尝试为 Android 设置 Gmail API 时，应用程序崩溃并出现 NullPointerException 错误

Question

我一直在尝试为 android 应用程序设置 gmail api，但没有成功。我在官方文档中运行ning 这个快速入门代码 Java Quickstart

按照页面上的三个步骤，Gmail api 已在控制台中启用，credentials.json 文件也已下载并添加到 android studio 的资产文件夹中

当我运行本应在我的电子邮件中显示标签的快速启动代码时，应用程序崩溃并且出现此错误

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)

主要活动

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'
  }
}

这是 Android G Suite API 的 API 页面 Android 但它没有提供任何关于如何设置 api 的说明，只提供了要包含的依赖项

Answer 1

为 Java 提供的文档似乎不适用于 Android。我在使用 G-Mail API 时遇到了同样的问题。使用这段代码获取凭证和服务：

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();

此外，您不能运行此代码在主线程上，因为它涉及网络操作。使用 AsyncTask 来执行这段代码。我使用此 github project 作为将 Gmail API 集成到我的 Android 应用程序的指南。

尝试为 Android 设置 Gmail API 时，应用程序崩溃并出现 NullPointerException 错误

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

java

android

gmail-api