CODEWITHSUNDEEP
google-app-enginepersistence
Arun
Arun

Reputation: 1789

Appengine/Java - persisting a String

My aim is to persist a String to Datastore

final static PersistenceManager pm = PMF.get().getPersistenceManager();
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException
    {
        String s = "test";
        System.out.println(pm.makePersistent(s));
        PrintWriter pw = resp.getWriter();
        pw.write("Data Saved");
    }

Not sure about the reason for following ERROR

The class "The class "java.lang.String" is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data/annotations for the class are not found." is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data for the class is not found.

Caused by:

org.datanucleus.jdo.exceptions.ClassNotPersistenceCapableException: The class "The class "java.lang.String" is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data/annotations for the class are not found." is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data for the class is not found. at org.datanucleus.jdo.NucleusJDOHelper.getJDOExceptionForNucleusException(NucleusJDOHelper.java:241) at org.datanucleus.jdo.JDOPersistenceManager.jdoMakePersistent(JDOPersistenceManager.java:674) at org.datanucleus.jdo.JDOPersistenceManager.makePersistent(JDOPersistenceManager.java:694) at com.arunraja101.SaveDataServlet.doGet(SaveDataServlet.java:91)

Is an String Object can't be persisted ?

Upvotes: 1

Views: 662

Answers (2)

Dan Holevoet
Dan Holevoet

Reputation: 9183

You can persist a String in the datastore, but as part of an entity. You describe the entities that are written to and read from the datastore by defining a class. For instance, this document describes how to define an entity class using JDO.

The class dictates which fields are stored when the entities are written to the datastore, and where they are written. Without such a class, App Engine will not know where to save the data when you call makePersistent (as is the case in your example code) nor how to read it back.

Upvotes: 1

MStodd
MStodd

Reputation: 4746

Check out the documentation:

http://db.apache.org/jdo/api20/apidocs/javax/jdo/PersistenceManager.html#makePersistent(java.lang.Object)

You need to give it an object that is persistence capable. When using JDO, there is an enhancer process that will actually rip your .class files open and modify them so that they can be persisted. Check out the GAE documentation for persisting your objects:

https://developers.google.com/appengine/docs/java/datastore/jdo/dataclasses

Upvotes: 1

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