如何在春季添加可选的依赖项类？

Question

I'm creating a common library and would like to add a service that uses ServletContext . But as the library should also be used in a cli environment, I have to find a way telling spring to ignore the servlet there.

@Component
public class MyService {
    //only available in a web environment
    @Autowired(required = false)
    private ServletContext servletContext;
}

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <optional>true</optional>
        </dependency>

In the implementation project, I want to run only a command line interface, thus I did not include the servlet dependency.

But when I try to launch, the following exception is thrown on startup: Caused by: java.lang.ClassNotFoundException: javax.servlet.ServletContext

Question: how can I make use of the ServletContext in the CommonService without having to add the dependency to each project that reuses that class?

Answer 1

Just include the servlet dependency and make the dependency optional as you've done. All the other options are worse.

It's a fairly light weight class - 93K for the jar on servlet spec 3.1.0.

But if you absolutely must run without the class on the classpath then you could do the following...

Firstly let me say that the reason you're having the issue is not because the ServletContext is a field but because it is an autowired field. I wrote a little test app and the type of the fields of a class are not required to be available at runtime.

So to get this to work you'll need a profile and a named bean.

Firstly you're going to need to inject the ServletContext by name - and not have it typed as ServletContext. This is not great as you're going to need to cast it to servlet context when you need to use it.

So

@Autowired(required = false)
@Qualifier("servletContext")
Object servletContext

is what you'll put here.

Then create another class that is going to provide the serlvet context - and this will be turned on via a profile - or possibly what @membersound referred though @ConditionalOnWebApplication is only available in Spring Boot.

@Configuration
@Profile("web") or @ConditionalOnWebApplication
public class ServletContextProvider {

  @Autowired
  ServletContext servletContext;

    @Bean("servletContext")
    public ServletContext getServletContext() {
      return servletContext;
    }
}

Like I said, just include ServletContext on your classpath.

Answer 2

Luckily there are the following annotations that could be directly added onto the class:

@ConditionalOnWebApplication
@ConditionalOnNotWebApplication

This way spring in a cli environment does not need to evaluate the ServletContext bean.

Of course this requires the creation of two service classes, where each of them has one of the annotations above.

public abstract class MyService {
       abstract String impl();
}

@Component
class MyServiceCLI {
       @Override
       String impl() {
             return "cli specific";
       }
}

@Component
class MyServiceWeb {
    //only available in a web environment
    @Autowired
    private ServletContext servletContext;

       @Override
       String impl() {
             //use servlet context...
             return "ctx specific";
       }

}

Then inject just @Autowired MyService .

By making the implementations package private , they are invisible to the users, but spring still sees and creates them.