使用Java中的AppEngine和EntityManager将记录存储到Google DataStore中时出错

Question

I am trying to create a webservice that allows a user to add new records into the DataStore and also showing a table filled with the results. When the submit button is pressed, the following error appears on the web browser:-

javax.persistence.PersistenceException: Invalid primary key for com.epware.gae.java.books.model.Book.  Cannot have a null primary key field if the field is unencoded and of type String.  Please provide a value or, if you want the datastore to generate an id on your behalf, change the type of the field to Long.
at org.datanucleus.api.jpa.NucleusJPAHelper.getJPAExceptionForNucleusException(NucleusJPAHelper.java:302)
at org.datanucleus.api.jpa.JPAEntityManager.close(JPAEntityManager.java:197)
at com.epware.gae.java.books.dao.Dao.add(Dao.java:27)
at com.democo.gae.java.books.ServletCreateTodo.doPost(ServletCreateTodo.java:32)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:127)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectRequest(DevAppServerModulesFilter.java:366)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectModuleRequest(DevAppServerModulesFilter.java:349)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:116)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:491)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: org.datanucleus.exceptions.NucleusFatalUserException: Invalid primary key for com.epware.gae.java.books.model.Book.  Cannot have a null primary key field if the field is unencoded and of type String.  Please provide a value or, if you want the datastore to generate an id on your behalf, change the type of the field to Long.
at com.google.appengine.datanucleus.StoreFieldManager.storeStringPKField(StoreFieldManager.java:599)
at com.google.appengine.datanucleus.StoreFieldManager.storeStringField(StoreFieldManager.java:158)
at org.datanucleus.state.AbstractStateManager.providedStringField(AbstractStateManager.java:1433)
at com.epware.gae.java.books.model.Book.jdoProvideField(Book.java)
at com.epware.gae.java.books.model.Book.jdoProvideFields(Book.java)
at org.datanucleus.state.AbstractStateManager.provideFields(AbstractStateManager.java:1515)
at com.google.appengine.datanucleus.DatastorePersistenceHandler.insertObjectsInternal(DatastorePersistenceHandler.java:241)
at com.google.appengine.datanucleus.DatastorePersistenceHandler.insertObject(DatastorePersistenceHandler.java:218)
at org.datanucleus.state.JDOStateManager.internalMakePersistent(JDOStateManager.java:2381)
at org.datanucleus.state.JDOStateManager.flush(JDOStateManager.java:3778)
at org.datanucleus.ObjectManagerImpl.flushInternalWithOrdering(ObjectManagerImpl.java:3888)
at org.datanucleus.ObjectManagerImpl.flushInternal(ObjectManagerImpl.java:3811)
at org.datanucleus.ObjectManagerImpl.flush(ObjectManagerImpl.java:3751)
at org.datanucleus.ObjectManagerImpl.preCommit(ObjectManagerImpl.java:4141)
at org.datanucleus.ObjectManagerImpl.transactionPreCommit(ObjectManagerImpl.java:428)
at org.datanucleus.TransactionImpl.internalPreCommit(TransactionImpl.java:398)
at org.datanucleus.TransactionImpl.commit(TransactionImpl.java:287)
at org.datanucleus.ObjectManagerImpl.close(ObjectManagerImpl.java:1090)
at org.datanucleus.api.jpa.JPAEntityManager.close(JPAEntityManager.java:193)
... 39 more

Currently, this is how the object is made:-

public class Book 
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;
    private String author;
    private String description;
    private String publisher;
    private String publishDate;

    public Book(String title, String authorFirst, String authorSecond, String description, String publisher, String publishDate) {
        this.title = title;
        this.author = authorFirst+" "+authorSecond;
        this.description = description;
        this.publisher = publisher;
        this.publishDate = publishDate;
    }

    //Setters and Getters Below
}

How can I fix it so that the data can be entered into the table? I don't fully understand what the error message is saying but i'm fairly sure its to do with how the object itself is being made. Any help on how to resolve this is much appreciated.

Answer 1

entities must have a default constractor.

so in order to solve this you need to add this to you're code:

public Book()
{
}