Java REST Client Using Spring RestTemplate

There are many ways to create Java REST clients such as using the built-in java.net.URL package, leveraging client API of RESTful frameworks like Jersey, Resteasy, Spring Rest, etc. In this tutorial, we’d like to show how to create a Java REST client using Spring RestTemplate.

1. Preparation

Let’s assume that we have a RESTful web service with several APIs as follows:

1.1. Get all books

GET http://localhost:8080/v1/books

1	GET http://localhost:8080/v1/books

Responses: application/json

1.2. Create a new book

POST http://localhost:8080/v1/books

1	POST http://localhost:8080/v1/books

Request

Example:

{
    "name": "Into the Water",
    "author": "Celeste Ng"
}

{

"name": "Into the Water",

"author": "Celeste Ng"

}

Responses: application/json

Example:

{
    "id": 5
    "name": "Into the Water",
    "author": "Celeste Ng"
}

{

"id": 5

"name": "Into the Water",

"author": "Celeste Ng"

}

STATUS 201 if the book is created successfully.

1.3. Update a book

PUT http://localhost:8080/v1/books/{id}

1	PUT http://localhost:8080/v1/books/{id}

Request

Example:

{
    "id": 5
    "name": "Into the Water: A Novel",
    "author": "Paul Hawkins"
}

{

"id": 5

"name": "Into the Water: A Novel",

"author": "Paul Hawkins"

}

Responses: application/json

STATUS 200 if the book is updated successfully.

STATUS 400 if there is no book with given id

1.4. Delete a book

DELETE http://localhost:8080/v1/books/{id}

1	DELETE http://localhost:8080/v1/books/{id}

Responses: application/json

STATUS 204 if the book is deleted successfully.

STATUS 400 if there is no book with given id or cannot delete the book.

1.5. Source code

The source code presented in this tutorial is available on the Github.

About the library, we’re going to use some libraries:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.7.5</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
    <version>2.6.3</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>4.2.6.RELEASE</version>
</dependency>

<groupId>com.fasterxml.jackson.core</groupId>

<artifactId>jackson-databind</artifactId>

</dependency>

<groupId>com.fasterxml.jackson.dataformat</groupId>

<artifactId>jackson-dataformat-xml</artifactId>

</dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-web</artifactId>

<version>4.2.6.RELEASE</version>

</dependency>

1.6. POJO

Let’s define a POJO class “Book” to map with the above JSON:

public class Book {
    private Long id;
    private String name;
    private String author;

    //getters and setters
}

public class Book {

private Long id;

private String name;

private String author;

//getters and setters

}

2. Create A Java REST Client Using Spring RestTemplate

Let’s get to some examples of creating Java REST client using Spring RestTemplate. We’re going query, create, update and delete resources from the above REST service.

2.1. Using Spring RestTemplate to make a GET request

Firstly, let’s see how we use the Spring RestTemplate to issue an HTTP request to the first API to get all books:

import java.util.Arrays;
import java.util.List;
import org.springframework.web.client.RestTemplate;
import com.howtoprogram.domain.Book;

public class BookRepositoryImplSpring {

  private static final String URI_BOOK = "http://localhost:8080/v1/books";
  private RestTemplate restTemplate = new RestTemplate();;

  public Book[] getAllBooks() {
    Book[] books = restTemplate.getForObject(URI_BOOK, Book[].class);
    return books;
  }
}

import java.util.Arrays;

import java.util.List;

import org.springframework.web.client.RestTemplate;

import com.howtoprogram.domain.Book;

public class BookRepositoryImplSpring {

private static final String URI_BOOK = "http://localhost:8080/v1/books";

private RestTemplate restTemplate = new RestTemplate();;

public Book[] getAllBooks() {

Book[] books = restTemplate.getForObject(URI_BOOK, Book[].class);

return books;

}

We just need to specify the URL of the REST service and the ResponseType which is Book[].class in the example, and the Spring RestTemplate will convert the response into an array of Books.

We can test with a simple main method as below:

  public static void main(String[] args) {
    BookRepositoryImplSpring bookRepository = new BookRepositoryImplSpring();
    Book[] books = bookRepository.getAllBooks();
    System.out.println(Arrays.asList(books));
  }

public static void main(String[] args) {

BookRepositoryImplSpring bookRepository = new BookRepositoryImplSpring();

Book[] books = bookRepository.getAllBooks();

System.out.println(Arrays.asList(books));

}

2.2. Using Spring RestTemplate to make a POST request

Secondly, we’re going to use the Spring RestTemplate to issue a request to the 2nd API to create a book:

import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;
import com.howtoprogram.domain.Book;

public class BookRepositoryImplSpring {

  private static final String URI_BOOK = "http://localhost:8080/v1/books";
  private RestTemplate restTemplate = new RestTemplate();

  public Book createBook(Book book) {
    Book createdBook = restTemplate.postForObject(URI_BOOK, book, Book.class);
    return createdBook;
  }

}

import org.springframework.http.ResponseEntity;

import org.springframework.web.client.RestTemplate;

import com.howtoprogram.domain.Book;

public class BookRepositoryImplSpring {

private static final String URI_BOOK = "http://localhost:8080/v1/books";

private RestTemplate restTemplate = new RestTemplate();

public Book createBook(Book book) {

Book createdBook = restTemplate.postForObject(URI_BOOK, book, Book.class);

return createdBook;

}

We need to specify the URL of the RESTful service, the entity to be created and the ResponseType. Note that the API returns the STATUS 201 and the newly created book (includes the generated Id field) as JSON; however, we have used the postForObject method, we can not get the returned HTTP status. If we want to obtain the Http Status, we can use the postForEntity method as below:

public class BookRepositoryImplSpring {

  private static final String URI_BOOK = "http://localhost:8080/v1/books";
  private RestTemplate restTemplate = new RestTemplate();

  public Book createBook(Book book) {
    // Book createdBook = restTemplate.postForObject(URI_BOOK, book, Book.class);
    ResponseEntity<Book> responseEntity = restTemplate.postForEntity(URI_BOOK, book, Book.class);
    Book createdBook = null;
    if (responseEntity.getStatusCode() == HttpStatus.CREATED) {
      createdBook = responseEntity.getBody();
    }
    return createdBook;
  }

}

public class BookRepositoryImplSpring {

private static final String URI_BOOK = "http://localhost:8080/v1/books";

private RestTemplate restTemplate = new RestTemplate();

public Book createBook(Book book) {

// Book createdBook = restTemplate.postForObject(URI_BOOK, book, Book.class);

ResponseEntity<Book> responseEntity = restTemplate.postForEntity(URI_BOOK, book, Book.class);

Book createdBook = null;

if (responseEntity.getStatusCode() == HttpStatus.CREATED) {

createdBook = responseEntity.getBody();

}

return createdBook;

}

If the RESTful API returns a URI for the newly created object, we can use another method: postForLocation of the Spring RestTemplate.

URI uri =  restTemplate.postForLocation(URI_BOOK, book);

1	URI uri = restTemplate.postForLocation(URI_BOOK, book);

To test the method “createBook “, we can create a simple main method.

  public static void main(String[] args) {
    Book book = new Book(null, "Effective Java", "Joshua Bloch");
    BookRepositoryImplSpring bookRepository = new BookRepositoryImplSpring();
    Book createdBook = bookRepository.createBook(book);
    System.out.println(createdBook);
  }

public static void main(String[] args) {

Book book = new Book(null, "Effective Java", "Joshua Bloch");

BookRepositoryImplSpring bookRepository = new BookRepositoryImplSpring();

Book createdBook = bookRepository.createBook(book);

System.out.println(createdBook);

}

The output console is:

Book [id=4, name=Effective Java, author=Joshua Bloch]

1	Book [id=4, name=Effective Java, author=Joshua Bloch]

Note that the response includes the new field: id=4.

2.3. Using Spring RestTemplate to make a PUT request

And next, in the following example, we’re going to use the Spring RestTemplate to update a remote resource (Book) by making a PUT request to the RESTful API:

public class BookRepositoryImplSpring {

  private static final String URI_BOOK = "http://localhost:8080/v1/books";
  private RestTemplate restTemplate = new RestTemplate();

  public void updateBook(Book book) {
    restTemplate.put(URI_BOOK + "/{id}", book, book.getId());
  }
}

public class BookRepositoryImplSpring {

private static final String URI_BOOK = "http://localhost:8080/v1/books";

private RestTemplate restTemplate = new RestTemplate();

public void updateBook(Book book) {

restTemplate.put(URI_BOOK + "/{id}", book, book.getId());

}

We have called the put method provided by the Spring RestTemplate class. And we can update the main method to test the updateBook method.

  public static void main(String[] args) {
    BookRepositoryImplSpring repository = new BookRepositoryImplSpring();
    // Getting the first book from the RESTful service
    Book book = repository.getAllBooks()[0];
    // Change the name
    book.setName(book.getName() + " 3rd");
    // Then update the book
    repository.updateBook(book);
  }

public static void main(String[] args) {

BookRepositoryImplSpring repository = new BookRepositoryImplSpring();

// Getting the first book from the RESTful service

Book book = repository.getAllBooks()[0];

// Change the name

book.setName(book.getName() + " 3rd");

// Then update the book

repository.updateBook(book);

}

2.4. Using Spring RestTemplate to make a DELETE request

And finally, we’re going to use the Spring RestTemplate to delete or remove a remote resource by making a DELETE request:

public class BookRepositoryImplSpring {

  private static final String URI_BOOK = "http://localhost:8080/v1/books";
  private RestTemplate restTemplate = new RestTemplate();

  public void deleteBook(Long id) {
    restTemplate.delete(URI_BOOK + "/{id}", id);
  }
}

public class BookRepositoryImplSpring {

private static final String URI_BOOK = "http://localhost:8080/v1/books";

private RestTemplate restTemplate = new RestTemplate();

public void deleteBook(Long id) {

restTemplate.delete(URI_BOOK + "/{id}", id);

}

We need to call the delete method of the Spring RestTemplate class, specify the URL and URL variables.

The main method will be updated to test as below:

  public static void main(String[] args) {
    BookRepositoryImplSpring repository = new BookRepositoryImplSpring();
    // Getting the first book from the RESTful service
    Book book = repository.getAllBooks()[0];
    // Try to delete the book
    repository.deleteBook(book.getId());
  }

public static void main(String[] args) {

BookRepositoryImplSpring repository = new BookRepositoryImplSpring();

// Getting the first book from the RESTful service

Book book = repository.getAllBooks()[0];

// Try to delete the book

repository.deleteBook(book.getId());

}

3. Conclusion

The tutorial has illustrated us how to create a Java REST client using Spring RestTemplate. The tutorial just only covers some very basic methods provided by Spring RestTemplate. If you want to get to know more about other methods of the RestTemplate, you can refer to Spring RestTemplate site. In near future, I will provide more examples of implementing Java REST client with other libraries, frameworks.

Below are other related tutorials:

Simple Java REST Client Using java.net.URL package(using the built-in package java.net.URL of the JDK)

Java REST Client Using Resteasy Client

Java REST Client Using Jersey Client

Java REST Client Using Resteasy Client Proxy Framework

Java REST Client Using Netflix Feign

Java REST Client Using Apache HttpClient

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