If you want to access the olatcore source code you might want to check it out. Here is an explanation of how to use the source code in your Eclipse project instead of the olatcore.jar file. This is especially useful for debugging and to get a better understanding of the olat core functionalities.
Select the Repository view, expand HEAD and right click on olatcore. Choose Check Out.

In order to remove the olatcore from the libraries go back to the Java view and right click on the olat3 project. Select Properties.

Then select the olatcore jar-file from the Java Build Path Libraries list.

Press Remove.
As a next step we will link the olatcore source code into the olat3 project. In the properties window select the source tab. Then choose Link Source... Browse to olatcore/src/main/java and hit Choose. Name the Folder to something more descriptive like corejava or just olatcore.

Repeat the same thing for the source code at olatcore/src/main/resources (Link Source... -> Browse... -> Choose -> Finish). In the Java Build Path Source list you should now have five entries:

What we also have to do is to get rid of the jar-file in the WEB-INF/lib directory. Right click on olatcore-1.0-SNAPSHOT.jar and delete it.

There is one more thing to do. Open the file olat.local.properties and set the property olatcore.src to the path of your workspace followed by /workspace/olatcore/src/main/java/ . In order for the properties to take effect, run mvn install.
That's it. Now you should be able to restart the Tomcat server and run OLAT from source.