Java REST Client Examples Using OkHttp

This tutorial covers how to develop some Java REST client examples using OkHttp, an HTTP & HTTP/2 client for Android and Java applications.

1. Preparation

Let’s assume that we have an RESTful web service with several API as below. And we’re going to use OkHttp client to create several basic Java REST client examples which will communicate with this RESTful web service.

Assume that we have an RESTful web services with several API as below:

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": "Java How To Program",
    "author": "Paul Deitel"
}

{

"name": "Java How To Program",

"author": "Paul Deitel"

}

Responses: application/json

Example:

{
    "id": 5
    "name": "Java How To Program",
    "author": "Paul Deitel"
}

{

"id": 5

"name": "Java How To Program",

"author": "Paul Deitel"

}

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": 1,
    "name": "Java How To Program 2nd",
    "author": "Paul Deitel"
}

{

"id": 1,

"name": "Java How To Program 2nd",

"author": "Paul Deitel"

}

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 can not delete the book.

1.5 Source code

The demo source code can be found on the Github or you can download it here: Java-Examples.zip

Let’s define a POJO to map with the Book response as below:

public class Book {
  private Long id;
  private String name;
  private String author;
  // All getters and setters
}

public class Book {

private Long id;

private String name;

private String author;

// All getters and setters

}

We use OkHttp client to communicate with the RESTful service above and we use Jackson to convert JSON responses to Java objects and vice versa.

<dependency>
  <groupId>com.squareup.okhttp3</groupId>
  <artifactId>okhttp</artifactId>
  <version>3.4.1</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.7.5</version>
</dependency>

<groupId>com.squareup.okhttp3</groupId>

<artifactId>okhttp</artifactId>

</dependency>

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

<artifactId>jackson-databind</artifactId>

</dependency>

2. Java REST client examples using OkHttp

In this section, we’re going to use OkHttp library to create, update, query, and delete resources from REST API.

2.1. Make a HTTP GET request to the RESTful web service (Get all books)

public class BookRepositoryImplOkHttp {

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

    public Book[] getAllBooks() throws Exception {
        Book[] books = null;
        // create new client
        OkHttpClient httpclient = new OkHttpClient();
        Request request = new Request.Builder().url(URI_BOOK).get().build();
        try (Response response = httpclient.newCall(request).execute()) {
            // converts response into an array of books
            ObjectMapper mapper = new ObjectMapper();
            books = mapper.readValue(response.body().bytes(), Book[].class);
        }
        return books;
    }
}

public class BookRepositoryImplOkHttp {

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

public Book[] getAllBooks() throws Exception {

Book[] books = null;

// create new client

OkHttpClient httpclient = new OkHttpClient();

Request request = new Request.Builder().url(URI_BOOK).get().build();

try (Response response = httpclient.newCall(request).execute()) {

// converts response into an array of books

ObjectMapper mapper = new ObjectMapper();

books = mapper.readValue(response.body().bytes(), Book[].class);

}

return books;

}

At first, we need to create an OkHttpClient object and create a Request with the URL is our REST API. After the client executes the request, we get back the response which is in JSON format and will be converted to Java objects by using Jackson.

2.2. Make a HTTP POST request to the RESTful web service (create a book)

public class BookRepositoryImplOkHttp {

    private static final MediaType MediaTypeJSON = MediaType
            .parse("application/json; charset=utf-8");
    private static final String URI_BOOK = "http://localhost:8080/v1/books";

    public Book createBook(Book book) throws Exception {
        Book createdBook = null;
        // creates new client
        OkHttpClient httpclient = new OkHttpClient();
        // convert the book to JSON by Jackson
        ObjectMapper mapper = new ObjectMapper();
        String jsonBook = mapper.writeValueAsString(book);

        // build a request
        Request request = new Request.Builder().url(URI_BOOK)
                .post(RequestBody.create(MediaTypeJSON, jsonBook)).build();
        try (Response response = httpclient.newCall(request).execute()) {

            if (response.isSuccessful()) {
                // Get back the response and convert it to a Book object
                createdBook = mapper.readValue(response.body().bytes(), Book.class);

            }
        }
        return createdBook;
    }

}

public class BookRepositoryImplOkHttp {

private static final MediaType MediaTypeJSON = MediaType

.parse("application/json; charset=utf-8");

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

public Book createBook(Book book) throws Exception {

Book createdBook = null;

// creates new client

OkHttpClient httpclient = new OkHttpClient();

// convert the book to JSON by Jackson

ObjectMapper mapper = new ObjectMapper();

String jsonBook = mapper.writeValueAsString(book);

// build a request

Request request = new Request.Builder().url(URI_BOOK)

.post(RequestBody.create(MediaTypeJSON, jsonBook)).build();

try (Response response = httpclient.newCall(request).execute()) {

if (response.isSuccessful()) {

// Get back the response and convert it to a Book object

createdBook = mapper.readValue(response.body().bytes(), Book.class);

}

return createdBook;

}

In this example, before creating request, we have to convert the Book which we want to created, into JSON to match with the requirement of REST API. We also need to set the content type of an HTTP request as application/json. In similar to the HTTP GET request, after executing the request, we get back to response and use Jackson convert it into a Java object.

2.3. Make a HTTP PUT request to the RESTful web service (Update a book)

public class BookRepositoryImplOkHttp {

    private static final MediaType MediaTypeJSON = MediaType
            .parse("application/json; charset=utf-8");
    private static final String URI_BOOK = "http://localhost:8080/v1/books";

    public Book updateBook(Book book) throws Exception {

        Book updatedBook = null;
        OkHttpClient httpclient = new OkHttpClient();
        //converts the book to JSON first
        ObjectMapper mapper = new ObjectMapper();
        String jsonBook = mapper.writeValueAsString(book);
        Request request = new Request.Builder().url(URI_BOOK + "/" + book.getId())
                .put(RequestBody.create(MediaTypeJSON, jsonBook)).build();
        try (Response response = httpclient.newCall(request).execute()) {
            if (response.isSuccessful()) {
                // convert the reponse into a Book object
                updatedBook = mapper.readValue(response.body().bytes(), Book.class);

            }
        }
        return updatedBook;
    }

}

public class BookRepositoryImplOkHttp {

private static final MediaType MediaTypeJSON = MediaType

.parse("application/json; charset=utf-8");

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

public Book updateBook(Book book) throws Exception {

Book updatedBook = null;

OkHttpClient httpclient = new OkHttpClient();

//converts the book to JSON first

ObjectMapper mapper = new ObjectMapper();

String jsonBook = mapper.writeValueAsString(book);

Request request = new Request.Builder().url(URI_BOOK + "/" + book.getId())

.put(RequestBody.create(MediaTypeJSON, jsonBook)).build();

try (Response response = httpclient.newCall(request).execute()) {

if (response.isSuccessful()) {

// convert the reponse into a Book object

updatedBook = mapper.readValue(response.body().bytes(), Book.class);

}

return updatedBook;

}

In this example, we have just issued an HTTP PUT request to the REST API. In similar to the HTTP POST above, we have to specify clearly the HTTP method that the REST API required. Before sending the request we have to convert the Book to JSON and after getting back the response, we convert it to a Java object.

2.4. Make a HTTP DELETE request to the RESTful web service (delete a book)

public class BookRepositoryImplOkHttp {

    private static final MediaType MediaTypeJSON = MediaType
            .parse("application/json; charset=utf-8");
    private static final String URI_BOOK = "http://localhost:8080/v1/books";

    public void deleteBook(Long id) throws Exception {
        OkHttpClient httpclient = new OkHttpClient();
        Request request = new Request.Builder().url(URI_BOOK + "/" + id).delete().build();
        try (Response response = httpclient.newCall(request).execute()) {
            if (!response.isSuccessful()) {
                throw new RuntimeException("Failed to delete book with id:" + id);

            }
        }
    }

}

public class BookRepositoryImplOkHttp {

private static final MediaType MediaTypeJSON = MediaType

.parse("application/json; charset=utf-8");

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

public void deleteBook(Long id) throws Exception {

OkHttpClient httpclient = new OkHttpClient();

Request request = new Request.Builder().url(URI_BOOK + "/" + id).delete().build();

try (Response response = httpclient.newCall(request).execute()) {

if (!response.isSuccessful()) {

throw new RuntimeException("Failed to delete book with id:" + id);

}

As required from the REST API, to delete the book, we have to issue an HTTP DELETE method to the resource URI.

4. Conclusions

We have just implemented several Java REST client examples using OkHttp library, an HTTP & HTTP/2 client for Android and Java applications powered by Square. OkHttp is introduced as a very efficiency Http client with HTTP/2 support or connection pooling reduces request latency, etc. In future posts, we will explore more feature of this library. Meanwhile, there are other tutorials related to the Java REST client for your references:

OkHttp Post Examples

Java REST Client Example With Retrofit 2

Java REST Client Using Netflix Feign

Java REST Client Using Apache HttpClient

Java REST Client Using Spring RestTemplate

Simple Java REST Client

Java REST Client With Jersey Client

Java REST Client Using Resteasy Client

Java REST Client Using Resteasy Client Proxy Framework

Java REST Client Using Apache CXF Proxy based API

Basic Authentication with OkHttp Example

Download a File with OkHttp

Upload a File with OkHttp

Set Timeout with OkHttp

WebSocket Client Example with OkHttp

How to Cache Response with OkHttp

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Java REST Client Examples Using OkHttp

1. Preparation

1.1 Get all books

1.2 Create a new book

1.3 Update a book

1.4 Delete a book

1.5 Source code

2. Java REST client examples using OkHttp

2.1. Make a HTTP GET request to the RESTful web service (Get all books)

2.2. Make a HTTP POST request to the RESTful web service (create a book)

2.3. Make a HTTP PUT request to the RESTful web service (Update a book)

2.4. Make a HTTP DELETE request to the RESTful web service (delete a book)

4. Conclusions

howtoprogram