Using SOAP with J2EE
SOAP was originally an acronym for Simple Object Access Protocol. (Now it's just a name.) SOAP 1.1 is the standard messaging protocol used by J2EE Web Services, and is the de facto standard for Web services in general. SOAP's primary application is Application-to-Application (A2A) communication. Specifically, it's used in Business-to-Business (B2B) and Enterprise Application Integration (EAI), which are two sides of the same coin: Both focus on integrating software applications and sharing data. To be truly effective in B2B and EAI, a protocol must be platform-independent, flexible, and based on standard, ubiquitous technologies. Unlike earlier B2B and EAI technologies, such as CORBA and EDI, SOAP meets these requirements, enjoys widespread use, and has been endorsed by most enterprise software vendors and major standards organizations (W3C, WS-I, OASIS, etc.).
Despite all the hoopla, however, SOAP is just another XML markup language accompanied by rules that dictate its use. SOAP has a clear purpose: exchanging data over networks. Specifically, it concerns itself with encapsulating and encoding XML data and defining the rules for transmitting and receiving that data. In a nutshell, SOAP is a network application protocol.
A SOAP XML document instance, which is called a SOAP message,1 is usually carried as the payload of some other network protocol. For example, the most common way to exchange SOAP messages is via HTTP (HyperText Transfer Protocol), used by Web browsers to access HTML Web pages. The big difference is that you don't view SOAP messages with a browser as you do HTML. SOAP messages are exchanged between applications on a network and are not meant for human consumption. HTTP is just a convenient way of sending and receiving SOAP messages.
SOAP messages can also be carried by e-mail using SMTP (Simple Mail Transfer Protocol) and by other network protocols, such as FTP (File Transfer Protocol) and raw TCP/IP (Transmission Control Protocol/Internet Protocol). At this time, however, the WS-I Basic Profile 1.0 sanctions the use of SOAP only over HTTP. Figure 4-1 illustrates how SOAP can be carried by various protocols between software applications on a network.
Figure 4-1. SOAP over HTTP, SMTP, and Raw TCP/IP
Web services can use One-Way messaging or Request/Response messaging. In the former, SOAP messages travel in only one direction, from a sender to a receiver. In the latter, a SOAP message travels from the sender to the receiver, which is expected to send a reply back to the sender. Figure 4-2 illustrates these two forms of messaging.
Figure 4-2. One-Way versus Request/Response Messaging
SOAP defines how messages can be structured and processed by software in a way that is independent of any programming language or platform, and thus facilitates interoperability between applications written in different programming languages and running on different operating systems. Of course, this is nothing new: CORBA IIOP and DCE RPC also focused on cross-platform interoperability. These legacy protocols were never embraced by the software industry as a whole, however, so they never became pervasive technologies. SOAP, on the other hand, has enjoyed unprecedented acceptance, and adoption by virtually all the players in distributed computing, including Microsoft, IBM, Sun Microsystems, BEA, HP, Oracle, and SAP, to name a few.
The tidal wave of support behind SOAP is interesting. One of the main reasons is probably its grounding in XML. The SOAP message format is defined by an XML schema, which exploits XML namespaces to make SOAP very extensible. Another advantage of SOAP is its explicit definition of an HTTP binding, a standard method for HTTP tunneling. HTTP tunneling is the process of hiding another protocol inside HTTP messages in order to pass through a firewall unimpeded. Firewalls will usually allow HTTP traffic through port 80, but will restrict or prohibit the use of other protocols and ports.
A port is a communication address on a computer that complements the Internet address. Each network application on a computer uses a different port to communicate. By convention, Web servers use port 80 for HTTP requests, but application servers can use any one of thousands of other ports.
The power that comes from XML's extensibility and the convenience of using the ubiquitous, firewall-immune HTTP protocol partly explain SOAP's success. It's difficult to justify SOAP's success purely on its technical merits, which are good but less than perfect. Another factor in SOAP's success is the stature of its patrons. SOAP is the brainchild of Dave Winner, Don Box, and Bob Atkinson. Microsoft and IBM supported it early, which sent a strong signal to everyone else in the industry: “If you want to compete in this arena, you better jump aboard SOAP.” The event that secured industry-wide support for SOAP was its publication by the World Wide Web Consortium as a Note2 in May of 2000, making it the de facto standard protocol for A2A messaging. Overnight, SOAP became the darling of distributed computing and started the biggest technology shift since the introduction of Java in 1995 and XML in 1998. SOAP is the cornerstone of what most people think of as Web services today, and will be for a long time to come.
Recently, the W3C has defined a successor to SOAP 1.1. SOAP 1.2 does a decent job of tightening up the SOAP processing rules and makes a number of changes that will improve interoperability. SOAP 1.2 is very new and has not yet been widely adopted, however, so it's not included in the WS-I Basic Profile 1.0. This exclusion is bound to end when the BP is updated, but for now J2EE 1.4 Web Services, which adheres to the WS-I Basic Profile 1.0, does not support the use of SOAP 1.2.
4.1 The Basic Structure of SOAP
As you now know, a SOAP message is a kind of XML document. SOAP has its own XML schema, namespaces, and processing rules. This section focuses on the structure of SOAP messages and the rules for creating and processing them.
A SOAP message is analogous to an envelope used in traditional postal service. Just as a paper envelope contains a letter, a SOAP message contains XML data. For example, a SOAP message could enclose a purchaseOrder element, as in Listing 4-1. Notice that XML namespaces are used to keep SOAP-specific elements separate from purchaseOrder elements—the SOAP elements are shown in bold.
Listing 4-1 A SOAP Message That Contains an Instance of Purchase Order Markup
<?xml version="1.0" encoding="UTF-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" > <soap:Body> <po:purchaseOrder orderDate="2003-09-22" xmlns:po="http://www.Monson-Haefel.com/jwsbook/PO"> <po:accountName>Amazon.com</po:accountName> <po:accountNumber>923</po:accountNumber> <po:address> <po:name>AMAZON.COM</po:name> <po:street>1850 Mercer Drive</po:street> <po:city>Lexington</po:city> <po:state>KY</po:state> <po:zip>40511</po:zip> </po:address> <po:book> <po:title>J2EE Web Services</po:title> <po:quantity>300</po:quantity> <po:wholesale-price>24.99</po:wholesale-price> </po:book> </po:purchaseOrder> </soap:Body> </soap:Envelope>
This message is an example of a SOAP message that contains an arbitrary XML element, the purchaseOrder element. In this case, the SOAP message will be One-Way; it will be sent from the initial sender to the ultimate receiver with no expectation of a reply. Monson-Haefel Books' retail customers will use this SOAP message to submit a purchase order, a request for a shipment of books. In this example, Amazon.com is ordering 300 copies of this book for sale on its Web site.
A SOAP message may have an XML declaration, which states the version of XML used and the encoding format, as shown in this snippet from Listing 4-1.
<?xml version="1.0" encoding="UTF-8"?>
If an xml declaration is used, the version of XML must be 1.0 and the encoding must be either UTF-8 or UTF-16. If encoding is absent, the assumption is that the SOAP message is based on XML 1.0 and UTF-8. An XML declaration isn't mandatory. Web services are required to accept messages with or without them.BP (Remember that I said I'd use a superscript BP to signal a BP-conformance rule.)
Every XML document must have a root element, and in SOAP it's the Envelope element. Envelope may contain an optional Header element, and must contain a Body element. If you use a Header element, it must be the immediate child of the Envelope element, and precede the Body element. The Body element contains, in XML format, the actual application data being exchanged between applications. The Body element delimits the application-specific data. Listing 4-2 shows the structure of a SOAP message.
Listing 4-2 The Structure of a SOAP Message
<?xml version="1.0" encoding="UTF-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"> <soap:Header> <!-- Header blocks go here --> </soap:Header> <soap:Body> <!-- Application data goes here --> </soap:Body> </soap:Envelope>
A SOAP message adheres to the SOAP 1.1 XML schema, which requires that elements and attributes be fully qualified (use prefixes or default namespaces). A SOAP message may have a single Body element preceded, optionally, by one Header element. The Envelope element cannot contain any other children.
Because SOAP doesn't limit the type of XML data carried in the SOAP Body, SOAP messages are extremely flexible; they can exchange a wide spectrum of data. For example, the application data could be an arbitrary XML element like a purchaseOrder, or an element that maps to the arguments of a procedure call.
The Header element contains information about the message, in the form of one or more distinct XML elements, each of which describes some aspect or quality of service associated with the message. Figure 4-3 illustrates the structure of a basic SOAP message.
Figure 4-3. The Structure of a Basic SOAP Message
The Header element can contain XML elements that describe security credentials, transaction IDs, routing instructions, debugging information, payment tokens, or any other information about the message that is important in processing the data in the Body element.
For example, we may want to attach a unique identifier to every SOAP message, to be used for debugging and logging. Although unique identifiers are not an integral part of the SOAP protocol itself, we can easily add an identifier to the Header element as in Listing 4-3.
Listing 4-3 A SOAP Message with a Unique Identifier
<?xml version="1.0" encoding="UTF-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mi="http://www.Monson-Haefel.com/jwsbook/message-id" > <soap:Header> <mi:message-id>11d1def534ea:b1c5fa:f3bfb4dcd7:-8000</mi:message-id> </soap:Header> <soap:Body> <!-- Application-specific data goes here --> </soap:Body> </soap:Envelope>
The message-id element is called a header block, and is an arbitrary XML element identified by its own namespace. A header block can be of any size and can be very extensive. For example, the header for an XML digital signature, shown in bold in Listing 4-4, is relatively complicated.
Listing 4-4 A SOAP Message with an XML Digital-Signature Header Block
<?xml version="1.0" encoding="UTF-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:sec="http://schemas.xmlsoap.org/soap/security/2000-12" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:mi="http://www.Monson-Haefel.com/jwsbook/message-id"> <soap:Header> <mi:message-id>11d1def534ea:b1c5fa:f3bfb4dcd7:-8000</mi:message-id> <sec:Signature > <ds:Signature> <ds:SignedInfo> <ds:CanonicalizationMethod Algorithm= "http://www.w3.org/TR/2000/CR-xml-c14n-20001026"/> <ds:SignatureMethod Algorithm= "http://www.w3.org/2000/09/xmldsig#dsa-sha1"/> <ds:Reference URI="#Body"> <ds:Transforms> <ds:Transform Algorithm= "http://www.w3.org/TR/2000/CR-xml-c14n-20001026"/> </ds:Transforms> <ds:DigestMethod Algorithm= "http://www.w3.org/2000/09/xmldsig#sha1"/> <ds:DigestValue>u29dj93nnfksu937w93u8sjd9= </ds:DigestValue> </ds:Reference> </ds:SignedInfo> <ds:SignatureValue>CFFOMFCtVLrklR…</ds:SignatureValue> </ds:Signature> </sec:Signature> </soap:Header> <soap:Body sec:id="Body"> <!-- Application-specific data goes here --> </soap:Body> </soap:Envelope>
You can place any number of header blocks in the Header element. The example above contains both the message-id and XML digital signature header blocks, each of which would be processed by appropriate functions. Header blocks are discussed in more detail in Section 4.3.