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SAAJ: No strings attached

Send and receive binary Web services content using SAAJ 1.2

By Frank Sommers, JavaWorld.com, 09/12/03

At the time of this writing, most Web services consist of simple message exchanges: A client contacts a Web service and sends a message to that service. The Web service, in turn, processes that request and then sends back a reply to the client. That simple request/response pattern models the way the HTTP protocol facilitates client/Web server interactions. As with HTTP, Web service message exchanges often must include binary content, such as images, documents, or sound clips. This article introduces sending and receiving binary Web service content using SOAP (Simple Object Access Protocol) with Attachments API for Java (SAAJ) 1.2.

Before diving into the intricacies of transferring binary Web service content, it's worth pointing out that a simple request/response-style Web service contrasts with services that fashion client/server interaction as remote procedure calls, or RPCs. In an RPC, a server exposes an interface that resembles an API. In turn, a client invokes such a service by making remote calls on the service's API, passing the required parameters, and receiving the values the call produces.

XML-based RPC resembles the way you invoke objects in an object-oriented (OO) system. Indeed, when working with the Java API for XML-based RPC (JAX-RPC), you seldom become aware you are working with XML documents, not Java objects. JAX-RPC lets you think of Web services as remote objects, much as you would with Java RMI (Remote Method Invocation). The JAX-RPC runtime translates the high-level, OO method calls to the XML documents expected by the remote Web service. While RPC-style Web services often provide a more convenient programming model, RPC calls must also rely on a lower-level messaging layer to exchange the XML messages that make up the remote call.

For some Web services, it is often useful to directly program to that lower-level messaging layer. For instance, if you wish to invoke a Web service that consumes a purchase order document and returns a receipt, you can easily model that document exchange as a single request/response message exchange. Instead of making remote method invocations, you would construct XML messages, send those messages directly to a Web service, and process the service's XML response, if any exists. Since SOAP defines the common message format for Web service messages, you would need to construct SOAP-conformant messages, and, once the service responds, parse those SOAP response messages back into a format your program understands.

SAAJ provides a convenient library to construct and read SOAP messages, and also lets you send and receive SOAP messages across the network. SAAJ defines the namespace javax.xml.soap. The classes that reside in that package initially formed part of the Java API for XML Messaging (JAXM), but were recently separated into their own API. JAXM relies on SAAJ for SOAP message construction and manipulation, and adds message reliability and other features specific to XML messaging. Whereas SAAJ is a required component of J2EE (Java 2 Platform, Enterprise Edition) 1.4, JAXM is not. This article focuses on one of SAAJ's most useful aspects: the ability to attach binary content to a SOAP message.