web.xml vs Initializer with Spring – web.xml与Spring的初始化器

最后修改: 2016年 9月 22日

1. Overview


In this article we’ll cover three different approaches of configuring a DispatcherServlet available in recent versions of the Spring Framework:


  1. We’ll start with an XML configuration and a web.xml file
  2. Then we’ll migrate the Servlet declaration from the web.xml file to Java config, but we’ll leave any other configuration in XML
  3. Finally in the third and final step of the refactoring, we’ll have a 100% Java-configured project

2. The DispatcherServlet


One of the core concepts of Spring MVC is the DispatcherServlet. The Spring documentation defines it as:

Spring MVC的核心概念之一是DispatcherServletSpring文档将其定义为。

A central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers or HTTP-based remote service exporters. Dispatches to registered handlers for processing a web request, providing convenient mapping and exception handling facilities.

HTTP请求处理程序/控制器的中央调度器,例如Web UI控制器或基于HTTP的远程服务输出器。派遣到注册的处理程序来处理一个web请求,提供方便的映射和异常处理设施。

Basically the DispatcherServlet is the entry point of every Spring MVC application. Its purpose is to intercept HTTP requests and to dispatch them to the right component that will know how to handle it.

基本上,DispatcherServlet是每个Spring MVC应用程序的入口点。它的目的是拦截HTTP请求,并将其分派给知道如何处理的正确组件。

3. Configuration With web.xml


If you deal with legacy Spring projects it is very common to find XML configuration and until Spring 3.1 the only way to configure the DispatcherServlet was with the WEB-INF/web.xml file. In this case there are two steps required.


Let’s see an example configuration – the first step is the Servlet declaration:



With this block of XML we are declaring a servlet that:


  1. Is named “dispatcher
  2. Is an instance of org.springframework.web.servlet.DispatcherServlet
  3. Will be initialized with a parameter named contextConfigLocation which contains the path to the configuration XML

load-on-startup is an integer value that specifies the order for multiple servlets to be loaded. So if you need to declare more than one servlet you can define in which order they will be initialized. Servlets marked with lower integers are loaded before servlets marked with higher integers.


Now our servlet is configured. The second step is declaring a servlet-mapping:



With the servlet mapping we are bounding it by its name to a URL pattern that specifies what HTTP requests will be handled by it.

通过servlet映射,我们将它的名字与URL pattern绑定,指定它将处理哪些HTTP请求。

4. Hybrid Configuration


With the adoption of the version 3.0 of Servlet APIs, the web.xml file has become optional, and we can now use Java to configure the DispatcherServlet.

随着3.0版Servlet APIs的采用,web.xml文件变得可有可无,我们现在可以使用Java来配置DispatcherServlet

We can register a servlet implementing a WebApplicationInitializer. This is the equivalent of the XML configuration above:


public class MyWebAppInitializer implements WebApplicationInitializer {
    public void onStartup(ServletContext container) {
        XmlWebApplicationContext context = new XmlWebApplicationContext();

        ServletRegistration.Dynamic dispatcher = container
          .addServlet("dispatcher", new DispatcherServlet(context));


In this example we are:


  1. Implementing the WebApplicationInitializer interface
  2. Overriding the onStartup method we create a new XmlWebApplicationContext configured with the same file passed as contextConfigLocation to the servlet in the XML example
  3. Then we are creating an instance of DispatcherServlet with the new context that we just instantiated
  4. And finally we are registering the servlet with a mapping URL pattern

So we used Java to declare the servlet and bind it to a URL mapping but we kept the configuration in a separated XML file: dispatcher-config.xml.


5. 100% Java Configuration


With this approach our servlet is declared in Java, but we still need an XML file to configure it. With WebApplicationInitializer you can achieve a 100% Java configuration.


Let’s see how we can refactor the previous example.


The first thing we will need to do is create the application context for the servlet.


This time we will use an annotation based context so that we can use Java and annotations for configuration and remove the need for XML files like dispatcher-config.xml:


AnnotationConfigWebApplicationContext context
  = new AnnotationConfigWebApplicationContext();

This type of context can then be configured registering a configuration class:



Or setting an entire package that will be scanned for configuration classes:



Now that our application context is created, we can add a listener to the ServletContext that will load the context:


container.addListener(new ContextLoaderListener(context));

The next step is creating and registering our dispatcher servlet:

下一步是创建和注册我们的dispatcher servlet。

ServletRegistration.Dynamic dispatcher = container
  .addServlet("dispatcher", new DispatcherServlet(context));


Now our WebApplicationInitializer should look like this:


public class MyWebAppInitializer implements WebApplicationInitializer {
    public void onStartup(ServletContext container) {
        AnnotationConfigWebApplicationContext context
          = new AnnotationConfigWebApplicationContext();

        container.addListener(new ContextLoaderListener(context));

        ServletRegistration.Dynamic dispatcher = container
          .addServlet("dispatcher", new DispatcherServlet(context));

Java and annotation configuration offers many advantages. Usually it leads to shorter and more concise configuration and annotations provide more context to declarations, as it’s co-located with the code that they configure.


But this is not always a preferable or even possible way. For example some developers may prefer keeping their code and configuration separated, or you may need to work with third party code that you can’t modify.


6. Conclusion


In this article we covered different ways to configure a DispatcherServlet in Spring 3.2+ and it’s up to you to decide which one to use based on your preferences. Spring will accommodate to your decision whatever you choose.

在这篇文章中,我们介绍了在Spring 3.2+中配置DispatcherServlet的不同方法,由你根据自己的喜好决定使用哪种方法。Spring将适应你的决定,无论你选择什么。

You can find the source code from this article on Github here and here.

你可以在Github 这里这里找到本文的源代码。