使用Gson从MongoDB解析日期

Question

In my MongoDB a document looks like this:

{ 
"_id" : ObjectId("5613700bc00eac21886b6a51"), 
"firstname" : "Marc", 
"lastname" : "Anonymous", 
"email" : "marc.XXXXXX@hotmail.com", 
"phone" : "+41/12/345678", 
"timestamp" : ISODate("2015-10-06T06:54:03.905+0000"), 
"state" : "waiting"
}

I am using gson to parse the json into my Java class User which has a variable timestamp and is a Date but I get the following error:

Caused by: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 179 path $.timestamp

I insert the timestamp into MongoDb via Date too. I don't know how to handle this. Should I just use a String and convert it every time I need in in Date form?

public class User {

public String firstname;
public String lastname;
public String email;
public String phone;
public Date timestamp;
public String state;

public Date getTimestamp() {
    return timestamp;
}

public void setTimestamp(Date timestamp) {
    this.timestamp = timestamp;
} 
//...

Edit

I get the json via the MongoDb Java driver. The error occurs when I want to deserialize the String with gson

public ArrayList<User> findUserByEmail(String email) {
    Gson g = new Gson();
    ArrayList<User> l = new ArrayList<>();
    MongoDatabase db = con.getDatabase("waitinglist");
    MongoCollection col = db.getCollection("users");

    MongoCursor<Document> f = col.find(eq("email", email)).iterator();
    while (f.hasNext()) {
        Document d = f.next();
        System.out.println(d.toJson());
        l.add(g.fromJson(d.toJson(), User.class));
    }
    return l;
}

d.toJson() returns

{
"_id": {
"$oid": "5613700bc00eac21886b6a51"
},
"firstname": "Marc",
"lastname": "xxxx",
"email": "marc.xxxxxx@hotmail.com",
"phone": "+41/12/345678",
"timestamp": {
   "$date": 1444114443905
 },
"state": "waiting"
}

The error is being thrown because the Date gets convertetd into an own document and when I want to deseriazable it with Gson the Json does not fit with my class.

Answer 1

You can solve this problem by using Jongo and changing your code to the following:

public ArrayList<User> findUserByEmail(String email) {
    Gson g = new Gson();
    ArrayList<User> l = new ArrayList<>();
    Jongo jongo = con.getDB("waitinglist");

    Iterator<User> users = jongo.getCollection("users").find("{email: #}", email).as(User.class).iterator();
    while(users.hasNext()){
        l.add(users.next());
    }
    return l;
}

Otherwise you're going to have to do something like the following...

Make a wrapper class like this

public static class GsonCompatibleDate {
    @SerializedName("$date")
    public Long date;

    public GsonCompatibleDate(Long date) {
        this.date = date;
    }

    public Date getDate() {
        return new Date(date);
    }

    public void setDate(Date date) {
        this.date = date.getTime();
    }
}

• Note the annotation @SerializedName("$date") . This just lets you use a nicer Java-style name for your variable;

Update your POJO to this or something similar:

public class User {

    public String firstname;
    public String lastname;
    public String email;
    public String phone;
    public GsonCompatibleDate timestamp;
    public String state;

    public Date getTimestamp() {
          return timestamp.getDate();
    }
}

Your de-serialization code would stay exactly as it was before. The updated POJO now matches your Mongo JSON output.

I tested both methods and they work for me. I also highly recommend using a library like Jongo, Morphia, etc... instead of using the Mongo driver itself, unless absolutely necessary.