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Chapter 25. Handlers
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## Chapter 25. Handlers

## Writing Custom Handlers

The Handler is the Jetty component that deals with received requests.

Many users of Jetty never need to write a Jetty Handler, but instead use the Servlet API. You can reuse the existing Jetty handlers for context, security, sessions and servlets without the need for extension. However, some users might have special requirements or footprint concerns that prohibit the use of the full servlet API. For them implementing a Jetty handler is a straight forward way to provide dynamic web content with a minimum of fuss.

See the section on Jetty Architecture to understand more about Handlers vs. Servlets.

### The Handler API

The Handler interface provides Jetty’s core of content generation or manipulation. Classes that implement this interface are used to coordinate requests, filter requests and generate content.

The core API of the Handler interface is:

    public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
        throws IOException, ServletException

An implementation of this method can handle a request and pass the request onto another handler (or servlet), or it can modify and/or wrap the request before passing it on. This gives three styles of handler:

- Coordinating Handlers - Handlers that route requests to other handlers (`HandlerCollection`, `ContextHandlerCollection`)
- Filtering Handlers - Handlers that augment a request and pass it on to other handlers (`HandlerWrapper`, `ContextHandler`, `SessionHandler`)
- Generating Handlers - Handlers that produce content (`ResourceHandler` and `ServletHandler`)

#### The Target

The target of a handler is an identifier for the resource that should handle the passed request. This is normally the URI that is parsed from an HTTP Request. However, in two key circumstances the target may differ from the URI of the passed request:

- If the request has been dispatched to a named resource, such as a named servlet, the target is the name of that resource.
- If the request is being made by a call to `RequestDispatcher`, the target is the URI of the included resource and is different to the URI of the actual request.

#### The Request and Response

The request and response objects used in the signature of the handle method are `ServletRequest` and `ServletResponse`. These are the standard APIs and are moderately restricted in what they can do to the request and response. More often than not, access to the Jetty implementations of these classes is required: `Request` and `Response`. However, as the request and response may be wrapped by handlers, filters and servlets, it is not possible to pass the implementation directly. The following mantra retrieves the core implementation objects from under any wrappers:

    Request base_request = request instanceof Request ? (Request)request : HttpConnection.getCurrentConnection().getHttpChannel().getRequest();
    Response base_response = response instanceof Response ? (Response)response : HttpConnection.getCurrentConnection().getHttpChannel().getResponse();

Notice that if the handler passes the request on to another handler, it should use the Request/Response objects passed in, and not the base objects. This is to preserve any wrapping done by up stream handlers.

#### The Dispatch

The dispatch argument indicates the state of the handling of the call and may be:

- `REQUEST == 1` - An original request received from a connector.
- `FORWARD == 2` - A request being forwarded by a RequestDispatcher.
- `INCLUDE == 4` - A request being included by a RequestDispatcher.
- `ERROR == 8` - A request being forwarded to a error handler by the container.

These mostly have significance for servlet and related handlers. For example, the security handler only applies authentication and authorization to REQUEST dispatches.

### Handling Requests

A Handler may handle a request by:

- Generating a Response
- Filtering the Request and/or Response
- Passing the Request and Response to Another Handler

#### Generating a Response

The `OneHandler` embedded example shows how a simple handler can generate a response.

You can use the standard servlet response API, which will typically set some status, content headers and then write out the content:

     response.setContentType("text/html");
     response.setStatus(HttpServletResponse.SC_OK);
     response.getWriter().println("<h1>Hello OneHandler</h1>");

It is also very important that a handler indicate that it has completed handling the request and that the request should not be passed to other handlers:

     Request base_request = (request instanceof Request) ? (Request)request:HttpConnection.getCurrentConnection().getHttpChannel().getRequest();
     base_request.setHandled(true);

#### Filtering the Request and/or Response

Once the base request or response object is obtained, you can modify it. Typically you would make modifications to accomplish:

- Breaking the URI into contextPath, servletPath and pathInfo components.
- Associating a resource base with a request for static content.
- Associating a session with a request.
- Associating a security principal with a request.
- Changing the URI and paths during a request dispatch forward to another resource.

You can also update the context of the request:

- Setting the current threads context classloader.
- Setting thread locals to identify the current `ServletContext`.

Typically Jetty passes a modified request to another handler and undoes modifications in a finally block afterwards:

     try
     {
        base_request.setSession(a_session);
        next_handler.handle(target,request,response,dispatch);
     }
     finally
     {
        base_request.setSession(old_session);
     }

The classes that implement the `HandlerWrapper` class are typically handler filters of this style.

#### Passing the Request and Response to Another Handler

A handler might simply inspect the request and use the target, request URI or other information to select another handler to pass the request to. These handlers typically implement the `HandlerContainer` interface.

Examples include:

- Class `HandlerCollection` - A collection of handlers, where each handler is called regardless of the state of the request. This is typically used to pass a request to a `ContextHandlerCollection`, and then the `RequestLogHandler`.
- `HandlerList` - A list of handlers that are called in turn until the request state is set as handled.
- `ContextHandlerCollection` - A collection of Handlers, of which one is selected by best match for the context path.

### More About Handlers

See the latest Jetty JavaDoc for detailed information on each Jetty handler.

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