How to use Jersey + Jackson + jQuery

• How to create Restful web service using Jersey - an implementation of JAX-RS.
• How to convert a Java object to JSON using Jackson - a library to deal with JSON Serialization.
• How to call Restful service from client using jQuery.

Creating Restful Web Service with Jersey

1. Start Eclipse Mars (Even older versions will do).
2. Create a new Dynamic Web Project called catalog.
3. We need an implementation of jax-rs. We use Jersey. Download Jersey 2.23.1 by going to their website https://jersey.java.net/download.html and clicking on Jersey JAX-RS 2.0 RI Bundle.
4. When jaxrs-ri-2.23.1.zip is downloaded, extract it into a folder like d:\java\jaxrs-ri-2.23.1.
5. Get into that folder and copy all .jar files from lib, ext and api folders into WEB-INF/lib folder of our project in Eclipse.
6. Create the following Java classes that are used in Restful service.

   Book.java

   Book class represents a book. It contains id, title and price properties.

   package rest;
   import javax.xml.bind.annotation.XmlRootElement;
   public class Book {
   	private int id;
   	private String title;
   	private double price;
   	public Book() {
   	}
   	public Book(int id, String title, double price) {
   		super();
   		this.id = id;
   		this.title = title;
   		this.price = price;
   	}
   	public int getId() {
   		return id;
   	}
   	public void setId(int id) {
   		this.id = id;
   	}
   	public String getTitle() {
   		return title;
   	}
   	public void setTitle(String title) {
   		this.title = title;
   	}
   	public double getPrice() {
   		return price;
   	}
   	public void setPrice(double price) {
   		this.price = price;
   	}
   }
   

   Books.java

   Books class creates a set of Book objects. To keep it simple, I am creating a few objects and putting them in an ArrayList.

   package rest;
   import java.util.ArrayList;
   import java.util.List;
   
   public class Books {
   	private static ArrayList<Book> books = new ArrayList<>();
   	static {
   		books.add(new Book(1, "The Outliers", 500));
   		books.add(new Book(2, "World Is Flat", 550));
   	}
   	public static List<Book> getBooks() {
   		return books;
   	}
   }
   

7. Now, create BookService.java, which is a Restful service with the following characteristics.
   • It supports URLs /books and /books/id
   • It throws NotFoundException when the given book id is not found
   • Method getBooks() returns a List of Book objects and getBook() returns a single Book object based on the given id
   • As this project uses Jackson, it will convert these Java objects to JSON before sending data to client

   BookService.java

   package rest;
   import java.util.List;
   import javax.ws.rs.GET;
   import javax.ws.rs.NotFoundException;
   import javax.ws.rs.Path;
   import javax.ws.rs.PathParam;
   import javax.ws.rs.Produces;
   import javax.ws.rs.core.MediaType;
   
   @Path("/books")
   public class BookService {
   	List<Book> books;
   	public BookService() {
   		books = Books.getBooks();
   	}
   	@GET
   	@Produces(MediaType.APPLICATION_JSON)
   	public List<Book> getBooks() {
   		return books;
   	}
   	@Path("{id}")
   	@GET
   	@Produces(MediaType.APPLICATION_JSON)
   	public Book getBook(@PathParam("id") int id) {
   		  for(Book b : books) {
   			  if ( b.getId() == id)
   				   return b;
   		  }
   		  // book with the given id is not found, so throw 404 error
   		  throw new NotFoundException(); 
   	}
   }
   

8. To use Jersey, we need to add Jersey servlet configuration to web.xml. So lets first create web.xml
   • Select project (catalog) and right click to get popup menu
   • Select Java EE Tools -> Generate Deployment Descriptor Stub. It will create WEB-INF/web.xml
   • Add the following servlet configuration to web.xml as follows:

     <?xml version="1.0" encoding="UTF-8"?>
     <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     	xmlns="http://xmlns.jcp.org/xml/ns/javaee"
     	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
     	version="3.1">
     	<display-name>catalog</display-name>
     	<servlet>
     		<servlet-name>Jersey REST Service</servlet-name>
     		<servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>
     
     		<init-param>
     			<param-name>jersey.config.server.provider.packages</param-name>
     			<param-value>rest</param-value>
     		</init-param>
     		<load-on-startup>1</load-on-startup>
     	</servlet>
     	<servlet-mapping>
     		<servlet-name>Jersey REST Service</servlet-name>
     		<url-pattern>/rest/*</url-pattern>
     	</servlet-mapping>
     </web-app>
     

9. Now our Restful Service is ready to be accessed from client. But it still cannot convert Java objects to JSON. To convert Book object to JSON we need to add Jackson libraries to our project.

Adding Jackson to Project

• jackson-core-2.8.1.jar
• jackson-databind-2.8.1.jar
• jackson-anotations-2.8.1.jar
• jackson-jars-base-2.8.1.jar
• jackson-jars-json-provider-2.8.1.jar
• jackson-module-jaxb-annotations-2.8.1.jar
• jersey-entity-filtering-2.23.1.jar
• jersey-media-json-jackson-2.23.1.jar

Testing Service

Creating Client using jQuery

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Books</title>
<script type="text/javascript" src="jquery-3.1.0.js">
</script>
<script>
  function getDetails() {
	     // getJSON returns Promise object, which contains done(), always() and fail() methods
	     // use fail() to respond to failed request like 404
	     var promise = $.getJSON("http://localhost:8888/catalog/rest/books/" + $("#id").val(),{}, showBook);
	     promise.fail(showError);   // getJSON returns promise object.
  }
  function showBook(book)
  {
	  $("#title").text(book.title);
	  $("#price").text(book.price);
  }
  
  function showError()
  {
	  $("#title").text("Sorry! Book not found!");
	  $("#price").text("");
  }
</script>
</head>
<body>
	<h2>Books Client</h2>
 	Book Id : <br/>
	<input type="text" id="id" /> 
	<button onclick="getDetails()">Details </button>
	<p />
        <div id="title"></div>
	<p />
        <div id="price"></div>
</body>
</html>