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Android HTTP Client: GET, POST, Download, Upload, Multipart Request

Topics covered

HTTP connection

Post request

Download data

Upload data

Multipart request

Often Android apps have to exchange information with a remote server. The easiest way is to use the HTTP protocol as base to transfer information. There are several scenarios where the HTTP protocol is very useful like downloading an image from a remote server or uploading some binary data to the server. Android app performs GET or POST request to send data. In this post, we want to analyze how to use HttpURLConnection to communicate with a remote server.
As a server we will use three simple Servlet running inside Tomcat 7.0. We won't cover how to create a Servlet using API 3.0, you can find the source code here.

GET and POST requests

GET and POST requests are the base blocks in HTTP protocol. To make this kind of requests we need first to open a connection toward the remove server:

HttpURLConnection con = (HttpURLConnection) ( new URL(url)).openConnection();
con.setRequestMethod("POST");
con.setDoInput(true);
con.setDoOutput(true);
con.connect();

In the first line we get the HttpURLConnection, while in the line 2 we set the method and at the end we connect to the server.
Once we have opened the connection we can write on it using the OutputStream.

con.getOutputStream().write( ("name=" + name).getBytes());

As we already know parameters are written using key value pair.
The last step is reading the response, using the InputStream:

InputStream is = con.getInputStream();
byte[] b = new byte[1024];
while ( is.read(b) != -1)
  buffer.append(new String(b));
con.disconnect();

Everything is very simple by now, but we have to remember one thing: making an HTTP connection is a time consuming operation that could require long time sometime so we can't run it in the main thread otherwise we could get a ANR problem. To solve it we can use an AsyncTask.

private class SendHttpRequestTask extends AsyncTask<String, Void, String>{
  
  @Override
  protected String doInBackground(String... params) {
   String url = params[0];
   String name = params[1];
   String data = sendHttpRequest(url, name);
   return data;
  }

  @Override
  protected void onPostExecute(String result) {
   edtResp.setText(result);
   item.setActionView(null);   
  }
}

Running the app we get:


android_httpclient_post_get_1android_httpclient_post_get_2

As we can see we post a name to the server and it responds with the classic ‘Hello….’. On the server side we can check that the server received correctly our post parameter:

android_tomcat_post_log



Download data from server

One of the most common scenario is when an Android App has to download some data from a remote sever. We can suppose that we want to download an image from the server. In this case we have always to use an AsyncTask to complete our operation, the code is shown below:
public byte[] downloadImage(String imgName) {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    try {
        System.out.println("URL ["+url+"] - Name ["+imgName+"]");
        
        HttpURLConnection con = (HttpURLConnection) ( new URL(url)).openConnection();
        con.setRequestMethod("POST");
        con.setDoInput(true);
        con.setDoOutput(true);
        con.connect();
        con.getOutputStream().write( ("name=" + imgName).getBytes());
        
        InputStream is = con.getInputStream();
        byte[] b = new byte[1024];
        
        while ( is.read(b) != -1)
            baos.write(b);
        
        con.disconnect();
    }
    catch(Throwable t) {
        t.printStackTrace();
    }
    
    return baos.toByteArray();
}

This method is called in this way:
private class SendHttpRequestTask extends AsyncTask<String, Void, byte[]> {

    
    @Override
    protected byte[] doInBackground(String... params) {
        String url = params[0];
        String name = params[1];
        
        HttpClient client = new HttpClient(url);
        byte[] data = client.downloadImage(name);
        
        return data;
    }

    @Override
    protected void onPostExecute(byte[] result) {
        Bitmap img = BitmapFactory.decodeByteArray(result, 0, result.length);
        imgView.setImageBitmap(img);
        item.setActionView(null);
        
    }
   
}

Running the app we have:

android_httpclient_post_download

Upload data to the server using MultipartRequest


This the most complex part in handling http connection. Natively HttpURLConnection doesn’t handle this type of request. It can happen that an Android App has to upload some binary data to the server. It can be that an app has to upload an image for example. In this case the request get more complex, because a “normal” request isn’t enough. We have to create a MultipartRequest.

A MultipartRequest is a request that is made by different parts like parameters and binary data. How can we handle this request?

Well the first step is opening a connection informing the server we wants to send some binary info:
public void connectForMultipart() throws Exception {
    con = (HttpURLConnection) ( new URL(url)).openConnection();
    con.setRequestMethod("POST");
    con.setDoInput(true);
    con.setDoOutput(true);
    con.setRequestProperty("Connection", "Keep-Alive");
    con.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
    con.connect();
    os = con.getOutputStream();
}

In the line 6 and 7 we specify the request content-type and another field called boundary. This field is a char sequence used to separate different parts.

For each part we want to add we need to specify if it is text part like post parameter or it is a file (so binary data).
public void addFormPart(String paramName, String value) throws Exception {
  writeParamData(paramName, value);
}

private void writeParamData(String paramName, String value) throws Exception {
    os.write( (delimiter + boundary + "\r\n").getBytes());
    os.write( "Content-Type: text/plain\r\n".getBytes());
    os.write( ("Content-Disposition: form-data; name=\"" + paramName + "\"\r\n").getBytes());;
    os.write( ("\r\n" + value + "\r\n").getBytes());
    
}

where
private String delimiter = "--";
private String boundary =  "SwA"+Long.toString(System.currentTimeMillis())+"SwA";
To add a file part we can use:
public void addFilePart(String paramName, String fileName, byte[] data) throws Exception {
    os.write( (delimiter + boundary + "\r\n").getBytes());
    os.write( ("Content-Disposition: form-data; name=\"" + paramName +  "\"; filename=\"" + fileName + "\"\r\n"  ).getBytes());
    os.write( ("Content-Type: application/octet-stream\r\n"  ).getBytes());
    os.write( ("Content-Transfer-Encoding: binary\r\n"  ).getBytes());
    os.write("\r\n".getBytes());
   
    os.write(data);
    
    os.write("\r\n".getBytes());
}

So in our app we have:
private class SendHttpRequestTask extends AsyncTask<String, Void, String> {

    
    @Override
    protected String doInBackground(String... params) {
        String url = params[0];
        String param1 = params[1];
        String param2 = params[2];
        Bitmap b = BitmapFactory.decodeResource(UploadActivity.this.getResources(), R.drawable.logo);
        
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        b.compress(CompressFormat.PNG, 0, baos);

        try {
            HttpClient client = new HttpClient(url);
            client.connectForMultipart();
            client.addFormPart("param1", param1);
            client.addFormPart("param2", param2);
            client.addFilePart("file", "logo.png", baos.toByteArray());
            client.finishMultipart();
            String data = client.getResponse();
        }
        catch(Throwable t) {
            t.printStackTrace();
        }
        
        return null;
    }

    @Override
    protected void onPostExecute(String data) {            
        item.setActionView(null);
        
    }
    
    
    
}

Running it we have:


android_tomcat_post_upload_logandroid_httpclient_post_upload
Source code @ github (for Android App)
Source code for server side servlet here

24 comments:

  1. It would be a lot more efficient to googles network library Volley for these tasks.Its very easy to use to.

    ReplyDelete
  2. I HAVE A QUESTION...Can I use this library for implementing a simple control remoto control (Android-android) between the intranet ?

    ReplyDelete
  3. survivingwithandroidJune 19, 2013 at 10:31 PM

    Usually this kind of protocol is used in a client-sever scenario, especially when you have firewall. If you are in an intranet this lib can be used too but i suggest to look for something else. First of all because you don't have fw second because you had to implement an HTTP server somewhere (i.e. in one of the two android device).

    ReplyDelete
  4. Rameshbabu PalanisamyJuly 25, 2013 at 6:45 AM

    i want to get data from server and show it in the map. How can i do that. I am doing android application. please suggest some answers. Example in taxi booking application every user get the details about taxi and location of that taxi standing. How can i do like this.

    ReplyDelete
  5. how to post the parameter(data) like this format. pls help me

    http://xxxxx/xxx/abc.php?id=xx&name=xxx&data=[{.....}] and the output result

    "local" :[{"id" :"123a", "ph" : "1234444"]}

    ReplyDelete
  6. survivingwithandroidSeptember 6, 2013 at 2:31 PM

    Use a POST method and instead of using
    con.getOutputStream().write( ("name=" + name).getBytes());
    use
    write..("name="+name+"&param1=....&param2="...").getBytes().
    Remember to encode the data.

    ReplyDelete
  7. hi...thnx for reply ..I have tried but not working as local..[{...}] this format. my code output like this format.{"local" :{ "id" : "123",...} .how can i do that local parameter as array. below is my code
    1) httpclass:

    HttpParams myParams = new BasicHttpParams();

    HttpConnectionParams.setConnectionTimeout(myParams, 10000);

    HttpConnectionParams.setSoTimeout(myParams, 10000);

    HttpClient httpclient = new DefaultHttpClient();

    String jSON = jObj.toString();

    HttpPost httppost = new HttpPost(url + "action=add_entry");

    httppost.setHeader("Content-type", "application/json");

    StringEntity se = new StringEntity(jSON);

    se.setContentEncoding(new BasicHeader(HTTP.CONTENT_TYPE, "application/json"));

    httppost.setEntity(se);

    // Checking Response



    HttpResponse response = httpclient.execute(httppost);

    String temp = EntityUtils.toString(response.getEntity());

    2) activity side

    JSONObject jTest = new JSONObject();

    httpcilentclass.postData(jTest.put("local", jObjMain));





    ===========================================
    pls reply ASAP possible with code..pls

    ReplyDelete
  8. Hi, I have a problem, When I Download a image this is dowloded only foe 30% end in the next part of image I have the blue ball. Do you have any suggest?

    ReplyDelete
  9. survivingwithandroidNovember 7, 2013 at 9:46 AM

    Is This image very big? Try to check if there are any error

    ReplyDelete
  10. I don't know Why this problem occurs, but I have find a solution, I have used the InputStream "is" and I have decoded the image with BitmapFactory.decodeStream(is); next at onPostExecute I have passed the img directly... I hope this procedure will is usefull for other user the will have my same problem

    ReplyDelete
  11. survivingwithandroidDecember 4, 2013 at 2:21 PM

    I've written a post about it look at this link if you like http://www.survivingwithandroid.com/2013/11/android-volley-tutorial-post-download.html

    ReplyDelete
  12. The variable buffer is never defined.

    ReplyDelete
  13. Why does some of this code use HttpClient (from Apache, org.apache.http.client.HttpClient) and some of the code use HttpURLConnection (from Java,

    java.net.HttpURLConnection)?
    You should use either, not both.

    ReplyDelete
  14. What can I do to get an string and not an image?
    Thanks for the tut!

    ReplyDelete
  15. survivingwithandroidMarch 13, 2014 at 9:18 AM

    If you want to get a String you simply read the response.

    ReplyDelete
  16. Please provide servlet or php code as well for better understating

    ReplyDelete
  17. hey can i also provide server side code as well .... it would be really helpful......thnks in advance.....

    ReplyDelete
  18. That would be better if you publish the 'DownloadServlet' codes.

    ReplyDelete
  19. survivingwithandroidApril 17, 2014 at 9:49 PM

    Here there is the link https://github.com/survivingwithandroid/Surviving-with-android/tree/master/misc

    ReplyDelete
  20. survivingwithandroidApril 17, 2014 at 9:49 PM

    https://github.com/survivingwithandroid/Surviving-with-android/tree/master/misc

    ReplyDelete
  21. Just great, thanks! Could you please, show me how to send a json string to a server that requires authentication?

    Thank you in advance.

    ReplyDelete
  22. hello how do I add a progressbar?

    ReplyDelete
  23. hello how do I add a progressbar?

    ReplyDelete
  24. Dear Sir,
    I need help in sending data to server with android code.I have an api example
    http://getepass.com/api.php?action=login&&email=aaa@gmail.com&&password=root
    I want to send email and password to server from mobile text and receive few parameter image ,name,compid etc and then show in app.
    I am using a lib for this but I want to code it myself If possible please send me the example to post and read the json from server in android.
    Thanks!
    Prachi Gupta

    ReplyDelete

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