Esper is free software available under the GNU General Public License (GPL)
The GPL license is sufficiently flexible to allow the use of Esper in both open source and your own projects. The GPL guarantees that Esper and any modifications made to Esper will stay open source, protecting our and your work.
Using Esper (by importing Esper's public interfaces in your Java code), and extending Esper (by subclassing or implementation of an extension interface) is considered by the authors of Esper to be dynamic linking. Hence our interpretation of the GPL is that the use of the unmodified Esper source does not affect the license of your application code.
The use of the unmodified Esper binary of course never affects the license of your application or distribution. The GPL also forbids distribution for profit, so you will never have to pay money to any commercial entity for using or integrating Esper.
If you modify Esper and redistribute your modifications, the GPL applies. Please submit any modifications to the issue tracking system.