Servlet类源码说明

servlet是学习java web不可不懂的一个类，网上各种教程都参杂太多，每次理解都感觉像把别人吐出来的食物再放在嘴里咀嚼，小编一怒之下，直接打开源码，原汁原味的芬芳扑面而来：

/**
 * Defines methods that all servlets must implement.
 *
 * <p>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. 
 *
 * <p>To implement this interface, you can write a generic servlet
 * that extends
 * <code>javax.servlet.GenericServlet</code> or an HTTP servlet that
 * extends <code>javax.servlet.http.HttpServlet</code>.
 *
 * <p>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:
 * <ol>
 * <li>The servlet is constructed, then initialized with the <code>init</code> method.
 * <li>Any calls from clients to the <code>service</code> method are handled.
 * <li>The servlet is taken out of service, then destroyed with the 
 * <code>destroy</code> method, then garbage collected and finalized.
 * </ol>
 *
 * <p>In addition to the life-cycle methods, this interface
 * provides the <code>getServletConfig</code> method, which the servlet 
 * can use to get any startup information, and the <code>getServletInfo</code>
 * method, which allows the servlet to return basic information about itself,
 * such as author, version, and copyright.
 *
 * @author     Various
 *
 * @see     GenericServlet
 * @see     javax.servlet.http.HttpServlet
 *
 */

public interface Servlet {

 1、Servlet是一个接口，定义的所有方法将被所有子类servlet实现；

2、一个Servlet是一个小java程序，必须与Web server一起运行；

3、Servlet从Web客户端获取请求并响应请求，通常通过Http(超文本传输协议）；

4、通过实现这个Servlet接口，你可以写一个通用的servlet（继承于javax.servlet.GenericServlet），或者写一个支持HTTP协议的servlet(继承javax.servlet.http.HttpServlet）；

5、这个接口定义一些方法去初始化一个servlet，去响应请求，并且可以从server中销毁servlet；

6、这些被称为生命周期的方法，以下面的顺序执行：

①：构造一个servlet实例，调用 init 方法进行初始化；

②：调用service方法处理客户端发过来的任何请求；

③：调用destroy方法销毁servlet，并由垃圾回收器终结这个servlet对象。

    /**
     * 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;

1、被servlet容器调用来表明是一个servlet，并且这个servlet会用来实现你的业务逻辑；

2、在servlet实例创建以后 init 方法被调用，并且只被调用一次；

3、在servlet处理各种请求之前，init方法必需成功运行，否则当抛出ServletException时，servlet将不会接收到任何请求；

4、可以在init方法中传入ServletConfig初始化参数

     /**
     *
     * 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();

 1、这个方法返回一个ServletConfig（一个接口）类的对象，包含了初始化servlet时的参数；

2、ServletConfig这个接口的实现负责存储servlet的初始化参数。

     /**
     * 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;

 1、这个方法在init()方法调用成功后调用，此方法是servlet容器负责调用来授予servlet对请求进行响应；

2、Servlets通常运行在多线程的servlet容器内部，可以同时处理多个请求。开发人员必须认识到同步访问任何共享资源，如文件、网络连接，以及servlet的类变量和实例变量。

 /**
     * 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();

 1、此方法返回关于servlet的信息，如作者、版本、版权；

2、这个方法返回的必须是纯字符串文本，不能带有html、xml等标记

 /**
     *
     * 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、此方法被servlet容器调用，指示了这个servlet已经完成业务实现，可以被销毁了；

2、此方法只被调用一次，在servlet的service方法中的线程都退出或者时间超时时调用；该servlet将不再调用service方法；

3、这个方法给servlet一个机会去清理所占用的资源（内存、文件、线程），确保与servlet的当前内存状态保持同步的持续状态。

读完源码，大脑立马清晰多了。釉木友同感？