JSP 2.0 specification 对
isThreadSafe 属性有说明
Note: The Servlet 2.4 speci?cation deprecates
SingleThreadModel, which is the most common
mechanism for JSP containers to implement isThreadSafe.
Page authors are advised against using isThreadSafe, as
the generated Servlet may contain deprecated code.
Tomcat 5.5是Servlet 2.4 / JSP2.0的实现。当isThreadSafe=false时，生成的JSP class中实现了 SingleThreadModel接口，但这个接口已被废弃，tomcat的源码中没有使用到这个接口，所以isThreadSafe也相当于废弃无用了