Javaライブラリ「OkHttp」の簡単なサンプル

事前に、ローカルにサーバーを立てて置いて下さい。

MainActivity.java

package com.example.test.testokhttp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.io.IOException;

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

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        OkHttpClient client = new OkHttpClient();

        Request request = new Request.Builder()
                .url("http://192.168.43.201:3000/users")
                .build();

        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                System.out.println("response: " + response.body().string());
            }
        });

    }
}

build.gradle

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:24.1.1'
    compile 'com.squareup.okhttp3:okhttp:3.3.1'
}

headerを設定したい場合は、こういう書き方でもいい

MainActivity.java

package com.example.test.testokhttp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.io.IOException;

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

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
        httpClient.addInterceptor(new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                Request original = chain.request();

                //header設定
                Request request = original.newBuilder()
                        .header("Accept", "application/json")
                        .method(original.method(), original.body())
                        .build();

                Response response = chain.proceed(request);

                return response;
            }
        });

        OkHttpClient client = httpClient.build();

        Request request = new Request.Builder()
                .url("http://192.168.43.201:3000/users")
                .build();

        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                System.out.println("response: " + response.body().string());
            }
        });

    }
}