Seems like the slow Tomcat 7 startup problem can be resolved with "metadata-complete" set to "true" in the web.xml, like so:
<?xml version="1.0" encoding="UTF-8"?>
<web-app metadata-complete="true" id="WebApp_ID" version="3.0"...
The problem is that Tomcat scans for annotations at startup, and this significantly slows it down. My time is cut down from 25 secs to 5 secs. (More info here: Tomcat and Servlet 3.0 Web Configuration)
However, I have some annotations in my code, like:
@ManagedBean
@RequestScoped
@Override
...
I am confused - will my code work after I have set metadata-complete="true"? Do I have to remove annotations and move everything into web.xml?
The slow startup is caused because every single class file in every single JAR file in /WEB-INF/lib is also scanned for Servlet 3.0 specific annotations. You apparently have a lot of (large) JAR files in /WEB-INF/lib.
The metadata-complete="true" indicates that the JAR files in /WEB-INF/lib doesn't need to be scanned for Servlet 3.0 specific annotations, but the webapp's own classes will still be scanned.
Note that you listed there two JSF annotations and one Java SE annotation, not any Servlet 3.0 annotations. The Servlet 3.0 annotations are listed in the javax.servlet.annotation package. JSF will only scan for annotations when the JAR file contains a JSF 2.0 compatible /META-INF/faces-config.xml file. It won't immediately scan every single class in every JAR file. The Java SE @Override annotation is not a runtime annotation, but a compile-time aid only.
See also:
Here is what Java Servlet 3.0 / 3.1 Specification has to say:
The web application deployment descriptor contains a metadata-complete attribute on the web-app element. The metadata-complete attribute defines whether the web.xml descriptor is complete, or whether other sources of metadata used by the deployment process should be considered. Metadata may come from the web.xml file, web-fragment.xml files, annotations on class files in WEB-INF/classes, and annotations on classes in jar files in the WEB-INF/lib directory. If metadata-complete is set to "true", the deployment tool only examines the web.xml file and must ignore annotations such as @WebServlet, @WebFilter, and @WebListener present in the class files of the application, and must also ignore any web-fragment.xml descriptor packaged in a jar file in WEB-INF/lib. If the metadata-complete attribute is not specified or is set to "false", the deployment tool must examine the class files and web-fragment.xml files for metadata,as previously specified.
This being said and to answer your question: Yes, in order to optimise the start up time of Tomcat you need to use metadata-complete="true" and put every Servlet or Filter or Listener in your deployment descriptor.
来源：https://stackoverflow.com/questions/9820379/what-to-do-with-annotations-after-setting-metadata-complete-true-which-resolv