Spring Data- MongoRepository：我们可以将二进制类型用作_id吗？

Question

I have a class that represents a Mongo collection called "archive"

@Document(collection = "archive")

public class Message{
    @Id
    private byte[] messageId;

    private String from;
    private String to;
    // more stuff

}

The interface MessagesRepository extends MongoRepository:

public interface MessagesRepository extends MongoRepository<Message, String>{

}

Through an API call, I get a findMessage request, which gives me a messageId in String . I will then need to encode it into byte[] and then invoke messagesRepository.findOne() method. ( Remember the ID is byte[] ).

It fails. It returns null . I guess because the byte [] stored in Mongo would be different from the byte[] within the findOne() method since different strings even with same values produce different byte[] arrays.

How can I make this work? Or is it really possible to work with _id in binary?

Answer 1

you are not doing yourself a favor by using byte[] as an id.

you say you get a messageId as a String - why not use that? otherwise you might just end up casting between string and bytes types (unless the use of byte[] is not by your own choice, but an constraint given to you). you noticed yourself that the bytes generated even from equal strings don't match.

short example:

public static void main(String[] args) throws Exception {
    String s1 = "hello";
    String s2 = "hello";

    System.out.println("our two strings: ");
    System.out.println(s1);
    System.out.println(s2);

    // one of the first thing we learned when starting with
    // java was that this is not equal
    System.out.println();
    System.out.println("compare with ==");
    if(s1==s2) System.out.println("equal");
    else System.out.println("not equal");

    // but this is equal
    System.out.println("compare with equals()");
    if(s1.equals(s2)) System.out.println("equal");
    else System.out.println("not equal");

    // create the byte arrays and compare them
    byte[] b1 = s1.getBytes();
    byte[] b2 = s2.getBytes();

    System.out.println();
    System.out.println("byte array 1: " + b1.toString());
    System.out.println("byte array 2: " + b2.toString());

    // same as for strings
    System.out.println("compare with ==");
    if(b1==b2) System.out.println("equal");
    else System.out.println("not equal");

    // not equal, unlike the strings from which we
    // created the byte arrays
    System.out.println("compare with equals()");
    if(b1.equals(b2)) System.out.println("equal");
    else System.out.println("not equal");


    // create string out of the bytes again and compare
    String ss1 = new String(b1, "UTF-8");
    String ss2 = new String(b2, "UTF-8");

    System.out.println();
    System.out.println("re-created string 1: " + ss1);
    System.out.println("re-created string 2: " + ss2);

    // this is equal again
    System.out.println("compare re-created strings with equals()");
    if(ss1.equals(ss2)) System.out.println("equal");
    else System.out.println("not equal");
}

that's why i ask why it has to be bytes; it's making everything HARDER, not easier.

Answer 2

Well, it still worked. I was able to have a byte[] as the _id . I was able to successfully insert and retrieve stuff. The problem with find() is in MongoRepository. It needs to be adjusted to make it work.

See : http://pastebin.com/xHVQJYfN

EDIT : I got the latest driver and it worked straight away.