Android Javamail Attachment throws java.lang.NullPointerExce

Question

大家好，我正在做一个电子邮件发送器应用程序发送电子邮件正常但是当我试图添加附件时它显示NullPointerException

这里是错误

W/System.err: java.lang.NullPointerException
    W/System.err:     at com.example.milingona.anew.MainActivity.addAttachment(MainActivity.java:120)
    W/System.err:     at com.example.milingona.anew.MainActivity.onActivityResult(MainActivity.java:96)
    W/System.err:     at android.app.Activity.dispatchActivityResult(Activity.java:5322)
    W/System.err:     at android.app.ActivityThread.deliverResults(ActivityThread.java:3363)
    W/System.err:     at android.app.ActivityThread.handleSendResult(ActivityThread.java:3410)
    W/System.err:     at android.app.ActivityThread.access$1100(ActivityThread.java:141)
    W/System.err:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1304)
    W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:99)
    W/System.err:     at android.os.Looper.loop(Looper.java:137)
    W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5103)
    W/System.err:     at java.lang.reflect.Method.invokeNative(Native Method)
    W/System.err:     at java.lang.reflect.Method.invoke(Method.java:525)
    W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
    W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    W/System.err:     at dalvik.system.NativeStart.main(Native Method)

这里是代码

import android.Manifest;
import android.app.ProgressDialog;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Environment;
import android.provider.MediaStore;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.format.DateFormat;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import java.io.File;
import java.io.UnsupportedEncodingException;
import java.util.Date;
import java.util.Properties;
import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.BodyPart;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
public class MainActivity extends AppCompatActivity {
    Button btn;
    ImageView imageView;
    String root;
    String imageFolderPath;
    String imageName;
    Uri fileUri;
    private static final String username = "[e-mail]";
    private static final String password = "*********";
    private Multipart _multipart;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn = (Button) findViewById(R.id.captureImage);
        imageView= (ImageView) findViewById(R.id.imageView);
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[] { Manifest.permission.CAMERA, Manifest.permission.WRITE_EXTERNAL_STORAGE, Manifest.permission.ACCESS_NETWORK_STATE, Manifest.permission.INTERNET, Manifest.permission_group.STORAGE }, 0);
        }
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                root = Environment.getExternalStorageDirectory()
                        + "/ANew";
                // Creating folders for Image
                imageFolderPath = root + "/Images";
                File imagesFolder = new File(imageFolderPath);
                imagesFolder.mkdirs();
                Date d = new Date();
                CharSequence s = DateFormat.format("hh-mm-ss", d.getTime());
                // Generating file name
                imageName = "img-" + s + ".jpg";
                // Creating image here
                File image = new File(imageFolderPath, imageName);
                fileUri = Uri.fromFile(image);
                Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                takePictureIntent.putExtra(MediaStore.EXTRA_OUTPUT, fileUri);
                startActivityForResult(takePictureIntent, 1);
            }
        });
    }
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        try {
            addAttachment(fileUri.getPath());
        } catch (Exception e) {
            e.printStackTrace();
        }
sendMail("[e-mail]","Subject","MESSAGE");
    }
    private void sendMail(String email, String subject, String messageBody) {
        Session session = createSessionObject();
        try {
            Message message = createMessage(email, subject, messageBody, session);
            new SendMailTask().execute(message);
        } catch (MessagingException | UnsupportedEncodingException e) {
            e.printStackTrace();
        }
    }
    public void addAttachment(String filename) throws Exception {
        BodyPart messageBodyPart = new MimeBodyPart();
        DataSource source = new FileDataSource(filename);
        messageBodyPart.setDataHandler(new DataHandler(source));
        messageBodyPart.setFileName(filename);
        _multipart.addBodyPart(messageBodyPart);
    }
    private Message createMessage(String email, String subject, String messageBody, Session session) throws MessagingException, UnsupportedEncodingException {
        Message message = new MimeMessage(session);
        message.setFrom(new InternetAddress("[e-mail]", "Alban Gashi"));
        message.addRecipient(Message.RecipientType.TO, new InternetAddress(email, email));
        message.setSubject(subject);
        message.setText(messageBody);
        return message;
    }
    private Session createSessionObject() {
        Properties properties = new Properties();
        properties.put("mail.smtp.auth", "true");
        properties.put("mail.smtp.starttls.enable", "true");
        properties.put("mail.smtp.host", "smtp.gmail.com");
        properties.put("mail.smtp.port", "587");
        return Session.getInstance(properties, new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });
    }
    private class SendMailTask extends AsyncTask<Message, Void, Void> {
        private ProgressDialog progressDialog;
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressDialog = ProgressDialog.show(MainActivity.this, "Please wait", "Sending mail", true, false);
        }
        @Override
        protected void onPostExecute(Void aVoid) {
            super.onPostExecute(aVoid);
            progressDialog.dismiss();
        }
        @Override
        protected Void doInBackground(Message... messages) {
            try {
                Transport.send(messages[0]);
            } catch (MessagingException e) {
                e.printStackTrace();
            }
            return null;
        }
    }
}

顺便说一下，我在manifest

中添加了权限

<uses-feature android:name="android.hardware.camera2"></uses-feature>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.INTERNET"/>

Answer 1

您没有在任何地方初始化_multipart变量，因此当您试图访问MainActivity的第120行.addBodyPart方法时，您会得到NullPointerException。