5.1 The Structure of a Response

Java Servlet Programming, 2nd Edition > 5. Sending HTML Information > 5.1 The Structure of a Response

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An HTTP servlet can return three kinds of things to the client: a single status code, any number of HTTP headers, and a response body. A status code is an integer value that describes, as you would expect, the status of the response. The status code can indicate success or failure, or it can tell the client software to take further action to finish the request. The numerical status code is often accompanied by a reason phrase that describes the status in prose better understood by a human. Usually, a status code works behind the scenes and is interpreted by the browser software. Sometimes, especially when things go wrong, a browser may show the status code to the user. The most famous status code is probably the 404 Not Found code, sent by a web server when it cannot locate a requested URL.

We saw HTTP headers in the previous chapter when clients used them to send extra information along with a request. In this chapter, we'll see how a servlet can send HTTP headers as part of its response.

The response body is the main content of the response. For an HTML page, the response body is the HTML itself. For a graphic, the response body contains the bytes that make up the image. A response body can be of any type and of any length; the client knows what to expect by reading and interpreting the HTTP headers in the response.

A generic servlet is much simpler than an HTTP servlet it returns only a response body to its client. It's possible, however, for a subclass of GenericServlet to present an API that divides this single response body into a more elaborate structure, giving the appearance of returning multiple items. In fact, this is exactly what HTTP servlets do. At the lowest level, a web server sends its entire response as a stream of bytes to the client. Any methods that set status codes or headers are abstractions above that.

It's important to understand this because even though a servlet programmer doesn't have to know the details of the HTTP protocol, the protocol does affect the order in which a servlet can call its methods. Specifically, the HTTP protocol specifies that the status code and headers must be sent before the response body. A servlet, therefore, must be careful to always set its status code and headers before sending any response body to the client. A servlet can buffer its response body to give it some extra flexibility, but once any response body has been sent the response is considered committed and the status code and headers cannot be altered.

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