Contract-first web services refer to a development approach where the service contract, or the interface, is defined before the implementation. It means that developers first write the WSDL (Web Services Description Language) and then use it as a starting point to create the actual service. This approach ensures that the service adheres to the agreed-upon contract, and any changes made to it are made explicitly.
Java is a popular language for developing web services, and this article will provide an example of creating a contract-first web service in Java.
Step 1: Define the Service Contract
The first step in creating a contract-first web service is to define the service contract. In Java, this is done using the WSDL file. The WSDL file describes the service, including its operations, input/output messages, and data types. Here`s an example of a WSDL file for a simple calculator service:
“`xml
“`
The WSDL file defines an operation named “addition” that takes two integers as input and returns their sum. The input and output messages are defined using XML schema elements.
Step 2: Generate Java Code from the WSDL
Once the WSDL is defined, the next step is to generate the Java code from it. This is done using the “wsimport” command-line tool that comes with the Java Development Kit (JDK).
To generate the Java code, navigate to the directory where the WSDL file is saved and run the following command:
“`
wsimport CalculatorService.wsdl
“`
This command generates Java classes for the service interface, request and response messages, and any complex types defined in the WSDL.
Step 3: Implement the Service
Now that we have the Java code generated from the WSDL, we can start implementing the service. In this example, we`ll implement the Calculator service that performs addition.
First, let`s define an implementation class for the service interface:
“`java
package com.example.calculator;
import javax.jws.WebService;
@WebService(endpointInterface = “com.example.calculator.CalculatorPortType”)
public class Calculator implements CalculatorPortType {
@Override
public AdditionResponse addition(AdditionRequest parameters) {
int result = parameters.getA() + parameters.getB();
AdditionResponse response = new AdditionResponse();
response.setResult(result);
return response;
}
}
“`
The class implements the “CalculatorPortType” interface generated from the WSDL and provides an implementation for the “addition” operation.
Next, let`s create a servlet that publishes the service:
“`java
package com.example.calculator;
import javax.servlet.ServletException;
import javax.xml.ws.Endpoint;
import javax.xml.ws.WebServiceException;
public class CalculatorServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private Endpoint endpoint;
@Override
public void init() throws ServletException {
try {
Calculator calculator = new Calculator();
endpoint = Endpoint.publish(“/CalculatorService”, calculator);
} catch (WebServiceException e) {
throw new ServletException(“Failed to publish web service endpoint: ” + e.getMessage(), e);
}
}
@Override
public void destroy() {
endpoint.stop();
}
}
“`
The servlet creates an instance of the Calculator service implementation and publishes it at the “/CalculatorService” endpoint.
Step 4: Deploy the Service
To deploy the service, package the Java classes and the WSDL into a WAR file and deploy it to a web server like Tomcat or Jetty.
Conclusion
In this article, we have seen an example of creating a contract-first web service in Java. The approach of defining the service contract first before implementation ensures that the service adheres to the agreement, and any changes made to it are made explicitly. With Java, we can use tools like “wsimport” to generate Java code from the WSDL and implement the service using the generated code.
