Servlet使用基本步骤及功能实现

Servlet（Server Applet）是Java Servlet的简称，称为服务器端小程序，用Java代码进行编写，具有独立于平台和协议的特性，主要功能在于交互式地浏览和生成数据，生成动态Web内容。

Servlet是指任何实现了这个Servlet接口的类，运行于支持Java的应用服务器中。

源码：

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package javax.servlet;

import java.io.IOException;

/**
 * Defines methods that all servlets must implement.
 *
 * 

 * A servlet is a small Java program that runs within a Web server. Servlets
 * receive and respond to requests from Web clients, usually across HTTP, the
 * HyperText Transfer Protocol.
 *
 * 

 * To implement this interface, you can write a generic servlet that extends
 * javax.servlet.GenericServlet or an HTTP servlet that extends
 * javax.servlet.http.HttpServlet.
 *
 * 

 * This interface defines methods to initialize a servlet, to service requests,
 * and to remove a servlet from the server. These are known as life-cycle
 * methods and are called in the following sequence:
 * 

 * 
The servlet is constructed, then initialized with the init
 * method.
 * 
Any calls from clients to the service method are handled.
 * 
The servlet is taken out of service, then destroyed with the
 * destroy method, then garbage collected and finalized.
 * 
 *
 * 

 * In addition to the life-cycle methods, this interface provides the
 * getServletConfig method, which the servlet can use to get any
 * startup information, and the getServletInfo method, which allows
 * the servlet to return basic information about itself, such as author,
 * version, and copyright.
 *
 * @see GenericServlet
 * @see javax.servlet.http.HttpServlet
 */
public interface Servlet {
     

    /**
     * Called by the servlet container to indicate to a servlet that the servlet
     * is being placed into service.
     *
     * 

     * The servlet container calls the init method exactly once
     * after instantiating the servlet. The init method must
     * complete successfully before the servlet can receive any requests.
     *
     * 

     * The servlet container cannot place the servlet into service if the
     * init method
     * 

     * 
Throws a ServletException
     * 
Does not return within a time period defined by the Web server
     * 
     *
     *
     * @param config
     *            a ServletConfig object containing the servlet's
     *            configuration and initialization parameters
     *
     * @exception ServletException
     *                if an exception has occurred that interferes with the
     *                servlet's normal operation
     *
     * @see UnavailableException
     * @see #getServletConfig
     */
    public void init(ServletConfig config) throws ServletException;//对象的初始化创建

    /**
     *
     * Returns a {@link ServletConfig} object, which contains initialization and
     * startup parameters for this servlet. The ServletConfig
     * object returned is the one passed to the init method.
     *
     * 

     * Implementations of this interface are responsible for storing the
     * ServletConfig object so that this method can return it. The
     * {@link GenericServlet} class, which implements this interface, already
     * does this.
     *
     * @return the ServletConfig object that initializes this
     *         servlet
     *
     * @see #init
     */
    public ServletConfig getServletConfig();//拿到当前web对象的配置文件

    /**
     * Called by the servlet container to allow the servlet to respond to a
     * request.
     *
     * 

     * This method is only called after the servlet's init() method
     * has completed successfully.
     *
     * 

     * The status code of the response always should be set for a servlet that
     * throws or sends an error.
     *
     *
     * 

     * Servlets typically run inside multithreaded servlet containers that can
     * handle multiple requests concurrently. Developers must be aware to
     * synchronize access to any shared resources such as files, network
     * connections, and as well as the servlet's class and instance variables.
     * More information on multithreaded programming in Java is available in 
     * the Java tutorial on multi-threaded programming.
     *
     *
     * @param req
     *            the ServletRequest object that contains the
     *            client's request
     *
     * @param res
     *            the ServletResponse object that contains the
     *            servlet's response
     *
     * @exception ServletException
     *                if an exception occurs that interferes with the servlet's
     *                normal operation
     *
     * @exception IOException
     *                if an input or output exception occurs
     */
    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException;

    /**
     * Returns information about the servlet, such as author, version, and
     * copyright.
     *
     * 

     * The string that this method returns should be plain text and not markup
     * of any kind (such as HTML, XML, etc.).
     *
     * @return a String containing servlet information
     */
    public String getServletInfo();//当前servlet的基本信息


    /**
     * Called by the servlet container to indicate to a servlet that the servlet
     * is being taken out of service. This method is only called once all
     * threads within the servlet's service method have exited or
     * after a timeout period has passed. After the servlet container calls this
     * method, it will not call the service method again on this
     * servlet.
     *
     * 

     * This method gives the servlet an opportunity to clean up any resources
     * that are being held (for example, memory, file handles, threads) and make
     * sure that any persistent state is synchronized with the servlet's current
     * state in memory.
     */
    public void destroy();//对象销毁
}

Servlet使用步骤

1. 编写⼀个Java类实现Servlet的接口
2. 在web.xml中，配置Servlet
3. 访问servlet

//1. 编写⼀个Java类实现Servlet的接口

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class HelloServlet implements Servlet {
     
    @Override
    public void init(ServletConfig config) throws ServletException {
     

    }

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

    @Override
/**
 * ⽤来处理客户的请求，并且对请求作出响应的⽅法
 */
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
     
        System.out.println("⼀个servlet程序。。");
        //把服务器的数据 返回给浏览器显示
        // 向⻚⾯输出⼀个HelloServlet字符串:
        servletResponse.getWriter().print("hello servlet");
    }

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

    @Override
    public void destroy() {
     

    }
    //省略剩下⽅法，采⽤默认实现
}

//2. 在web.xml中，配置Servlet
<?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_4_0.xsd"
         version="4.0">
    <servlet>
        <servlet-name>helloServlet</servlet-name>
        <servlet-class>com.byc.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>helloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>        <!--匹配路径-->
    </servlet-mapping>
</web-app>