Java™ - Retrieve Contents of an Email Remotely with Gmail SMTPS

What Will I Learn?

In this tutorial, I will continue my previous lesson about how to manipulate email via Java™ application that all classes are available in the javax.mail package. Previously we already know how to send an email with attachments, now we will try to retrieve and read all existing emails on our Gmail account.

• Java™ I/O Stream
• Java™ Mail
• Java™ Functional Interfaces

Requirements

• Any current JDK.
• IntelliJ IDEA Community.
• Target google account.
  Before we get started, make sure to change the Allow less secure apps: ON setting on the target account here.
  

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Difficulty

• Basic

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Tutorial Contents

1. Prepare Connection To the Target Folder

   In this case, we will retrieve and read all emails in the Inbox folder.

   
      public static void retrieve(String username, String password) {
          Session session = Session.getDefaultInstance(GMAIL);
          try {
              Store store = session.getStore("imaps");
              store.connect(GMAIL.getProperty("mail.smtp.host"), username, password);
              Folder inbox = store.getFolder("Inbox");
              inbox.open(Folder.READ_ONLY);
              /*
               * Right now, the folder is ready
               */
              {
                  inbox.getMessages();
              }
              inbox.close(false);
              store.close();
          }
          catch(Throwable e) { e.printStackTrace(); }
      }
      

2. Read Headers of Email

   We will read From, To, Cc, Bcc and the Subject of Email.

   
      private static void print(Part part) throws Throwable {
          if(part instanceof Message) {
              Message message = (Message) part;
              // From
              System.out.println("From: [" + print(message.getFrom()) + "]");
              // To
              System.out.println("  To: [" + print(message.getRecipients(Message.RecipientType.TO)) + "]");
              // CC
              System.out.println("  Cc: [" + print(message.getRecipients(Message.RecipientType.CC)) + "]");
              // BCC
              System.out.println(" Bbc: [" + print(message.getRecipients(Message.RecipientType.BCC)) + "]");
              // Subject
              System.out.println("Subject: " + message.getSubject());
          }
      }
      
   
      private static String print(Address[] accounts) {
          if(accounts == null) return "";
          StringBuilder plain = new StringBuilder(accounts[0].toString());
          for(int i = 0, n = accounts.length; ++i < n;)
              plain.append(";" + accounts[i]);
          return plain.toString();
      }
      

3. Define Functions for Handling Any Types of Message Bodies

   We will handle the type of message bodies: image, multipart and plain string as the default. Next, we will try to handle the other types.
   The content of an email (usually known as Part) is available in various formats:

   • As a DataHandler - using the getDataHandler() method. The content of a Part is also available through a javax.activation.DataHandler object. The DataHandler object allows clients to discover the operations available on the content, and to instantiate the appropriate component to perform those operations.
   • As an InputStream - using the getInputStream() method. Any mail-specific encodings are decoded before this stream is returned.
   • As a Java Object - using the getContent() method. This method returns the content as a Java object. The returned object is of course dependent on the content itself. In particular, a multipart Part's content is always a Multipart or subclass thereof. That is, getContent() on a multipart type Part will always return a Multipart (or subclass) object.
     _References
     • javax.mail.Part - The common base interface for Messages and BodyParts.
     • javax.mail.Multipart - The container that holds multiple body parts. Multipart provides methods to retrieve and set its subparts.
     • javax.mail.internet.MimeMultipart - An implementation of the abstract Multipart class that uses MIME conventions for the multipart data.

   
      private static final Map<String, Function<Object, Void>> FUNCTIONS = new java.util.HashMap<>();
      private static final String IMG_JPEG = "image/jpeg";
      private static final String MULTIPART = "multipart/*";
      
   
      static {
          //Default action
          FUNCTIONS.put(null, content -> {
              System.out.println(content);
              return null;
          });
          FUNCTIONS.put(MULTIPART, content -> {
              Multipart multiPart;
              try { multiPart = (Multipart) content; }
              catch(Throwable e) {
                  System.out.println("Parsing content was unsuccessful");
                  e.printStackTrace();
                  return null;
              }
              int i, n;
              try {
                  n = multiPart.getCount();
              } catch(Throwable e) {
                  System.out.println("Getting count of this content was unsuccessful");
                  n = 0;
              }
              for(i = 0; i < n; i++)
                  try { print(multiPart.getBodyPart(i)); }
                  catch(Throwable e) { e.printStackTrace(); }
              return null;
          });
          FUNCTIONS.put(IMG_JPEG, content -> {
              InputStream in;
              try { in = (InputStream) content; }
              catch(Throwable e) {
                  System.out.println("Parsing content was unsuccessful");
                  e.printStackTrace();
                  return null;
              }
              final byte[] BYTES = new byte[2048];
              System.out.println("Enter image path and name of this current content: ");
              String name;
              try(BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
                  if((name = reader.readLine()) == null || name.length() < 1)
                      throw new UnsupportedOperationException("Name should not be empty");
              } catch(Throwable e) {
                  System.out.println("Scanning name was unsuccessful");
                  e.printStackTrace();
                  return null;
              }
              try(FileOutputStream writer = new FileOutputStream(name)) {
                  for(int length; ; writer.write(BYTES, 0, length))
                      if((length = in.read(BYTES)) < 0)
                          break;
              } catch(Throwable e) { e.printStackTrace(); }
              return null;
          });
      }
      
   
      private static void print(Part part) throws Throwable {
          if(part instanceof Message) {
              Message message = (Message) part;
              System.out.println("From: [" + print(message.getFrom()) + "]");
              System.out.println("  To: [" + print(message.getRecipients(Message.RecipientType.TO)) + "]");
              System.out.println("  Cc: [" + print(message.getRecipients(Message.RecipientType.CC)) + "]");
              System.out.println(" Bbc: [" + print(message.getRecipients(Message.RecipientType.BCC)) + "]");
              System.out.println("Subject: " + message.getSubject());
          }
          /*
           * Additional logic
           */
          if(part.isMimeType("text/plain") || part.isMimeType("message/rfc822"))
              FUNCTIONS.get(null).apply(part.getContent());
          else if(part.isMimeType(MULTIPART))
              FUNCTIONS.get(MULTIPART).apply(part.getContent());
          else if(part.isMimeType(IMG_JPEG) || part.getContentType().contains("image/"))
              FUNCTIONS.get(IMG_JPEG).apply(part.getContent());
          else {
              Object content = part.getContent();
              if(content instanceof InputStream) {
                  InputStream in = (InputStream) content;
                  for(int c; (c = in.read()) > -1; )
                      System.out.write(c);
              } else
                  System.out.println(content.toString());
              System.out.println("*** THE END ***");
          }
          
      }
      

Testing

_Main

public static void main(String[] args) {
    String username = "rezanas************";
    String password = "*******************";
    retrieve(username, password);
}


public static void retrieve(String username, String password) {
    Session session = Session.getDefaultInstance(GMAIL);
    try {
        Store store = session.getStore("imaps");
        store.connect(GMAIL.getProperty("mail.smtp.host"), username, password);
        Folder inbox = store.getFolder("Inbox");
        inbox.open(Folder.READ_ONLY);
        /*
         * Listen input from the user
         */
        String line;
        int i = 0, n = inbox.getMessageCount();
        try(BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            do {
                print(inbox.getMessage(n -= (i + 1)));
                System.out.print("To stop, type \"0\" (zero) or type any positive number to jump: ");
                line = reader.readLine();
                try{ i = Integer.parseInt(line); }
                catch(Throwable e) { i = 0; }
            }
            while(!line.equals("0"));
        }
        inbox.close(false);
        store.close();
    }
    catch(Throwable e) { e.printStackTrace(); }
}

_Result

Final Source Code

public class Mail {
    public static void main(String[] args) {
        String username = "rezanas***********";
        String password = "******************";
        retrieve(username, password);
    }
    

    public static void retrieve(String username, String password) {
        Session session = Session.getDefaultInstance(GMAIL);
        try {
            Store store = session.getStore("imaps");
            store.connect(GMAIL.getProperty("mail.smtp.host"), username, password);
            Folder inbox = store.getFolder("Inbox");
            inbox.open(Folder.READ_ONLY);
            String line;
            int i = 0, n = inbox.getMessageCount();
            try(BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
                do {
                    print(inbox.getMessage(n -= (i + 1)));
                    System.out.print("To stop, type \"0\" (zero) or type any positive number to jump: ");
                    line = reader.readLine();
                    try{ i = Integer.parseInt(line); }
                    catch(Throwable e) { i = 0; }
                }
                while(!line.equals("0"));
            }
            inbox.close(false);
            store.close();
        }
        catch(Throwable e) { e.printStackTrace(); }
    }
    

    private static final Map<String, Function<Object, Void>> FUNCTIONS = new java.util.HashMap<>();
    private static final String IMG_JPEG = "image/jpeg";
    private static final String MULTIPART = "multipart/*";
    

    static {
        //Default action
        FUNCTIONS.put(null, content -> {
            System.out.println(content);
            return null;
        });
        FUNCTIONS.put(MULTIPART, content -> {
            Multipart multiPart;
            try { multiPart = (Multipart) content; }
            catch(Throwable e) {
                System.out.println("Parsing content was unsuccessful");
                e.printStackTrace();
                return null;
            }
            int i, n;
            try {
                n = multiPart.getCount();
            } catch(Throwable e) {
                System.out.println("Getting count of this content was unsuccessful");
                n = 0;
            }
            for(i = 0; i < n; i++)
                try { print(multiPart.getBodyPart(i)); }
                catch(Throwable e) { e.printStackTrace(); }
            return null;
        });
        FUNCTIONS.put(IMG_JPEG, content -> {
            InputStream in;
            try { in = (InputStream) content; }
            catch(Throwable e) {
                System.out.println("Parsing content was unsuccessful");
                e.printStackTrace();
                return null;
            }
            final byte[] BYTES = new byte[2048];
            System.out.println("Enter image path and name of this current content: ");
            String name;
            try(BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
                if((name = reader.readLine()) == null || name.length() < 1)
                    throw new UnsupportedOperationException("Name should not be empty");
            } catch(Throwable e) {
                System.out.println("Scanning name was unsuccessful");
                e.printStackTrace();
                return null;
            }
            try(FileOutputStream writer = new FileOutputStream(name)) {
                for(int length; ; writer.write(BYTES, 0, length))
                    if((length = in.read(BYTES)) < 0)
                        break;
            } catch(Throwable e) { e.printStackTrace(); }
            return null;
        });
    }
    

    private static void print(Part part) throws Throwable {
        if(part instanceof Message) {
            Message message = (Message) part;
            System.out.println("From: [" + print(message.getFrom()) + "]");
            System.out.println("  To: [" + print(message.getRecipients(Message.RecipientType.TO)) + "]");
            System.out.println("  Cc: [" + print(message.getRecipients(Message.RecipientType.CC)) + "]");
            System.out.println(" Bbc: [" + print(message.getRecipients(Message.RecipientType.BCC)) + "]");
            System.out.println("Subject: " + message.getSubject());
        }
        if(part.isMimeType("text/plain") || part.isMimeType("message/rfc822"))
            FUNCTIONS.get(null).apply(part.getContent());
        else if(part.isMimeType(MULTIPART))
            FUNCTIONS.get(MULTIPART).apply(part.getContent());
        else if(part.isMimeType(IMG_JPEG) || part.getContentType().contains("image/"))
            FUNCTIONS.get(IMG_JPEG).apply(part.getContent());
        else {
            Object content = part.getContent();
            if(content instanceof InputStream) {
                InputStream in = (InputStream) content;
                for(int c; (c = in.read()) > -1; )
                    System.out.write(c);
            } else
                System.out.println(content.toString());
            System.out.println("*** THE END ***");
        }
    }
    

    private static String print(Address[] accounts) {
        if(accounts == null) return "";
        StringBuilder plain = new StringBuilder(accounts[0].toString());
        for(int i = 0, n = accounts.length; ++i < n;)
            plain.append(";" + accounts[i]);
        return plain.toString();
    }
}

Thank you!

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Curriculum

• Java™ - How to Zip the Path (Files or Directory) and Send Them as Attachment Over Network Using Gmail SMTP

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Posted on Utopian.io - Rewarding Open Source Contributors

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