上一篇看了Servlet接口，现在来看下我们经常涉及的Httpservlet:
/** * * Provides an abstract class to be subclassed to create * an HTTP servlet suitable for a Web site. A subclass of * <code>HttpServlet</code> must override at least * one method, usually one of these: * * <ul> * <li> <code>doGet</code>, if the servlet supports HTTP GET requests * <li> <code>doPost</code>, for HTTP POST requests * <li> <code>doPut</code>, for HTTP PUT requests * <li> <code>doDelete</code>, for HTTP DELETE requests * <li> <code>init</code> and <code>destroy</code>, * to manage resources that are held for the life of the servlet * <li> <code>getServletInfo</code>, which the servlet uses to * provide information about itself * </ul> * * <p>There's almost no reason to override the <code>service</code> * method. <code>service</code> handles standard HTTP * requests by dispatching them to the handler methods * for each HTTP request type (the <code>do</code><i>XXX</i> * methods listed above). * * <p>Likewise, there's almost no reason to override the * <code>doOptions</code> and <code>doTrace</code> methods. * * <p>Servlets typically run on multithreaded servers, * so be aware that a servlet must handle concurrent * requests and be careful to synchronize access to shared resources. * Shared resources include in-memory data such as * instance or class variables and external objects * such as files, database connections, and network * connections. * See the * <a href="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html"> * Java Tutorial on Multithreaded Programming</a> for more * information on handling multiple threads in a Java program. * * @author Various */ public abstract class HttpServlet extends GenericServlet implements java.io.Serializable { private static final String METHOD_DELETE = "DELETE"; private static final String METHOD_HEAD = "HEAD"; private static final String METHOD_GET = "GET"; private static final String METHOD_OPTIONS = "OPTIONS"; private static final String METHOD_POST = "POST"; private static final String METHOD_PUT = "PUT"; private static final String METHOD_TRACE = "TRACE"; private static final String HEADER_IFMODSINCE = "If-Modified-Since"; private static final String HEADER_LASTMOD = "Last-Modified"; private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings"; private static ResourceBundle lStrings = ResourceBundle.getBundle(LSTRING_FILE);
HttpServlet是一个抽象类，它是为了实现http协议的servlet，所有继承此抽象类的servlet必须实现以下方法中的一种：
doGet;
doPost;
doPut;
doDelete;
init 和 destroy;
没有必要去重写service方法，service处理标准的http请求，根据不同的HTTP请求类型分发给以上的doXXX方法进行处理；
其他的描述和上一篇的servlet一样，在此就不多费篇幅咯。
下面重点看看doGet方法和doPost方法：
1、doGet:
/** * * Called by the server (via the <code>service</code> method) to * allow a servlet to handle a GET request. * * <p>Overriding this method to support a GET request also * automatically supports an HTTP HEAD request. A HEAD * request is a GET request that returns no body in the * response, only the request header fields. * * <p>When overriding this method, read the request data, * write the response headers, get the response's writer or * output stream object, and finally, write the response data. * It's best to include content type and encoding. When using * a <code>PrintWriter</code> object to return the response, * set the content type before accessing the * <code>PrintWriter</code> object. * * <p>The servlet container must write the headers before * committing the response, because in HTTP the headers must be sent * before the response body. * * <p>Where possible, set the Content-Length header (with the * {@link javax.servlet.ServletResponse#setContentLength} method), * to allow the servlet container to use a persistent connection * to return its response to the client, improving performance. * The content length is automatically set if the entire response fits * inside the response buffer. * * <p>When using HTTP 1.1 chunked encoding (which means that the response * has a Transfer-Encoding header), do not set the Content-Length header. * * <p>The GET method should be safe, that is, without * any side effects for which users are held responsible. * For example, most form queries have no side effects. * If a client request is intended to change stored data, * the request should use some other HTTP method. * * <p>The GET method should also be idempotent, meaning * that it can be safely repeated. Sometimes making a * method safe also makes it idempotent. For example, * repeating queries is both safe and idempotent, but * buying a product online or modifying data is neither * safe nor idempotent. * * <p>If the request is incorrectly formatted, <code>doGet</code> * returns an HTTP "Bad Request" message. * * * @param req an {@link HttpServletRequest} object that * contains the request the client has made * of the servlet * * @param resp an {@link HttpServletResponse} object that * contains the response the servlet sends * to the client * * @exception IOException if an input or output error is * detected when the servlet handles * the GET request * * @exception ServletException if the request for the GET * could not be handled * * * @see javax.servlet.ServletResponse#setContentType * */ protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_get_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } }
^此方法由服务器调用允许一个servlet去处理一个GET请求；
^重写此方法去支持一个GET请求，同时也支持一个HEAD请求，一个HEAD请求是一个得到响应中没有返回任何主体的GET请求，只有请求头字段；
^当载入此方法时，读取请求数据，写入响应头，获取response的writer对象或者输出流对象，最终写入response数据，最好包含内容类型和编码方式；
^servlet容器必须在提交response之前写头文件，因为在HTTP协议中文件头必须在response body体之前提交；
^set方法是安全的，也就是说这个方法没有任何需要用户负责任的副作用；比如：大多数的查询都没有副作用，当你试图改变数据库的数据时，则需要使用其他方法。也就是说GET方法只用于查询某些简单的数据；
2、doPost方法：
/** * * Called by the server (via the <code>service</code> method) * to allow a servlet to handle a POST request. * * The HTTP POST method allows the client to send * data of unlimited length to the Web server a single time * and is useful when posting information such as * credit card numbers. * * <p>When overriding this method, read the request data, * write the response headers, get the response's writer or output * stream object, and finally, write the response data. It's best * to include content type and encoding. When using a * <code>PrintWriter</code> object to return the response, set the * content type before accessing the <code>PrintWriter</code> object. * * <p>The servlet container must write the headers before committing the * response, because in HTTP the headers must be sent before the * response body. * * <p>Where possible, set the Content-Length header (with the * {@link javax.servlet.ServletResponse#setContentLength} method), * to allow the servlet container to use a persistent connection * to return its response to the client, improving performance. * The content length is automatically set if the entire response fits * inside the response buffer. * * <p>When using HTTP 1.1 chunked encoding (which means that the response * has a Transfer-Encoding header), do not set the Content-Length header. * * <p>This method does not need to be either safe or idempotent. * Operations requested through POST can have side effects for * which the user can be held accountable, for example, * updating stored data or buying items online. * * <p>If the HTTP POST request is incorrectly formatted, * <code>doPost</code> returns an HTTP "Bad Request" message. * * * @param req an {@link HttpServletRequest} object that * contains the request the client has made * of the servlet * * @param resp an {@link HttpServletResponse} object that * contains the response the servlet sends * to the client * * @exception IOException if an input or output error is * detected when the servlet handles * the request * * @exception ServletException if the request for the POST * could not be handled * * * @see javax.servlet.ServletOutputStream * @see javax.servlet.ServletResponse#setContentType * * */ protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_post_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } }
此时你会发现与上面的doGet方法的描述非常类似。唯一的不同是doPost会处理修改数据的请求。
其他的四种方法我就不一一列举了，大同小异，大家自己看一下。