App crashes and gives NullPointerException error when trying to set up Gmail API for Android

Question

I've been trying to set up gmail api for an android app but it has been unsuccessful. I'm running this quickstart code on the official doc Java Quickstart

Following the three steps on the page, the Gmail api has been enabled in console, credentials.json file has also been downloaded and added to assets folder in android studio

When I run the quickstart code which is suppose to display the labels in my email, the app crashes and i get this error

Process: com.example.receiveemail, PID: 20339
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.receiveemail/com.example.receiveemail.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2724)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2789)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1527)
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:203)
    at android.app.ActivityThread.main(ActivityThread.java:6251)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)
 Caused by: java.lang.NullPointerException
    at com.google.api.client.repackaged.com.google.common.base.Preconditions.checkNotNull(Preconditions.java:770)
    at com.google.api.client.util.Preconditions.checkNotNull(Preconditions.java:127)
    at com.google.api.client.auth.oauth2.AuthorizationCodeFlow$Builder.setTransport(AuthorizationCodeFlow.java:545)
    at com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow$Builder.setTransport(GoogleAuthorizationCodeFlow.java:254)
    at com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow$Builder.setTransport(GoogleAuthorizationCodeFlow.java:152)
    at com.google.api.client.auth.oauth2.AuthorizationCodeFlow$Builder.<init>(AuthorizationCodeFlow.java:494)
    at com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow$Builder.<init>(GoogleAuthorizationCodeFlow.java:195)
    at com.example.receiveemail.MainActivity.getCredentials(MainActivity.java:111)
    at com.example.receiveemail.MainActivity.onCreate(MainActivity.java:69)
    at android.app.Activity.performCreate(Activity.java:6666)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2677)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2789) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1527) 
    at android.os.Handler.dispatchMessage(Handler.java:110) 
    at android.os.Looper.loop(Looper.java:203) 
    at android.app.ActivityThread.main(ActivityThread.java:6251) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)

MainActivity

package com.example.receiveemail;

import android.content.res.Resources;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.Label;
import com.google.api.services.gmail.model.ListLabelsResponse;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.security.GeneralSecurityException;
import java.util.Collections;
import java.util.List;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.security.GeneralSecurityException;
import java.util.Collections;
import java.util.List;
public class MainActivity extends AppCompatActivity {

private static final String TAG = "MainActivity";


private static final String APPLICATION_NAME = "Gmail API Java Quickstart";
private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();
private static final String TOKENS_DIRECTORY_PATH = "tokens";

/**
 * Global instance of the scopes required by this quickstart.
 * If modifying these scopes, delete your previously saved tokens/ folder.
 */
private static final List<String> SCOPES = Collections.singletonList(GmailScopes.GMAIL_LABELS);
private static final String CREDENTIALS_FILE_PATH = "/assets/credentials.json";


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

    // Build a new authorized API client service.
    NetHttpTransport HTTP_TRANSPORT = null;

    try {
        HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
    } catch (GeneralSecurityException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    Gmail service = null;

    try {
        service = new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, getCredentials(HTTP_TRANSPORT))
                .setApplicationName(APPLICATION_NAME)
                .build();
    } catch (IOException e) {
        e.printStackTrace();
    }


    // Print the labels in the user's account.
    String user = "me";
    ListLabelsResponse listResponse = null;

    try {
        listResponse = service.users().labels().list(user).execute();
    } catch (IOException e) {
        e.printStackTrace();
    }

    List<Label> labels = listResponse.getLabels();
    if (labels.isEmpty()) {
        Log.v(TAG, "No labels found.");

    } else {
        Log.v(TAG, "Labels:");
        for (Label label : labels) {
            Log.v(TAG, label.getName());
        }
    }
}

/**
 * Creates an authorized Credential object.
 * @param HTTP_TRANSPORT The network HTTP Transport.
 * @return An authorized Credential object.
 * @throws IOException If the credentials.json file cannot be found.
 */
private static Credential getCredentials(final NetHttpTransport HTTP_TRANSPORT) throws IOException {
    // Load client secrets.
    InputStream in = MainActivity.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
    GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));

    // Build flow and trigger user authorization request.
    GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
            HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
            .setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
            .setAccessType("offline")
            .build();
    LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(8888).build();
    return new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");
}
}

build.gradle

apply plugin: 'com.android.application'

android {
compileSdkVersion 28
defaultConfig {
    applicationId "com.example.receiveemail"
    minSdkVersion 15
    targetSdkVersion 28
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
    }
 }
}

dependencies {
  implementation fileTree(dir: 'libs', include: ['*.jar'])
  implementation 'com.android.support:appcompat-v7:28.0.0'
  implementation 'com.android.support.constraint:constraint-layout:1.1.3'
  testImplementation 'junit:junit:4.12'
  androidTestImplementation 'com.android.support.test:runner:1.0.2'
  implementation 'com.google.android.gms:play-services-auth:15.0.1'
  implementation 'pub.devrel:easypermissions:0.3.0'
  implementation('com.google.api-client:google-api-client-android:1.23.0') {
    exclude group: 'org.apache.httpcomponents'
  }
  implementation('com.google.apis:google-api-services-gmail:v1-rev98-1.25.0') {
    exclude group: 'org.apache.httpcomponents'
  }
  implementation 'com.google.oauth-client:google-oauth-client-jetty:1.23.0'

  androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

 android {
   packagingOptions {
     exclude 'META-INF/DEPENDENCIES'
  }
}

Here is the API page for Android G Suite APIs for Android but it doesn't provide any instruction on how to set up the api, only the dependencies to include

What is line 111 in MainActivity? Something there is null that shouldn't be — Nathan Headley
GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in)); — johnson
i'm thinking maybe the credential.json file is not properly linked. Can you help me check if the file path is correct. I mean the private static final String CREDENTIALS_FILE_PATH — johnson

Charan M Charan M · Accepted Answer · 2019-01-22T15:03:59

The documentation provided for Java doesn't seem to work well for Android. I faced the same issues while working on G-Mail API. Use this piece of code to get credential and service:

mCredential = GoogleAccountCredential.usingOAuth2(getApplicationContext(), Arrays.asList(SCOPES)).setBackOff(new ExponentialBackOff());
HttpTransport transport = AndroidHttp.newCompatibleTransport();
JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
mService = new com.google.api.services.gmail.Gmail.Builder(
        transport, jsonFactory, credential)
        .setApplicationName(mContext.getResources().getString(R.string.app_name))
        .build();

Further, you can't run this code on the main thread since it involves network operations. Use AsyncTask to execute this code. I used this github project as a guide to integrate Gmail API into my Android app.

App crashes and gives NullPointerException error when trying to set up Gmail API for Android

1 Answers