Why I don't Need toplink-essentials.jar in classpath

I use TopLink bundled inside Glassfish. Compiling and running java persistence application is easy, even when done from command line, as you can see from the previous post.

At compile time, I only have toplink-essentials-agent.jar in classpath. This jar only contains a java agent class:
oracle/toplink/essentials/internal/ejb/cmp3/JavaSECMPInitializerAgent.clasBut its META-INF/MANIFEST.MF contains a Class-Path entry , in addition to a Premain-Class entry:

Premain-Class: oracle.toplink.essentials.internal.ejb.cmp3.JavaSECMPIn
Class-Path: toplink-essentials.jar
<other entries omitted here>
So the class loader will follow this reference and load toplink-essentials.jar in the same directory. toplink-essentials.jar contains all the Java Persistence API and implementation classes.

At runtime, I don't even have any toplink jar in classpath. Because we use -javaagent:toplink-essentials-agent.jar, and any agent jars are automatically appended to the classpath. I wrote this post on -javaagent: option. So runtime classpath only needs to include application classes and jdbc driver jar.

In NetBeans 5.5, I found I do need to include both toplink-essentials-agent.jar and toplink-essentials.jar in project library. Otherwise, NetBeans can't find JPA classes.