Using non-blocking handlers with servlet

When using servlet deployments in Undertow it is possible to mix and match servlets and Undertow native handlers.

This is achieved via the io.undertow.servlet.ServletExtension interface. This interface allows you to customise a servlet deployment before it is deployed, including wrapping the servlet handler chain with your own handlers.

Lets get started. First we need a ServletExtension implementation:

package io.undertow.example.nonblocking;

import io.undertow.Handlers;
import io.undertow.server.HandlerWrapper;
import io.undertow.server.HttpHandler;
import io.undertow.server.handlers.PathHandler;
import io.undertow.servlet.ServletExtension;
import io.undertow.servlet.api.DeploymentInfo;

import javax.servlet.ServletContext;

public class NonBlockingHandlerExtension implements ServletExtension {
    @Override
    public void handleDeployment(final DeploymentInfo deploymentInfo, final ServletContext servletContext) {
        deploymentInfo.addInitialHandlerChainWrapper(new HandlerWrapper() {
            @Override
            public HttpHandler wrap(final HttpHandler handler) {
                return Handlers.path()
                        .addPrefixPath("/", handler)
                        .addPrefixPath("/hello", new HelloWorldHandler());
            }
        });
    }
}

Now we need a handler:

public class HelloWorldHandler implements HttpHandler {
    @Override
    public void handleRequest(final HttpServerExchange exchange) throws Exception {
        exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
        exchange.getResponseSender().send("Hello World");
    }
}

We now need to register this extension. This uses to standard java service loader mechanism, so we need to create a WEB-INF/classes/META-INF/services/io.undertow.servlet.ServletExtension file that contains the name of our extension class.

Now when you deploy your war to Wildfly you should be able to navigate to /mywar/hello and your custom handler will be invoked.

Lets see exactly what is going on here. When the deployment is about to deploy the handleDeployment method is invoked. This method is passed the io.undertow.servlet.api.DeploymentInfo structure, that contains a complete description of the deployment. One of the things that this contains is a list of handler chain wrappers. These wrappers allow you to add additional handlers before the servlet handler.

In our wrapper we create an io.undertow.server.handlers.PathHandler, which is a handler provided by undertow that maps handlers to paths. We register two different handlers into the path handler, our custom handler under /hello, and the servlet handler under /. These paths are relative to the root of the servlet context (technically they are relative to the last resolved path, so if you chain two path handlers together the second paths will be resolved relative to the first one).

In our handler we simply set a Content-Type header and then send a "Hello World" response via async IO.

A slightly more complex example

Say we are serving up an application, and we decide that we would like to serve all our .js and .css files using an async handler, as we want to avoid the overhead of a servlet request.

To do this we are going to create a handler that checks the extension on the incoming request, and if it is .js or .css then it will serve the file directly, bypassing servlet all together.

This will bypass all servlet handlers, including security handlers, so security rules will not be applied for these handlers.

public class NonBlockingHandlerExtension implements ServletExtension {
    @Override
    public void handleDeployment(final DeploymentInfo deploymentInfo, final ServletContext servletContext) {
        deploymentInfo.addInitialHandlerChainWrapper(new HandlerWrapper() {
            @Override
            public HttpHandler wrap(final HttpHandler handler) {

                final ResourceHandler resourceHandler = new ResourceHandler()
                        .setResourceManager(deploymentInfo.getResourceManager());

                PredicateHandler predicateHandler = new PredicateHandler(Predicates.suffixs(".css", ".js"), resourceHandler, handler);

                return predicateHandler;
            }
        });
    }
}

Lets go through this line by line:

final ResourceHandler resourceHandler = new ResourceHandler()
    .setResourceManager(deploymentInfo.getResourceManager());

A resource handler is a handler provided by Undertow that serves resources from a resource manager. This is basically just an abstraction that allows us to re-use the file serving code no matter where a file in coming from. For example undertow provides several default resource manager implementations:

io.undertow.server.handlers.resource.FileResourceManager: A resource manager that serves files from the file system
io.undertow.server.handlers.resource.ClassPathResourceManager: A resource manager that serves files from the class path
io.undertow.server.handlers.resource.CachingResourceManager: A resource manger that wraps another resource manger, and provides caching.

You do not need to worry about what type of resource manager is in use here, all you need to know is that this is the resource manager that is being used by the default servlet, so serving files from this resource manager will mirror the behaviour of the default servlet.

We now need to wire up our resource handler so it is only used for .js and .css. We could simply write a handler that checks the file extension and delegates accordingly, however Undertow already provides us with one:

PredicateHandler predicateHandler = new PredicateHandler(Predicates.suffixs(".css", ".js"), resourceHandler, handler);

A PredicateHandler chooses between two different handlers based on the result of a predicate that is applied to the exchange. In this case we are using a suffix predicate, that will return true if the request ends with .js or .css.

When this predicate returns true our resource handler will be invoked, otherwise the request will be delegated to the servlet container as normal.