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本内容来自尚硅谷课程，此处在知识库做了个人理解

4、原理-手写IoC

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。

4.1、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;
    }
    @Override
    public 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
    @Test
    public void test01() throws Exception {
        //1 类名.class
        Class 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
    @Test
    public 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  private
        Constructor[] 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private
        Constructor 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
    @Test
    public 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
    @Test
    public 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 toString
            if(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 run
            if(m.getName().equals("run")) {
                m.setAccessible(true);
                m.invoke(car);
            }
        }
    }
}

4.2、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 {
    @Override
    public 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;
    @Override
    public 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<>();
    @Override
    public 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;
    @Override
    public 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value
                                        System.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value
                                        System.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 {
    @Test
    public 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 {
    @Di
    private UserDao userDao;
    @Override
    public 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;
    @Override
    public 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value
                                        System.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value
                                        System.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

SpringIoC原理

4、原理-手写IoC

4.1、回顾Java反射

4.2、实现Spring的IoC

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