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Java and XML: SOAP
Pages: 1, 2, 3, 4, 5, 6, 7

Encoding

The second major element that SOAP brings to the table is a simple means of encoding user-defined data types. In RPC (and XML-RPC), encoding can only occur for a predefined set of datatypes: those that are supported by whatever XML-RPC toolkit you download. Encoding other types requires modifying the actual RPC server and clients themselves. With SOAP, however, XML schemas can be used to easily specify new datatypes (using the complexType structure discussed way back in Chapter 2), and those new types can be easily represented in XML as part of a SOAP payload. Because of this integration with XML Schema, you can encode any datatype in a SOAP message that you can logically describe in an XML schema.



Invocation

The best way to understand how SOAP invocation works is to compare it with something you already know, such as XML-RPC. If you recall, an XML-RPC call would look something like the code fragment shown in Example 12-2.

Example 12-2: Invocation in XML-RPC

// Specify the XML parser to use
XmlRpc.setDriver("org.apache.xerces.parsers.SAXParser");
 
// Specify the server to connect to
XmlRpcClient client =
  new XmlRpcClient("http://rpc.middleearth.com");
 
// Create the parameters
Vector params = new Vector( );
params.addElement(flightNumber);
params.addElement(numSeats);
params.addElement(creditCardType);
params.addElement(creditCardNum);
 
// Request reservation
Boolean boughtTickets =
  (Boolean)client.execute("ticketCounter.buyTickets", params);
 
// Deal with the response

I've coded up a simple ticket counter-style application. Now, look at Example 12-3, which shows the same call in SOAP.

Example 12-3: Invocation in SOAP

// Create the parameters
Vector params = new Vector( );
params.addElement(
  new Parameter("flightNumber", Integer.class, flightNumber, null));
params.addElement(
  new Parameter("numSeats", Integer.class, numSeats, null));
params.addElement(
  new Parameter("creditCardType", String.class, creditCardType, null));
params.addElement(
  new Parameter("creditCardNumber", Long.class, creditCardNum, null));
 
// Create the Call object
Call call = new Call( );
call.setTargetObjectURI("urn:xmltoday-airline-tickets");
call.setMethodName("buyTickets");
call.setEncodingStyleURI(Constants.NS_URI_SOAP_ENC);
call.setParams(params);
 
// Invoke
Response res = call.invoke(new URL("http://rpc.middleearth.com"), "");
 
// Deal with the response

As you can see, the actual invocation itself, represented by the Call object, is resident in memory. It allows you to set the target of the call, the method to invoke, the encoding style, the parameters, and more not shown here. It is more flexible than the XML-RPC methodology, allowing you to explicitly set the various parameters that are determined implicitly in XML-RPC. You'll see quite a bit more about this invocation process in the rest of the chapter, including how SOAP provides fault responses, an error hierarchy, and of course the returned results from the call.

With that brief introduction, you probably know enough to want to get on with the fun stuff. Let me show you the SOAP implementation I'm going to use, explain why I made that choice, and get to some code.

Setting Up

Now that you have some basic concepts down, it's time to get going on the fun part, the code. You need a project or product for use, which turns out to be simpler to find than you might think. If you want a Java-based project providing SOAP capability, you don't have to look that far. There are two groups of products out there: commercial and free. As in most of the rest of the book, I'm steering away from covering commercial products. This isn't because they are bad (on the contrary, some are wonderful); it's because I want every reader of this book to be able to use every example. That calls for accessibility, something commercial products don't provide; you have to pay to use them, or download them and at some point the trial period runs out.

That brings us to open source projects. In that realm, I see only one available: Apache SOAP. Located online at http://xml.apache.org/soap, this project seeks to provide a SOAP toolkit in Java. Currently in a 2.2 versioned release, you can download it from the Apache website. That's the version and project I use for the examples throughout this chapter.

Other Options

Before moving on to the installation and setup of Apache SOAP, I will answer a few questions that might be rattling around in your head. It's probably clear why I'm not using a commercial product. However, you may be thinking of a couple of other open source or related options that you might want to use, and wondering why I am not covering those.

What about IBM SOAP4J?

First on the list of options is IBM's SOAP implementation, IBM SOAP4J. IBM's work is actually the basis of the current Apache SOAP project, much as IBM XML4J fed into what is now the Apache Xerces XML parser project. Expect the IBM implementation to resurface, wrapping the Apache SOAP project's implementation. This is similar to what is happening with IBM's XML4J; it currently just provides IBM packaging over Xerces. This makes some additional levels of vendor-backing available to the open source version, although the two (Apache and IBM) projects are using the same codebase.

Isn't Microsoft a player?

Yes. Without a doubt, Microsoft and their SOAP implementation, as well as the whole .NET initiative (covered more in the next chapter), are very important. In fact, I wanted to spend some time covering Microsoft's SOAP implementation in detail, but it only supports COM objects and the like, without Java support. For this reason, coverage of it doesn't belong in a book on Java and XML. However, Microsoft (despite the connotations we developers tend to have about the company) is doing important work in web services, and you'd be making a mistake in writing them off, at least in this particular regard. If you need to communicate with COM or Visual Basic components, I highly recommend checking out the Microsoft SOAP toolkit, found online along with a lot of other SOAP resources.

What's Axis?

Those of you who monitor activity in Apache may have heard of Apache Axis. Axis is the next-generation SOAP toolkit, also being developed under the Apache XML umbrella. With SOAP (the specification, not a specific implementation) undergoing fairly fast and radical change these days, tracking it is difficult. Trying to build a version of SOAP that meets current requirements and moves with new development is also awfully tough. As a result, the current Apache SOAP offering is somewhat limited in its construction. Rather than try to rearchitect an existing toolkit, the Apache folks started fresh with a new codebase and project; thus, Axis was born. Additionally, the naming of SOAP was apparently going to change, from SOAP to XP and then to XMLP. As a result, the name of this new SOAP project was uncoupled from the specification name; thus, you have "Axis." Of course, now it looks like the W3C is going back to calling the specification SOAP (Version 1.2, or Version 2.0), so things are even more confusing!

Think of IBM SOAP4J as architecture 1 of the SOAP toolkit. Following that is Apache SOAP (covered in this chapter), which is architecture 2. Finally, Axis provides a next-generation architecture, architecture 3. This project is driven by SAX, while Apache SOAP is based upon DOM. Additionally, Axis provides a more user-friendly approach in header-interaction, something missing in Apache SOAP. With all of these improvements, you're probably wondering why I'm not covering Axis. It's simply too early. Axis is presently trying to get together a 0.51 release. It's not a beta, or even an alpha, really; it's very early on. While I'd love to cover all the new Axis features, there's no way your boss is going to let you put in a pre-alpha release of open source software in your mission-critical systems, now is there? As a result, I've chosen to focus on something you can use, today: Apache SOAP. I'm sure when Axis does finalize, I'll update this chapter in a subsequent revision of the book. Until then, let's focus on a solution you can use.

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