Servlet

servlet 是运行在 Web 服务器中的小型 Java 程序（即：服务器端的小应用程序）。servlet 通常通过 HTTP（超文本传输协议）接收和响应来自 Web 客户端的请求。

编写一个servlet程序

1. 写一个java类，实现servlet接口

import javax.servlet.*;
import java.io.IOException;

public class ServletDemo1 implements Servlet {

    @Override
    public void init(ServletConfig servletConfig) throws ServletException {

    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("hello servlet");
    }

    @Override
    public String getServletInfo() {
        return null;
    }

    @Override
    public void destroy() {

    }
}

2. 修改web.xml文件，给servlet提供一个可访问的URI地址

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
    <!--创建一个servlet实例-->
    <servlet>
        <servlet-name>servletDemo1</servlet-name>
        <servlet-class>com.ljb.ServletDemo1</servlet-class>
    </servlet>
    <!-- 给servlet提供一个可供客户端访问的URI -->
    <servlet-mapping>
        <servlet-name>servletDemo1</servlet-name>
        <url-pattern>/demo1</url-pattern>
    </servlet-mapping>

</web-app>

3. 部署应用到Tomcat服务器
4. 测试：http://localhost:8080/servlet/demo1

执行过程

执行过程.png

Servlet生命周期

实例化-->初始化-->服务->销毁
出生：（实例化-->初始化）第一次访问Servlet就出生（默认情况下）
--如何让servlet在服务器启动时就创建

    <servlet>
        <servlet-name>servletDemo1</servlet-name>
        <servlet-class>com.ljb.ServletDemo1</servlet-class>
        <!--数字越小先启动-->
        <load-on-startup>2</load-on-startup>
    </servlet>

活着：（服务）应用活着，servlet就活着
死亡：（销毁）应用卸载了servlet就销毁

public class ServletDemo1 implements Servlet {

    //在第一次访问时调用
    public ServletDemo1() {
        System.out.println("-----ServletDemo1----");
    }
    //在第一次访问时调用
    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("-----------init--------");
    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }
    //每次访问时调用
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
//        System.out.println("hello servlet");
        System.out.println("-----------servlet--------");
    }

    @Override
    public String getServletInfo() {
        return null;
    }

    //应用卸载的时候
    @Override
    public void destroy() {
        System.out.println("-----------destroy--------");
    }
}

输出结果

-----ServletDemo1----
-----------init--------
-----------servlet--------
-----------destroy--------

Servlet的三种创建方式

1. 实现javax.servlet.Servlet接口（如上）
2. 继承javax.servet.GenericServlet类(适配器模式)

public class ServletDemo2 extends GenericServlet {

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("GenericServlet");
    }
}

3. 继承javax.servlet.http.HttpServlet类（模板方法设计模式）
   (开发中常用方式)

public class ServletDemo3 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        super.doGet(req, resp);
        System.out.println("doget");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        super.doPost(req, resp);
        System.out.println("dopost");
    }
}

Servlet --> GenericServlet --> HttpServlet --> (继承HttpServlet)
曾祖父 --> 爷爷 --> 爸爸 --> 孙子

servlet映射细节

<servlet-mapping>
        <servlet-name>servletDemo3</servlet-name>
        <url-pattern>/demo3</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>servletDemo3</servlet-name>
        <url-pattern>/demo33</url-pattern>
    </servlet-mapping>

2. 通配符* 代表任意字符串
   url-pattern: .do 以.字符串的请求都可以访问 注：不要加/
   url-pattern: /* 任意字符串都可以访问
   url-pattern： /action/* 以/action开头的请求都可以访问
   匹配规则：
   优先级：从高到低
   绝对匹配--> /开头匹配 --> 扩展名方式匹配
   如果url-pattern的值是/，表示执行默认映射。所有资源都是servlet

Servlet的线程安全

单实例：每次访问多线程
解决线程安全问题的最佳办法，不要写全局变量，而写局部变量。

Servlet获取配置信息

ServletConfig的使用
作用1：可以获取servlet配置信息

@WebServlet(name = "ServletConfigDemo1", urlPatterns = {"/confdemo1"}, initParams = {@WebInitParam(name = "encoding",value = "GBK")})
public class ServletConfigDemo1 extends HttpServlet {

    private ServletConfig config;
    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        this.config = config;
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String s = config.getInitParameter("encoding");
        System.out.println(s);
    }
}

   protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        String s = config.getInitParameter("encoding");

        String s = this.getInitParameter("encoding");
        System.out.println(s);

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        String s = config.getInitParameter("encoding");

//        String s = this.getInitParameter("encoding");
        
        String s = this.getServletConfig().getInitParameter("encoding");
        
        System.out.println(s);
        

    }

作用2：可以获得ServletContext对象

ServletContext

ServletContext: 代表的是整个应用。一个应用只有一个ServletContext对象。单实例。

作用

1.域对象：在一定范围内（当前应用），使多个Servlet共享数据。
常用方法：
void setAttribute(String name,object value);//向ServletContext对象的map中添加数据
Object getAttribute(String name);//从ServletContext对象的map中取数据
void rmoveAttribute(String name);//根据name去移除数据

@WebServlet(name = "ServletContextDemo1", urlPatterns = "/context1")
public class ServletContextDemo1 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        servletContext.setAttribute("name","ljb");
    }
}
-----------------------
@WebServlet(name = "ServletContextDemo2", urlPatterns = "/context2")
public class ServletContextDemo2 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String name = (String) this.getServletContext().getAttribute("name");
        System.out.println(name);
    }
}

2.获取全局配置信息

<!--配置全局信息-->
    <context-param>
        <param-name>encoding</param-name>
        <param-value>UTF-8</param-value>
    </context-param>

        String name = this.getServletContext().getInitParameter("encoding");
        System.out.println(name);

3. 获取资源路径
   String getRealPath(String path);//根据资源名称得到资源的绝对路径.
   可以得到当前应用任何位置的任何资源。
4. 实现Servlet的转发

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        ServletContext servletContext = this.getServletContext();
//        servletContext.setAttribute("name","ljb");

        System.out.println("我要转发了");
        ServletContext application = this.getServletContext();
        application.getRequestDispatcher("/context2").forward(request,response);
        System.out.println("转发结束");
    }