javaee 中servlet的简单教学，虽说j2ee已经是遥远时代，但是java web开发依然无法离开servlet，从底层学起，详细了解web实际的运行过程，会对将来学习框架更有帮助

Servlet总结

1.Servlet接口

创建HelloServlet实现Servlet接口,作为实例代码

1.Servlet生命周期方法:以下方法都是Servlet容器负责调用(tomcat就是一个Servlet容器)

1.构造方法

只要第一次请求Servlet时,创建实例,调用构造器.说明Servlet是单实例的

    public HelloServlet() {
        // TODO Auto-generated constructor stub
        System.out.println("HelloServlet's constructor");
    }

2.init()方法:只被调用一次,在创建好实例后立即被调用.用于初始化当前的构造方法

  @Override
    public void init(ServletConfig config) throws ServletException {
    }

3.service()方法:被调用多次,每次请求都会调用service方法.实际用于响应请求

    @Override
    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        System.out.println("service");
        
    }

4.destroy()方法:只被调用一次,在当前Servlet所在的WEB应用被卸载前调用.用于释放当前Servlet所占用的资源

@Override
    public void destroy() {
        // TODO Auto-generated method stub
        System.out.println("destroy");
    }

2.servlet配置web.xml

每创建一个Servlet需要在web.xml中配置相关信息

<servlet>
	<!-- Servlet的名字 -->
    <servlet-name>helloServlet</servlet-name>
    <!-- Servlet的java文件的路径 -->
    <servlet-class>com.kingkiller.HelloServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <!-- Servlet的名字,需和上面的相同 -->
    <servlet-name>helloServlet</servlet-name>
      <!-- Servlet的url路径 -->
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

load-on-startup参数:

1.配置在servlet节点中 2.可以指定Servlet被创建的时机，若为负数，则在第一次请求时被创建，若为0或正数，则在当前WEB应用被Servlet容器加载时创建实例，且数值越小越早被创建

  <servlet>
    <servlet-name>helloServlet</servlet-name>
    <servlet-class>com.kingkiller.HelloServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
  </servlet>

3..Servlet映射的细节

1.同一个Servlet可以被映射到多个URL上，即多个元素的子元素的设置可以是同一个Servlet的注册名。 2.在Servlet映射到URL中也可以使用通配符，但是只能有两种固定的格式，一种格式是".拓展名"，另一种格式是以正斜杠(/)开头并以"/*“结尾

2.ServletConfig

封装了Servlet的配置信息,并且可以获取ServletContext对象

1.获取参数的方法

1.getInitParameter(String name):获取指定参数名的初始化参数

2.getInitParameterNames():获取参数名组成的Enumeration对象

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
        String user = config.getInitParameter("user");
        System.out.println("user:"+user);
        Enumeration<String> names = config.getInitParameterNames();
        while (names.hasMoreElements()) {
            String name = names.nextElement();
            System.out.println("name"+name);
        }
  	}

2.获取Servlet名字的方法

1.getServletName:获得注册的Servlet的名字

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		String servletName = config.getServletName();
         System.out.println(servletName);
  	}

3.获取Context的方法

1.getServletContext:返回这个Servlet的ServletContext对象。

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		ServletContext servletContext = config.getServletContext();	
  	}

3.ServletContext接口

1.可以由servletConfig获取 2.对象代表当前WEB应用：可以认为ServletContext是当前WEB应用的一个大管家，可以从中获取到WEB应用的各个方面的信息

1.获取当前WEB应用的初始化参数

1.在web.xml配置当前WEB应用的初始化参数

<servlet>
    <servlet-name>helloServlet</servlet-name>
    <servlet-class>com.kingkiller.HelloServlet</servlet-class>
    <init-param>
      <param-name>user</param-name>
      <param-value>root</param-value>
    </init-param>
    <init-param>
      <param-name>password</param-name>
      <param-value>1234</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>

注意放在load-on-startup前

2.常用方法:

getInitParameter(): Returns a String containing the value of the named context-wide initialization parameter, or null if the parameter does not exist.

getInitParameterNames(): Returns the names of the context’s initialization parameters as an Enumeration of String objects, or an empty Enumeration if the context has no initialization parameters.

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		ServletContext servletContext = config.getServletContext();
         String driver = servletContext.getInitParameter("driver");
         System.out.println("driver:"+driver);
         Enumeration<String> names2 = servletContext.getInitParameterNames();
         while (names2.hasMoreElements()) {
            String name = names2.nextElement();
            System.out.println(" -->" + name);
         }
  	}

2.获取当前web应用的某一个文件在服务器上的绝对路径,而不是部署前的路径

1.getRealPath(String path): Returns a String containing the real path for a given virtual path.

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		ServletContext servletContext = config.getServletContext();
         String realPath = servletContext.getRealPath("/note.txt");
         System.out.println(realPath);
  	}

3.获取当前WEB应用的名称

1.getContextPath(): Returns the context path of the web application.

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		ServletContext servletContext = config.getServletContext();
         String contextPath = servletContext.getContextPath();
         System.out.println(contextPath);
  	}

4.获取当前WEB应用的某一个文件对应的输入流

1.getResourceAsStream(String):Returns the resource located at the named path as an InputStream object.

 @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
		ServletContext servletContext = config.getServletContext();
          try {
            ClassLoader classLoader = getClass().getClassLoader();
            InputStream is = classLoader.getResourceAsStream("jdbc.properties");
            System.out.println("1." + is);
        } catch (Exception e) {
            // TODO: handle exception
        }
        try {
            InputStream is2 = servletContext.getResourceAsStream("/WEB-INF/classes/jdbc/properties");
            System.out.println("2." + is2);
        } catch (Exception e) {
            // TODO: handle exception
        }
  	}

path的”/“为当前WEB应用的根目录

5.和attribute相关的方法

4.在Servlet中获取请求信息

Servlet的service()方法用于应答请求:因为每次请求都会调用service()方法 ServletRequest:封装了请求信息.可以从中获取到任何的请求信息. ServletResponse:封装了响应信息,如果想给用户什么响应,具体可以使用该借口的方法实现这两个接口的实现类都是服务器给予实现的,并在服务器调用service方法时传入

下面将会详细讲述ServletRequest和ServletResponse这两个接口

创建login.html做测试

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>

	<form action="longinServlet" method="post">
		user:<input type="text" name="user"/>
		password:<input type="password" name="password"/>
		<br><br>
		interesting:
		<input type="checkbox" name="interesting" value="reading"/>Reading
		<input type="checkbox" name="interesting" value="game"/>Game
		<input type="checkbox" name="interesting" value="party"/>Party
		<input type="checkbox" name="interesting" value="shopping"/>Shopping
		<input type="checkbox" name="interesting" value="sport"/>Sport
		<input type="checkbox" name="interesting" value="tv"/>TV
		<input type="submit" value="提交"/>
	</form>
</body>
</html>

5.ServletRequest接口

1.获取请求参数

1.String getParameter(String name):以一个String返回指定的参数的值,如果这个参数不存在返回空值.但若接收的参数值为多个时,只能接收第一个的值

2.Enumeration getParameterNames():返回所有的参数名的String对象列表,如果没有输入参数,则该返回一个空值

3.String[] getParameterValues(String name):通过一个String对象的数组返回指定参数的值,如果这个参数不存在,该方法返回一个空值

4.Map getParameterMap():返回请求参数的键值对,key:参数名,value:参数值.

    @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        System.out.println("请求来了");
        System.out.println(request);
        String user = request.getParameter("user");
        String password = request.getParameter("password");
        System.out.println(user+":"+password);
        String interesting = request.getParameter("interesting");
        System.out.println(interesting);
        String[] interestings = request.getParameterValues("interesting");
        for (String interest : interestings) {
            System.out.println(interest);
        }
        Enumeration<String> names = request.getParameterNames();
        while (names.hasMoreElements()) {
            String name = names.nextElement();
            String val = request.getParameter(name);//因为这里用的是getParameter所有在传入多个参数时只获取到第一个
            System.out.println(name + ":" + val);
        }
        Map<String, String[]> map = request.getParameterMap();
        for (Entry<String, String[]>  entry : map.entrySet()) {
            System.out.println(entry.getKey() + ":" + Arrays.asList(entry.getValue()));
        }
     }

2.获取请求的url

需要强转成其子类HttpServletRequest,再调用getRequestURI()方法

    @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
		HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        String requestURI = httpServletRequest.getRequestURI();
        System.out.println(requestURI);
     }

3.获取请求方式

1.getMethod:也是HttpServletRequest里的方法

 @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
		HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        String method = httpServletRequest.getMethod();
        System.out.println(method);
     }

4.若是一个GET请求,获取请求参数对应的字符串

即url中?后的部分

getQueryString()

 @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
		HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        String queryString = httpServletRequest.getQueryString();
        System.out.println(queryString);
     }

5.获取Servlet的请求路径

即url-pattern

getServletPath()

 @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
		HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        String servletPath = httpServletRequest.getServletPath();
        System.out.println(servletPath);
     }

6.和attribute相关的几个方法

6.ServletResponse接口

1.getWriter()

返回PrintWriter对象,调用该对象的print方法

 @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        PrintWriter out = response.getWriter();
        out.println("helloworld");
     }

2.设置响应的内容类型:

responseContentType()

 @Override
    public void service(ServletRequest request, ServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        response.setContentType("application/msword");//设置响应类型为WORD文档,发送request请求后,会直接下载一个含有response内容的word文档
        PrintWriter out = response.getWriter();
        out.println("helloworld");
     }

3.请求重定向

void sendRedirect(String location):此方法定义在HttpServletResponse中定义

7.GenericServlet

1.是一个Servlet,是Servlet接口和ServletConfig接口的实现类.但是是一个抽象类,其中的service方法为抽象方法 2.如果新建的Servlet程序直接继承GenericServlet会使开发更加的简洁 3.具体实现: 1.在GenericServlet里面,声明了一个ServletConfig类型的成员变量, 在init(ServletConfig)方法中对其初始化 2.利用了servletConfig成员变量的方法实现了ServletConfig接口的方法 3.还定义了一个init()方法,在init(ServletConfig)方法中对其调用,子类可以直接重写init(),在其中实现对Servlet的初始化 4.不建议直接覆盖init(Servlet),因为如果忘记编写super(ServletConfig),则会出现空指针异常 5.新建的init()并非Servlet的生命周期方法,而init(ServletConfig)是生命周期方法

下面我们自已创建一个MyGenericServlet(不包含两个日志方法)

import java.io.IOException;
import java.util.Enumeration;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public abstract class MyGenericServlet implements Servlet,ServletConfig{
    /**
     * 下面是实现Servlet的方法
     */
    private ServletConfig config;
    @Override
    public void destroy() {
        // TODO Auto-generated method stub
    }
    @Override
    public ServletConfig getServletConfig() {
        // TODO Auto-generated method stub
        return config;
    }
    @Override
    public String getServletInfo() {
        // TODO Auto-generated method stub
        return null;
    }
    @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
        this.config = config;
        init();
    }
    private void init() {
        // TODO Auto-generated method stub
    }
    @Override
    public abstract void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException;
    /**
             * 下面是实现ServletConfig的方法
     */
    @Override
    public String getInitParameter(String name) {
        // TODO Auto-generated method stub
        return config.getInitParameter(name);
    }
    @Override
    public Enumeration<String> getInitParameterNames() {
        // TODO Auto-generated method stub
        return config.getInitParameterNames();
    }
    @Override
    public ServletContext getServletContext() {
        // TODO Auto-generated method stub
        return config.getServletContext();
    }
    @Override
    public String getServletName() {
        // TODO Auto-generated method stub
        return config.getServletName();
    }
}

8.HttpServlet

1.是一个Servlet,继承自GenericServlet,针对HTTP协议定制 2.在service()方法中直接把ServletRequest和ServetResponse转为HttpServletRequest和HttpServletResponse 并调用了重载的service(HttpServletRequest,HttpServletResponse)方法在service(HttpServletRequest,HttpServletResponse)获取了侵权方式:request.getMethod()根据请求方式又创建 doPost,doGet方法 3.实际开发中,直接继承HttpServlet,并根据请求方式覆写doXxx()方法接口 4.好处:直接由针对性的覆盖doXxx()方法;直接使用HttpServletRequest合HttpServletResponse,不再需要强转

下面我们创建自己的MyHttpServlet进行测试

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * 针对HTTP协议定义的一个Servlet基类
 * @author Administrator
 *
 */
public class MyHttpServlet extends MyGenericServlet {
    @Override
    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        if (req instanceof HttpServletRequest) {
            HttpServletRequest request = (HttpServletRequest)req;
            if (res instanceof HttpServletResponse) {
                HttpServletResponse response = (HttpServletResponse) res;
                service(request, response);
            }
        }
    }
    public void service(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException {
        //1.获取请求方式
        String method = request.getMethod();
        //2.根据请求方式再调用对应的处理方法
        if ("GET".equalsIgnoreCase(method)) {
            doGet(request,response);
        }
        if ("POST".equalsIgnoreCase(method)) {
            doPost(request,response);
        }
    }
    public void doPost(HttpServletRequest request,
            HttpServletResponse response) {
        // TODO Auto-generated method stub
    }

    public void doGet(HttpServletRequest request,
            HttpServletResponse response) {
        // TODO Auto-generated method stub      
    }
}

Servlet总结
@ kingkiller | Thursday, Jan 28, 2021 | 6 minutes read | Update at Thursday, Jan 28, 2021