Javaweb —— HttpServlet类

心已赠人 2024-04-19 09:43 12阅读 0赞

## Servlet应用 —— HTTPServlet类 ##

由于大多数Web应用都是通过HTTP协议和客户端进行交互，因此，在Servlet接口中，提供了一个抽象类javax.http.HttpServlet，它是GenericServlet的子类，专门用于创建应用于HTTP协议的Servlet。为了更好的了解HTTPServlet，接下来，看一下HTTPServlet类的源码，

public abstract class HttpServlet extends GenericServlet{
    	 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);
            }
        }
      public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException
        {
            HttpServletRequest  request;
            HttpServletResponse response;
            
            if (!(req instanceof HttpServletRequest &&
                    res instanceof HttpServletResponse)) {
                throw new ServletException("non-HTTP request or response");
            }
    
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
    
            service(request, response);
        }
    
    }

**doGet()和doPost()**  
在HttpServlet的service(HttpServletRequest,HttpServletResponse)方法会去判断当前请求  
是GET还是POST。如果是GET请求，那么会去调用本类的doGet()方法，  
如果是POST请求会去调用doPost()方法，这说明我们在子类中去覆盖doGet()或doPost()方法即可。

public class AServlet extends HttpServlet {
                public void doGet(HttpServletRequest request, HttpServletResponse response)
                        throws ServletException, IOException {
                    System.out.println("hello doGet()...");
                }
            }
    
            public class BServlet extends HttpServlet {
                public void doPost(HttpServletRequest request, HttpServletResponse response)
                        throws ServletException, IOException {
                    System.out.println("hello doPost()...");
                }
            }