用300行代码手写1个Spring框架，麻雀虽小五脏俱全

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本文节选自《Spring 5核心原理》

1 自定义配置

1.1 配置application.properties文件

为了解析方便，我们用application.properties来代替application.xml文件，具体配置内容如下：

scanPackage=com.tom.demo

1.2 配置web.xml文件

大家都知道，所有依赖于Web容器的项目都是从读取web.xml文件开始的。我们先配置好web.xml中的内容：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:javaee="http://java.sun.com/xml/ns/javaee"
   xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
   xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
   version="2.4">
   <display-name>Gupao Web Application</display-name>
   <servlet>
      <servlet-name>gpmvc</servlet-name>
      <servlet-class>com.tom.mvcframework.v1.servlet.GPDispatcherServlet</servlet-class>
      <init-param>
         <param-name>contextConfigLocation</param-name>
         <param-value>application.properties</param-value>
      </init-param>
      <load-on-startup>1</load-on-startup>
   </servlet>
   <servlet-mapping>
      <servlet-name>gpmvc</servlet-name>
      <url-pattern>/*</url-pattern>
   </servlet-mapping>
</web-app>

其中的GPDispatcherServlet是模拟Spring实现的核心功能类。

1.3 自定义注解

@GPService注解如下：

package com.tom.mvcframework.annotation;
import java.lang.annotation.*;
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface GPService {
    String value() default "";
}

@GPAutowired注解如下：

package com.tom.mvcframework.annotation;
import java.lang.annotation.*;
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface GPAutowired {
    String value() default "";
}

@GPController注解如下：

package com.tom.mvcframework.annotation;
import java.lang.annotation.*;
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface GPController {
    String value() default "";
}

@GPRequestMapping注解如下：

package com.tom.mvcframework.annotation;
import java.lang.annotation.*;
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface GPRequestMapping {
    String value() default "";
}

@GPRequestParam注解如下：

package com.tom.mvcframework.annotation;
import java.lang.annotation.*;
@Target({ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface GPRequestParam {
    String value() default "";
}

1.4 配置注解

配置业务实现类DemoService：

package com.tom.demo.service.impl;
import com.tom.demo.service.IDemoService;
import com.tom.mvcframework.annotation.GPService;
/**
 * 核心业务逻辑
 */
@GPService
public class DemoService implements IDemoService{
   public String get(String name) {
      return "My name is " + name;
   }
}

配置请求入口类DemoAction：

package com.tom.demo.mvc.action;
import java.io.IOException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.tom.demo.service.IDemoService;
import com.tom.mvcframework.annotation.GPAutowired;
import com.tom.mvcframework.annotation.GPController;
import com.tom.mvcframework.annotation.GPRequestMapping;
import com.tom.mvcframework.annotation.GPRequestParam;
@GPController
@GPRequestMapping("/demo")
public class DemoAction {
   @GPAutowired private IDemoService demoService;
   @GPRequestMapping("/query")
   public void query(HttpServletRequest req, HttpServletResponse resp,
                 @GPRequestParam("name") String name){
      String result = demoService.get(name);
      try {
         resp.getWriter().write(result);
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
   @GPRequestMapping("/add")
   public void add(HttpServletRequest req, HttpServletResponse resp,
               @GPRequestParam("a") Integer a, @GPRequestParam("b") Integer b){
      try {
         resp.getWriter().write(a + "+" + b + "=" + (a + b));
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
   @GPRequestMapping("/remove")
   public void remove(HttpServletRequest req,HttpServletResponse resp,
                  @GPRequestParam("id") Integer id){
   }
}

至此，配置全部完成。

2 容器初始化1.0版本

所有的核心逻辑全部写在init()方法中，代码如下：

package com.tom.mvcframework.v1.servlet;
import com.tom.mvcframework.annotation.GPAutowired;
import com.tom.mvcframework.annotation.GPController;
import com.tom.mvcframework.annotation.GPRequestMapping;
import com.tom.mvcframework.annotation.GPService;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.*;

public class GPDispatcherServlet extends HttpServlet {
    private Map<String,Object> mapping = new HashMap<String, Object>();
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req,resp);}
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {}
   
    @Override
    public void init(ServletConfig config) throws ServletException {
        InputStream is = null;
        try{
            Properties configContext = new Properties();
            is = this.getClass().getClassLoader().getResourceAsStream(config.getInitParameter ("contextConfigLocation"));
            configContext.load(is);
            String scanPackage = configContext.getProperty("scanPackage");
            doScanner(scanPackage);
            for (String className : mapping.keySet()) {
                if(!className.contains(".")){continue;}
                Class<?> clazz = Class.forName(className);
                if(clazz.isAnnotationPresent(GPController.class)){
                    mapping.put(className,clazz.newInstance());
                    String baseUrl = "";
                    if (clazz.isAnnotationPresent(GPRequestMapping.class)) {
                        GPRequestMapping requestMapping = clazz.getAnnotation (GPRequestMapping.class);
                        baseUrl = requestMapping.value();
                    }
                    Method[] methods = clazz.getMethods();
                    for (Method method : methods) {
                        if(!method.isAnnotationPresent(GPRequestMapping.class)){  continue; }
                        GPRequestMapping requestMapping = method.getAnnotation (GPRequestMapping.class);
                        String url = (baseUrl + "/" + requestMapping.value()).replaceAll("/+", "/");
                        mapping.put(url, method);
                        System.out.println("Mapped " + url + "," + method);
                    }
                }else if(clazz.isAnnotationPresent(GPService.class)){
                        GPService service = clazz.getAnnotation(GPService.class);
                        String beanName = service.value();
                        if("".equals(beanName)){beanName = clazz.getName();}
                        Object instance = clazz.newInstance();
                        mapping.put(beanName,instance);
                        for (Class<?> i : clazz.getInterfaces()) {
                            mapping.put(i.getName(),instance);
                        }
                }else {continue;}
            }
            for (Object object : mapping.values()) {
                if(object == null){continue;}
                Class clazz = object.getClass();
                if(clazz.isAnnotationPresent(GPController.class)){
                    Field [] fields = clazz.getDeclaredFields();
                    for (Field field : fields) {
                        if(!field.isAnnotationPresent(GPAutowired.class)){continue; }
                        GPAutowired autowired = field.getAnnotation(GPAutowired.class);
                        String beanName = autowired.value();
                        if("".equals(beanName)){beanName = field.getType().getName();}
                        field.setAccessible(true);
                        try {
                            field.set(mapping.get(clazz.getName()),mapping.get(beanName));
                        } catch (IllegalAccessException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        } catch (Exception e) {
        }finally {
            if(is != null){
                try {is.close();} catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        System.out.print("GP MVC Framework is init");
    }
    private void doScanner(String scanPackage) {
        URL url = this.getClass().getClassLoader().getResource("/" + scanPackage.replaceAll ("\\.","/"));
        File classDir = new File(url.getFile());
        for (File file : classDir.listFiles()) {
            if(file.isDirectory()){ doScanner(scanPackage + "." +  file.getName());}else {
                if(!file.getName().endsWith(".class")){continue;}
                String clazzName = (scanPackage + "." + file.getName().replace(".class",""));
                mapping.put(clazzName,null);
            }
        }
    }
}

3 请求执行

重点实现doGet()和doPost()方法，实际上就是在doGet()和doPost()方法中调用doDispatch()方法，具体代码如下：

public class GPDispatcherServlet extends HttpServlet {
    private Map<String,Object> mapping = new HashMap<String, Object>();
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req,resp);}
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            doDispatch(req,resp);
        } catch (Exception e) {
            resp.getWriter().write("500 Exception " + Arrays.toString(e.getStackTrace()));
        }
    }
    private void doDispatch(HttpServletRequest req, HttpServletResponse resp) throws Exception {
        String url = req.getRequestURI();
        String contextPath = req.getContextPath();
        url = url.replace(contextPath, "").replaceAll("/+", "/");
        if(!this.mapping.containsKey(url)){resp.getWriter().write("404 Not Found!!");return;}
        Method method = (Method) this.mapping.get(url);
        Map<String,String[]> params = req.getParameterMap();
        method.invoke(this.mapping.get(method.getDeclaringClass().getName()),new Object[]{req,resp,params.get("name")[0]});
    }
    @Override
    public void init(ServletConfig config) throws ServletException {
        ...
    }

}

4 优化并实现2.0版本

在1.0版本上进行优化，采用常用的设计模式（工厂模式、单例模式、委派模式、策略模式），将init()方法中的代码进行封装。按照之前的实现思路，先搭基础框架，再“填肉注血”，具体代码如下：

//初始化阶段
@Override
public void init(ServletConfig config) throws ServletException {

    //1. 加载配置文件
    doLoadConfig(config.getInitParameter("contextConfigLocation"));

    //2. 扫描相关的类
    doScanner(contextConfig.getProperty("scanPackage"));
    
    //3. 初始化扫描到的类，并且将它们放入IoC容器中
    doInstance();
    
    //4. 完成依赖注入
    doAutowired();

    //5. 初始化HandlerMapping
    initHandlerMapping();

    System.out.println("GP Spring framework is init.");

}

声明全局成员变量，其中IoC容器就是注册时单例的具体案例：

//保存application.properties配置文件中的内容
private Properties contextConfig = new Properties();

//保存扫描的所有的类名
private List<String> classNames = new ArrayList<String>();

//传说中的IoC容器，我们来揭开它的神秘面纱
//为了简化程序，暂时不考虑ConcurrentHashMap
//主要还是关注设计思想和原理
private Map<String,Object> ioc = new HashMap<String,Object>();

//保存url和Method的对应关系
private Map<String,Method> handlerMapping = new HashMap<String,Method>();

实现doLoadConfig()方法：

//加载配置文件
private void doLoadConfig(String contextConfigLocation) {
    //直接通过类路径找到Spring主配置文件所在的路径
    //并且将其读取出来放到Properties对象中
    //相当于将scanPackage=com.tom.demo保存到了内存中
    InputStream fis = this.getClass().getClassLoader().getResourceAsStream(contextConfigLocation);
    try {
        contextConfig.load(fis);
    } catch (IOException e) {
        e.printStackTrace();
    }finally {
        if(null != fis){
            try {
                fis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

实现doScanner()方法：

//扫描相关的类
private void doScanner(String scanPackage) {
    //scanPackage = com.tom.demo ，存储的是包路径
    //转换为文件路径，实际上就是把.替换为/
    URL url = this.getClass().getClassLoader().getResource("/" + scanPackage.replaceAll ("\\.","/"));
    File classPath = new File(url.getFile());
    for (File file : classPath.listFiles()) {
        if(file.isDirectory()){
            doScanner(scanPackage + "." + file.getName());
        }else{
            if(!file.getName().endsWith(".class")){ continue;}
            String className = (scanPackage + "." + file.getName().replace(".class",""));
            classNames.add(className);
        }
    }
}

实现doInstance()方法，doInstance()方法就是工厂模式的具体实现：

private void doInstance() {
    //初始化，为DI做准备
    if(classNames.isEmpty()){return;}

    try {
        for (String className : classNames) {
            Class<?> clazz = Class.forName(className);

            //什么样的类才需要初始化呢？
            //加了注解的类才初始化，怎么判断？
            //为了简化代码逻辑，主要体会设计思想，只用@Controller和@Service举例，
            //@Componment等就不一一举例了
            if(clazz.isAnnotationPresent(GPController.class)){
                Object instance = clazz.newInstance();
                //Spring默认类名首字母小写
                String beanName = toLowerFirstCase(clazz.getSimpleName());
                ioc.put(beanName,instance);
            }else if(clazz.isAnnotationPresent(GPService.class)){
                //1. 自定义的beanName
                GPService service = clazz.getAnnotation(GPService.class);
                String beanName = service.value();
                //2. 默认类名首字母小写
                if("".equals(beanName.trim())){
                    beanName = toLowerFirstCase(clazz.getSimpleName());
                }

                Object instance = clazz.newInstance();
                ioc.put(beanName,instance);
                //3. 根据类型自动赋值，这是投机取巧的方式
                for (Class<?> i : clazz.getInterfaces()) {
                    if(ioc.containsKey(i.getName())){
                        throw new Exception("The “" + i.getName() + "” is exists!!");
                    }
                    //把接口的类型直接当成key
                    ioc.put(i.getName(),instance);
                }
            }else {
                continue;
            }

        }
    }catch (Exception e){
        e.printStackTrace();
    }

}

为了处理方便，自己实现了toLowerFirstCase()方法，来实现类名首字母小写，具体代码如下：

//将类名首字母改为小写
private String toLowerFirstCase(String simpleName) {
    char [] chars = simpleName.toCharArray();
    //之所以要做加法，是因为大、小写字母的ASCII码相差32
    //而且大写字母的ASCII码要小于小写字母的ASCII码
    //在Java中，对char做算术运算实际上就是对ASCII码做算术运算
    chars[0] += 32;
    return String.valueOf(chars);
}

实现doAutowired()方法：

//自动进行依赖注入
private void doAutowired() {
    if(ioc.isEmpty()){return;}

    for (Map.Entry<String, Object> entry : ioc.entrySet()) {
        //获取所有的字段，包括private、protected、default类型的
        //正常来说，普通的OOP编程只能获得public类型的字段
        Field[] fields = entry.getValue().getClass().getDeclaredFields();
        for (Field field : fields) {
            if(!field.isAnnotationPresent(GPAutowired.class)){continue;}
            GPAutowired autowired = field.getAnnotation(GPAutowired.class);

            //如果用户没有自定义beanName，默认就根据类型注入
            //这个地方省去了对类名首字母小写的情况的判断，这个作为课后作业请“小伙伴们”自己去实现
            String beanName = autowired.value().trim();
            if("".equals(beanName)){
                //获得接口的类型，作为key，稍后用这个key到IoC容器中取值
                beanName = field.getType().getName();
            }

            //如果是public以外的类型，只要加了@Autowired注解都要强制赋值
            //反射中叫作暴力访问
            field.setAccessible(true);

            try {
                //用反射机制动态给字段赋值
                field.set(entry.getValue(),ioc.get(beanName));
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }


        }

    }


}

实现initHandlerMapping()方法，HandlerMapping就是策略模式的应用案例：

//初始化url和Method的一对一关系
private void initHandlerMapping() {
    if(ioc.isEmpty()){ return; }

    for (Map.Entry<String, Object> entry : ioc.entrySet()) {
        Class<?> clazz = entry.getValue().getClass();

        if(!clazz.isAnnotationPresent(GPController.class)){continue;}


        //保存写在类上面的@GPRequestMapping("/demo")
        String baseUrl = "";
        if(clazz.isAnnotationPresent(GPRequestMapping.class)){
            GPRequestMapping requestMapping = clazz.getAnnotation(GPRequestMapping.class);
            baseUrl = requestMapping.value();
        }

        //默认获取所有的public类型的方法
        for (Method method : clazz.getMethods()) {
            if(!method.isAnnotationPresent(GPRequestMapping.class)){continue;}

            GPRequestMapping requestMapping = method.getAnnotation(GPRequestMapping.class);
            //优化
            String url = ("/" + baseUrl + "/" + requestMapping.value())
                        .replaceAll("/+","/");
            handlerMapping.put(url,method);
            System.out.println("Mapped :" + url + "," + method);

        }


    }


}

到这里初始化的工作完成，接下来实现运行的逻辑，来看doGet()和doPost()方法的代码：

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    this.doPost(req,resp);
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    //运行阶段
    try {
        doDispatch(req,resp);
    } catch (Exception e) {
        e.printStackTrace();
        resp.getWriter().write("500 Exection,Detail : " + Arrays.toString(e.getStackTrace()));
    }
}

doPost()方法中用了委派模式，委派模式的具体逻辑在doDispatch()方法中实现：

private void doDispatch(HttpServletRequest req, HttpServletResponse resp)throws Exception {
    String url = req.getRequestURI();
    String contextPath = req.getContextPath();
    url = url.replaceAll(contextPath,"").replaceAll("/+","/");
    if(!this.handlerMapping.containsKey(url)){
        resp.getWriter().write("404 Not Found!!");
        return;
    }
    Method method = this.handlerMapping.get(url);
    //第一个参数：方法所在的实例
    //第二个参数：调用时所需要的实参

    Map<String,String[]> params = req.getParameterMap();
    //投机取巧的方式
    String beanName = toLowerFirstCase(method.getDeclaringClass().getSimpleName());
    method.invoke(ioc.get(beanName),new Object[]{req,resp,params.get("name")[0]});
    //System.out.println(method);
}

在以上代码中，doDispatch()虽然完成了动态委派并进行了反射调用，但对url参数的处理还是静态的。要实现url参数的动态获取，其实有些复杂。我们可以优化doDispatch()方法的实现，代码如下：

private void doDispatch(HttpServletRequest req, HttpServletResponse resp)throws Exception {
    String url = req.getRequestURI();
    String contextPath = req.getContextPath();
    url = url.replaceAll(contextPath,"").replaceAll("/+","/");
    if(!this.handlerMapping.containsKey(url)){
        resp.getWriter().write("404 Not Found!!");
        return;
    }

    Method method = this.handlerMapping.get(url);
    //第一个参数：方法所在的实例
    //第二个参数：调用时所需要的实参
    Map<String,String[]> params = req.getParameterMap();
    //获取方法的形参列表
    Class<?> [] parameterTypes = method.getParameterTypes();
    //保存请求的url参数列表
    Map<String,String[]> parameterMap = req.getParameterMap();
    //保存赋值参数的位置
    Object [] paramValues = new Object[parameterTypes.length];
    //根据参数位置动态赋值
    for (int i = 0; i < parameterTypes.length; i ++){
        Class parameterType = parameterTypes[i];
        if(parameterType == HttpServletRequest.class){
            paramValues[i] = req;
            continue;
        }else if(parameterType == HttpServletResponse.class){
            paramValues[i] = resp;
            continue;
        }else if(parameterType == String.class){

            //提取方法中加了注解的参数
            Annotation[] [] pa = method.getParameterAnnotations();
            for (int j = 0; j < pa.length ; j ++) {
                for(Annotation a : pa[i]){
                    if(a instanceof GPRequestParam){
                        String paramName = ((GPRequestParam) a).value();
                        if(!"".equals(paramName.trim())){
                            String value = Arrays.toString(parameterMap.get(paramName))
                                    .replaceAll("\\[|\\]","")
                                    .replaceAll("\\s",",");
                            paramValues[i] = value;
                        }
                    }
                }
            }

        }
    }
    //投机取巧的方式
    //通过反射获得Method所在的Class，获得Class之后还要获得Class的名称
    //再调用toLowerFirstCase获得beanName
    String beanName = toLowerFirstCase(method.getDeclaringClass().getSimpleName());
    method.invoke(ioc.get(beanName),new Object[]{req,resp,params.get("name")[0]});
}

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