OkHttp Get Request Java Example

In this post, we will create an OkHttp GET HTTP request example in Java.
OkHTTP is an open source project designed to be an efficient HTTP client for Android and Java applications.
OkHttp supports Android 5.0+ (API level 21+) and Java 1.8+. In this article, we will write a code using Java 1.8+.

OkHttp Get Request Examples

We will create the following different GET request examples:
  1. OkHttp GET Request Java Example
  2. Synchronous GET with OkHttp
  3. Asynchronous GET with OkHttp
  4. GET Request with Query Parameters
  5. Accessing Http Request Headers

Maven Dependency

Let’s first add the library as a dependency into the pom.xml:
<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>3.9.0</version>
</dependency>
To see the latest dependency of this library check out the page on Maven Central.

1. OkHttp GET Request Java Example

In this example, we will make a GET HTTP client request for spring boot CRUD example project. This spring boot crud example project is deployed and up and running. Refer to this article - https://www.javaguides.net/2018/09/spring-boot-2-hibernate-5-mysql-crud-rest-api-tutorial.html
package com.javaguides.okhttp.tutorial.crud;

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpGet {

    OkHttpClient client = new OkHttpClient();

    public String run(String url) throws IOException {
        Request request = new Request.Builder().url(url).build();

        try (Response response = client.newCall(request).execute()) {
            return response.body().string();
        }
    }

    public static void main(String[] args) throws IOException {
        OkHttpGet example = new OkHttpGet();
        String response = example.run("http://localhost:8080/api/v1/employees/1");
        System.out.println(response);
    }
}
Below diagram shows the screenshot of source code as well as output:

2. Synchronous GET with OkHttp

To send synchronous GET request we need to build a Request object based on a URL and make a Call. After its execution we get back an instance of Response:
package com.javaguides.okhttp.tutorial;

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpGetExample {

    OkHttpClient client = new OkHttpClient();

    public String run(String url) throws IOException {
        Request request = new Request.Builder().url(url).build();

        try (Response response = client.newCall(request).execute()) {
            return response.body().string();
        }
    }

    public static void main(String[] args) throws IOException {
        OkHttpGetExample example = new OkHttpGetExample();
        String response = example.run("https://raw.github.com/square/okhttp/master/README.md");
        System.out.println(response);
    }
}
Output:
OkHttp
======

An HTTP & HTTP/2 client for Android and Java applications. For more information see [the
website][website] and [the wiki][wiki].

.....

3. Asynchronous GET with OkHttp

Now, to make an asynchronous GET we need to enqueue a Call. A Callback allows us to read the response when it is readable. This happens after the response headers are ready.
Reading the response body may still block. OkHttp doesn’t currently offer any asynchronous APIs to receive a response body in parts:
package com.javaguides.okhttp.tutorial;

import java.io.IOException;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpAsynchronousGetRequest {

    OkHttpClient client = new OkHttpClient();

    public static void main(String[] args) throws IOException {
        OkHttpAsynchronousGetRequest example = new OkHttpAsynchronousGetRequest();
        example.run("https://raw.github.com/square/okhttp/master/README.md");
    }

    public void run(String url) throws IOException {
        Request request = new Request.Builder().url(url).build();

        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            public void onResponse(Call call, Response response) throws IOException {
                System.out.println("on Success");
                System.out.println(response.toString());
            }

            public void onFailure(Call call, IOException e) {
                System.out.println("onFailure");
            }
        });
    }
}
Output:
on Success
Response{protocol=http/1.1, code=200, message=OK, url=https://raw.githubusercontent.com/square/okhttp/master/README.md}

4. GET Request with Query Parameters

Finally, to add query parameters to our GET request we can take advantage of the HttpUrl.Builder.
After the URL is built we can pass it to our Request object:
package com.javaguides.okhttp.tutorial;

import java.io.IOException;

import okhttp3.HttpUrl;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpGetQueryParametersExample {
    // avoid creating several instances, should be singleon
    OkHttpClient client = new OkHttpClient();

    public static void main(String[] args) throws IOException {

        OkHttpGetQueryParametersExample example = new OkHttpGetQueryParametersExample();
        HttpUrl.Builder urlBuilder = HttpUrl.parse("https://api.github.com").newBuilder();
        urlBuilder.addQueryParameter("v", "3.0");
        urlBuilder.addQueryParameter("user", "RameshMF");
        String url = urlBuilder.build().toString();
        System.out.println("Print URL : " + url);
        System.out.println(example.run(url));
    }

    // Common method to execute and return response
    public String run(String url) throws IOException {
        Request request = new Request.Builder().url(url).build();
        try (Response response = client.newCall(request).execute()) {
            return response.body().string();
        }
    }
}
Output:
Print URL : https://api.github.com/?v=3.0&user=RameshMF
{"current_user_url":"https://api.github.com/user","current_user_authorizations_html_url":
....

5. Accessing Http Request Headers

package com.javaguides.okhttp.tutorial;

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpHeadersExample {

    private final OkHttpClient client = new OkHttpClient();

    public static void main(String[] args) throws Exception {
        OkHttpHeadersExample example = new OkHttpHeadersExample();
        example.run();
    }

    public void run() throws Exception {
        Request request = new Request.Builder().url("https://api.github.com/repos/square/okhttp/issues")
            .header("User-Agent", "OkHttp Headers.java").addHeader("Accept", "application/json; q=0.5")
            .addHeader("Accept", "application/vnd.github.v3+json").build();

        try (Response response = client.newCall(request).execute()) {
            if (!response.isSuccessful())
                throw new IOException("Unexpected code " + response);

            System.out.println("Server: " + response.header("Server"));
            System.out.println("Date: " + response.header("Date"));
            System.out.println("Vary: " + response.headers("Vary"));
        }
    }
}
Output:
Server: GitHub.com
Date: Mon, 27 May 2019 06:59:49 GMT
Vary: [Accept]

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