Listener、Filter、Servlet的初始化顺序

Listener、Filter、Servlet都有一个初始化的过程，对应的方法分别为：

contextInitialized(ServletContextEvent arg0)

init(FilterConfig filterConfig)

init(ServletConfig config)

那么它们的初始化顺序是什么呢？

Listener > Filter > Servlet

TestServlet.java

importjava.io.IOException;
importjavax.servlet.GenericServlet;
importjavax.servlet.ServletConfig;
importjavax.servlet.ServletException;
importjavax.servlet.ServletRequest;
importjavax.servlet.ServletResponse;

publicclassTestServletextendsGenericServlet{
@Override
publicvoidinit(ServletConfigconfig){
System.out.println("Servlet初始化。。。");
}
@Override
publicvoidservice(ServletRequestarg0,ServletResponsearg1)throwsServletException,IOException{
System.out.println("Servletservice。。。");
}
}

TestFilter.java

importjava.io.IOException;
importjavax.servlet.Filter;
importjavax.servlet.FilterChain;
importjavax.servlet.FilterConfig;
importjavax.servlet.ServletException;
importjavax.servlet.ServletRequest;
importjavax.servlet.ServletResponse;

publicclassTestFilterimplementsFilter{
publicvoidinit(FilterConfigfilterConfig)throwsServletException{
System.out.println("Filter初始化。。。");
}
publicvoiddoFilter(ServletRequestrequest,ServletResponseresponse,FilterChainchain)throwsIOException,ServletException{
System.out.println("doFilter。。。");
chain.doFilter(request,response);
}
publicvoiddestroy(){
System.out.println("Filter销毁。。。");
}
}

TestListener.java

importjavax.servlet.ServletContextEvent;
importjavax.servlet.ServletContextListener;

publicclassTestListenerimplementsServletContextListener{
publicvoidcontextInitialized(ServletContextEventarg0){
System.out.println("Listener初始化。。。");
}
publicvoidcontextDestroyed(ServletContextEventarg0){
System.out.println("Listener销毁。。。");
}
}

web.xml

<?xmlversion="1.0"encoding="UTF-8"?>
<web-appversion="2.5"xmlns="http://java.sun.com/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/javaeehttp://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>s</servlet-name>
<servlet-class>com.cos.TestServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
<filter>
<filter-name>f</filter-name>
<filter-class>com.cos.TestFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>f</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<listener>
<listener-class>com.cos.TestListener</listener-class>
</listener>
</web-app>

启动tomcat后，在控制台里以此打印出：

Listener 初始化。。。

Filter 初始化。。。

信息: Server startup in 675 ms

可以看出Listener的初始化最早，Filter次之。他俩的初始化都是在容器启动完成之前初始化的。

Servlet没有初始化，原因是没有匹配的请求进来。

如果想要servlet自动初始化，需要在指定的servlet中配置<load-on-startup>参数，没有此标签，默认启动时servlet不进行初始化。

初始化的顺序跟Listener、Filter、Servlet在web.xml中的顺序无关

而多个Filter或多个Servlet的时候，谁的mapping在前面，谁先初始化。

如果web.xml中配置了<context-param>，初始化顺序：

context-param > Listener > Filter > Servlet