Ali P7 summary: Spring MVC works, the benefit reading

This article will delve into some of --Spring Web MVC powerful features and inner workings of the Spring framework.

Installation Project

In this article, we will use the latest and best Spring Framework 5. We will focus on the classic Spring Web stack, which will emerge from the first version of the framework, and are still the main way is to use Spring to build Web applications.

For starters, in order to install the test project, it is best to use Spring Boot and beginners dependencies; also need to define parent:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.0.0.M5</version>
    <relativePath/>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
</dependencies>

Please note that in order to use Spring 5, we also need to use Spring Boot 2.x. As of the time of writing, it is still a milestone release available in Spring Milestone Repository Locate. Let's add this repository to your Maven project:

<repositories>
    <repository>
        <id>spring-milestones</id>
        <name>Spring Milestones</name>
        <url>https://repo.spring.io/milestone</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
    </repository>
</repositories>

You can Maven Central to view the current version of the Spring Boot.

The sample project

Spring Web MVC in order to understand how it works, we will implement a simple application through a login page. In order to display the login page, we need to create @Controller annotation class InternalController with GET mapping for the context root.

hello () method without parameters. It returns a Spring MVC as interpreted by the view name String (login.html template is in the example):

import org.springframework.web.bind.annotation.GetMapping;
@GetMapping("/")
public String hello() {
    return "login";
}

In order to process user login, registration data need to create another method of using POST request process. Then the results page will redirect the user to the success or failure.

Note, login () method receives as a parameter and returns the domain objects ModelAndView object:

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.servlet.ModelAndView;
@PostMapping("/login")
public ModelAndView login(LoginData loginData) {
    if (LOGIN.equals(loginData.getLogin()) 
      && PASSWORD.equals(loginData.getPassword())) {
        return new ModelAndView("success", 
          Collections.singletonMap("login", loginData.getLogin()));
    } else {
        return new ModelAndView("failure", 
          Collections.singletonMap("login", loginData.getLogin()));
    }
}

ModelAndView is the holder of two different objects:

• Model-- rendering the page data-value mapping
• Page templates View-- filling model data

These connections are for convenience, such a method may return them once the controller.

To render HTML pages, using Thymeleaf as the view template engine, which has an integrated and reliable out of the box with the Spring.

Servlet as the basis for Java Web applications

Then, when your browser to http: // localhost: 8080 /, press Enter, and then request reaches the Web server, what actually happened? How do you see the Web browser forms from this request?

Given that the project is a simple Spring Boot application, so you can run it through Spring5Application.

Spring Boot using Apache Tomcat default. Therefore, when you run the application, you may see the following message in the log:

2017-10-16 20:36:11.626  INFO 57414 --- [main] 
  o.s.b.w.embedded.tomcat.TomcatWebServer  : 
  Tomcat initialized with port(s): 8080 (http)
2017-10-16 20:36:11.634  INFO 57414 --- [main] 
  o.apache.catalina.core.StandardService   : 
  Starting service [Tomcat]
2017-10-16 20:36:11.635  INFO 57414 --- [main] 
  org.apache.catalina.core.StandardEngine  : 
  Starting Servlet Engine: Apache Tomcat/8.5.23

Since a Tomcat Servlet container is thus sent to each server HTTP Tomcat Web request by the Java servlet natural process. So Spring Web application entry point is a servlet, which is not surprising.

Simply put, servlet is the core component of any Java Web applications; it is low-level, not many requirements in a particular programming model MVC like that.

The servlet can receive an HTTP an HTTP request, processed in some way, and then sends back a response.

Moreover, from the beginning of Servlet 3.0 API, you can now go beyond the XML configuration, and start using the Java configuration (only small restrictions).

DispatcherServlet as the core of Spring MVC

As a developer of Web applications, we really want to do is abstract and less cumbersome template of tasks, and focus on useful business logic:

• The HTTP request is mapped to a processing method
• The HTTP request header data and parsed into data transfer object (the DTO) or domain objects
• Model - View - Controller Integrated
• Generating a response from the DTO, and the like domain object

The Spring  the DispatcherServlet can provide these. It is the Web the Spring  the MVC framework core; the core component receives all requests to the application.

As you can see, DispatcherServlet is very scalable. For example, it allows you to insert a different existing or new adapter large number of tasks:

• Maps the request to be processed or the method of its class (HandlerMapping implementation)
• Processing requests using a specific pattern, such as a conventional the servlet, (HandlerAdapter implement interface) MVC more complex workflows or POJO bean Method
• By name resolution view, allowing you to use a different template engine, XML, XSLT, or any other view technology (to achieve ViewResolver interface)
• By using the default Apache Commons to parse multipart file upload request to implement or write your own MultipartResolver
• Any LocaleResolver a settlement locales, including the cookie, session, Accept HTTP header, or any other user to determine the desired locale-sensitive manner

HTTP request handling

First, we will simply process the HTTP request to a tracking control method in the layer, and then returned to the browser / client.

DispatcherServlet has a long inheritance hierarchy; top-down one by one to understand these are valuable. Request processing method is most interesting to us.

Understand HTTP request, either locally or remotely in standards development are key to understanding part of the MVC architecture.

GenericServlet

GenericServlet is part of the Servlet specification does not directly concern HTTP. It defines the service () method receives incoming requests and generating responses.

Notice how, ServletRequest and ServletResponse method parameters independent of the HTTP protocol:

public abstract void service(ServletRequest req, ServletResponse res) 
  throws ServletException, IOException;

This is the ultimate method is invoked on any request to the server, including a simple GET request.

HttpServlet

As the name suggests, HttpServlet class is implemented on HTTP Servlet is defined in the specification.

More practical to say, the HttpServlet is an abstract class, there is a service () method implementation, service () method is implemented by dividing the type of HTTP request method, substantially as follows:

protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {
    String method = req.getMethod();
    if (method.equals(METHOD_GET)) {
        // ...
        doGet(req, resp);
    } else if (method.equals(METHOD_HEAD)) {
        // ...
        doHead(req, resp);
    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);
        // ...
    }

HttpServletBean

接下来，HttpServletBean是层次结构中第一个Spring-aware类。它使用从web.xml或WebApplicationInitializer接收到的servlet init-param值来注入bean的属性。

在请求应用程序的情况下，doGet()，doPost()等方法应特定的HTTP请求而调用。

FrameworkServlet

FrameworkServlet集成Servlet功能与Web应用程序上下文，实现了ApplicationContextAware接口。但它也能够自行创建Web应用程序上下文。

正如你已经看到的，HttpServletBean超类注入init-params为bean属性。所以，如果在servlet的contextClass init-param中提供了一个上下文类名，那么这个类的一个实例将被创建为应用程序上下文。否则，将使用默认的XmlWebApplicationContext类。

由于XML配置现在已经过时，Spring Boot默认使用AnnotationConfigWebApplicationContext配置DispatcherServlet。但是你可以轻松更改。

例如，如果你需要使用基于Groovy的应用程序上下文来配置Spring Web MVC应用程序，则可以在web.xml文件中使用以下DispatcherServlet配置：

dispatcherServlet
        org.springframework.web.servlet.DispatcherServlet
        contextClass
        org.springframework.web.context.support.GroovyWebApplicationContext

使用WebApplicationInitializer类，可以用更现代的基于Java的方式来完成相同的配置。

DispatcherServlet：统一请求处理

HttpServlet.service()实现，会根据HTTP动词的类型来路由请求，这在低级servlet的上下文中是非常有意义的。然而，在Spring MVC的抽象级别，方法类型只是可以用来映射请求到其处理程序的参数之一。

因此，FrameworkServlet类的另一个主要功能是将处理逻辑重新加入到单个processRequest()方法中，processRequest()方法反过来又调用doService()方法：

@Override
protected final void doGet(HttpServletRequest request, 
  HttpServletResponse response) throws ServletException, IOException {
    processRequest(request, response);
}
@Override
protected final void doPost(HttpServletRequest request, 
  HttpServletResponse response) throws ServletException, IOException {
    processRequest(request, response);
}
// …

DispatcherServlet：丰富请求

最后，DispatcherServlet实现doService()方法。在这里，它增加了一些可能会派上用场的有用对象到请求：Web应用程序上下文，区域解析器，主题解析器，主题源等：

request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, 
  getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

另外，doService()方法准备输入和输出Flash映射。Flash映射基本上是一种模式，该模式将参数从一个请求传递到另一个紧跟的请求。这在重定向期间可能非常有用（例如在重定向之后向用户显示一次性信息消息）：

FlashMap inputFlashMap = this.flashMapManager
  .retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
    request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, 
      Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());

然后，doService()方法调用负责请求调度的doDispatch()方法。

DispatcherServlet：调度请求

dispatch()方法的主要目的是为请求找到合适的处理程序，并为其提供请求/响应参数。处理程序基本上是任何类型的object，不限于特定的接口。这也意味着Spring需要为此处理程序找到适配器，该处理程序知道如何与处理程序“交谈”。

为了找到匹配请求的处理程序，Spring检查HandlerMapping接口的注册实现。有很多不同的实现可以满足你的需求。

SimpleUrlHandlerMapping允许通过URL将请求映射到某个处理bean。例如，可以通过使用java.util.Properties实例注入其mappings属性来配置，就像这样：

/welcome.html=ticketController
/show.html=ticketController

可能处理程序映射最广泛使用的类是RequestMappingHandlerMapping，它将请求映射到@Controller类的@ RequestMapping注释方法。这正是使用控制器的hello()和login()方法连接调度程序的映射。

请注意，Spring-aware方法使用@GetMapping和@PostMapping进行注释。这些注释依次用@RequestMapping元注释标记。

dispatch()方法还负责其他一些HTTP特定任务：

• 在资源未被修改的情况下，GET请求的短路处理
• 针对相应的请求应用多部分解析器
• 如果处理程序选择异步处理该请求，则会短路处理该请求

处理请求

现在Spring已经确定了请求的处理程序和处理程序的适配器，是时候来处理请求了。下面是HandlerAdapter.handle()方法的签名。请注意，处理程序可以选择如何处理请求：

• 自主地编写数据到响应对象，并返回null
• 返回由DispatcherServlet呈现的ModelAndView对象

@Nullable
ModelAndView handle(HttpServletRequest request, 
                    HttpServletResponse response, 
                    Object handler) throws Exception;

有几种提供的处理程序类型。以下是SimpleControllerHandlerAdapter如何处理Spring MVC控制器实例（不要将其与@ Controller注释POJO混淆）。

注意控制器处理程序如何返回ModelAndView对象，并且不自行呈现视图：

public ModelAndView handle(HttpServletRequest request, 
  HttpServletResponse response, Object handler) throws Exception {
    return ((Controller) handler).handleRequest(request, response);
}

第二个是SimpleServletHandlerAdapter，它将常规的Servlet作为请求处理器。

Servlet不知道任何有关ModelAndView的内容，只是简单地自行处理请求，并将结果呈现给响应对象。所以这个适配器只是返回null而不是ModelAndView：

public ModelAndView handle(HttpServletRequest request, 
  HttpServletResponse response, Object handler) throws Exception {
    ((Servlet) handler).service(request, response);
    return null;
}

我们碰到的情况是，控制器是有若干@RequestMapping注释的POJO，所以任何处理程序基本上是包装在HandlerMethod实例中的这个类的方法。为了适应这个处理器类型，Spring使用RequestMappingHandlerAdapter类。

处理参数和返回处理程序方法的值

注意，控制器方法通常不会使用HttpServletRequest和HttpServletResponse，而是接收和返回许多不同类型的数据，例如域对象，路径参数等。

此外，要注意，我们不需要从控制器方法返回ModelAndView实例。可能会返回视图名称，或ResponseEntity，或将被转换为JSON响应等的POJO。

RequestMappingHandlerAdapter确保方法的参数从HttpServletRequest中解析出来。另外，它从方法的返回值中创建ModelAndView对象。

在RequestMappingHandlerAdapter中有一段重要的代码，可确保所有这些转换魔法的发生：

ServletInvocableHandlerMethod invocableMethod 
  = createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
    invocableMethod.setHandlerMethodArgumentResolvers(
      this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
    invocableMethod.setHandlerMethodReturnValueHandlers(
      this.returnValueHandlers);
}

argumentResolvers对象是不同的HandlerMethodArgumentResolver实例的组合。

有超过30个不同的参数解析器实现。它们允许从请求中提取任何类型的信息，并将其作为方法参数提供。这包括URL路径变量，请求主体参数，请求标头，cookies，会话数据等。

returnValueHandlers对象是HandlerMethodReturnValueHandler对象的组合。还有很多不同的值处理程序可以处理方法的结果来创建适配器所期望的ModelAndViewobject。

例如，当你从hello()方法返回字符串时，ViewNameMethodReturnValueHandler处理这个值。但是，当你从login()方法返回一个准备好的ModelAndView时，Spring会使用ModelAndViewMethodReturnValueHandler。

渲染视图

到目前为止，Spring已经处理了HTTP请求并接收了ModelAndView对象，所以它必须呈现用户将在浏览器中看到的HTML页面。它基于模型和封装在ModelAndView对象中的选定视图来完成。

另外请注意，我们可以呈现JSON对象，或XML，或任何可通过HTTP协议传输的其他数据格式。我们将在即将到来的REST-focused部分接触更多。

让我们回到DispatcherServlet。render()方法首先使用提供的LocaleResolver实例设置响应语言环境。假设现代浏览器正确设置了Accept头，并且默认使用AcceptHeaderLocaleResolver。

在渲染过程中，ModelAndView对象可能已经包含对所选视图的引用，或者只是一个视图名称，或者如果控制器依赖于默认视图，则什么都没有。

由于hello()和login()方法两者都指定所需的视图为String名称，因此必须用该名称查找。所以，这是viewResolvers列表开始起作用的地方：

for (ViewResolver viewResolver : this.viewResolvers) {
    View view = viewResolver.resolveViewName(viewName, locale);
    if (view != null) {
        return view;
    }
}

这是一个ViewResolver实例列表，包括由thymeleaf-spring5集成库提供的ThymeleafViewResolver。该解析器知道在哪里搜索视图，并提供相应的视图实例。

在调用视图的render()方法后，Spring最终通过发送HTML页面到用户的浏览器来完成请求处理。

REST支持

除了典型的MVC场景之外，我们还可以使用框架来创建REST Web服务。

简而言之，我们可以接受Resource作为输入，指定POJO作为方法参数，并使用@RequestBody对其进行注释。也可以使用@ResponseBody注释方法本身，以指定其结果必须直接转换为HTTP响应：

import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.ResponseBody;
@ResponseBody
@PostMapping("/message")
public MyOutputResource sendMessage(
  @RequestBody MyInputResource inputResource) {
    return new MyOutputResource("Received: "
      + inputResource.getRequestMessage());
}

归功于Spring MVC的可扩展性，这也是可行的。

为了将内部DTO编组为REST表示，框架使用HttpMessageConverter基础结构。例如，其中一个实现是MappingJackson2HttpMessageConverter，它可以使用Jackson库将模型对象转换为JSON或从JSON转换。

为了进一步简化REST API的创建，Spring引入了@RestController注解。默认情况下，这很方便地假定了@ResponseBody语义，并避免在每个REST控制器上的明确设置：

import org.springframework.web.bind.annotation.RestController;
@RestController
public class RestfulWebServiceController {
    @GetMapping("/message")
    public MyOutputResource getMessage() {
        return new MyOutputResource("Hello!");
    }
}

结论

在这篇文章中，我们详细了介绍在Spring MVC框架中请求的处理过程。了解框架的不同扩展是如何协同工作来提供所有魔法的，可以让你能够事倍功半地处理HTTP协议难题。

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