This tutorial shows you how to write contract-first Web services, that is, developing web services that start with the XML Schema/WSDL contract first followed by the Java code second. Spring-WS focuses on this development style, and this tutorial will help you get started. Note that the first part of this tutorial contains almost no Spring-WS specific information: it is mostly about XML, XSD, and WSDL. The second part focusses on implementing this contract using Spring-WS .

The most important thing when doing contract-first Web service development is to try and think in terms of XML. This means that Java-language concepts are of lesser importance. It is the XML that is sent across the wire, and you should focus on that. The fact that Java is used to implement the Web service is an implementation detail. An important detail, but a detail nonetheless.

In this tutorial, we will define a Web service that is created by a Human Resources department. Clients can send holiday request forms to this service to book a holiday.

In this section, we will focus on the actual XML messages that are sent to and from the Web service. We will start out by determining what these messages look like.

<Holiday xmlns="http://mycompany.com/hr/schemas">
    <StartDate>2006-07-03</StartDate>
    <EndDate>2006-07-07</EndDate>
</Holiday>

<Employee xmlns="http://mycompany.com/hr/schemas">
    <Number>42</Number>
    <FirstName>Arjen</FirstName>
    <LastName>Poutsma</LastName>
</Employee>

<HolidayRequest xmlns="http://mycompany.com/hr/schemas">
    <Holiday>
        <StartDate>2006-07-03</StartDate>
        <EndDate>2006-07-07</EndDate>
    </Holiday>
    <Employee>
        <Number>42</Number>
        <FirstName>Arjen</FirstName>
        <LastName>Poutsma</LastName>
    </Employee>
</HolidayRequest>

Now that we have seen some examples of the XML data that we will use, it makes sense to formalize this into a schema. This data contract defines the message format we accept. There are four different ways of defining such a contract for XML:

DTDs have limited namespace support, so they are not suitable for Web services. Relax NG and Schematron certainly are easier than XML Schema. Unfortunately, they are not so widely supported across platforms. We will use XML Schema.

By far the easiest way to create an XSD is to infer it from sample documents. Any good XML editor or Java IDE offers this functionality. Basically, these tools use some sample XML documents, and generate a schema from it that validates them all. The end result certainly needs to be polished up, but it's a great starting point.

Using the sample described above, we end up with the following generated schema:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified"
        targetNamespace="http://mycompany.com/hr/schemas"
        xmlns:hr="http://mycompany.com/hr/schemas">
    <xs:element name="HolidayRequest">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="hr:Holiday"/>
                <xs:element ref="hr:Employee"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="Holiday">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="hr:StartDate"/>
                <xs:element ref="hr:EndDate"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="StartDate" type="xs:NMTOKEN"/>
    <xs:element name="EndDate" type="xs:NMTOKEN"/>
    <xs:element name="Employee">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="hr:Number"/>
                <xs:element ref="hr:FirstName"/>
                <xs:element ref="hr:LastName"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="Number" type="xs:integer"/>
    <xs:element name="FirstName" type="xs:NCName"/>
    <xs:element name="LastName" type="xs:NCName"/>
</xs:schema>

This generated schema obviously can be improved. The first thing to notice is that every type has a root-level element declaration. This means that the Web service should be able to accept all of these elements as data. This is not desirable: we only want to accept a <HolidayRequest/>. By removing the wrapping element tags (thus keeping the types), and inlining the results, we can accomplish this.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xmlns:hr="http://mycompany.com/hr/schemas"
        elementFormDefault="qualified"
        targetNamespace="http://mycompany.com/hr/schemas">
    <xs:element name="HolidayRequest">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="Holiday" type="hr:HolidayType"/>
                <xs:element name="Employee" type="hr:EmployeeType"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:complexType name="HolidayType">
        <xs:sequence>
            <xs:element name="StartDate" type="xs:NMTOKEN"/>
            <xs:element name="EndDate" type="xs:NMTOKEN"/>
        </xs:sequence>
    </xs:complexType>
    <xs:complexType name="EmployeeType">
        <xs:sequence>
            <xs:element name="Number" type="xs:integer"/>
            <xs:element name="FirstName" type="xs:NCName"/>
            <xs:element name="LastName" type="xs:NCName"/>
        </xs:sequence>
    </xs:complexType>
</xs:schema>

The schema still has one problem: with a schema like this, you can expect the following messages to validate:

<HolidayRequest xmlns="http://mycompany.com/hr/schemas">
    <Holiday>
        <StartDate>this is not a date</StartDate>
        <EndDate>neither is this</EndDate>
    </Holiday>
    <!-- ... -->
</HolidayRequest>

Clearly, we must make sure that the start and end date are really dates. XML Schema has an excellent built-in date type which we can use. We also change the NCNames to strings. Finally, we change the sequence in <HolidayRequest/> to all. This tells the XML parser that the order of <Holiday/> and <Employee/> is not significant. Our final XSD now looks like this:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xmlns:hr="http://mycompany.com/hr/schemas"
        elementFormDefault="qualified"
        targetNamespace="http://mycompany.com/hr/schemas">
    <xs:element name="HolidayRequest">
        <xs:complexType>
            <xs:all>
                <xs:element name="Holiday" type="hr:HolidayType"/>                       
                <xs:element name="Employee" type="hr:EmployeeType"/>
            </xs:all>
        </xs:complexType>
    </xs:element>
    <xs:complexType name="HolidayType">
        <xs:sequence>
            <xs:element name="StartDate" type="xs:date"/>
            <xs:element name="EndDate" type="xs:date"/>                                  
        </xs:sequence>                                                                   
    </xs:complexType>
    <xs:complexType name="EmployeeType">
        <xs:sequence>
            <xs:element name="Number" type="xs:integer"/>
            <xs:element name="FirstName" type="xs:string"/>
            <xs:element name="LastName" type="xs:string"/>                               
        </xs:sequence>                                                                   
    </xs:complexType>
</xs:schema>

	all tells the XML parser that the order of <Holiday/> and <Employee/> is not significant.
	We use the xsd:date data type, which consist of a year, month, and day, for <StartDate/> and <EndDate/>.
	xsd:string is used for the first and last name.

We store this file as hr.xsd.

A service contract is generally expressed as a WSDL file. Note that in Spring-WS, writing the WSDL by hand is not required. Based on the XSD and some conventions, Spring-WS can create the WSDL for you, as explained in the section entitled Section 3.6, “Implementing the Endpoint”. You can skip to the next section if you want to; the remainder of this section will show you how to write your own WSDL by hand.

We start our WSDL with the standard preamble, and by importing our existing XSD. To separate the schema from the definition, we will use a separate namespace for the WSDL definitions: http://mycompany.com/hr/definitions.

<wsdl:definitions xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
                  xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
                  xmlns:schema="http://mycompany.com/hr/schemas"
                  xmlns:tns="http://mycompany.com/hr/definitions"
                  targetNamespace="http://mycompany.com/hr/definitions">
    <wsdl:types>
        <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <xsd:import namespace="http://mycompany.com/hr/schemas" schemaLocation="hr.xsd"/>
        </xsd:schema>
    </wsdl:types>

Next, we add our messages based on the written schema types. We only have one message: one with the <HolidayRequest/> we put in the schema:

    <wsdl:message name="HolidayRequest">
        <wsdl:part element="schema:HolidayRequest" name="HolidayRequest"/>
    </wsdl:message>

We add the message to a port type as an operation:

    <wsdl:portType name="HumanResource">
        <wsdl:operation name="Holiday">
            <wsdl:input message="tns:HolidayRequest" name="HolidayRequest"/>
        </wsdl:operation>
    </wsdl:portType>

That finished the abstract part of the WSDL (the interface, as it were), and leaves the concrete part. The concrete part consists of a binding, which tells the client how to invoke the operations you've just defined; and a service, which tells it where to invoke it.

Adding a concrete part is pretty standard: just refer to the abstract part you defined previously, make sure you use document/literal for the soap:binding elements (rpc/encoded is deprecated), pick a soapAction for the operation (in this case http://mycompany.com/RequestHoliday, but any URI will do), and determine the location URL where you want request to come in (in this case http://mycompany.com/humanresources):

<wsdl:definitions xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
                  xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
                  xmlns:schema="http://mycompany.com/hr/schemas"
                  xmlns:tns="http://mycompany.com/hr/definitions"
                  targetNamespace="http://mycompany.com/hr/definitions">
    <wsdl:types>
        <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <xsd:import namespace="http://mycompany.com/hr/schemas"                      
                schemaLocation="hr.xsd"/>
        </xsd:schema>
    </wsdl:types>
    <wsdl:message name="HolidayRequest">                                                 
        <wsdl:part element="schema:HolidayRequest" name="HolidayRequest"/>               
    </wsdl:message>
    <wsdl:portType name="HumanResource">                                                 
        <wsdl:operation name="Holiday">
            <wsdl:input message="tns:HolidayRequest" name="HolidayRequest"/>             
        </wsdl:operation>
    </wsdl:portType>
    <wsdl:binding name="HumanResourceBinding" type="tns:HumanResource">                  
        <soap:binding style="document"                                                   
            transport="http://schemas.xmlsoap.org/soap/http"/>                           
        <wsdl:operation name="Holiday">
            <soap:operation soapAction="http://mycompany.com/RequestHoliday"/>           
            <wsdl:input name="HolidayRequest">
                <soap:body use="literal"/>                                               
            </wsdl:input>
        </wsdl:operation>
    </wsdl:binding>
    <wsdl:service name="HumanResourceService">
        <wsdl:port binding="tns:HumanResourceBinding" name="HumanResourcePort">          
            <soap:address location="http://localhost:8080/holidayService/"/>             
        </wsdl:port>
    </wsdl:service>
</wsdl:definitions>

	We import the schema defined in Section 3.3, “Data Contract”.
	We define the HolidayRequest message, which gets used in the portType.
	The HolidayRequest type is defined in the schema.
	We define the HumanResource port type, which gets used in the binding.
	We define the HumanResourceBinding binding, which gets used in the port.
	We use a document/literal style.
	The literal http://schemas.xmlsoap.org/soap/http signifies a HTTP transport.
	The soapAction attribute signifies the SOAPAction HTTP header that will be sent with every request.
	The http://localhost:8080/holidayService/ address is the URL where the Web service can be invoked.

This is the final WSDL. We will describe how to implement the resulting schema and WSDL in the next section.

In this section, we will be using Maven2 to create the initial project structure for us. Doing so is not required, but greatly reduces the amount of code we have to write to setup our HolidayService.

The following command creates a Maven2 web application project for us, using the Spring-WS archetype (that is, project template)

mvn archetype:create -DarchetypeGroupId=org.springframework.ws \
  -DarchetypeArtifactId=spring-ws-archetype \
  -DarchetypeVersion=1.5.9 \
  -DgroupId=com.mycompany.hr \
  -DartifactId=holidayService

This command will create a new directory called holidayService. In this directory, there is a 'src/main/webapp' directory, which will contain the root of the WAR file. You will find the standard web application deployment descriptor 'WEB-INF/web.xml' here, which defines a Spring-WS MessageDispatcherServlet and maps all incoming requests to this servlet:

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
             http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
         version="2.4">

    <display-name>MyCompany HR Holiday Service</display-name>

    <!-- take especial notice of the name of this servlet -->
    <servlet>
        <servlet-name>spring-ws</servlet-name>
        <servlet-class>org.springframework.ws.transport.http.MessageDispatcherServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>spring-ws</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

</web-app>

In addition to the above 'WEB-INF/web.xml' file, you will also need another, Spring-WS-specific configuration file, named 'WEB-INF/spring-ws-servlet.xml'. This file contains all of the Spring-WS-specific beans such as EndPoints, WebServiceMessageReceivers, and suchlike, and is used to create a new Spring container. The name of this file is derived from the name of the attendant servlet (in this case 'spring-ws') with '-servlet.xml' appended to it. So if you defined a MessageDispatcherServlet with the name 'dynamite', the name of the Spring-WS-specific configuration file would be 'WEB-INF/dynamite-servlet.xml'.

(You can see the contents of the 'WEB-INF/spring-ws-servlet.xml' file for this example in ???.)

In Spring-WS, you will implement Endpoints to handle incoming XML messages. There are two flavors of endpoints: message endpoints and payload endpoints. Message endpoints give access to the entire XML message, including SOAP headers. Typically, the endpoint will only be interested in the payload of the message, that is the contents of the SOAP body. In that case, creating a payload endpoint makes more sense.

3.6.1. Handling the XML Message

In this sample application, we are going to use JDom to handle the XML message. We are also using XPath, because it allows us to select particular parts of the XML JDOM tree, without requiring strict schema conformance. We extend our endpoint from AbstractJDomPayloadEndpoint, because that will give us a JDOM element to execute the XPath queries on.

package com.mycompany.hr.ws;

import java.text.SimpleDateFormat;
import java.util.Date;

import com.mycompany.hr.service.HumanResourceService;
import org.jdom.Element;
import org.jdom.JDOMException;
import org.jdom.Namespace;
import org.jdom.xpath.XPath;
import org.springframework.ws.server.endpoint.AbstractJDomPayloadEndpoint;

public class HolidayEndpoint extends AbstractJDomPayloadEndpoint {

    private XPath startDateExpression;

    private XPath endDateExpression;

    private XPath nameExpression;

    private final HumanResourceService humanResourceService;

    public HolidayEndpoint(HumanResourceService humanResourceService) throws JDOMException {
        this.humanResourceService = humanResourceService;
        Namespace namespace = Namespace.getNamespace("hr", "http://mycompany.com/hr/schemas");
        startDateExpression = XPath.newInstance("//hr:StartDate");
        startDateExpression.addNamespace(namespace);
        endDateExpression = XPath.newInstance("//hr:EndDate");
        endDateExpression.addNamespace(namespace);
        nameExpression = XPath.newInstance("concat(//hr:FirstName,' ',//hr:LastName)");
        nameExpression.addNamespace(namespace);
    }

    protected Element invokeInternal(Element holidayRequest) throws Exception {          
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        Date startDate = dateFormat.parse(startDateExpression.valueOf(holidayRequest));
        Date endDate = dateFormat.parse(endDateExpression.valueOf(holidayRequest));
        String name = nameExpression.valueOf(holidayRequest);

        humanResourceService.bookHoliday(startDate, endDate, name);
        return null;
    }
}

The HolidayEndpoint requires the HumanResourceService business service to operate, so we inject the dependency via the constructor. Next, we set up XPath expressions using the JDOM API. There are three expressions: //hr:StartDate for extracting the <StartDate> text value, //hr:EndDate for extracting the end date and concat(//hr:FirstName,' ',//hr:LastName) for extracting and concatenating the names of the employee.

The invokeInternal(..) method is a template method, which gets passed with the <HolidayRequest/> element from the incoming XML message. We use the XPath expressions to extract the string values from the XML messages, and convert these values to Date objects using a SimpleDateFormat. With these values, we invoke a method on the business service. Typically, this will result in a database transaction being started, and some records being altered in the database. Finally, we return null, which indicates to Spring-WS that we do not want to send a response message. If we wanted a response message, we could have returned a JDOM Element that represents the payload of the response message.

Using JDOM is just one of the options to handle the XML: other options include DOM, dom4j, XOM, SAX, and StAX, but also marshalling techniques like JAXB, Castor, XMLBeans, JiBX, and XStream. We chose JDOM because it gives us access to the raw XML, and because it is based on classes (not interfaces and factory methods as with W3C DOM and dom4j), which makes the code less verbose. We use XPath because it is less fragile than marshalling technologies: we don't care for strict schema conformance, as long as we can find the dates and the name.

Because we use JDOM, we must add some dependencies to the Maven pom.xml, which is in the root of our project directory. Here is the relevant section of the POM:

<dependencies>
    <dependency>
        <groupId>org.springframework.ws</groupId>
        <artifactId>spring-ws-core</artifactId>
        <version>1.5.9</version>
    </dependency>
    <dependency>
        <groupId>jdom</groupId>
        <artifactId>jdom</artifactId>
        <version>1.0</version>
    </dependency>
    <dependency>
        <groupId>jaxen</groupId>
        <artifactId>jaxen</artifactId>
        <version>1.1</version>
    </dependency>
    <dependency>
        <groupId>javax.xml.soap</groupId>
        <artifactId>saaj-api</artifactId>
        <version>1.3</version>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>com.sun.xml.messaging.saaj</groupId>
        <artifactId>saaj-impl</artifactId>
        <version>1.3</version>
        <scope>runtime</scope>
    </dependency>
</dependencies>

Here is how we would configure these classes in our spring-ws-servlet.xml Spring XML configuration file:

<beans xmlns="http://www.springframework.org/schema/beans">

    <bean id="holidayEndpoint" class="com.mycompany.hr.ws.HolidayEndpoint">
        <constructor-arg ref="hrService"/>
    </bean>

    <bean id="hrService" class="com.mycompany.hr.service.StubHumanResourceService"/>

</beans>

<bean class="org.springframework.ws.server.endpoint.mapping.PayloadRootQNameEndpointMapping">
    <property name="mappings">
        <props>
            <prop key="{http://mycompany.com/hr/schemas}HolidayRequest">holidayEndpoint</prop>
        </props>
    </property>
    <property name="interceptors">
        <bean class="org.springframework.ws.server.endpoint.interceptor.PayloadLoggingInterceptor"/>
    </property>
</bean>

Finally, we need to publish the WSDL. As stated in Section 3.4, “Service contract”, we don't need to write a WSDL ourselves; Spring-WS can generate one for us based on some conventions. Here is how we define the generation:

<bean id="holiday" class="org.springframework.ws.wsdl.wsdl11.DefaultWsdl11Definition">   
    <property name="schema" ref="schema"/>                                               
    <property name="portTypeName" value="HumanResource"/>                                
    <property name="locationUri" value="/holidayService/"/>                              
    <property name="targetNamespace" value="http://mycompany.com/hr/definitions"/>       
</bean>

<bean id="schema" class="org.springframework.xml.xsd.SimpleXsdSchema">                   
    <property name="xsd" value="/WEB-INF/hr.xsd"/>
</bean>

	The bean id determines the URL where the WSDL can be retrieved. In this case, the bean id is holiday, which means that the WSDL can be retrieved as holiday.wsdl in the servlet context. The full URL will typically be http://localhost:8080/holidayService/holiday.wsdl.
	The schema property refers to the human resource schema we defined in Section 3.3, “Data Contract”, wrapped in a SimpleXsdSchema. We simply placed the schema in the WEB-INF directory of the application.
	Next, we define the WSDL port type to be HumanResource.
	We set the location where the service can be reached: /holidayService/. We use a a relative URI and we instruct the framework to transform it dynamically to an absolute URI. Hence, if the service is deployed to different contexts we don't have to change the URI manually. For more information, please refer to Section 5.3.1.1, “Automatic WSDL exposure”
	Finally, we define the target namespace for the WSDL definition itself. Setting these is not required. If not set, we give the WSDL the same namespace as the schema.

You can create a WAR file using mvn install. If you deploy the application (to Tomcat, Jetty, etc.), and point your browser at this location, you will see the generated WSDL. This WSDL is ready to be used by clients, such as soapUI, or other SOAP frameworks.