load-on-startup标签

Servlets are initialized either lazily at request processing time or eagerly during
deployment. In the latter case, they are initialized in the order indicated by
their load-on-startup elements.

在web容器启动的时候，可以采用lazily加载的方式和eagerly的方式。

load-on-startup中的值决定了进行哪种方式。

If the value is a negative integer, or the element is not present, the
container is free to load the servlet whenever it chooses. If the value is a positive
integer or 0, the container must load and initialize the servlet as the application is
deployed.

如果这个元素没有出现，或者出现了但是里面的值是负的，容器可以按照自己的需要选择加载Servlet的时机。

如果里面的值是正数或者0，容器必须保证在容器启动的时候加载和初始化这个servlet

The container must guarantee that servlets marked with lower integers
are loaded before servlets marked with higher integers.

这个值越小，优先级越高，容器优先加载。

The container may choose
the order of loading of servlets with the same load-on-startup value.

如果里面的值是一样的，那么加载的顺序由容器来决定（不同实现可能不同）

参考

  1. Java Servlet Specification 3.0