Posted by : Munish Gogna Sunday, May 8, 2011

This post is dedicated to my friend jagdish salgotra who wanted me to write something on RESTful web services and to all starters in this area.

In the REST(REpresentational State Transfer) world, information on the server side is considered a resource, which anyone can access in a uniform way using web URIs (Uniform Resource Identifiers) and HTTP. Because REST uses HTTP as the communication protocol, the REST style is constrained to a stateless client/server architecture. We can map the HTTP methods POST, GET, PUT, and DELETE to create, read, update and delete (CRUD) operations.

There are many ways to write RESTful web services:
> Sun offers a reference implementation for JAX-RS code-named Jersey.
> Spring provides RestTemplate for the same purpose

In this sample example (very basic and naive just to give u all a gentle kick) I will try to explain how we can use DEPARTMENT as representational state in REST using GET method. We will try to map following urls to the particular state we are interested in using Jersey implementation.

/api/departments/Get all departments
/api/departments/{id}Get details of a particular department
/api/departments/employeesGet all departments with employee details
/api/departments/{id}/employeesGet employees of particular department

NOTE: All employees related data will be available under department node (by design)

The output of the Restful web services can be a plain text, html, xml, json or any media type, For our example We will use XML as content type of our service output and will rely on JAXB annotations to provide the required marshaling/unmarshaling services.

Let's start with the root XML element. I chose to call mine GetDepartmentResponse, and use it as a container for a collection of Department objects. This is just to do with the convention I usually follow (no hard fast rules behind this).
package com.mg.rest.hr.resources;

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class GetDepartmentResponse {

 private List<Department> departments;

 @XmlElement(name = "department")
@XmlElementWrapper(name = "departments")
 public List<Department> getAllDepartments() {
  if (departments == null) {
   departments = new ArrayList<Department>();
  }
  return departments;
 }

}

Notice the @XmlElementWrapper annotation on the Department collection. This makes JAXB wrap all of the department XML elements inside of an departments XML element. Also notice that I placed the @XmlElement annotations on the getter methods instead of on the private fields. When placed on the private fields, JAXB will give you an error unless you add @XmlAccessorType(XmlAccessType.FIELD) at the class level.

Next we define our Resources - Department and Employee

package com.mg.rest.hr.resources;

import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlType;

/**
 * 
 * @author Munish Gogna
 *
 */
@XmlType(name = "department")
public class Department {

 private String deptId;
 private String deptName;
 private List<Employee> employees;

 public Department() {
 }

 @XmlElement
 public String getDeptId() {
  return deptId;
 }

 @XmlElement(name = "employee")
 @XmlElementWrapper(name = "employees")
 public List<Employee> getEmployees() {
  return employees;
 }

 @XmlElement
 public String getDeptName() {
  return deptName;
 }

 public void setDeptId(String deptId) {
  this.deptId = deptId;
 }

 public void setEmployees(List<Employee> employees) {
  this.employees = employees;
 }

 public void setDeptName(String deptName) {
  this.deptName = deptName;
 }

}

In the example above, we use @XmlType at the class level instead of @XmlRootElement because it is not the root element

package com.mg.rest.hr.resources;

import java.math.BigDecimal;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlType;

/**
 * 
 * @author Munish Gogna
 * 
 */
@XmlType(propOrder = { "empId", "empName", "salary" })
public class Employee {

 private String empId;
 private String empName;
 private BigDecimal salary;

 public Employee() {
 }

 public Employee(String empId, String empName, BigDecimal salary) {
  super();
  this.empId = empId;
  this.empName = empName;
  this.salary = salary;
 }

 @XmlElement
 public BigDecimal getSalary() {
  return salary;
 }

 @XmlElement
 public String getEmpId() {
  return empId;
 }

 @XmlElement
 public String getEmpName() {
  return empName;
 }

 public void setEmpId(String empId) {
  this.empId = empId;
 }

 public void setEmpName(String empName) {
  this.empName = empName;
 }

 public void setSalary(BigDecimal salary) {
  this.salary = salary;
 }

}


Now lets create a JAX-RS RESTful web service that can return above defined object graph in the responses we have defined in the beginning of this article.

JAX-RS defines a resource as any Java class (POJO) that uses JAX-RS annotations to implement a web resource. The annotation @Path identifies a Java class as a resource class as shown below.

package com.mg.rest.hr;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;

import com.mg.rest.hr.resources.Department;
import com.mg.rest.hr.resources.GetDepartmentResponse;

/**
 * Service to handle Department and related resources.
 * Just a mock implementation, nothing fancy.
 * @author Munish Gogna
 *
 */
@Path("/api")
public class DepartmentResource {

 @GET
 @Produces("text/xml")
 @Path("/departments/employees")
 public GetDepartmentResponse getAllDepartmentsWithEmployees() {
  GetDepartmentResponse response = new GetDepartmentResponse();
  response.getAllDepartments().addAll(
    FromHeaven.getDeptsWithEmps().values());
  return response;
 }

 @GET
 @Produces("text/xml")
 @Path("/departments/{id}")
 public GetDepartmentResponse getDepartment(@PathParam("id") String id) {
  GetDepartmentResponse response = new GetDepartmentResponse();
  Department dept = FromHeaven.getDepts().get(id);
  response.getAllDepartments().add(dept);
  return response;
 }

 @GET
 @Produces("text/xml")
 @Path("/departments/{id}/employees")
 public GetDepartmentResponse getEmployeesForDepartment(
   @PathParam("id") String id) {
  GetDepartmentResponse response = new GetDepartmentResponse();
  Department dept = FromHeaven.getDeptsWithEmps().get(id);
  response.getAllDepartments().add(dept);
  return response;

 }

 @GET
 @Produces("text/xml")
 @Path("/departments")
 public GetDepartmentResponse getAllDepartments() {
  GetDepartmentResponse response = new GetDepartmentResponse();
  response.getAllDepartments().addAll(FromHeaven.getDepts().values());
  return response;

 }
}


Notice the @Produces("text/xml") annotation, and that the method returns a GetDepartmentResponse object. Since the GetDepartmentResponse is annotated with JAXB annotations, JAX-RS will automatically marshal the response to XML.
Also note that FromHeaven is a utility class that provides dummy departmental data(This class has 2 hard coded departments D1 and D2 with 2 employees in D1 and only single employee in D2)

Let's try some URIs now:

1. http://localhost:8080/rest/api/departments

 
  
   D2
   Human Resource
  
  
   D1
   Finance
  
 


2. http://localhost:8080/rest/api/departments/D1

 
  
   D1
   Finance
  
 


3. http://localhost:8080/rest/api/departments/employees

 
  
   D1
   Finance
   
    
     e1
     Munish Gogna
     1000
    
    
     e2
     Jagdish Salgotra
     2000
    
   
  
  
   D2
   Human Resource
   
    
     e3
     Sahil Gogna
     5000
    
   
  
 


4.http://localhost:8080/rest/api/departments/D1/employees

 
  
   D1
   Finance
   
    
     e1
     Munish Gogna
     1000
    
    
     e2
     Jagdish Salgotra
     2000
    
   
  
 


In order for Jersey to work, we need to configure the JAX-RS Servlet in the web.xml as shown below:


 
  com.mg.rest.hr
  JAX-RS REST Servlet
  com.sun.jersey.spi.container.servlet.ServletContainer
  
   com.sun.jersey.config.property.packagescom.mg.rest.hr
  1
 
 
  JAX-RS REST Servlet
  /*
 


Notice that we have to provide the package name (com.mg.rest.hr) of the resource handler classes to the servlet class.

That's all for now, later some day we will try make these RESTful services secure so that only authorized callers can use these resources.

Please don't forget to provide your valuable feedback, especially you Jagi :)

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