了解容器执行Servlet API 中ServletContext、ServletContextListener、Filter、Servlet的顺序。
容器启动加载顺序:
启动一个WEB项目的时候，WEB容器会去读取它的配置文件web.xml，
1.加载产生Servlet上下文实例，ServletContext
这个web项目的所有部分都将共享这个上下文。
容器将<context-param>转换为键值对，并交给servletContext。
<context-param>
<param-name>paramName</param-name>
<param-value>paramValue</param-value>
</context-param>
2.依次加载Servlet的事件监听器，ServletContextListener
并依次调用public void contextInitialized(ServletContextEvent sce)方法。
加载和调用多个Listener的顺序由在web.xml中配置的依次顺序决定的。
下面的配置依次调用MyListener和MyListener1的contextInitialized方法
<listener>
<description>this is myListener</description>
<display-name>myListener</display-name>
<listener-class>com.zghw.servlet.demo.MyListener</listener-class>
</listener>
<listener>
<listener-class>com.zghw.servlet.demo.MyListener1</listener-class>
</listener>
3.依次加载Servlet的过滤器，Filter
并依次调用public void init(FilterConfig filterConfig) throws ServletException;方法
加载和调用多个filter的顺序由在web.xml中配置的依次顺序决定的。
下面的配置依次调用MyFilter的init方法，然后调用MyFilter1的init方法
<filter>
<description>this is a myFilter</description>
<display-name>myFilter</display-name>
<filter-name>myFilter</filter-name>
<filter-class>com.zghw.servlet.demo.MyFilter</filter-class>
<init-param>
<param-name>name</param-name>
<param-value>value</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>myFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>myFilter1</filter-name>
<filter-class>com.zghw.servlet.demo.MyFilter1</filter-class>
</filter>
<filter-mapping>
<filter-name>myFilter1</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
4.依次加载Servlet
如果在web.xml中的servlet配置了<load-on-startup>1</load-on-startup>
则会调用Servlet类中的方法public void init() 方法
Load-on-startup元素在web应用启动的时候指定了servlet被加载的顺序，它的值必须是一个整数。如果它的值是一个负整数或是这个元素不存在，那么容器会在该servlet被调用的时候，加载这个servlet。如果值是正整数或零，容器在配置的时候就加载并初始化这个servlet，容器必须保证值小的先被加载。如果值相等，容器可以自动选择先加载谁。
在servlet的配置当中，<load-on-startup>5</load-on-startup>的含义是：
标记容器是否在启动的时候就加载这个servlet。
当值为0或者大于0时，表示容器在应用启动时就加载这个servlet；
当是一个负数时或者没有指定时，则指示容器在该servlet被选择时才加载。
正数的值越小，启动该servlet的优先级越高。
下面的依次调用MyServlet、MyServlet1的init方法:
<servlet>
<servlet-name>myServlet</servlet-name>
<servlet-class>com.zghw.servlet.demo.MyServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>myServlet</servlet-name>
<url-pattern>/ms/*</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>myServlet1</servlet-name>
<servlet-class>com.zghw.servlet.demo.MyServlet1</servlet-class>
<load-on-startup>2</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>myServlet1</servlet-name>
<url-pattern>/ms1/*</url-pattern>
</servlet-mapping>
输出结果：
初始化参数：paramName 值： paramValue
执行MyListener contextInitialized
执行MyListener1 contextInitialized
执行MyFilter init
执行MyFilter1 init
执行Servlet init()
执行Servlet1 init()
以上是Web容器在启动时加载的顺序，启动加载只会加载一次。
首先可以肯定的是，加载顺序与它们在 web.xml 文件中的先后顺序无关。即不会因为 filter 写在 listener的前面而会先加载 filter。最终得出的结论是：ServletContext-> listener ->filter -> servlet
同时还存在着这样一种配置节：context-param，它用于向 ServletContext提供键值对，即应用程序上下文信息。我们的 listener, filter 等在初始化时会用到这些上下文中的信息，那么context-param 配置节是不是应该写在 listener 配置节前呢？实际上 context-param配置节可写在任意位置，因此真正的加载顺序为：context-param -> listener-> filter -> servlet
对于某类配置节而言，与它们出现的顺序是有关的。以filter 为例，web.xml 中当然可以定义多个 filter，与 filter 相关的一个配置节是filter-mapping，这里一定要注意，对于拥有相同 filter-name 的 filter 和 filter-mapping配置节而言，filter-mapping 必须出现在 filter 之后，否则当解析到 filter-mapping 时，它所对应的filter-name 还未定义。web 容器启动时初始化每个 filter 时，是按照 filter配置节出现的顺序来初始化的，当请求资源匹配多个 filter-mapping 时，filter 拦截资源是按照filter-mapping 配置节出现的顺序来依次调用 doFilter() 方法的。
servlet 同 filter类似，此处不再赘述。
由此，可以看出，web.xml 的加载顺序是：ServletContext-> context-param ->listener -> filter -> servlet，而同个类型之间的实际程序调用的时候的顺序是根据对应的 mapping 的顺序进行调用的。
容器启动后就可以http访问，当访问Servlet时，发生的顺序。
1.依次执行过滤器filter的方法public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain)，这个方法应用了责任链模式，当在该方法中使用chain.doFilter(request, response);则这个过滤器就调用下一个过滤器，直到过滤器链条完成调用，进入Servlet处理，这个时候doFilter并未执行完成，仅仅在servlet之前进行一连串的过滤处理。
2.进入相应Servlet并调用public void service(ServletRequest req, ServletResponse res)方法，
或者说是GET和POST方法。public void doGet(HttpServletRequest request, HttpServletResponse respose)
进行请求响应的业务处理。
3.Servlet处理完成后，执行chain.doFilter(request, response);执行其他过滤器链条的后置过滤处理，然后执行自己的后置处理。
以上Filter和Servlet的执行顺序有点像Spring AOP 的前置通知和后置通知与业务方法关系。在Filter的doFilter方法中的chain.doFilter(request, response);之前做的业务逻辑就像前置通知，之后的逻辑像后置通知。业务方法是Sevlet中的public void service(ServletRequest req, ServletResponse res)方法。并且可以由多个有序的过滤链条进行Servlet的过滤。
Filter的过滤请求的Servlet的范围与配置有关,Filter在每次访问Servlet时都会拦截过滤。
输出访问请求时执行的顺序结果；
执行MyFilter doFilter
执行MyFilter doFilter before
执行MyFilter1 doFilter
执行MyFilter1 doFilter before
执行Servlet service
执行MyFilter1 doFilter after
执行MyFilter doFilter after
上面的顺序只是简单的，你也可以在Filter中直接返回请求，或者不经过任何链条等等，根据业务灵活配置。
<web-app 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/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
version="3.0">
<context-param>
<param-name>paramName</param-name>
<param-value>paramValue</param-value>
</context-param>
<listener>
<description>this is myListener</description>
<display-name>myListener</display-name>
<listener-class>com.zghw.servlet.demo.MyListener</listener-class>
</listener>
<listener>
<listener-class>com.zghw.servlet.demo.MyListener1</listener-class>
</listener>
<filter>
<description>this is a myFilter</description>
<display-name>myFilter</display-name>
<filter-name>myFilter</filter-name>
<filter-class>com.zghw.servlet.demo.MyFilter</filter-class>
<init-param>
<param-name>name</param-name>
<param-value>value</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>myFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>myFilter1</filter-name>
<filter-class>com.zghw.servlet.demo.MyFilter1</filter-class>
</filter>
<filter-mapping>
<filter-name>myFilter1</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<servlet>
<servlet-name>myServlet</servlet-name>
<servlet-class>com.zghw.servlet.demo.MyServlet</servlet-class>
<init-param>
<param-name>initkey1</param-name>
<param-value>initvalue1</param-value>
</init-param>
<init-param>
<param-name>initkey2</param-name>
<param-value>initvalue2</param-value>
</init-param>
<load-on-startup>-1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>myServlet</servlet-name>
<url-pattern>/ms/*</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>myServlet1</servlet-name>
<servlet-class>com.zghw.servlet.demo.MyServlet1</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>myServlet1</servlet-name>
<url-pattern>/ms1/*</url-pattern>
</servlet-mapping>
</web-app>
package com.zghw.servlet.demo;
import java.util.Enumeration;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
ServletContext servletContext=sce.getServletContext();
Enumeration<String> params=servletContext.getInitParameterNames();
while(params.hasMoreElements()){
String name=params.nextElement();
System.out.println("初始化参数："+name+" 值： "+servletContext.getInitParameter(name));
}
System.out.println("执行MyListener contextInitialized");
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("执行MyListener contextDestroyed");
}
}
package com.zghw.servlet.demo;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyListener1 implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
System.out.println("执行MyListener1 contextInitialized");
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("执行MyListener1 contextDestroyed");
}
}
package com.zghw.servlet.demo;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class MyFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("执行MyFilter init");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("执行MyFilter doFilter");
System.out.println("执行MyFilter doFilter before");
chain.doFilter(request, response);
System.out.println("执行MyFilter doFilter after");
}
@Override
public void destroy() {
System.out.println("执行MyFilter destroy");
}
}
package com.zghw.servlet.demo;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class MyFilter1 implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("执行MyFilter1 init");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("执行MyFilter1 doFilter ");
System.out.println("执行MyFilter1 doFilter before");
chain.doFilter(request, response);
System.out.println("执行MyFilter1 doFilter after");
}
@Override
public void destroy() {
System.out.println("执行MyFilter1 destroy");
}
}
package com.zghw.servlet.demo;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public void init() throws ServletException {
System.out.println("执行Servlet init()");
}
public void destroy() {
System.out.println("执行Servlet destroy()");
}
public void doGet(HttpServletRequest request, HttpServletResponse respose)
throws ServletException, IOException {
System.out.println("执行Servlet service");
respose.setContentType("text/html;charset=UTF-8");
PrintWriter out = respose.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>");
out.println("this is a servlet out");
out.println("</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Hello Servlet ： " + getServletInfo() + "</h1>");
out.println("<p>");
out.println("<h3>ServletConfig</h3>");
out.println("Servlet名字:" + this.getServletName());
out.println("<br/>");
Enumeration<String> initParamNames = this.getInitParameterNames();
while (initParamNames.hasMoreElements()) {
String name = initParamNames.nextElement();
out.print("初始化参数： name = " + name + " ， value = "
+ this.getInitParameter(name));
out.println("<br/>");
}
ServletContext servletContext = this.getServletContext();
out.println("<h3>servletContext</h3>");
out.println("</p>");
out.println("</body>");
out.println("</html>");
}
}
package com.zghw.servlet.demo;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyServlet1 extends HttpServlet {
private static final long serialVersionUID = 1L;
public void init() throws ServletException {
System.out.println("执行Servlet1 init()");
}
public void destroy() {
System.out.println("执行Servlet1 destroy()");
}
public void doGet(HttpServletRequest request, HttpServletResponse respose)
throws ServletException, IOException {
System.out.println("执行Servlet1 service");
}
}
