通过url访问资源有三个步骤：

• 接收请求

• 处理请求

• 响应请求

web服务器：将某个主机上的资源映射为一个URL供外界访问，完成接收和响应请求

servlet容器：存放着servlet对象（由程序员编程提供），处理请求

Servlet接口

Servlet接口定义了5种方法：

• init()
• service()
• destroy()
• getServletConfig()
• getServletInfo()

package javax.servlet;

import java.io.IOException;


public interface Servlet {

    //tomcat反射创建servlet之后，调用init方法传入ServletConfig,对于每一个Servlet实例，init()方法只能被调用一次
    void init(ServletConfig config) throws ServletException;
    
    //该方法返回容器调用init()方法时传递给Servlet对象的ServletConfig对象，ServletConfig对象包含了Servlet的初始化参数。
    ServletConfig getServletConfig();
    
    //tomcat解析http请求，封装成对象传入,容器调用service()方法来处理客户端的请求
    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;
    
    String getServletInfo();
    
    void destroy();
    
}

ServletConfig：

封装了servlet的参数信息，从web.xml中获取，init-param标签的参数

<!--注册servlet-->
<servlet>
  <!--为Servlet注册一个友好的名字-->
  <servlet-name>HelloServlet</servlet-name>
  <!--指明为哪一个Servlet类起个友好的名字，名字要写全限定名-->
  <servlet-class>com.yoocar.servlet.HelloServlet</servlet-class>
  <!--配置参数-->
  <init-param>
    <param-name>user</param-name>
    <param-value>root</param-value>
  </init-param>
  <init-param>
    <param-name>pwd</param-name>
    <param-value>123456</param-value>
  </init-param>
  <!--标签中间写的值，必须是正整数，数字越小，优先级越高-->
  <load-on-startup>1</load-on-startup>
</servlet>

获取ServletConfig

public class HelloServlet extends HttpServlet {

    //在servlet的整个生命周期内，servlet的init（）方法，只会被调用一次，就是在第一次访问的时候
    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("HttpServlet-------------init");
    }

    /**
     *
     * @param request   获得浏览器请求
     * @param response  获得服务器响应
     * @throws ServletException
     * @throws IOException
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Object count = request.getSession().getAttribute("Count");
        System.out.println("count==="+count);
        //获取ServletConfig对象
        getServletConfig();
        try {
            String userId = request.getParameter("userId");
            System.out.println(userId);
            String servletPath = this.getServletContext().getRealPath("/");
            System.out.println(servletPath);
            //获取回应，以便向浏览器写数据
            OutputStream out = response.getOutputStream() ;
            //写数据
            out.write("hello Servlet".getBytes());
            //关闭流
            out.close();
        } catch (Exception e) {
            // TODO: handle exception
        }
    }

    //获取ServletConfig对象
    @Override
    public ServletConfig getServletConfig() {
        ServletConfig servletConfig = super.getServletConfig();
        Enumeration<String> initParameterNames = servletConfig.getInitParameterNames();
        while(initParameterNames.hasMoreElements()){
            //获取init-param标签配置的参数param-name
            String element = initParameterNames.nextElement();
            System.out.println("param-name--------"+element);
            String value = servletConfig.getInitParameter(element);
            System.out.println("param-value--------"+value);
        }
        return servletConfig;
    }

}

ServletRequest：

http请求到了tomcat后，tomcat通过字符串解析，把各个请求头（header），请求地址（URL），请求参数（queryString）都封装进Request。

ServletResponse：

Response在tomcat传给servlet时还是空的对象，servlet逻辑处理后，最终通过response.write()方法，将结果写入response内部的缓冲区,tomcat会在servlet处理结束后拿到response，获取里面的信息，组装成http响应给客户端

GenericServlet

改良版的servlet，抽象类,将ServletConfig提取出来,提升ServletConfig作用域，由局部变量变成全局变量

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {

    private static final long serialVersionUID = 1L;
    
    //将ServletConfig提取出来
    private transient ServletConfig config;

    public GenericServlet() {
    }

    //并不是销毁servlet的方法，而是销毁servlet前一定会调用的方法。默认空实现,可以借此关闭一些资源
    public void destroy() {
    }

    public String getInitParameter(String name) {
        return this.getServletConfig().getInitParameter(name);
    }

    public Enumeration<String> getInitParameterNames() {
        return this.getServletConfig().getInitParameterNames();
    }

	//初始化时已被赋值
    public ServletConfig getServletConfig() {
        return this.config;
    }

	//通过ServletConfig获取ServletContext
    public ServletContext getServletContext() {
        
        return this.getServletConfig().getServletContext();
    }

    public String getServletInfo() {
        return "";
    }

	//提升ServletConfig作用域，由局部变量变成全局变量
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        //提供给子类覆盖
        this.init();
    }

    public void init() throws ServletException {
    }

    public void log(String message) {
        this.getServletContext().log(this.getServletName() + ": " + message);
    }

    public void log(String message, Throwable t) {
        this.getServletContext().log(this.getServletName() + ": " + message, t);
    }

    //空实现
    public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    public String getServletName() {
        return this.config.getServletName();
    }
}

HttpServlet

GenericServlet的升级版，针对http请求所定制，在GenericServlet的基础上增加了service方法的实现，完成请求方法的判断

抽象类，用来被子类继承，得到匹配http请求的处理，子类必须重写以下方法中的一个

doGet，doPost，doPut，doDelete 未重写会报错（400,405）

service方法不应该重写,tomcat会为每一个HttpServlet创建单例

模板模式实现

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
    HttpServletRequest request;
    HttpServletResponse response;
    try {
        request = (HttpServletRequest)req;//强转成http类型，功能更强大
        response = (HttpServletResponse)res;
    } catch (ClassCastException var6) {
        throw new ServletException(lStrings.getString("http.non_http"));
    }
	//每次都调,关键代码
    this.service(request, response);
}

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
	//获取请求方式
    String method = req.getMethod();
    long lastModified;
    //判断逻辑，调用不同的处理方法
    if (method.equals("GET")) {
        lastModified = this.getLastModified(req);
        if (lastModified == -1L) {
            //本来业务逻辑应该直接写在这里，但是父类无法知道子类具体的业务逻辑，所以抽成方法让子类重写，父类的默认实现输出405，没有意义
            this.doGet(req, resp);
        } else {
            long ifModifiedSince;
            try {
                ifModifiedSince = req.getDateHeader("If-Modified-Since");
            } catch (IllegalArgumentException var9) {
                ifModifiedSince = -1L;
            }

            if (ifModifiedSince < lastModified / 1000L * 1000L) {
                this.maybeSetLastModified(resp, lastModified);
                this.doGet(req, resp);
            } else {
                resp.setStatus(304);
            }
        }
    } else if (method.equals("HEAD")) {
        lastModified = this.getLastModified(req);
        this.maybeSetLastModified(resp, lastModified);
        this.doHead(req, resp);
    } else if (method.equals("POST")) {
        this.doPost(req, resp);
    } else if (method.equals("PUT")) {
        this.doPut(req, resp);
    } else if (method.equals("DELETE")) {
        this.doDelete(req, resp);
    } else if (method.equals("OPTIONS")) {
        this.doOptions(req, resp);
    } else if (method.equals("TRACE")) {
        this.doTrace(req, resp);
    } else {
        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[]{method};
        errMsg = MessageFormat.format(errMsg, errArgs);
        resp.sendError(501, errMsg);
    }

}

一个类被声明为抽象的，一般有两个原因：

• 有抽象方法需要被实现
• 没有抽象方法，但是不希望被实例化

public abstract class HttpServlet extends GenericServlet

ServletContext

servlet上下文，代表web.xml文件，其实就是一个map，服务器会为每个应用创建一个servletContext对象：

• 创建是在服务器启动时完成
• 销毁是在服务器关闭时完成

javaWeb中的四个域对象：都可以看做是map，都有getAttribute()/setAttribute()方法。

• ServletContext域（Servlet间共享数据）

• Session域（一次会话间共享数据，也可以理解为多次请求间共享数据）

• Request域（同一次请求共享数据）

• Page域（JSP页面内共享数据）

servletConfig

servletConfig对象持有ServletContext的引用，Session域和Request域也可以得到ServletContext

五种方法获取：

* ServletConfig#getServletContext();

* GenericServlet#getServletContext();

* HttpSession#getServletContext();

* HttpServletRequest#getServletContext();

* ServletContextEvent#getServletContext();//创建ioc容器时的监听

Filter

不仅仅是拦截Request

拦截方式有四种：

Redirect和REQUEST/FORWARD/INCLUDE/ERROR最大区别在于：

重定向会导致浏览器发送2次请求，FORWARD们是服务器内部的1次请求

因为FORWARD/INCLUDE等请求的分发是服务器内部的流程，不涉及浏览器，REQUEST/FORWARD/INCLUDE/ERROR和Request有关，Redirect通过Response发起

通过配置，Filter可以过滤服务器内部转发的请求

Listener

ServletContextListener

public interface ServletContextListener extends EventListener {

    /**
     * Receives notification that the web application initialization
     * process is starting.
     *
     * <p>All ServletContextListeners are notified of context
     * initialization before any filters or servlets in the web
     * application are initialized.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * Receives notification that the ServletContext is about to be
     * shut down.
     *
     * <p>All servlets and filters will have been destroyed before any
     * ServletContextListeners are notified of context
     * destruction.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

在 Servlet API 中有一个 ServletContextListener 接口，它能够监听 ServletContext 对象的生命周期，实际上就是监听 Web 应用的生命周期。

　　当Servlet 容器启动或终止Web 应用时，会触发ServletContextEvent 事件，该事件由ServletContextListener 来处理。在 ServletContextListener 接口中定义了处理ServletContextEvent 事件的两个方法。

public class MyServletContextListener implements ServletContextListener {

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        System.out.println("MyServletContextListener Destoryed");

    }

    /**
     * servletContext初始化
     */
    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        System.out.println("MyServletContextListener Init");
        ArrayList<String> expressList=new ArrayList<String>();
        expressList.add("顺丰速递");
        expressList.add("如风达");
        expressList.add("宅急送");
        expressList.add("EMS");
        arg0.getServletContext().setAttribute("expressList", expressList);

    }

}

Servlet项目启动执行顺序(重要)

• ServletContainerInitializer.onStartup(Set<Class<?>> c, ServletContext ctx)
• ServletContextListener.contextInitialized(ServletContextEvent sce)
• Filter-------init(FilterConfig filterConfig)
• HttpServlet-------------init()
• Filter-------doFilter 过滤前
• HttpServlet------------doget/dopost
• Filter-------doFilter 过滤后

ServletContainerInitializer------------onStartup(Set<Class<?>> c, ServletContext ctx)
ServletContextListener contextInitialized(ServletContextEvent sce)
Filter-------init(FilterConfig filterConfig)
HttpServlet-------------init()
Filter-------doFilter 过滤前
Filter-------doFilter 过滤后
Filter-------doFilter 过滤前
Filter-------doFilter 过滤后

Filter-------doFilter 过滤前
param-name--------pwd
param-value--------123456
param-name--------user
param-value--------root
Filter-------doFilter 过滤后

servlet映射器

每一个url要交给哪个servlet处理，由映射器决定

映射器在tomcat中就是Mapper类：

nternalMapWrapper方法定义了七种映射规则

private final void internalMapWrapper(ContextVersion contextVersion,
                                          CharChunk path,
                                          MappingData mappingData) throws IOException {

    int pathOffset = path.getOffset();
    int pathEnd = path.getEnd();
    boolean noServletPath = false;

    int length = contextVersion.path.length();
    if (length == (pathEnd - pathOffset)) {
        noServletPath = true;
    }
    int servletPath = pathOffset + length;
    path.setOffset(servletPath);

    // Rule 1 -- 精确匹配
    MappedWrapper[] exactWrappers = contextVersion.exactWrappers;
    internalMapExactWrapper(exactWrappers, path, mappingData);

    // Rule 2 -- 前缀匹配
    boolean checkJspWelcomeFiles = false;
    MappedWrapper[] wildcardWrappers = contextVersion.wildcardWrappers;
    if (mappingData.wrapper == null) {
        internalMapWildcardWrapper(wildcardWrappers, contextVersion.nesting,
                                   path, mappingData);
        if (mappingData.wrapper != null && mappingData.jspWildCard) {
            char[] buf = path.getBuffer();
            if (buf[pathEnd - 1] == '/') {
                mappingData.wrapper = null;
                checkJspWelcomeFiles = true;
            } else {
                // See Bugzilla 27704
                mappingData.wrapperPath.setChars(buf, path.getStart(),
                                                 path.getLength());
                mappingData.pathInfo.recycle();
            }
        }
    }

    if(mappingData.wrapper == null && noServletPath &&
       contextVersion.object.getMapperContextRootRedirectEnabled()) {
        // The path is empty, redirect to "/"
        path.append('/');
        pathEnd = path.getEnd();
        mappingData.redirectPath.setChars
            (path.getBuffer(), pathOffset, pathEnd - pathOffset);
        path.setEnd(pathEnd - 1);
        return;
    }

    // Rule 3 -- 扩展名匹配
    MappedWrapper[] extensionWrappers = contextVersion.extensionWrappers;
    if (mappingData.wrapper == null && !checkJspWelcomeFiles) {
        internalMapExtensionWrapper(extensionWrappers, path, mappingData,
                                    true);
    }

    ...

上面都不匹配，则交给DefaultServlet，就是简单地用IO流读取静态资源并响应给浏览器。如果资源找不到，报404错误

对于静态资源，Tomcat最后会交由一个叫做DefaultServlet的类来处理对于Servlet ，Tomcat最后会交由一个叫做 InvokerServlet的类来处理对于JSP，Tomcat最后会交由一个叫做JspServlet的类来处理

也就是说，servlet,/*这种配置，相当于把DefaultServlet、JspServlet以及我们自己写的其他Servlet都“短路”了，它们都失效了。

这会导致两个问题：

• JSP无法被编译成Servlet输出HTML片段（JspServlet短路）
• HTML/CSS/JS/PNG等资源无法获取（DefaultServlet短路）

DispatcherServlet配置/，会和DefaultServlet产生路径冲突，从而覆盖DefaultServlet。此时，所有对静态资源的请求，映射器都会分发给我们自己写的DispatcherServlet处理。遗憾的是，它只写了业务代码，并不能IO读取并返回静态资源。JspServlet的映射路径没有被覆盖，所以动态资源照常响应。

DispatcherServlet配置/*，虽然JspServlet和DefaultServlet拦截路径还是.jsp和/，没有被覆盖，但无奈的是在到达它们之前，请求已经被DispatcherServlet抢去，所以最终不仅无法处理JSP，也无法处理静态资源。

tomcat中conf/web.xml

相当于每个应用默认都配置了JSPServlet和DefaultServlet处理JSP和静态资源。

servlet SPI机制

要使用Java SPI，需要遵循如下约定：

从servlet3.0开始，web容器启动时为提供给第三方组件机会做一些初始化的工作，例如注册servlet或者filtes等，servlet规范中通过ServletContainerInitializer实现此功能。每个框架要使用ServletContainerInitializer就必须在对应的jar包的META-INF/services 目录创建一个名为javax.servlet.ServletContainerInitializer的文件，文件内容指定具体的ServletContainerInitializer实现类，那么，当web容器启动时就会运行这个初始化器做一些组件内的初始化工作。

注意一下,该机制的会调用是实现了ServletContainerInitializer的类/抽象类的onStartup方法,并且把被@HandlesTypes注解的对象,封装成onStartup中的set集合传入

/**
 * Interface which allows a library/runtime to be notified of a web
 * application's startup phase and perform any required programmatic
 * registration of servlets, filters, and listeners in response to it.
 *
 * <p>Implementations of this interface may be annotated with
 * {@link javax.servlet.annotation.HandlesTypes HandlesTypes}, in order to
 * receive (at their {@link #onStartup} method) the Set of application
 * classes that implement, extend, or have been annotated with the class
 * types specified by the annotation.
 * 
 * <p>If an implementation of this interface does not use <tt>HandlesTypes</tt>
 * annotation, or none of the application classes match the ones specified
 * by the annotation, the container must pass a <tt>null</tt> Set of classes
 * to {@link #onStartup}.
 *
 * <p>When examining the classes of an application to see if they match
 * any of the criteria specified by the <tt>HandlesTypes</tt> annontation
 * of a <tt>ServletContainerInitializer</tt>, the container may run into
 * classloading problems if any of the application's optional JAR
 * files are missing. Because the container is not in a position to decide
 * whether these types of classloading failures will prevent
 * the application from working correctly, it must ignore them,
 * while at the same time providing a configuration option that would
 * log them. 
 *
 * <p>Implementations of this interface must be declared by a JAR file
 * resource located inside the <tt>META-INF/services</tt> directory and
 * named for the fully qualified class name of this interface, and will be 
 * discovered using the runtime's service provider lookup mechanism
 * or a container specific mechanism that is semantically equivalent to
 * it. In either case, <tt>ServletContainerInitializer</tt> services from web
 * fragment JAR files excluded from an absolute ordering must be ignored,
 * and the order in which these services are discovered must follow the
 * application's classloading delegation model.
 *
 * @see javax.servlet.annotation.HandlesTypes
 *
 * @since Servlet 3.0
 */
public interface ServletContainerInitializer {

    /**
     * Notifies this <tt>ServletContainerInitializer</tt> of the startup
     * of the application represented by the given <tt>ServletContext</tt>.
     *
     * <p>If this <tt>ServletContainerInitializer</tt> is bundled in a JAR
     * file inside the <tt>WEB-INF/lib</tt> directory of an application,
     * its <tt>onStartup</tt> method will be invoked only once during the
     * startup of the bundling application. If this
     * <tt>ServletContainerInitializer</tt> is bundled inside a JAR file
     * outside of any <tt>WEB-INF/lib</tt> directory, but still
     * discoverable as described above, its <tt>onStartup</tt> method
     * will be invoked every time an application is started.
     *
     * @param c the Set of application classes that extend, implement, or
     * have been annotated with the class types specified by the 
     * {@link javax.servlet.annotation.HandlesTypes HandlesTypes} annotation,
     * or <tt>null</tt> if there are no matches, or this
     * <tt>ServletContainerInitializer</tt> has not been annotated with
     * <tt>HandlesTypes</tt>
     *
     * @param ctx the <tt>ServletContext</tt> of the web application that
     * is being started and in which the classes contained in <tt>c</tt>
     * were found
     *
     * @throws ServletException if an error has occurred
     */
    public void onStartup(Set<Class<?>> c, ServletContext ctx)
        throws ServletException; 
}

servlet实战

pom.xml

<dependencies>
    <!-- jsp start -->
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>3.1.0</version>
    </dependency>

    <!--使用maven tomcat插件时，当前依赖需要注释掉，不然会产生冲突。-->
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-core</artifactId>
        <version>8.5.31</version>
    </dependency>
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-jasper</artifactId>
        <version>8.5.31</version>
    </dependency>

</dependencies>

<build>
    <finalName>servlet-demo</finalName>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
        <plugins>
            <plugin>
                <artifactId>maven-clean-plugin</artifactId>
                <version>3.1.0</version>
            </plugin>
            <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_war_packaging -->
            <plugin>
                <artifactId>maven-resources-plugin</artifactId>
                <version>3.0.2</version>
            </plugin>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.0</version>
            </plugin>
            <plugin>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.22.1</version>
            </plugin>
            <plugin>
                <artifactId>maven-war-plugin</artifactId>
                <version>3.2.2</version>
            </plugin>
            <plugin>
                <artifactId>maven-install-plugin</artifactId>
                <version>2.5.2</version>
            </plugin>
            <plugin>
                <artifactId>maven-deploy-plugin</artifactId>
                <version>2.8.2</version>
            </plugin>
        </plugins>
    </pluginManagement>
</build>

web.xml

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>

  <display-name>Archetype Created Web Application</display-name>

  <!--<listener>
    <listener-class>com.yoocar.listener.HelloListener</listener-class>
  </listener>-->
  <!--<listener>
    <listener-class>com.yoocar.listener.MyListener</listener-class>
  </listener>-->

  <listener>
    <listener-class>com.yoocar.listener.MyServletContextListener</listener-class>
  </listener>

  <filter>
    <!--过滤器名称-->
    <filter-name>HelloFilter</filter-name>
      <!--过滤器类的包路径-->
    <filter-class>com.yoocar.filter.HelloFilter</filter-class>
  </filter>


  <!--过滤器映射-->
  <filter-mapping>
    <filter-name>HelloFilter</filter-name>
    <!--客户端请求访问任意资源文件时都要经过过滤器过滤，通过则访问文件，否则拦截。-->
    <url-pattern>/*</url-pattern>
  </filter-mapping>

  <!--注册servlet-->
  <servlet>
    <!--为Servlet注册一个友好的名字-->
    <servlet-name>HelloServlet</servlet-name>
    <!--指明为哪一个Servlet类起个友好的名字，名字要写全限定名-->
    <servlet-class>com.yoocar.servlet.HelloServlet</servlet-class>
    <!--配置参数-->
    <init-param>
      <param-name>user</param-name>
      <param-value>root</param-value>
    </init-param>
    <init-param>
      <param-name>pwd</param-name>
      <param-value>123456</param-value>
    </init-param>
    <!--标签中间写的值，必须是正整数，数字越小，优先级越高-->
    <load-on-startup>1</load-on-startup>
  </servlet>

  <!--映射关系，下面为地址，即在浏览器中输入的url -->
  <servlet-mapping>
    <!--指明为哪一个Servlet类配置对外访问路径-->
    <servlet-name>HelloServlet</servlet-name>
    <!--指定对外访问的路径-->
    <url-pattern>/helloServlet</url-pattern>
  </servlet-mapping>

  <!--同一个Servlet可以被映射到多个URL上，即多个 <servlet-mapping> 的<servlet-name> 的值，可以是同一个Servlet ；-->
  <servlet>
    <servlet-name>OrderServlet</servlet-name>
    <servlet-class>com.yoocar.servlet.OrderServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>OrderServlet</servlet-name>
    <url-pattern>/orderServlet</url-pattern>
  </servlet-mapping>

  <servlet-mapping>
    <servlet-name>OrderServlet</servlet-name>
    <url-pattern>/orderServlet2</url-pattern>
  </servlet-mapping>


</web-app>

Servlet

import javax.servlet.ServletConfig;
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.Enumeration;

public class HelloServlet extends HttpServlet {

    //在servlet的整个生命周期内，servlet的init（）方法，只会被调用一次，就是在第一次访问的时候
    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("HttpServlet-------------init");
    }

    //对于servlet的每次访问请求，都会调用一次servlet的service（）方法 ；
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.service(req, resp);
        System.out.println("HttpServlet-------------service");
    }

    /**
     *
     * @param request   获得浏览器请求
     * @param response  获得服务器响应
     * @throws ServletException
     * @throws IOException
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Object count = request.getSession().getAttribute("Count");
        System.out.println("count==="+count);
        //获取ServletConfig对象
        getServletConfig();
        try {
            String userId = request.getParameter("userId");
            System.out.println(userId);
            String servletPath = this.getServletContext().getRealPath("/");
            System.out.println(servletPath);
            //获取回应，以便向浏览器写数据
            OutputStream out = response.getOutputStream() ;
            //写数据
            out.write("hello Servlet".getBytes());
            //关闭流
            out.close();
        } catch (Exception e) {
            // TODO: handle exception
        }
    }

    //获取ServletConfig对象
    @Override
    public ServletConfig getServletConfig() {
        ServletConfig servletConfig = super.getServletConfig();
        Enumeration<String> initParameterNames = servletConfig.getInitParameterNames();
        while(initParameterNames.hasMoreElements()){
            //获取init-param标签配置的参数param-name
            String element = initParameterNames.nextElement();
            System.out.println("param-name--------"+element);
            String value = servletConfig.getInitParameter(element);
            System.out.println("param-value--------"+value);
        }
        return servletConfig;
    }

}

Filter

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;

public class HelloFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println(filterConfig.toString());
        System.out.println("Filter-------init");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("Filter-------doFilter 过滤前");
        chain.doFilter(request,response);
        System.out.println("Filter-------doFilter 过滤后");
    }

    @Override
    public void destroy() {
        System.out.println("Filter-------destroy");
    }

}

ServletContainerInitializer

import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import java.util.Set;

//spi机制
public class TestServletContainerInitializer implements ServletContainerInitializer{

    @Override
    public void onStartup(Set<Class<?>> c,ServletContext servletContext) {
        System.out.println("ServletContainerInitializer------------onStartup");
    }

}