Spring MVC：servlet中忽略了自动装配

Question

For some reasons, I can autowire in my controllers but not in a servlet I created.

This is the top part of my servlet:

@Component
public class MyServlet extends javax.servlet.http.HttpServlet implements javax.servlet.Servlet {

    @Autowired
    private CobiService cobiService;

In my web.xml this is the related configuration:

    <servlet>
        <servlet-name>convservlet</servlet-name>
        <servlet-class>com.gim.servlets.MyServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>convservlet</servlet-name>
        <url-pattern>/MyServlet</url-pattern>
    </servlet-mapping>

And this is the way I tell spring to scan for components:

    <context:component-scan base-package="com.gim" />

For some reason my autowired object cobiService is null. Did I forget anything? What should I change?

Answer 1

Servlets are not managed by Spring, they are managed by the Servlet container (like Tomcat). Therefore Spring cannot inject dependencies into Servlets in a normal Spring way. You can however do something like the following:

public class MyServlet extends javax.servlet.http.HttpServlet {

    private CobiService cobiService;

    @Override
    public void init() throws ServletException {
        super.init();
        ApplicationContext applicationContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        cobiService = applicationContext.getBean(CobiService.class);
    }

}

Answer 2

There are two servlets being created. One is being created by the servlet container/Java EE container when the application initializes and reads the configured <servlet> tag inside the web.xml file. The other would be created when the Spring IOC container performs its component scanning upon initialization.

In this case, the first instance is not able to participate in dependency injection because it was not created within the Spring IOC container. In order to participate in dependency injection, a bean must be managed by the Spring IOC container. When the servlet container/Java EE container instantiates the servlet it has no knowledge of Spring's IOC container.

Unfortunately, when a request comes in that satisfies the url-pattern specified in the web.xml file for the servlet, the request is redirected to the first instance created by the servlet container/Java EE container which is not a bean and not a candidate for autowiring.

If you were to remove the servlet from web.xml and add a @RequestMapping annotation to the servlet, the second instance (which is an actual bean able to utilize autowiring) would be utilized for requests fullfilling the specified url pattern in the @RequestMapping , however at that point you pretty much have a controller.

So in summary, my recommendation would be to adhere to Spring conventions and use a controller class. The controller class will match and exceed the functionality provided by the Servlet.

Answer 3

You can override your servlet init method, and do

SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext (this);

SpringBeanAutowiringSupport

There is no need to do

implements javax.servlet.Servlet , since you are extending HttpServlet