Sample code demo retrieve HTTP request content for Android Mashmallow

HttpRequestSample(API 23).java

package com.example.guinso.restclientdemo;

import android.os.AsyncTask;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

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

        //example bind event to Floating Action Button (you can bind with any control event)
        FloatingActionButton fbReload = (FloatingActionButton)findViewById(R.id.fbReload);
        fbReload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d("MainActivity", "Reload is triggered.");

                //make HTTP REST request in async task
                //(since Android constraint network execution must not run in main loop)
                AsyncTask<String, Void, String> tass = new AsyncTask<String, Void, String>() {
                    //this is where to run HTTP request execution
                    @Override
                    protected String doInBackground(String... params) {
                        //since this example only pass url, so definitely is first parameter
                        String raw = getHttpContentString(params[0]);

                        return raw;
                    }

                    //this is where you update your GUI content after retrieve from HTTP request
                    @Override
                    protected void onPostExecute(String result) {
                        Log.d("MainActivity", result);
                    }
                };

                //replace with your targeted url (this is just an example)
                tass.execute("http://192.168.0.44:7777/api/meals");
            }
        });
    }

    private String getHttpContentString(String requestUrl) {
        URL url = null;

        try {
            url = new URL(requestUrl);
        } catch (MalformedURLException e) {
            Log.e("mainActivity", String.format("URL %s is not valid", requestUrl));
            url = null;
        }

        //exit scope if URL is invalid
        if(url == null) {
            //TODO: notify failed to reload content
            return "";
        }

        //start pull content from REST server
        HttpURLConnection httpConn = null;
        String responseContent = "";

        try {
            httpConn = (HttpURLConnection) url.openConnection();
            httpConn.setRequestMethod("GET");

            //read HTTP response code and decide whether continue or not
            int responseCode = httpConn.getResponseCode();
            if(responseCode != 200)
            {
                Log.e("mainActivity", "Fail to get content from " + requestUrl);
                responseContent = "";
            } else {
                //get content character set (default set to UTF-8)
                String charset = httpConn.getContentEncoding();
                if(charset == null || charset.equals(""))
                    charset = "UTF-8";

                //decode input stream to string
                InputStream in = httpConn.getInputStream();

                ByteArrayOutputStream readBuffer = new ByteArrayOutputStream();
                byte[] buffer = new byte[1024];
                int readLen = 0;
                while((readLen = in.read(buffer)) != -1) {
                    readBuffer.write(buffer, 0, readLen);
                }

                responseContent = readBuffer.toString(charset);
            }
        }
        catch (IOException ex) {
            ex.printStackTrace();
            responseContent = "";
        } finally {
            if(httpConn != null)
                httpConn.disconnect();
        }

        return responseContent;
    }
}