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I would like to create an application that saves and retrieves records to the GAE server. I followed the tutorial "Deploying to Google App Engine" http://code.google.com/webtoolkit/doc/latest/tutorial/appengine.html to get started.

I have the StockWatcher application working now, but in my application I need to store a String that can be large (>10KB). I read that I can't use a Java String type to store large strings and need to use the Text data type instead.

I think by Text, they mean: com.google.appengine.api.datastore.Text, but it would be nice to confirm this is correct. ???

Regardless, I can't get Text to work. After some research it appears both the types Key and Text can only be used in the server code and not the client code. It seems that this is because the source code is not available for these classes and GWT needs the source to create the JavaScript code on the client's computer. At least that my current working hypothesis as to why I'm getting the following errors:

21:52:52.823 [ERROR] [myapp] Line 15: The import com.google.appengine.api.datastore cannot be resolved
21:52:52.951 [ERROR] [myapp] Line 103: Key cannot be resolved to a type
21:52:53.011 [ERROR] [myapp] Line 106: Text cannot be resolved to a type

I use the following fields in a class in a shared folder.

shared/MyDataRecord

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key id;

@Persistent
private Text description;

MyDataRecord class in a shared folder because I wanted to use to send back all the fields in one get method return rather than multiple individual field get methods. Here's how I use MyDataRecord class in my server/DataServiceImpl.java class

public class DataServiceImpl extends RemoteServiceServlet implements DataService
{
...
  @Override
  public MyDataRecord getDataRecord() throws NotLoggedInException
  {
    ...

I've seen some posted solutions suggest using non-standard, 3rd party libraries, like http://www.resmarksystems.com/code/. I couldn't get this one installed, but even if I could, I'd prefer a different solution. Storing Text must be such a common task that I'd prefer to solve this using what is considered a standard solution.

I could change my code to return each field in multiple get methods instead of an single return of a MyDataRecord instance. However, even if that works, that would be significantly more work and more difficult to maintain over time. However, if this is what is normally expected, then that's what I'll do.

I'd like to solve this using what is considered best practices by GWT and GAE. A simple example or tutorial would go a long way, but I can't find one.

Are there example programs/tutorials that show what GWT considers best practices for storing and retrieving large strings?

I am a newbie with both GWT and GAE (as well as web development), please consider this in any responses, thanks.

No Snark Please

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  • An example would be the easiest way to answer this. Commented Aug 30, 2011 at 3:36

2 Answers 2

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The serializable POJO. Note the NotPersistent annotation for description

package com.my.project.shared;

@PersistenceCapable(identityType=IdentityType.APPLICATION,detachable="true")
public class MyParent implements Serializable {

    @PrimaryKey
    @Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY)
    private Long id;
    @NotPersistent //Note the NotPersistent annotation. GAE won't persist this value in big table
    private String description;

}

The second POJO. Notice the package

package com.my.project.server;

@PersistenceCapable(identityType=IdentityType.APPLICATION,detachable="true")
public class MyChild implements Serializable{//Not really required to implement Serializable

    @PrimaryKey
    @Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY)
    private Long id;
    @Persistent
    private Long parentID;//Reference to the MyParent
    @Persistent
    private Text description;//The actual value of the description variable.
}

Notice the parent ID mapped in the child. While retrieving you will need to identify which child belongs to which parent. In pseudo code 1) Load parent from DB 2) Identify child for this parent, and load it 3) Convert child.description->parent.description 4) Now you have a fully constructed parent POJO which is serializable. Send it to the UI

Just reverse the procedure on the way back from UI to GAE.

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4 Comments

Thanks. That's clear-er. After looking at your suggestion, I may see 2 possible solutions, depending if I understand what you are suggesting. Both solutions require splitting the class into 2 classes, but in different ways.
One way, splits the class by placing all the data members that can only be used on the server, like Text and Key, into one class. In the other class put all the data members that can be shared on both the client and the server.
The other way is to split the classes by duplicating all data members in each classes. Just like the first way, use a Long to store a Key and a String to store Text in the shared class. The shared class will be a POJO as it will be not be persisted (No @Persistent Data). You will need to be able to convert between each class. The defined client's interfaces (DataService.java and DataServiceAsync.java) will only used the shared classes. The server's class (DataServiceImpl.java) implementation will use the user defined conversions when reading and writing to the database.
Without a full example, I may be misunderstanding what you are saying, so maybe I still don't understand. Since I have no experience using GAE, there may be other considerations I'm missing. Also, since I haven't tried either solution yet, there may be problems I can't foresee. I'd think the 2 full class solution would be better since it means only saving one record in the database per class instance.
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1) Define a NotPersistent field in your serializable POJO private String description 2) Define a new POJO on the server side which will have private Text description 3) When you persist/load the original POJO, retrieve the new POJO and populate the String description from the Text description

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3 Comments

So make 2 classes? One that is implements the java.io.Serializable interface and is a POJO. Another class that has @Persistent private Text description; and write code that converts both back and forth between all the fields? Seems like a lot of work to maintain 2 classes, but if that's what is expected, that's what I'll do. Thanks.
No. The second class with only has an ID which maps to the UID of the 1st class and the Text description. No need to have a full copy of the 1st POJO. Just abstract the Text
Okay, I don't understand what you are saying. Can you point to an example programs/tutorials that shows what you are saying?

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