17.2 The DispatcherServlet

综述：

　　Spring’s web MVC framework is, like many other web MVC frameworks, request-driven, designed around a central Servlet that dispatches requests to controllers and offers other functionality that facilitates the development of web applications. Spring’s DispatcherServlet however, does more than just that. It is completely integrated with the Spring IoC container and as such allows you to use every other feature that Spring has.

　　The request processing workflow of the Spring Web MVC DispatcherServlet is illustrated in the following diagram. The pattern-savvy reader will recognize that the DispatcherServlet is an expression of the "Front Controller" design pattern (this is a pattern that Spring Web MVC shares with many other leading web frameworks).

　　spring的web mvc框架，像其他的web mvc框架一样，请求驱动，围绕着一个中央servlet设计的，该中央servlet分发请求到控制器，并提供其他功能以促进web应用的发展。然而，spring的dispatcherServlet，做的不只这么多。它完全使用了spring ioc容器进行集成，正因如此才允许你使用spring拥有的所有特性

　　spring mvc dispatcherservlet的请求处理流程，列举在下面的图表里。pattren-savvy的读者将会识别出DispatcherServlet前置控制器的表达模式（这是一个模式，那些spring web mvc与很多其他领先的web框架共享的）。

The request processing workflow in Spring Web MVC (high level)

The DispatcherServlet is an actual Servlet (it inherits from the HttpServlet base class), and as such is declared in the web.xml of your web application. You need to map requests that you want the DispatcherServlet to handle, by using a URL mapping in the same web.xml file. This is standard Java EE Servlet configuration; the following example shows such a DispatcherServlet declaration and mapping:

spring web mvc的请求处理流程：

　　DispatcherServlet实际上就是一个servlet，它继承自HttpServlet这个基础类，并且可以在你的web项目的web.xml中声明。你只需要将该控制器处理的请求分发给它即可，在同一个web.xml文件中，通过使用一个url映射。这是一个标准的java ee servlet配置，下面的例子说明了该servlet的声明和映射：

<web-app>

    <servlet>
        <servlet-name>example</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>example</servlet-name>
        <url-pattern>/example/*</url-pattern>
    </servlet-mapping>

</web-app>

In the preceding example, all requests starting with /example will be handled by the DispatcherServlet instance named example. In a Servlet 3.0+ environment, you also have the option of configuring the Servlet container programmatically. Below is the code based equivalent of the above web.xml example:

　　上面的例子，所有以/example开头的请求都将会被名为example的DispatcherServlet处理。在servlet 3.0以上的环境中，你也可以通过编程方式代替配置该servlet的手段。下面的代码就是同上面web.xml示例相对应的代码。

public class MyWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext container) {
        ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet());
        registration.setLoadOnStartup(1);
        registration.addMapping("/example/*");
    }

}

　　WebApplicationInitializer is an interface provided by Spring MVC that ensures your code-based configuration is detected and automatically used to initialize any Servlet 3 container. An abstract base class implementation of this interface named AbstractDispatcherServletInitializer makes it even easier to register the DispatcherServlet by simply specifying its servlet mapping. See Code-based Servlet container initialization for more details.

　　The above is only the first step in setting up Spring Web MVC. You now need to configure the various beans used by the Spring Web MVC framework (over and above the DispatcherServlet itself).

　　As detailed in Section 5.15, “Additional Capabilities of the ApplicationContext”, ApplicationContext instances in Spring can be scoped. In the Web MVC framework, each DispatcherServlet has its own WebApplicationContext, which inherits all the beans already defined in the root WebApplicationContext. These inherited beans can be overridden in the servlet-specific scope, and you can define new scope-specific beans local to a given Servlet instance.

　　WebApplicationInitializer spring mvc提供的一个接口，确保你在编码中的配置能够被发现，并自动被使用以用来初始化servlet3容器。一个抽象的基础类实现该接口，被命名为
AbstractDispatcherServletInitializer使得它更容易去注册该DispatcherServlet通过简单的指定它的servlet映射。看Code-based Servlet container initialization有更多详细信息。
　　上面的内容仅仅是设置spring-mvc的第一步，现在，你需要配置spring mvc framework使用的各种beans（）。
　　就像在5.15中描述的一样，应用上下文添加的能力，spring的应用上下文实例是能够被审查的。在web mvc 框架中，每个DispatcherServlet都有它自己的webApplicationContext，该~继承了在基础WebApplicationContext中定义的所有beans。这些继承的beans能够在servlet-specific scope中重写，你能够定义新的scope-specific beans 给servlet实例。

Figure 17.2. Context hierarchy in Spring Web MVC

Upon initialization of a DispatcherServlet, Spring MVC looks for a file named [servlet-name]-servlet.xml in the WEB-INF directory of your web application and creates the beans defined there, overriding the definitions of any beans defined with the same name in the global scope.

　　在初始化DispatcherServlet之前，spring mvc在你的应用程序的WEB-INF目录中查找一个文件名servlet-name-servlet.xml，并根据定义的内容创建相应的beans，覆盖全局中同样名称的任何beans。

Consider the following DispatcherServlet Servlet configuration (in the web.xml file):

注意下面的DispatcherServlet配置

<web-app>
    <servlet>
        <servlet-name>golfing</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>golfing</servlet-name>
        <url-pattern>/golfing/*</url-pattern>
    </servlet-mapping>
</web-app

With the above Servlet configuration in place, you will need to have a file called /WEB-INF/golfing-servlet.xml in your application; this file will contain all of your Spring Web MVC-specific components (beans). You can change the exact location of this configuration file through a Servlet initialization parameter (see below for details).

用上面的servlet配置替换，你需要一个叫做/WEB-INF/golfing-servlet.xml的文件在你的系统中，这个文件将会包含所有你spring mvc 特定的组件。你可以通过servlet的初始化参数配置，改变这个文件存放的地方。如下：

<web-app>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/root-context.xml</param-value>
    </context-param>
    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value></param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
</web-app>

The WebApplicationContext is an extension of the plain ApplicationContext that has some extra features necessary for web applications. It differs from a normal ApplicationContext in that it is capable of resolving themes (see Section 17.9, “Using themes”), and that it knows which Servlet it is associated with (by having a link to the ServletContext). The WebApplicationContext is bound in the ServletContext, and by using static methods on the RequestContextUtils class you can always look up the WebApplicationContext if you need access to it.

WebApplicationContext是简单ApplicationContext的一个扩展，有一些额外的特性是应用程序所需要的。与一个普通的ApplicationContext不同，在WebApplicationContext中包含了解析主题。Web被绑定在ServletContext中，通过使用RequestContextUtils类的静态方法，当你需要使用它时，你总能找到它。