dao.impl;import com.atguigu.spring.dao.UserDao;public class UserDaoImpl implements UserDao {@Overridepublic void print() {System.out.println("Dao层执行结束");}
}

创建UserService接口

package com.atguigu.spring6.test.service;public interface UserService {public void out();
}

创建UserServiceImpl实现类

package com.atguigu.spring.test.service.impl;import com.atguigu.spring.core.annotation.Bean;
import com.atguigu.spring.service.UserService;@Bean
public class UserServiceImpl implements UserService {//    private UserDao userDao;@Overridepublic void out() {//userDao.print();System.out.println("Service层执行结束");}
}

③定义注解

我们通过注解的形式加载bean与实现依赖注入

bean注解

package com.atguigu.spring.core.annotation;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Bean {
}

依赖注入注解

package com.atguigu.spring.core.annotation;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Di {
}

说明：上面两个注解可以随意取名

④定义bean容器接口

package com.atguigu.spring.core;public interface ApplicationContext {Object getBean(Class clazz);
}

⑤编写注解bean容器接口实现

AnnotationApplicationContext基于注解扫描bean

package com.atguigu.spring.core;import java.util.HashMap;public class AnnotationApplicationContext implements ApplicationContext {//存储bean的容器private HashMap<Class, Object> beanFactory = new HashMap<>();@Overridepublic Object getBean(Class clazz) {return beanFactory.get(clazz);}/*** 根据包扫描加载bean* @param basePackage*/public AnnotationApplicationContext(String basePackage) {}
}

⑥编写扫描bean逻辑

我们通过构造方法传入包的base路径，扫描被@Bean注解的java对象，完整代码如下：

package com.atguigu.spring.core;import com.atguigu.spring.core.annotation.Bean;import java.io.File;
import java.util.HashMap;public class AnnotationApplicationContext implements ApplicationContext {//存储bean的容器private HashMap<Class, Object> beanFactory = new HashMap<>();private static String rootPath;@Overridepublic Object getBean(Class clazz) {return beanFactory.get(clazz);}/*** 根据包扫描加载bean* @param basePackage*/public AnnotationApplicationContext(String basePackage) {try {String packageDirName = basePackage.replaceAll("\\.", "\\\\");Enumeration<URL> dirs =Thread.currentThread().getContextClassLoader().getResources(packageDirName);while (dirs.hasMoreElements()) {URL url = dirs.nextElement();String filePath = URLDecoder.decode(url.getFile(),"utf-8");rootPath = filePath.substring(0, filePath.length()-packageDirName.length());loadBean(new File(filePath));}} catch (Exception e) {throw new RuntimeException(e);}}private  void loadBean(File fileParent) {if (fileParent.isDirectory()) {File[] childrenFiles = fileParent.listFiles();if(childrenFiles == null || childrenFiles.length == 0){return;}for (File child : childrenFiles) {if (child.isDirectory()) {//如果是个文件夹就继续调用该方法,使用了递归loadBean(child);} else {//通过文件路径转变成全类名,第一步把绝对路径部分去掉String pathWithClass = child.getAbsolutePath().substring(rootPath.length() - 1);//选中class文件if (pathWithClass.contains(".class")) {//    com.xinzhi.dao.UserDao//去掉.class后缀，并且把 \ 替换成 .String fullName = pathWithClass.replaceAll("\\\\", ".").replace(".class", "");try {Class<?> aClass = Class.forName(fullName);//把非接口的类实例化放在map中if(!aClass.isInterface()){Bean annotation = aClass.getAnnotation(Bean.class);if(annotation != null){Object instance = aClass.newInstance();//判断一下有没有接口if(aClass.getInterfaces().length > 0) {//如果有接口把接口的class当成key，实例对象当成valueSystem.out.println("正在加载【"+ aClass.getInterfaces()[0] +"】,实例对象是：" + instance.getClass().getName());beanFactory.put(aClass.getInterfaces()[0], instance);}else{//如果有接口把自己的class当成key，实例对象当成valueSystem.out.println("正在加载【"+ aClass.getName() +"】,实例对象是：" + instance.getClass().getName());beanFactory.put(aClass, instance);}}}} catch (ClassNotFoundException | IllegalAccessException | InstantiationException e) {e.printStackTrace();}}}}}}}

⑦java类标识Bean注解

@Bean
public class UserServiceImpl implements UserService

@Bean
public class UserDaoImpl implements UserDao 

⑧测试Bean加载

package com.atguigu.spring;import com.atguigu.spring.core.AnnotationApplicationContext;
import com.atguigu.spring.core.ApplicationContext;
import com.atguigu.spring.test.service.UserService;
import org.junit.jupiter.api.Test;public class SpringIocTest {@Testpublic void testIoc() {ApplicationContext applicationContext = new AnnotationApplicationContext("com.atguigu.spring.test");UserService userService = (UserService)applicationContext.getBean(UserService.class);userService.out();System.out.println("run success");}
}

控制台打印测试

⑨依赖注入

只要userDao.print();调用成功，说明就注入成功

package com.atguigu.spring.test.service.impl;import com.atguigu.spring.core.annotation.Bean;
import com.atguigu.spring.core.annotation.Di;
import com.atguigu.spring.dao.UserDao;
import com.atguigu.spring.service.UserService;@Bean
public class UserServiceImpl implements UserService {@Diprivate UserDao userDao;@Overridepublic void out() {userDao.print();System.out.println("Service层执行结束");}
}

执行第八步：报错了，说明当前userDao是个空对象

⑩依赖注入实现

package com.atguigu.spring.core;import com.atguigu.spring.core.annotation.Bean;
import com.atguigu.spring.core.annotation.Di;import java.io.File;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;public class AnnotationApplicationContext implements ApplicationContext {//存储bean的容器private HashMap<Class, Object> beanFactory = new HashMap<>();private static String rootPath;@Overridepublic Object getBean(Class clazz) {return beanFactory.get(clazz);}/*** 根据包扫描加载bean* @param basePackage*/public AnnotationApplicationContext(String basePackage) {try {String packageDirName = basePackage.replaceAll("\\.", "\\\\");Enumeration<URL> dirs =Thread.currentThread().getContextClassLoader().getResources(packageDirName);while (dirs.hasMoreElements()) {URL url = dirs.nextElement();String filePath = URLDecoder.decode(url.getFile(),"utf-8");rootPath = filePath.substring(0, filePath.length()-packageDirName.length());loadBean(new File(filePath));}} catch (Exception e) {throw new RuntimeException(e);}//依赖注入loadDi();}private  void loadBean(File fileParent) {if (fileParent.isDirectory()) {File[] childrenFiles = fileParent.listFiles();if(childrenFiles == null || childrenFiles.length == 0){return;}for (File child : childrenFiles) {if (child.isDirectory()) {//如果是个文件夹就继续调用该方法,使用了递归loadBean(child);} else {//通过文件路径转变成全类名,第一步把绝对路径部分去掉String pathWithClass = child.getAbsolutePath().substring(rootPath.length() - 1);//选中class文件if (pathWithClass.contains(".class")) {//    com.xinzhi.dao.UserDao//去掉.class后缀，并且把 \ 替换成 .String fullName = pathWithClass.replaceAll("\\\\", ".").replace(".class", "");try {Class<?> aClass = Class.forName(fullName);//把非接口的类实例化放在map中if(!aClass.isInterface()){Bean annotation = aClass.getAnnotation(Bean.class);if(annotation != null){Object instance = aClass.newInstance();//判断一下有没有接口if(aClass.getInterfaces().length > 0) {//如果有接口把接口的class当成key，实例对象当成valueSystem.out.println("正在加载【"+ aClass.getInterfaces()[0] +"】,实例对象是：" + instance.getClass().getName());beanFactory.put(aClass.getInterfaces()[0], instance);}else{//如果有接口把自己的class当成key，实例对象当成valueSystem.out.println("正在加载【"+ aClass.getName() +"】,实例对象是：" + instance.getClass().getName());beanFactory.put(aClass, instance);}}}} catch (ClassNotFoundException | IllegalAccessException | InstantiationException e) {e.printStackTrace();}}}}}}private void loadDi() {for(Map.Entry<Class,Object> entry : beanFactory.entrySet()){//就是咱们放在容器的对象Object obj = entry.getValue();Class<?> aClass = obj.getClass();Field[] declaredFields = aClass.getDeclaredFields();for (Field field : declaredFields){Di annotation = field.getAnnotation(Di.class);if( annotation != null ){field.setAccessible(true);try {System.out.println("正在给【"+obj.getClass().getName()+"】属性【" + field.getName() + "】注入值【"+ beanFactory.get(field.getType()).getClass().getName() +"】");field.set(obj,beanFactory.get(field.getType()));} catch (IllegalAccessException e) {e.printStackTrace();}}}}}}

执行第八步：执行成功，依赖注入成功