Android | Send JSON Content to the Cloud Using HttpUrlConnection & Kotlin Coroutine

Exploring Kotlin

Less than 100 pages covering Kotlin syntax and features in straight and to the point explanation.

get-location Sending data to the cloud is a common task in mobile applications. Here we will consume an open REST service to practice sending data in JSON format to a cloud server. We will use HttpURLConnection to make an HTTP POST request to the server. Then, we can verify that our request is successful by going to hmkcode-api.appspot.com/links.html page.

Objectives

  1. How to send HTTP POST request to the cloud with JSON content.
  2. How to check network connection?
  3. How to use kotlin coroutine to perform network operations on a separate thread?

Environment, Tools & Library

used in this post

About the app

We are going to build a sample app that stores links info in the cloud. The app takes three inputs from the user link title, url and optional tags. When the user click send, the app will build a JSON object holding the three inputs, add the built JSON object to the POST request body and finally execute the POST request.

( 1 ) Create new Android Project

Create new android application keep default options.

  • Application Name: post-json
  • Package Name: com.hmkcode
  • Language: Kotlin
  • Minimum SDK: API 19: Android 4.4 (KitKat)

( 2 ) Add Permissions

  • Add the following two lines to the AndroidMainifest.xml file
   <uses-permission android:name="android.permission.INTERNET" />
   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

( 3 ) Design App Layout

  • Add the following views to activity_main.xml.
  • TextView for displaying network connection info.
  • Three EditText for title, URL & tags account.

target_api

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout ...>
    <TextView
        android:id="@+id/tvIsConnected"
        ... />

    <com.google.android.material.textfield.TextInputLayout
        ...>
        <com.google.android.material.textfield.TextInputEditText
            android:id="@+id/etTitle"
            ...
            android:hint="Title..."/>
    </com.google.android.material.textfield.TextInputLayout>

    <com.google.android.material.textfield.TextInputLayout
       ...>
        <com.google.android.material.textfield.TextInputEditText
            android:id="@+id/etUrl"
            ...
            android:hint="example.com"/>
    </com.google.android.material.textfield.TextInputLayout>

    <com.google.android.material.textfield.TextInputLayout
        ...>
        <com.google.android.material.textfield.TextInputEditText
            android:id="@+id/etTags"
            ...
            android:hint="tag1;tag2;..."/>
    </com.google.android.material.textfield.TextInputLayout>

    <Button
        android:id="@+id/btnSend"
        android:text="Send"
        ...
        android:onClick="send"/>

    <TextView
        android:id="@+id/tvResult"
       .../>
</LinearLayout>
Dependencies
  • You need to include the following dependencies to build.gradle

build.gradle (Module: post-json)

dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'com.google.android.material:material:1.1.0-beta01'
    implementation 'androidx.core:core-ktx:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    // Coroutine
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.2"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.2"
    implementation "androidx.lifecycle:lifecycle-runtime-ktx:2.2.0-alpha05"
}

( 4 ) Send JSON to Server

MainActivity.kt is the only class where we need to code the following:

  • a. Check network connectivity.
  • b. Build httpPost() function with IO dispatcher.
  • c. Build send() function to launch httpPost() in lifecycleScope coroutine scope.

a. Check network connectivity

  • Before making network operation check notwork connectivity status.
  • To check network connectivity use getActiveNetworkInfo() & isConnected().
private fun checkNetworkConnection(): Boolean {
    val connMgr = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager

    val networkInfo = connMgr.activeNetworkInfo
    val isConnected: Boolean = if(networkInfo != null) networkInfo.isConnected() else false
    if (networkInfo != null && isConnected) {
        // show "Connected" & type of network "WIFI or MOBILE"
        tvIsConnected.text = "Connected " + networkInfo.typeName
        // change background color to red
        tvIsConnected.setBackgroundColor(-0x8333da)
    } else {
        // show "Not Connected"
        tvIsConnected.text = "Not Connected"
        // change background color to green
        tvIsConnected.setBackgroundColor(-0x10000)
    }
    return isConnected
}

b. Build httpPost() function with IO dispatcher.

  • Create httpPost(String url) method
  • httpPost() method does the following:
    1. Use withContext(Dispatcher.IO){} to run code on IO thread.
    2. Creates HttpURLConnection
    3. Builds JSONObject
    4. Adds JSON content to POST request body
    5. executes the POST request
    6. returns response message
Throws(IOException::class, JSONException::class)
    private suspend fun httpPost(myUrl: String): String {

        val result = withContext(Dispatchers.IO) {
            val url = URL(myUrl)
            // 1. create HttpURLConnection
            val conn = url.openConnection() as HttpsURLConnection
            conn.requestMethod = "POST"
            conn.setRequestProperty("Content-Type", "application/json; charset=utf-8")

            // 2. build JSON object
            val jsonObject = buidJsonObject()

            // 3. add JSON content to POST request body
            setPostRequestContent(conn, jsonObject)

            // 4. make POST request to the given URL
            conn.connect()

            // 5. return response message
            conn.responseMessage + ""
        }
        return result
    }

}
	

c. Build send() function to launch httpPost() in lifecycleScope coroutine scope.

  • Launch coroutine using activity lifecycleScope.
  • Show the result on the tvResult
 public fun send(view:View) {
        Toast.makeText(this, "Clicked", Toast.LENGTH_SHORT).show()
        // clear text result
        tvResult.setText("")

        if (checkNetworkConnection())
            lifecycleScope.launch {
                val result = httpPost("https://hmkcode-api.appspot.com/rest/link/add")
                tvResult.setText(result)
            }
        else
            Toast.makeText(this, "Not Connected!", Toast.LENGTH_SHORT).show()

    }

MainActivity.java complete code

package com.hmkcode
import android.content.Context
import android.net.ConnectivityManager
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.view.View
import android.widget.EditText
import android.widget.TextView
import android.widget.Toast
import androidx.lifecycle.lifecycleScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
import kotlinx.coroutines.withContext
import org.json.JSONException
import org.json.JSONObject
import java.io.BufferedWriter
import java.io.IOException
import java.io.OutputStreamWriter
import java.net.HttpURLConnection
import java.net.URL
import javax.net.ssl.HttpsURLConnection

class MainActivity : AppCompatActivity() {

    lateinit var tvIsConnected: TextView
    lateinit var etTitle: EditText
    lateinit var etUrl: EditText
    lateinit var etTags: EditText
    lateinit var tvResult: TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        tvIsConnected = findViewById<TextView>(R.id.tvIsConnected)
        etTitle = findViewById<EditText>(R.id.etTitle)
        etUrl = findViewById<EditText>(R.id.etUrl)
        etTags = findViewById<EditText>(R.id.etTags)
        tvResult = findViewById<TextView>(R.id.tvResult)
        checkNetworkConnection()
    }

    public fun send(view:View) {
        Toast.makeText(this, "Clicked", Toast.LENGTH_SHORT).show()
        // clear text result
        tvResult.setText("")

        if (checkNetworkConnection())
            lifecycleScope.launch {
                val result = httpPost("https://hmkcode-api.appspot.com/rest/link/add")
                tvResult.setText(result)
            }
        else
            Toast.makeText(this, "Not Connected!", Toast.LENGTH_SHORT).show()

    }

    @Throws(IOException::class, JSONException::class)
    private suspend fun httpPost(myUrl: String): String {

        val result = withContext(Dispatchers.IO) {
            val url = URL(myUrl)
            // 1. create HttpURLConnection
            val conn = url.openConnection() as HttpsURLConnection
            conn.requestMethod = "POST"
            conn.setRequestProperty("Content-Type", "application/json; charset=utf-8")

            // 2. build JSON object
            val jsonObject = buidJsonObject()

            // 3. add JSON content to POST request body
            setPostRequestContent(conn, jsonObject)

            // 4. make POST request to the given URL
            conn.connect()

            // 5. return response message
            conn.responseMessage + ""
        }
        return result
    }

    private fun checkNetworkConnection(): Boolean {
        val connMgr = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager

        val networkInfo = connMgr.activeNetworkInfo
        val isConnected: Boolean = if(networkInfo != null) networkInfo.isConnected() else false
        if (networkInfo != null && isConnected) {
            // show "Connected" & type of network "WIFI or MOBILE"
            tvIsConnected.text = "Connected " + networkInfo.typeName
            // change background color to red
            tvIsConnected.setBackgroundColor(-0x8333da)
        } else {
            // show "Not Connected"
            tvIsConnected.text = "Not Connected"
            // change background color to green
            tvIsConnected.setBackgroundColor(-0x10000)
        }
        return isConnected
    }

    @Throws(JSONException::class)
    private fun buidJsonObject(): JSONObject {

        val jsonObject = JSONObject()
        jsonObject.accumulate("title", etTitle.getText().toString())
        jsonObject.accumulate("url", etUrl.getText().toString())
        jsonObject.accumulate("tags", etTags.getText().toString())

        return jsonObject
    }

    @Throws(IOException::class)
    private fun setPostRequestContent(conn: HttpURLConnection, jsonObject: JSONObject) {

        val os = conn.outputStream
        val writer = BufferedWriter(OutputStreamWriter(os, "UTF-8"))
        writer.write(jsonObject.toString())
        Log.i(MainActivity::class.java.toString(), jsonObject.toString())
        writer.flush()
        writer.close()
        os.close()
    }
}

(5) Run & Check

Source Code @ GitHub

Exploring Kotlin

Less than 100 pages covering Kotlin syntax and features in straight and to the point explanation.