OC4J：
Oracle Containers for J2EE,OC4J provides all the containers, APIs, and services mandated by the J2EE specification.
OC4J可以作为独立J2EE Server存在，比如你开发的JSP和Servlet，打包成war/ear文件，然后部署到OC4J容器供用户调用。
Oracle EBS 11i的时候是使用JServ(Java servlet containers) 作为J2EE Server，到了R12，使用OC4J替代JServ来作为J2EE容器。
默认Oracle Application会创建3个OC4J Instance:
- OACore: 运行OA Framework应用；
- Forms: 运行Forms-base 应用；
- OAFM (Oracle Apps Fusion Middleware): 运行web services, mapviewer, ascontrol
JServ ：
one of the very first servlet engines（比较老的容器了）. It is now an open source project, consisting of two parts: a pure-java servlet engine, compliant with the Servlet spec version 2.0, and an Apache module written in C called mod_jserv that allows Apache to send servlet requests to the JServ JVM.
Tomcat：
a completely separate product, originally written by Sun, that is now also open-source and administered by Apache. It supports the Servlet spec version 2.2, and the JSP spec version 1.1. It also contains a pure Java servlet engine, and a number of connectors for several commercial web servers.
下面就是讲述mod_jserv和mod_jk的演变：
大致意思是，mod_jserv是 Jserv用来和Apache做连接的，tmocat也想搞一个，用来和Apache连接，本来打算那mod_jserv改改，重新写写，但是写了一段时间后发现，还是不行，最后干脆重新自己写个新的吧，就叫mod_jk，这个新的就是tomcat拿来和Apache 连接用的。功能还不错，支持SSL。。。
Here's the confusing part: originally, when it was time to write a connector from Apache to Tomcat, they started with the code base of mod_jserv. Although it has been rewritten and revised to the point where it is no longer compatible with the mod_jserv that comes with JServ, unfortunately, it is still called mod_jserv. This means that if you have the JServ mod_jserv installed, it will no twork with Tomcat, and vice versa（反之，也是如此）.
Fortunately（幸运的是）, the latest version of Tomcat comes with a new connector for Apache, called mod_jk. This connector supports Tomcat but not JServ. It is also more advanced, in that it supports Tomcat-style load balancing, and also supports SSL authentication (though this latter functionality is still a bit rough and may have bugs in actually *reporting* that the connection is secure).
So if you just use mod_jk and Tomcat, you should have no problems (at least as far as this connector issue is concerned).