java web学习: 三大组件之servlet基础

java web技术三大组件之servlet

　　在ava web应用之中核心组件有三个:servlet 、Filter、Listener。本篇文章主要介绍servlet相关的基础知识

servlet的定义

从代码的角度看，搜索的Servlet都是实现了servlet接口的实现类，如果的单纯的实现servlet接口，实现servlet接口会比较麻烦，有好多功能需要自己写，所以我们一般继承HttpServlet类。

下面是Servlet的结构图

继承HttpServlet后的MyServlet类就会有许多扩展的方法供我们使用，如下图

下面的代码就是一个实现例子

package com.web.servlet;

import java.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
   
    public MyServlet() {
        
        super();
        System.out.println("MyServlet() 构造器运行");
    }
    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        System.out.println("servlet 开始初始化");
    }
    
    @Override
    public void destroy() {
        super.destroy();
        System.out.println("servelt开始销毁");
    }
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("请求执行了");
    }

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

}

Servlet定义好之后，可以通过web.xml进行注册，也可以通过ServletContainerInitializer接口的实现类进行注册(SPI机制实现)，

https://www.cnblogs.com/cplinux/p/12312496.html

https://www.cnblogs.com/cplinux/p/12323316.html

Servlet 的生命周期

从之前的内容我们知道，我们编写的MyServlet其实是一个实现了Servlet接口的类，那么这个类要运行就必须加载进虚拟机中，

（1）Servlet默认的加载时机是在第一次访问时，

（2）但是有的Servlet比较特殊，需要在Tomcat启动时就加载并且实例化，比如DispatcherServlet。这个可以通过web.xml文件中的<load-on-startup>进行配置。

（3）Servlet实例化之后，会调用执行init方法，并且这个方法只执行一次

  (4)  init方法执行之后，就是执行service方法，或者doGet/doPost方法，这些方法时用来相应用户请求的

（5） 当服务器关闭时，destroy() 方法只会被调用一次，在 Servlet 生命周期结束时被调用。 destroy() 方法可以让您的 Servlet 关闭数据库连接、停止后台线程、把 Cookie 列表或点击计数器写入到磁盘，并执行其他类似的清理活动。

package javax.servlet;

import java.io.IOException;




public interface Servlet {

    /**
     * Called by the servlet container to indicate to a servlet that the 
     * servlet is being placed into service.
     *
     * <p>The servlet container calls the <code>init</code>
     * method exactly once after instantiating the servlet.
     * The <code>init</code> method must complete successfully
     * before the servlet can receive any requests.
     *
     * <p>The servlet container cannot place the servlet into service
     * if the <code>init</code> method
     * <ol>
     * <li>Throws a <code>ServletException</code>
     * <li>Does not return within a time period defined by the Web server
     * </ol>
     *
     *
     * @param config            a <code>ServletConfig</code> 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;
    
    

    /**
     *
     * Returns a {@link ServletConfig} object, which contains
     * initialization and startup parameters for this servlet.
     * The <code>ServletConfig</code> object returned is the one 
     * passed to the <code>init</code> method. 
     *
     * <p>Implementations of this interface are responsible for storing the 
     * <code>ServletConfig</code> object so that this 
     * method can return it. The {@link GenericServlet}
     * class, which implements this interface, already does this.
     *
     * @return        the <code>ServletConfig</code> object
     *            that initializes this servlet
     *
     * @see         #init
     *
     */

    public ServletConfig getServletConfig();
    
    

    /**
     * Called by the servlet container to allow the servlet to respond to 
     * a request.
     *
     * <p>This method is only called after the servlet's <code>init()</code>
     * method has completed successfully.
     * 
     * <p>  The status code of the response always should be set for a servlet 
     * that throws or sends an error.
     *
     * 
     * <p>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 
     * <a href="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">
     * the Java tutorial on multi-threaded programming</a>.
     *
     *
     * @param req     the <code>ServletRequest</code> object that contains
     *            the client's request
     *
     * @param res     the <code>ServletResponse</code> 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.
     * 
     * <p>The string that this method returns should
     * be plain text and not markup of any kind (such as HTML, XML,
     * etc.).
     *
     * @return         a <code>String</code> containing servlet information
     *
     */

    public String getServletInfo();
    
    

    /**
     *
     * 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
     * <code>service</code> method have exited or after a timeout
     * period has passed. After the servlet container calls this 
     * method, it will not call the <code>service</code> method again
     * on this servlet.
     *
     * <p>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();
}

Servlet的实例对象是单例的，在整个tomcat运行期间，一个Servlet只有一个实例，但是每个Http请求确有一个线程，如果一个Servlet被多次请求，那么就会产生多个线程。