ClassCastException during JNDI lookup for MongoDB, inside WAB, running Liberty

问题

I want to use JNDI lookup for MongoDB, inside WAB/OSGi, running Liberty Profile.

import com.mongodb.DB;

@WebServlet("/MongoServlet")
public class MongoServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;  

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        try {
            DB db = (DB) new InitialContext().lookup("java:comp/env/mongodb");
        } catch (NamingException e) {
            e.printStackTrace();
        }
    }
}

The JNDI lookup gives below exception.

[ERROR   ] SRVE0777E: Exception thrown by application class 'com.osgi.jndi.web.MongoServlet.doGet:57'
java.lang.ClassCastException: com.mongodb.DBApiLayer incompatible with com.mongodb.DB
com.mongodb.DBApiLayer is subclass of com.mongodb.DB. 

I believe that the ClassCastException is because of different classloaders being used by my application, and OSGi bootclassloader. But, I don't know how to troubleshoot it.

回答1:

Currently there is no way to use the mongodb-2.0 feature with an OSGi application.

Looking up a com.mongodb.DB resource will return you back an instance of DBApiLayer, but that's not the issue (since DBApiLayer extends DB). You would still get a ClassCastException even if you did this:

DBApiLayer db = (DBApiLayer) new InitialContext().lookup("java:comp/env/mongodb");

Would throw:

java.lang.ClassCastException: com.mongodb.DBApiLayer incompatible with com.mongodb.DBApiLayer

The reason for this limitation is that the mongodb-2.0 feature will use a Classloader from Liberty to load com.mongodb classes, and an OSGi application will use a separate Classloader to load com.mongodb classes, no matter how you configure things.

Normal Java EE applications can do things like this:

<library id="MongoLib">
    <file name="${server.config.dir}/lib/mongo-java-driver-2.11.4.jar"/>
</library>

<application name="myApp">
    <classloader commonLibraryRef="MongoLib"/>
</application>

However, <osgiApplication> elements do not have that capability. Unfortunately, the only solution here is to open an RFE with IBM (or upvote one if an RFE already exists for this).

来源：https://stackoverflow.com/questions/22447446/classcastexception-during-jndi-lookup-for-mongodb-inside-wab-running-liberty