Using gson to read simple JSON from an URL on Android

Question

I am trying to read very simple JSON from an URL (https://link.com/json.php) that looks like this

{
    "code":"asdfg"
}

I have tried many articles online, and this is what I could do, but it has does not run successfully and crashes the Android application. Is there something I'm doing wrong?

static class Page {
String code;
}

public static String checkIp() throws Exception {
String json = readUrl("https://link.com/json.php");
Gson gson = new Gson();
Page page = gson.fromJson(json, Page.class);
Log.d("MainActivity", page.code);
return (page.code);
}

private static String readUrl(String urlString) throws Exception {
BufferedReader reader = null;
try {
    URL url = new URL(urlString);
    reader = new BufferedReader(new InputStreamReader(url.openStream()));
    StringBuffer buffer = new StringBuffer();
    int read;
    char[] chars = new char[1024];
    while ((read = reader.read(chars)) != -1)
        buffer.append(chars, 0, read);

    return buffer.toString();
} finally {
    if (reader != null)
        reader.close();
}
}

Errors:

FATAL EXCEPTION: main
   Process: com.a.b.c, PID: 26961
   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.a.b.c/com.a.b.c.MainActivity}: android.os.NetworkOnMainThreadException
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2659)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2724)
       at android.app.ActivityThread.-wrap12(ActivityThread.java)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1473)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:154)
       at android.app.ActivityThread.main(ActivityThread.java:6123)
       at java.lang.reflect.Method.invoke(Native Method)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
    Caused by: android.os.NetworkOnMainThreadException
       at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1317)
       at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:86)
       at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:74)
       at java.net.InetAddress.getAllByName(InetAddress.java:752)
       at com.android.okhttp.internal.Network$1.resolveInetAddresses(Network.java:29)
       at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:187)
       at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:156)
       at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:98)
       at com.android.okhttp.internal.http.HttpEngine.createNextConnection(HttpEngine.java:345)
       at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:328)
       at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:246)
       at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:457)
       at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:405)
       at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:243)
       at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
       at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java)
       at com.a.b.c.MainActivity.parseJson(MainActivity.java:64)
       at com.a.b.c.MainActivity.onCreate(MainActivity.java:54)
       at android.app.Activity.performCreate(Activity.java:6672)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1140)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2612)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2724) 
       at android.app.ActivityThread.-wrap12(ActivityThread.java) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1473) 
       at android.os.Handler.dispatchMessage(Handler.java:102) 
       at android.os.Looper.loop(Looper.java:154) 
       at android.app.ActivityThread.main(ActivityThread.java:6123) 
       at java.lang.reflect.Method.invoke(Native Method) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)

Please post what type of error you get so we will be able to help you. — Supun Amarasinghe, Nov 12 '17 at 16:39
@KalharaAmarasinghe I included the errors on my question. Sorry about that — FrankFabregat3, Nov 12 '17 at 17:19

Shamsul Arafin Mahtab · Accepted Answer · 2017-11-12T17:39:09.857

To use GSON in android, remember to provide it on your build.gradle

dependencies {
    compile 'com.google.code.gson:gson:2.8.2'
}

Your GSON code here

static class Page {
    String code;
}

public static String checkIp() throws Exception {
    String json = readUrl("https://link.com/json.php");

    GsonBuilder gsonBuilder = new GsonBuilder();
    Gson gson = gsonBuilder.create();

    Page page = gson.fromJson(json, Page.class);
    Log.d("MainActivity", page.code);
    return Page.code;
}

I have just modified your GSON code. Your below method readURL is on your own. I hope that method has no problem. I usually use Volley Library to parse JSON. If you further face any problem you can use any HTTP request library like Volley.

Using gson to read simple JSON from an URL on Android

1 Answers1