Using a JNDI datasource created by another applica

问题:

I have a .properties file in my application which contains dataSource properties. I set up a JNDI reference to this dataSource using the following code :

// first I create MyDataSource from the properties found in the .properties file
//then :

Context initContext = new InitialContext();
initContext.createSubcontext("jdbc");
initContext.createSubcontext("jdbc/oracle");
initContext.rebind(jdbc/oracle/myDataSource, MyDataSource);

If I use a lookup in this application, the dataSource is found :

Context initContext = new InitialContext();
BasicDataSource dataSource = 
            (BasicDataSource) initContext.lookup("jdbc/oracle/myDataSource")
//everything works fine and I can use my dataSource to getConnection,
//requests, etc...

Now I would like to use this dataSource in another application. But if I do the same lookup than previously, I don't find myDataSource (whereas there is still the previous application in tomcat and the jndi binding is done on start-up with the help of a listener).

How can I get myDataSource in this second application, given that I can't use a Tomcat's resource in server.xml or a context.xml file (for different reasons I have to use this .properties file)?

Thanks

回答1:

"local" JDNI directories are read-only in Tomcat. Nevertheless, you can bind "global" JNDI resources in a LifecycleListener, and then "link" them to your context(s)(*):

You need to implement org.apache.catalina.LifecycleListener http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/LifecycleListener.html

Then register it in your server.xml like this (along with the other listeners):

<Listener className="yourlistener.YourLifecycleListener"/>

Your listener should await for 2 events:

public void lifecycleEvent(final LifecycleEvent event) {

    if (Lifecycle.START_EVENT.equals(event.getType())) {
      // Create your datasource instance...
      Context initContext = new InitialContext();
              initContext.createSubcontext("jdbc");
              initContext.createSubcontext("jdbc/oracle");
              initContext.rebind("jdbc/oracle/myDataSource", myDataSource);
    } else if (Lifecycle.STOP_EVENT.equals(event.getType())) {
      // unbind...
    }
}

Then you'll have to propagate resource accesses by "linking" them from "global" JNDI directory to "local" JNDI directory using ResourceLink element in your META-INF/context.xml:

<ResourceLink name="jdbc/oracle/myDataSource" global="jdbc/oracle/myDataSource"
    type="javax.sql.DataSource" />

That worked for me so far.

(*) Some notes:

There's an advantage on using lifecycle listeners. Since the order of context creation is not guaranteed. The advantage is that all of your contexts will see this object created.

If you need to create and configure datasource creation more dynamically that on lifecycle listener creation, note that you can bind a custom class implementing the Factory pattern.

To avoid classloading incompatibility problems, consider putting your listener, datasource, etc. classes in a jar file in the Tomcat lib directory, so they're included y the common classloader.

Regards.

回答2:

What you are trying to do is not going to work. J2EE applications are not allowed to modify the JNDI environment provided by the application server (J2EE spec, section 5.2.2) and the Tomcat JNDI documentation also states, that each web applications gets each own read-only JNDI environment. I'm not sure why binding/rebinding your datasource is not failing immediately and why it's working within the same web application, but even such application-internal usage of the JNDI environment is undocumented behaviour, which I would not rely on.

回答3:

A couple people have already commented on this, but I think the answer to your question is: Tomcat has a file called server.xml that you need to use. A good reference I have used before is below:

http://tomcat.apache.org/tomcat-5.5-doc/jndi-resources-howto.html

Resources defined here will be visible to all apps deployed (if set up correctly). If you set up a JNDI resource in your apps context or web xml file, it should only be available to your app.

回答4:

JNDI context are private to each webapp. Context created in one app can't be accessed by others.

Try to create an entry in GlobalNamingResources and links in both webapps using <ResourceLink> to see if it works.

I used this setup before to read from both apps but never tried to write from one. So not sure if it will work.

回答5:

Actually, it is possible to access others JNDI resources, if the servlet implements org.apache.catalina.ContainerServlet. This interface has a org.apache.catalina.Wrapper attribute, that is "populated" by the container itself.

through that, I created a simple application to monitor resources.

BUT, I would like to do that in a listener, so my resource monitor could start when the container starts. Anyone knows a way?