Access to Derby database from another project using EclipseLink

I have a dynamic web project that creates and writes Derby database. Everything works fine. Here is the persistence.xml I used:

<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
<persistence-unit name="alerts" transaction-type="RESOURCE_LOCAL">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <class>test.Alert</class>
    <properties>
        <property name="javax.persistence.jdbc.driver" value="org.apache.derby.jdbc.EmbeddedDriver" />
        <property name="javax.persistence.jdbc.url" value="jdbc:derby:databases/bDb;create=true" />
        <property name="javax.persistence.jdbc.user" value="user" />
        <property name="javax.persistence.jdbc.password" value="user-pwd" />
        <!-- EclipseLink should create the database schema automatically -->
        <property name="eclipselink.ddl-generation" value="create-tables" />
        <property name="eclipselink.ddl-generation.output-mode"
            value="database" />

        <property name="eclipselink.logging.level" value="FINEST" />
        <property name="eclipselink.logging.level.sql" value="FINEST" />
    </properties>
</persistence-unit>

Now I would like to create a java project that reads the same database. So I use the same persistence.xml (except that I change "create=true" to "create=false" since I don't want to create a new table). but I got this error:

[EL Severe]: ejb: 2014-12-28 22:27:20.379--ServerSession(1488953836)--Thread(Thread[DefaultQuartzScheduler_Worker-7,5,main])--Local Exception Stack: 
Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: Database 'databases/bDb' not found.
Error Code: 40000
    at org.eclipse.persistence.exceptions.DatabaseException.sqlException(DatabaseException.java:331)
    at org.eclipse.persistence.exceptions.DatabaseException.sqlException(DatabaseException.java:326)
    at org.eclipse.persistence.sessions.DefaultConnector.connect(DefaultConnector.java:138)
    at org.eclipse.persistence.sessions.DatasourceLogin.connectToDatasource(DatasourceLogin.java:162)
    at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.setOrDetectDatasource(DatabaseSessionImpl.java:204)
    at org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.loginAndDetectDatasource(DatabaseSessionImpl.java:741)
    at org.eclipse.persistence.internal.jpa.EntityManagerFactoryProvider.login(EntityManagerFactoryProvider.java:239)
    at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:685)
    at org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession(EntityManagerFactoryDelegate.java:204)
    at org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:304)
    at org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:336)
    at org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:302)

If I use "create=true", it will create a new database instance (it is different from the one I created from the web project). How do I fix this problem? Thanks a lot!

Solution

Since you're using a relative filename in your jdbc.url (databases/bDb), Derby is looking for the database relative to your application start directory.

And your two separate applications have two separate directories in which they are being run.

You could address this in a number of ways:

Arrange for each application to be run from the same directory
Use an absolute database path name, not a relative one, in your jdb.url value
Use the Derby Network Server configuration, not the Derby Embedded configuration

And there are undoubtedly other approaches.