Question

If one Servlet is mapped twice, using two different names (as shown bellow) how many instances of the Servlet will be created by the container?

One instance or two or it is container dependent?

<servlet>
    <servlet-name>servlet1</servlet-name>
    <servlet-class>com.me.servlet.MyServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>servlet1</servlet-name>
    <url-pattern>/page1.html</url-pattern>
</servlet-mapping>


<servlet>
    <servlet-name>servlet2</servlet-name>
    <servlet-class>com.me.servlet.MyServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>servlet2</servlet-name>
    <url-pattern>/page2.html</url-pattern>
</servlet-mapping>

Answer 1

<servlet>
    <servlet-name>FooServlet</servlet-name>
    <servlet-class>foo.FooServlet</servlet-class>
    <init-param>
        <param-name>foo</param-name>
        <param-value>bar</param-value>
    </init-param>
</servlet>

Since each servlet has its own configuration, there must be at least one servlet instance per servlet definition.

The Servlet 3.0 specification says:

For a servlet not hosted in a distributed environment (the default),
the servlet container must use only one instance per servlet
declaration. However, for a servlet implementing the
SingleThreadModel interface, the servlet container may instantiate
multiple instances to handle a heavy request load and serialize
requests to a particular instance.

In the case where a servlet was deployed as part of an application
marked in the deployment descriptor as distributable, a container may
have only one instance per servlet declaration per Java Virtual
Machine (JVM™). However, if the servlet in a distributable application
implements the SingleThreadModel interface, the container may
instantiate multiple instances of that servlet in each JVM of the
container.

SingleThreadModel should be avoided.

Assuming a non-distributed environment, the posted descriptor fragment will result in two servlet instances.