三种注册方式

第一种：Servlet

建WEB项目

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增加web.xml

<servlet>
        <servlet-name>partServlet</servlet-name>
        <servlet-class>servlet.asyn.PartServlet</servlet-class>
        <load-on-startup>0</load-on-startup>
        <init-param>
            <param-name>db</param-name>
            <param-value>mysql</param-value>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>partServlet</servlet-name>
        <url-pattern>/test11</url-pattern>
    </servlet-mapping>

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其中

1	<load-on-startup>0</load-on-startup>

表示servlet在何时初始化，Web容器启动时初始化，还是用户请求某个Servlet时对其初始化。

设置为0时，表示容器在应用启动时就加载并初始化这个servlet

写个servlet实现类

package servlet.asyn;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Collection;

public class PartServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        OutputStream os = response.getOutputStream();
        os.write("test".getBytes());
    }
}

访问

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第二种：使用Filter

web.xml配置

<filter>
        <filter-name>EncodingFilter</filter-name>
        <filter-class>filter.EncodingFilter</filter-class>
        <async-supported>false</async-supported>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>utf-8</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>EncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
    </filter-mapping>

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类实现注意使用javax.servlet的接口：

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第三种：使用Listener

1
2
3

<listener>
        <listener-class>listener.CustomServletContextListener</listener-class>
</listener>

实现ServletContextListener接口

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第四种：注解注册（Servlet3.0+）

@WebServlet(asyncSupported = false,
        urlPatterns = {"/ps"},
        loadOnStartup = 0,
        name = "PartServlet",
        initParams = {@WebInitParam(name="db",value="mysql")}
)

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第五种：ServletContext 动态注册

Servlet，Listener，Filter由ServletContext去加载，无论是使用xml配置还是使用Annotation注解配置，均由Web容器进行初始化，读取其中的配置属性，然后向Web容器中进行注册。Servlet 3.0 可以由ServletContext动态进行注册，因此需在Web容器初始化的时候（即建立ServletContext对象的时候）进行动态注册。

自定义ServletContextListener

改写其方法contextInitialized，在该方法中动态注册Servlet，Filter，Listener
Servlet
需要配置Servlet的参数为：loadOnStartup，urlMapping，initParameter，asyncSupport，MultipartConfig等。

javax.servlet.FilterRegistration.Dynamic filterRegistration = servletContext.addFilter("threedr3am", threedr3am);
filterRegistration.setInitParameter("encoding", "utf-8");
filterRegistration.setAsyncSupported(false);
filterRegistration.addMappingForUrlPatterns(java.util.EnumSet.of(javax.servlet.DispatcherType.REQUEST), false, new String[]{"/*"});