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Writing Session Bean Web Services 113 WSDL and the XML/Java Mapping You have seen the WSDL description of the HelloWorld Web Service already. If you are building new Web Services, you can start with a WSDL description of your service and write WSDL directly and then use a WSDL compiler to generate the service endpoint interface in Java. Alternatively, almost all Web Services platforms and SOAP toolkits provide tools to derive WSDL descriptions automatically from Java endpoint interfaces. See the source code for this book for examples of generating WSDL from Java. Packaging and Deploying a Web Service Session Bean The packaging of a Web Service implementation as a stateless session bean is an extension of the packaging for regular stateless session beans, that is, an ejbjar archive. This file contains the usual set of Java classes, plus the service endpoint interface class. The EJB server requires extra information to be able to dispatch incoming SOAP messages to an implementation of the service endpoint interface. First, it needs to know the Java class that will handle these calls. Additionally, it needs the WSDL file with the endpoint address that it should listen on. The WSDL file is provided in the META-INF directory of the ejb-jar archive. The other information is provided in an additional descriptor file, the webservices. xml file, which is also added to the ejb-jar archive s META-INF directory. Your specific J2EE product may provide vendor-specific deployment tools to generate this file. The webservices.xml file for our HelloWorld service is reproduced here: HelloWorldWS HelloWorldWS META-INF/wsdl/HelloWorldWS.wsdl META-INF/wsdl/mapping.xml
HelloWS
HelloWS wsdl-port_ns__: HelloInterfacePort examples.HelloInterface
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112 Chapter 5 { public String hello() throws java.rmi.RemoteException; } The service endpoint interface is required by the Java APIs for XML-based Remote Procedure Calls (JAX-RPC) that works behind the scenes in the container to invoke the bean. The JAX-RPC specification requires that the service endpoint interface follow these rules: The interface must extend java.rmi.Remote either directly or indirectly. All methods must throw java.rmi.RemoteException. The method parameters and return types must be the Java types sup ported by JAX-RPC. Service endpoint interfaces must not include constants (as public final static declarations). The JAX-RPC specification defines a mapping between a set of supported Java types and WSDL/XML types. The Java types directly supported by JAXRPC are the primitive types boolean, byte, double, float, int, long, short, and arrays of these types. In addition, the following non-primitive types are directly supported by JAX-RPC: java.lang.Boolean java.lang.Byte java.lang.Double java.lang.Float java.lang.Integer java.lang.Long java.lang.Short java.lang.String java.math.BigDecimal java.math.BigInteger java.net.URI java.util.Calendar java.util.Date JAX-RPC also provides hooks for customized type mappers that extend the standard type mapping provided by JAX-RPC. In our HelloWorld example, the only data type that is transmitted is java.lang.String, which is mapped to the XML string type without additional effort.
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Writing Session Bean Web Services 111 Container Service Implementation Bean Port Components SEI Service Implementation Bean SEI Figure 5.2 EJB container with port components. The great news is that (almost) no additional coding is required. The only thing that our session bean is missing is a slightly different form of remote interface, which we will add. After that, exposing the bean as a Web Service requires only repackaging and redeploying the application, which now contains a port component. The EJB container will know how to dispatch incoming SOAP messages to our bean implementation and how to map incoming XML data types to Java. The same will happen on the way back: The container just knows how to map our Java return values back into XML, how to build a SOAP response message, and where to send it. Figure 5.2 depicts the additional ports that the container now supports. The JAX-RPC Service Endpoint Interface The simple convention that we mentioned previously is to provide a Java interface to your bean that lists all the business methods supported by the bean. This interface is called the Service Endpoint Interface (SEI) and shown in the following block of code. The remaining HelloBean implementation does not have to be changed to actually implement this interface. All that is required is that the HelloBean support business methods with the same name and signature as those in the remote interface, as shown in the following example: package examples; /** This is the Hello service endpoint interface. */ public interface HelloInterface extends java.rmi.Remote
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110 Chapter 5 Applications are not as flexibly extensible as we would like. With IIOP-based request messages, all parties must have complete type information because they are not able to demarshal messages otherwise. There is no skipping of unknown parts of a message in IIOP. These restrictions do not exist with interfaces written in XML and with XML messages. XML also enables you to write extensible specifications (after all, that s the X in XML): data types in interface definitions can contain extensibility points from the outset. These extensibility points make use of a wildcard any type and, optional elements in sequences, and so on. Future versions of a service, while still servicing the clients written to the original interface, may fill in complex data records in these places for the benefit of more current client applications. If your end of the application does not rely on it, you don t need to care. To summarize this approach more generally, you could say that Web Services leave many details open for mutual agreement between the parties that will be actually involved whereas other middleware systems, such as CORBA, have sought to define stricter, inherent semantics as part of their models. This means that to use Web Services successfully in practice, you have to fill in these details. It also means that there is more room for refinement and thus wider applicability. Implementing a Web Service The J2EE model for Web Services provides a seamless Java perspective on Web Services, both for the service implementations and its clients. The model is relatively simple to use and allows you to deal with SOAP in the same way you deal with RMI or RMI/IIOP, which is to entrust all the details to the lower transport layers and happily ignore them for your business logic. The first thing to note is that your Web Services, like your beans, are managed for you completely by the container. The JSR 921 specification Implementing Enterprise Web Services defines the programming model for Web Services. This specification uses the term port component for the server-side view of a Web Service. A port component is a portable Java implementation of a service interface (a port) and comprises a Java mapping of the service interface and an implementation bean. Port components are deployed into and live in containers. Writing a Web Service using EJB requires creating one or more port components as stateless session beans. This involves writing (and generating) some more XML descriptors. A big advantage of the way the Web Services programming model is defined is that you can also expose existing session beans as Web Services. This is what we will do in the remainder of this chapter. To leverage the large investments that we made in Chapter 3, we will take our HelloWorld session bean and make it available as a Web Service at no extra cost.
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Writing Session Bean Web Services 109 Again, there is the HTTP header and an XML document that contains a SOAP envelope. This time, the SOAP body represents the result of the operation call, which is a single result element of type string. The two messages reproduced here serve to illustrate another key term that is often used in the context of Web Services. The SOAP protocol is extremely lightweight in the sense that it is very simple to use and does not make many assumptions about the behavior of clients and services. The SOAP protocol is not lightweight in terms of compactness and high performance. If uncompressed, there is a large transmission overhead when compared to binary representations, for example in CORBA s IIOP protocol. The XML parsing required to marshal and demarshal messages can also become CPU-intensive for larger messages. But this is beside the point: Web Services are not designed to deliver hitherto unknown performance but to enable integration where high-performance middleware is much less useful than lightweight protocols that can be implemented easily by simple scripts. (For an interesting discussion of scripting languages as middleware, refer to Steve Vinoski s article on Middleware Dark Matter available at www.iona.com/hyplan/vinoski/.) XML Artifacts and Platform Independence Web Services help with the integration of heterogeneous, distributed systems by using standardized XML documents for many different aspects of service design, deployment, lookup, and usage that leverages a broad array of open standards and sophisticated tools that are widely available. Many of the tools, like Apache Axis SOAP, IBM WSDL4J toolkit, and JBoss Application Server, are also in the open source arena. In a sense, the XML usage that we just looked at is perhaps the biggest technological advantage here because many of the practical virtues, like loose coupling and platform independence, follow from XML itself and the way the different XML technologies are combined. XML documents are also self- describing in that they contain a description of their structure in their markup tags. This does not mean that you will be able to understand arbitrary documents without any prior knowledge. What it does mean is that you can easily skip parts of a message that you are not concerned with and don t understand, and just deal with those parts that do concern you. This may sound trivial at first, but it has important consequences in that this enables the decoupling of applications and middleware. To understand this point, recall that clients of your beans have to use a fixed component interface. If that interface changes because a parameter is added to a method signature, you will have to rebuild, reassemble, and redeploy not only your beans, but your clients will also have to be recompiled. This is not loose coupling because you cannot develop the different components of your application individually. If one piece changes, the others have to change, too.
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108 Chapter 5 SOAP The SOAP protocol defines an XML message format for Web Services and their clients. Until version 1.1, SOAP was an acronym for Simple Object Access Protocol, but it was turned into a proper name for version 1.2 of the standard. That SOAP starts with the three letters SOA is sheer coincidence. As we just mentioned, the targets of SOAP messages (both services and clients) are not objects in the object-oriented sense, so the acronym was a misnomer anyway. The SOAP message format is very simple. In a message exchange between a client and the HelloWorld service, the request message would look like this: POST /HelloBean HTTP/1.1 Content-Type: text/xml; charset= utf-8 Content-Length: 398 SOAPAction: Host: falcon:8080 This is an actual message as sent over the wire. As you can see, the message has two parts, an HTTP POST request header, and an XML document in the HTTP payload. This XML document is a SOAP envelope, which represents a request. The envelope contains a body element, which in turn contains the hello element that represents the operation call. The reply message is just as simple: HTTP/1.1 200 OK SOAPAction: Content-Type: text/xml;charset=utf-8 Transfer-Encoding: chunked Hello, World!
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Writing Session Bean Web Services 107

Some good news first before we look at some of the details: Relax, you don t have to write this XML document yourself. This interface description was automatically generated from a Java interface using a generator tool: the wscompile tool that comes with the J2EE SDK. A number of things are worth noting about the HelloWorld WSDL: The number of language concepts used here is larger than in Java. We have a service that provides one or more ports at an address. Ports represent the service interfaces and have bindings to protocols. The service description includes an endpoint address. The WSDL is thus like a Java interface and an object reference joined together. In other words, Web Services do not have distinct identities. They are not objects and must be viewed as modules. There is no client-visible state, and you cannot compare two references for equality! Operations are specified in terms of input and output messages rather than parameters and return values. These have to be represented as elements ( parts ) of input and output messages. Here, there is only the part that is actually transferred inside a message: the result part of type xsd:string in the HelloInterface_helloResponse message. The binding for the service is a SOAP binding. There can be other bindings in theory, but in practice SOAP is the only available option today. Also note that the soap:binding has an attribute style= rpc , so there must be other possible styles. Currently, the only other style for exchanging SOAP messages is document-style, which means that there is no specific representation of a called operation in the SOAP message s body.
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106 Chapter 5 Web Services Standards The set of de-facto standards that make up Web Services today can be summarized in a simple equation: Web Services = WSDL + SOAP + UDDI Let s take a quick look at WSDL and SOAP. We won t cover UDDI here because it is not necessarily required: Note that the actual service usage in Figure 5.1 does not depend on the existence of UDDI. The requestor may actually have known the service and its endpoint address without the registry. Also note that the registry is not simply a naming service but supports queries for services that obey a given predicate. At this stage in the life of Web Services, however, it is unclear whether dynamic service lookups in UDDI registry will ever happen on a larger scale than within enterprises. It did not happen with similar concepts that were available earlier, such as CORBA Trading Service. If you have been around in distributed computing for a while some of the technology in the Web Services arena will appear like a d ja vu. Figure 5.1, for example, looks a lot like the RM-ODP trader and later CORBA Trading Service. Many aspects that Web Services address are not new per se but have simply not been solved on a larger scale. WSDL To give you a first impression of a service description in WSDL, here is the definition of the HelloWorld component that we used in Chapter 3:

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Writing Session Bean Web Services 105 SOAs emphasize modularity through standardized interfaces, flexibility through looser coupling, and extensibility through using XML. All of this is important in the B2B scenarios, which are the primary targets of Web Services. Web Services are not just another RPC mechanism for your intranet applications but rather a great help in settings where no single middleware platform is applicable. As an example, consider the B2B relationships between a car manufacturer and its suppliers. Each of these companies has its own IT infrastructure and set of applications, such as payroll, inventory, order processing, and so on. Also, each supplier builds parts for more than just a single car manufacturer, and each manufacturer buys parts from different suppliers. In a situation like this, it is highly unlikely that any of the involved parties will be able to switch to a specific middleware for the sake of the business relationship with just a single partner. For any given supplier, building a middleware X adapter (for example CORBA) to its order processing application to interoperate with customer A, and then building another adapter Y (say, MQSeries) to interoperate with customer B, and so on is going to be too much effort and too expensive. This is what standardization efforts in the past (such as EDI) tried but failed to tackle on a larger scale. Web Services can thus be seen as a new attempt at building universally agreed upon standards that hide the differences behind standardized interfaces. This time, the standards are going to be based on XML and on established Internet protocols. So why do we talk about integration and interoperability so much in the context of Web Services? Aren t EJBs interoperable already, thanks to the standardization of the RMI/IIOP protocol and the container and bean APIs? EJBs are interoperable in the sense of vendor and platform independence: there are J2EE/EJB products from many different vendors that run on different platforms and still talk to each other. These containers can host your beans no matter which product they were written for, so you also get portability. But there is language dependency: EJBs are coded in Java and nothing else, so you cannot create interoperate bean implementations written in different languages. On the one hand, this is great because of Java s portability (write once run anywhere). On the other hand, portability is not always an issue, and you may actually need a specific language for your project if you wanted to leverage, say, a large amount of C++ or COBOL code for business objects that your company has investments in. With EJB, a common approach is to build wrapper beans that talk to an adapter in C++, most likely based on CORBA. Another way of putting this is to say that EJBs prescribe not only the component interfaces and client contracts, but also an implementation model. With Web Services, there is no single implementation framework; a contract with a Web Service involves only its interface. Web Services interfaces are defined in the Web Services Description Language (WSDL). Web Services can be implemented in any language. Of course, we will be building them with EJB in this book, so they will be written in Java.
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104 Chapter 5 Service Registry Service Requestor Service Provider SOAP WSDL + UDDI WSDL + UDDI Bind PublishFind Service Description Service Description Service Figure 5.1 Service-oriented architecture with Web Services. A service provider creates an abstract service definition that it publishes in a service registry. With Web Services, the description is a Web Services Definition Language (WSDL) file, and the registry follows the Universal Description, Discovery, and Integration (UDDI) standard. A service requestor can find the service description, possibly using a set of selection criteria to query the registry. If a suitable description is found, the requestor can bind to the service. You can find simple examples of Web Services collected on Web sites such xmethods. org, for example a service to determine if a given Internet domain name is taken, or to convert temperature values from Fahrenheit to Celsius. More realistic Web Services are built today in larger-scale, in-house architectures that interconnect existing, heterogeneous applications, for example, a billing application and a report generator. A service interface is similar to an object or bean interface, but the contract between the interface and its clients is more flexible and the client and the service implementation are less closely coupled than in EJB or other distribution platforms. This looser coupling allows client and service implementations to run on very different platforms, for example, a Microsoft .NET client could access a service running in a J2EE application server. Also, services are generally coarser-grained entities than objects are. From a client perspective, their life cycles are more static because services don t just pop up and go away but stay around longer than your average object, even if services are implemented using object technology.
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