这是5.5.15最新的stable版本了。
官方提到：
Apache Tomcat 5.5.x. Apache Tomcat 5.5 is the current focus of development. While it supports the same Servlet and JSP Specification versions as Apache Tomcat 5.0.x, there are significant changes in many areas under the hood, resulting in improved performance, stability, and total cost of ownership. Please refer to the Apache Tomcat 5.5 Changelog for details.
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