SpringMVC: ServletContext

[wittyResry]

Sevlet 接口

• Servlet介绍
• Servlet主要的接口

  public void init(ServletConfig config) throws ServletException;
  public void service(ServletRequest req, ServletResponse res)
  throws ServletException, IOException;
  public void destroy();

• GenericServlet 抽象类没有实现最重要的service()方法
• HttpServlet 抽象类实现了service()方法，根据一定的条件分发到doGet()或者doPost()方法中。

[wittyResry]

ServletContext

ServletConfig 对象中维护了 ServletContext 对象的引用，通过ServletConfig.getServletContext 方法获得ServletContext对象。

1. 一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。
2. String url = context.getInitParameter("url");

   <context-param>    
   <param-name>url</param-name>     
   <param-value>jdbc:mysql://localhost:3306</param-value>    
   </context-param>    

3. Servlet转发

   RequestDispatcher  rd  = context.getRequestDispatcher("/index.jsp");     
   rd.forward(request, response);  

• 根据Http请求方式的不同，分发的doPost和doGet。注意不需要重写该方法，只需求重写 doGet，doPost 等方法即可。

javax.servlet.http.HttpServlet#service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)

public abstract class HttpServlet extends GenericServlet {

    private static final long serialVersionUID = 1L;

    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";
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 {
                ...
            }
        } 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);
        }
    }