Seasar was reborn to DIContainer with AOP(S2Container) from Version2. It enables features such as transaction auto-management, which is easier to use POJO(Plain Old Java Object) with combined connection pooling. You may not need a heavy application server any longer. You only have to execute S2Container on the Servlet container such as Tomcat and Jetty. The theme of S2 are,
-Dismantlement and restructuring of J2EE
-Ease and gentleness for all users
Like transaction management or connection pooling, these features could not use without an application server. For this reason, it was hard to do test with such in-container approach. S2 provides the transaction management and connectio pool as a plain component. You can use it without the application server. S2 is created to remove the complexity of J2EE, and to use only the goodness of J2EE features.