05-HttpServlet类

前情提要

在前面我们学习了Servlet和它的5个接口，学习了ServletConfig和ServletContext对象，下面介绍web开发中实际用到的HttpServlet接口。

详解HttpServlet

public abstract class HttpServlet extends GenericServlet

HttServlet继承自GenericServlet，它重写了service方法

public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {

        HttpServletRequest  request;
        HttpServletResponse response;

        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException(lStrings.getString("http.non_http"));
        }
        service(request, response);
    }

将ServletRequest和ServletResponse对象向下转型成HttpServletRequest和HttpServletResponse，然后执行自己的重载的service方法。

 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;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // 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);
        }
    }

而重载的service方法遵循了模板类设计模式的思想，首先通过getMethod方法，判断当前浏览器的请求是什么类型，包括GET，POST， DELETE等七种类型，对于不同的类型走不同的方法，比如若是GET则走doGet方法。
这些方法是需要子类的复现的，HttpServlet规定好了流程，而流程的子过程需要子类自己实现。

在浏览器键入地址，点击超链接这种都是GET方法，那就需要实现doGet方法。如果前端是一个GET请求，而后端servlet中没有对象的doGet方法，就会报405错误。

欢迎页的设置

web.xml中可以设置欢迎页

  <welcome-file-list>
        <welcome-file>index.htmlwelcome-file>
        <welcome-file>index.htmwelcome-file>
        <welcome-file>index.jspwelcome-file>
    welcome-file-list>

优先级从上到下对应从高到低。
Tomcat的config目录下也有个web.xml文件，其中它的欢迎页配置就如上所示。
现在我们键入http://localhost:8080/项目名，那么就会自动跳转到欢迎页。