[FIXED] HandlerInterceptor has an Object hander parameter, what types will get passed into the interceptor?

Issue

The HandlerInterceptor interface has a parameter Object handler, which means implementing code has to do type checking on the handler object in order to use it, and cast it as appropriate.

Code snippets I've found seem to assume the handler is always a HandlerMethod object and return true if this isn't the case, but I want to understand why this seems to be a common implementation to make a robust implementation.

A standard way of implementing this interface seems to be:

import org.springframework.web.method.HandlerMethod;
import org.springframework.web.servlet.HandlerInterceptor;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        if (handler instanceof HandlerMethod) {
            // ...
            return x; // whether or not to proceed with execution
        }
        return true; // fallback value in case handler wasn't a HandlerMethod
    }
}

The Spring javadoc seems to ignore the fact that the interface has an Object type in it, it's very unhelpful for my understanding of this interface.

Some questions that might aid my understanding of how this interface is supposed to be implemented:

• Why is true a sensible default if we get a handler that's not the object we expect it to be?
• Which types can an HandlerInterceptor take for its handler parameter?
• Under what circumstances could handler be a different type, if any?
• Why is it an Object, not a HandlerMethod, parameter?

Solution

Basically the handler argument can be any type for which a HandlerAdapter exists. Most commonly used will be the RequestMappingHandlerAdapter utilizing a HandlerMethod.

But it could be a regular class, a servlet, or even a function (when using the functional approach). Spring Web Services also has an implementation as well as Spring Integration.

Spring itself will support the following out-of-the-box

• HandlerMethod
• HandlerFunction
• Controller
• Servlet
• HttpRequestHandler
• WsdlDefinition (Spring Web Services)
• XsdDefinition (Spring Web Services)

So no it isn't always a HandlerMethod but it will be for most users.

Answered By - M. Deinum
Answer Checked By - Senaida (JavaFixing Volunteer)