I’m back from a recent trip out of the country. While the facility I was staying at would have an Internet connection, the price for Internet access for about a week was a little over 100 USD. I’d rather go without a connection. While I didn’t have access to the Internet, I did have access to a local network. I considered options on how to bring media with me. Rather than bring a few movies and songs here and there I wanted to indiscriminately copy what I could to a drive. In addition to myself, there were three other people with me that might also want to view the media. It made since to host the media on a pocket-sized web server. I setup a Raspberry Pi to do just this and took it with me on the trip.

After the trip was over, I thought to myself that there should be a way to do the same thing in a more compact package. I started to look at what the smallest Raspberry Pi Computer Module based setup would look like, and as I mentally constructed a solution in my mind, I realized it was converging to the same form factor as a phone. I’ve got plenty of old phones lying about. While I wouldn’t suggest this as a general solution (phones are a lot more expensive than a Pi) it is what I decided to have fun with.

There are various ways to run NodeJS on a phone and some other apps in the app store that let you host a web server on your phone. I didn’t use any of these. I am reinventing the wheel simply because I find enjoyment in creating. It was a Sunday night, I was watching my TV lineup, and decided to make a simple proof of concept. I only wanted the PoC to listen for incoming request and send a hard coded HTML page back to the client. I had that working in no time! I’ll build upon this to give it the ability to host static files and media files when I do a future update on this. I’m taking a moment to talk about how I build this first.

I created a new Android project. Before writing code, I declared a few permissions. I like to do this first so that later on I don’t have to wonder why a specific call failed. The permissions I added are for Internet access, accessing the WiFi state, and access to the Wake Lock to keep the device from completely suspending. For what I show here, only Internet capabilities are going to be used. You can choose to omit the other two permissions for this version of the program.

With the permissions in place, I started writing the code. There are only three classes used in the web server (counting an interface as a class).

• WebServer – Listens for incoming request and passes them off to be handled as they com in
• ClientSocketHandler – Processes an incoming class and gives the response
• IStatusUpdater – used for passing status information back to the UI

The WebServer class accepts in its constructor the port on which it should run and a Context object, which is needed for some other calls. A WebServer instance does not begin to listen for connections until the start() method is called. Once it is started, the device retrieves the address of the WiFi adapter and creates a socket that is bound to this address. A status message is also sent to the UI so that it can show the device’s IP address. The class then creates the thread that will listen for incoming connections.

In the function listenerThread(), the class waits for an incoming connection. As soon as it receives one, it creates a new ClientSocketHandler with the socket and lets the ClientSocketHandler process the request and immediately goes back to listening for another connection. It doesn’t wait for the ClientSocketHandler to finish before waiting for the next connection.

package net.j2i.webserver.Service
import android.content.Context
import android.net.wifi.WifiManager
import android.text.format.Formatter
import java.net.InetSocketAddress
import java.net.ServerSocket
import java.net.Socket
import kotlin.concurrent.thread
class WebServer {
    companion object {
    }
    val port:Int;
    lateinit var receiveThread:Thread
    lateinit var  listenerSocket:ServerSocket;
    var keepRunning = true;
    val context:Context;
    var statusReceiver:IStatusUpdateReceiver
    constructor(port:Int, context: Context) {
        this.port = port;
        this.context = context;
        this.statusReceiver = object : IStatusUpdateReceiver {
            override fun updateStatus(ipAddress: String, clientCount: Int) {
                fun updateStatus(ipAddress: String, clientCount: Int) {
                }
            }
        }
    }
    fun start() {
        keepRunning = true;
        val wifiManager:WifiManager =
            this.context.getSystemService(Context.WIFI_SERVICE) as WifiManager;
        val wifiIpAddress:String = Formatter.formatIpAddress(wifiManager.connectionInfo.ipAddress);
        this.statusReceiver.updateStatus(wifiIpAddress, 0)
        this.listenerSocket = ServerSocket();
        this.listenerSocket.reuseAddress = true;
        this.listenerSocket.bind(InetSocketAddress(wifiIpAddress, this.port))
        this.receiveThread = thread(start = true) {
                this.listenerThread()
        }
        //this.receiveThread.start()
    }
    fun listenerThread() {
        while(keepRunning) {
            var clientSocket: Socket = this.listenerSocket.accept()
            val clientSocketHandler = ClientSocketHandler(clientSocket)
            clientSocketHandler.respondAsync()
        }
    }
}

In ClientSocketHandler, the class grabs the input stream (to read the request from the remote client) and the OutputStream (to send data back to the client). Now I haven’t implemented the HTTP protocol. But in HTTP, the client will send a one or more lines that make up the request followed by a blank line. For now, my client handler reads from input stream until that blank line is encountered. Once received, it composes a response.

I’ve got the HTML string that the client is going to return hardcoded into the application. In the response string is converted to a byte array. The size of this array is needed for one of the response headers. The client will receive the size of the response in the header Content-Length. The header for the response is constructed as a string and converted to a byte array. Then the two arrays are sent back to the client (first the header, then the content). After the response is sent, the client has done its work.

package net.j2i.webserver.Service
import android.util.Log
import java.lang.StringBuilder
import java.net.Socket
import kotlin.concurrent.thread
class ClientSocketHandler {
    companion object {
        val TAG = "ClientSocketHandler"
    }
    private val clientSocket: Socket;
    private val responseThread:Thread
    constructor(sourceClientSocket:Socket) {
        this.clientSocket = sourceClientSocket;
        this.responseThread = thread( start = false) {
                this.respond()
        }
    }
    public fun respondAsync() {
        this.responseThread.run()
    }
    private fun respond() {
        val inputStream = this.clientSocket.getInputStream()
        val outputStream = this.clientSocket.getOutputStream()
        var requestReceived = false;
        while(inputStream.available()>0 &&  !requestReceived) {
            val requestLine = inputStream.bufferedReader().readLine()
            Log.i(ClientSocketHandler.TAG, requestLine)
            if(processRequestLine(requestLine)) {
            requestReceived = true;}
        }
        val sb:StringBuilder = StringBuilder()
        val sbHeader = StringBuilder()
        sb.appendLine(
            "<html>"+
                    "<head><title>Test</title></head>" +
                    "<body>Test Response;lkj;ljkojiojioijoij</body>"+
                   "</html>")
        sb.appendLine()
        val responseString = sb.toString()
        val responseBytes = responseString.toByteArray(Charsets.UTF_8)
        val responseSize = responseBytes.size
        sbHeader.appendLine("HTTP/1.1 200 OK");
        sbHeader.appendLine("Content-Type: text/html");
        sbHeader.append("Content-Length: ")
        sbHeader.appendLine(responseSize)
        sbHeader.appendLine()
        val responseHeaderString = sbHeader.toString()
        val responseHeaderBytes = responseHeaderString.toByteArray(Charsets.UTF_8)
        outputStream.write(responseHeaderBytes)
        outputStream.write(responseBytes)
        outputStream.flush()
        outputStream.close()
    }
    fun processRequestLine(requestLine:String): Boolean {
        if(requestLine == "") {
            return true;
        }
        return false;
    }
}

The interface that I mentioned, IStatusUpdateReceiver, is currently only being used to communicate the IP address on which the server is listening back to the UI.

package net.j2i.webserver.Service
interface IStatusUpdateReceiver {
    fun updateStatus(ipAddress:String, clientCount:Int);
}

Since the server runs on a different thread, before updating the UI I must make sure that UI related calls are being performed on the main thread. If you look in the class for MainActivity you will see that I created the  WebServer instance in the activity. I’m only doing this because it is a PoF. If you make your own application, implement this as a service.  I set the statusReceiver member of the WebServer to an annonymous class instance that does nothing more than update the IP address displayed in the UI. The call to set the text in the UI is wrapped in a runOnUiThread block. After this is setup, I call start() on the webserver to get things going.

package net.j2i.webserver
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import net.j2i.webserver.Service.IStatusUpdateReceiver
import net.j2i.webserver.Service.WebServer
class MainActivity : AppCompatActivity() {
    lateinit var webServer:WebServer
    lateinit var txtIpAddress:TextView
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        this.txtIpAddress = findViewById(R.id.txtIpAddress)
        this.webServer = WebServer(8888, this)
        this.webServer.statusReceiver = object:IStatusUpdateReceiver {
            override fun updateStatus(ipAddress:String, clientCount:Int) {
                runOnUiThread {
                    txtIpAddress.text = ipAddress
                }
            }
        }
        this.webServer.start()
    }
}

I was happy that my proof of concept worked. I haven’t yet decided if I am going to throw this away or continue working from this. In either case, there are a few things that I want to have in whatever my next version is. I do absolutely no exception handling or cleanup in this code. It needs to be able to timeout a connection and refuse connections if it gets inundated. I also want my next version to do actual processing of the incoming HTTP request and serve up content that has been saved to the device’s memory, such as a folder on the device’s memory card. While I am making this to serve up static content, I might add a few services to the server/device side, such as a socket server. That will require a lot more thought.

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