模板方法模式在Servlet中的应用

在 Servlet 中，每一个 Servlet 类都必须要实现 HttpServlet 接口。GenericServlet 是个通用的、不特定于任何协议的 Servlet，它实现了 Servlet 接口。而 HttpServlet 继承于 GenericServlet，为 Servlet 接口提供了处理 HTTP 协议的通用实现，所以我们定义 Servlet 时只需要继承 HttpServlet 即可。

HttpServlet 源码如下：

public abstract class HttpServlet extends GenericServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ...
    }

    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 {
            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);
        }
    }

    // ...省略...
}

在 HttpServlet 的 service 方法中，首先获取请求的方法名，然后根据方法名调用对应的 doXXX 方法，比如说请求方法为 GET，那么就去调用 doGet 方法；请求方法为 POST，那么就去调用 doPost 方法。

这里，HttpServlet 就相当于定义了一套处理 HTTP 请求的模板。service 方法为模板方法，定义了处理 HTTP 请求的基本流程；doXXX 等方法为基本方法，根据请求方法做相应的处理，子类可重写这些方法。HttpServletRequest 中的 Method 则起到钩子方法的作用。

比如实现一个输出 Hello World 的简单 Servlet，代码如下。

public class HelloWorld extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h1>Hello World!</h1>");
    }
}

该自定义的 Servlet 重写了 doGet 方法，当客户端发起 GET 请求时将得到 <h1>Hello World!</h1>。