Spring Boot: Inject Bean into HttpServlet - spring

Question

I've got a Spring Boot where I've autoconfigured a Router bean.
This all works perfect but it becomes a problem when I want to inject that bean into a custom servlet:
public class MembraneServlet extends HttpServlet {
#Autowired
private Router router;
#Override
public void init() throws ServletException {
SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this);
}
#Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
new HttpServletHandler(req, resp, router.getTransport()).run();
}
}
This should be the way to go, but
SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this);
won't autowire the Router because the WebapplicationContext is always null. The application is running in an MVC environment.

Answer 1

Assuming you Spring Application Context is wired to the Servlet Context, you might want to pass ServletContext to SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext
public class MembraneServlet extends HttpServlet {
#Autowired
private Router router;
#Override
public void init() throws ServletException {
SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this, getServletContext());
}
#Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
new HttpServletHandler(req, resp, router.getTransport()).run();
}
}

Answer 2

What about injecting 'Router' as constructor parameter.
So you would have this:
public class MembraneServlet extends HttpServlet {
private Router router;
public MembraneServlet(Router router){
this.router = router;
}
#Override
public void init() throws ServletException {
SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this);
}
#Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
new HttpServletHandler(req, resp, router.getTransport()).run();
}
}
then you can programatically create servlet registration like this:
#Bean
public ServletRegistrationBean membraneServletRegistrationBean(){
return new ServletRegistrationBean(new MembraneServlet(),"/*");
}

Answer 3

Embedded server
You can annotate with #WebServlet your servlet class:
#WebServlet(urlPatterns = "/example")
public class ExampleServlet extends HttpServlet
And enable #ServletComponentScan on base class:
#ServletComponentScan
#EntityScan(basePackageClasses = { ExampleApp.class, Jsr310JpaConverters.class })
#SpringBootApplication
public class ExampleApp
But injection with #ServletComponentScan will work only with embedded server:
Enables scanning for Servlet components (filters, servlets, and
listeners). Scanning is only performed when using an embedded web
server.
More info: The #ServletComponentScan Annotation in Spring Boot
External server
When using external server, mark HttpServlet class as #Component:
#Component
public class ExampleServlet extends HttpServlet
And create configuration class:
#Configuration
#ComponentScan(value = "com.example.servlet.package")
public class ServletConfig {
#Autowired
private ExampleServlet exampleServlet;
#Bean
public ServletRegistrationBean resetServletRegistrationBean(){
return new ServletRegistrationBean(exampleServlet, "/example");
}
}