servlet学习总结一

1.什么是servlet:

Java Servlet 是运行在 Web 服务器或应用服务器上的程序，它是作为来自 Web 浏览器或其他 HTTP 客户端的请求和 HTTP 服务器上的数据库或应用程序之间的中间层。

其实我的理解，servlet就是在 遵循JAVAEE规范的服务器上运行的一小段用java编写的小程序.

2.servlet的实现方式

1) 直接implements servlet

2) 继承GenericSevlet

3) 继承HttpServlet

先看一下目录结构

当我看到这个目录结构的时候很费解，扒源码发现那个facesServlet类是final修饰的

不太明白这样设计的目的，希望对这方面了解的能交流一下。

3.servlet 的生命周期

我们先看一下API的方法

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 configa <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.     *     * @returnthe <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();}

1）初始化 调用init()方法 并且创建servlet实例

servlet是单例多线程模式，在服务器启动之后，如果当客户端第一次请求的时候，开始运行init()方法 并创建servlet实例，多个请求会自动创建多线程，对于每个线程来说，servlet实例都是单例的。在servlet的生命周期中，init()方法仅执行一次。

如果想在服务器启动的时候就运行init()方法，创建servlet实例 ,需要在web.xml 里配置  增加 <load-on-startup>1</load-on-startup> 即可

<span style="white-space:pre"></span><servlet>  <servlet-name>MyServlet</servlet-name><servlet-class>demo.com.cn.ssm.controller.UserServlet</servlet-class>  <load-on-startup>1</load-on-startup>  </servlet>  <servlet-mapping>  <servlet-name>MyServlet</servlet-name>  <url-pattern>/myServlet</url-pattern>  </servlet-mapping>

2）调用  调用service() 方法

我们来看一下开发的时候 直接extends HttpServlet 这种方式 ，  HttpServlet 的 service() 方法源码

protected void service(HttpServletRequest req, HttpServletResponse resp)        throws ServletException, IOException    {        String method = req.getMethod();        if (method.equals(METHOD_GET)) {            long lastModified = getLastModified(req);            if (lastModified == -1) {                // servlet doesn't support if-modified-since, no reason                // to go through further expensive logic                doGet(req, resp);            } else {                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);                if (ifModifiedSince < lastModified) {                    // If the servlet mod time is later, call doGet()                    // Round down to the nearest second for a proper compare                    // A ifModifiedSince of -1 will always be less                    maybeSetLastModified(resp, lastModified);                    doGet(req, resp);                } else {                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);                }            }        } else if (method.equals(METHOD_HEAD)) {            long lastModified = getLastModified(req);            maybeSetLastModified(resp, lastModified);            doHead(req, resp);        } else if (method.equals(METHOD_POST)) {            doPost(req, resp);                    } else if (method.equals(METHOD_PUT)) {            doPut(req, resp);                    } else if (method.equals(METHOD_DELETE)) {            doDelete(req, resp);                    } else if (method.equals(METHOD_OPTIONS)) {            doOptions(req,resp);                    } else if (method.equals(METHOD_TRACE)) {            doTrace(req,resp);                    } else {            //            // Note that this means NO servlet supports whatever            // method was requested, anywhere on this server.            //            String errMsg = lStrings.getString("http.method_not_implemented");            Object[] errArgs = new Object[1];            errArgs[0] = method;            errMsg = MessageFormat.format(errMsg, errArgs);                        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);        }    }

它的实现实际就是判断不同的提交方式来跳转不同的方法，所有开发的时候我们一般不去重写这个方法，只需根据情况重写 doGet() 和 doPost()等

public class UserServlet extends HttpServlet{/**  */private static final long serialVersionUID = -7018671373592622333L;@Overridepublic void doGet(HttpServletRequest request,HttpServletResponse response){System.out.println("doGet ~~~");}@Overridepublic void doPost(HttpServletRequest request,HttpServletResponse response){System.out.println("doPost ~~");}@Overridepublic void init(ServletConfig servletConfig) throws ServletException {System.out.println("初始化 servlet实例");}@Overridepublic void destroy() {System.out.println("销毁 servlet实例");}}

3）销毁 调用 destroy() 方法

调用 destroy()方法，即 开始恢复一些初始化的变量，释放资源，线程正常退出等。之后等待JVM垃圾回收。

4）回收 等待JVM回收