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？
