尝试为Android设置Gmail API时，应用程序崩溃并给出NullPointerException错误 [英] App crashes and gives NullPointerException error when trying to set up Gmail API for Android
本文介绍了尝试为Android设置Gmail API时，应用程序崩溃并给出NullPointerException错误的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我一直在尝试为Android应用设置gmail api，但未成功.我正在官方文档上运行此快速入门代码 Java快速入门
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
按照页面上的三个步骤，已在控制台中启用了Gmail api，还已下载了notes.json文件并将其添加到android studio中的资产文件夹中
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
当我运行假定在电子邮件中显示标签的快速入门代码时，应用程序崩溃，并且出现此错误
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
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
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的API页面适用于Android的G Suite API ，但是它没有提供有关如何设置api的任何说明，仅包含要包含的依赖项
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
推荐答案
为Java提供的文档似乎不适用于Android.在使用G-Mail API时，我遇到了同样的问题.使用这段代码来获取证书和服务:
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();
此外，您无法在主线程上运行此代码，因为它涉及网络操作.使用AsyncTask执行此代码.我使用了 github项目作为将Gmail API集成到我的Android应用中的指南.
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.
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