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JSP服务器响应(Response)

当Web服务器响应一个HTTP请求到浏览器中,反应通常由一个状态行,一些响应头,一个空行,这个文件。一个典型的反应看起来像这样:

HTTP/1.1 200 OK
Content-Type: text/html
Header2: ...
...
HeaderN: ...
  (Blank Line)
<!doctype ...>
<html>
<head>...</head>
<body>
...
</body>
</html>

状态行包含HTTP版本(HTTP/1.1中的例子),一个状态码(200中的例子),以及相应的状态码(确定在本例中),在很短的消息。

以下是回到浏览器从Web服务器端,你会使用它们非常频繁在网络编程中最有用的HTTP1.1响应头的摘要:

Header 描述
Allow This header specifies the request methods (GET, POST, etc.) that the server supports.
Cache-Control This header specifies the circumstances in which the response document can safely be cached. It can have values public, private orno-cache etc. Public means document is cacheable, Private means document is for a single user and can only be stored in private (nonshared) caches and no-cache means document should never be cached.
Connection This header instructs the browser whether to use persistent in HTTP connections or not. A value of close instructs the browser not to use persistent HTTP connections and keep-alive means using persistent connections.
Content-Disposition This header lets you request that the browser ask the user to save the response to disk in a file of the given name.
Content-Encoding This header specifies the way in which the page was encoded during transmission.
Content-Language This header signifies the language in which the document is written. For example en, en-us, ru, etc.
Content-Length This header indicates the number of bytes in the response. This information is needed only if the browser is using a persistent (keep-alive) HTTP connection.
Content-Type This header gives the MIME (Multipurpose Internet Mail Extension) type of the response document.
Expires This header specifies the time at which the content should be considered out-of-date and thus no longer be cached.
Last-Modified This header indicates when the document was last changed. The client can then cache the document and supply a date by an If-Modified-Since request header in later requests.
Location This header should be included with all responses that have a status code in the 300s. This notifies the browser of the document address. The browser automatically reconnects to this location and retrieves the new document.
Refresh This header specifies how soon the browser should ask for an updated page. You can specify time in number of seconds after which a page would be refreshed.
Retry-After This header can be used in conjunction with a 503 (Service Unavailable) response to tell the client how soon it can repeat its request.
Set-Cookie This header specifies a cookie associated with the page.

HttpServletResponse 对象:

响应对象是javax.servlet.http.HttpServletRequest对象的一个实例。正如服务器创建的请求对象,它还会创建一个对象来表示响应给客户端。

响应对象还定义了处理创造新的HTTP标头的接口。通过这个对象的JSP程序员可以添加新的Cookie或日期标记,HTTP状态代码等。

有以下方法可用于设置HTTP响应报头中的Servlet程序。这些方法可与HttpServletResponse对象表示服务器响应。

S.N. 方法 & 描述
1 String encodeRedirectURL(String url)
Encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged.
2 String encodeURL(String url)
Encodes the specified URL by including the session ID in it, or, if encoding is not needed, returns the URL unchanged.
3 boolean containsHeader(String name)
Returns a boolean indicating whether the named response header has already been set.
4 boolean isCommitted()
Returns a boolean indicating if the response has been committed.
5 void addCookie(Cookie cookie)
Adds the specified cookie to the response.
6 void addDateHeader(String name, long date)
Adds a response header with the given name and date-value.
7 void addHeader(String name, String value)
Adds a response header with the given name and value.
8 void addIntHeader(String name, int value)
Adds a response header with the given name and integer value.
9 void flushBuffer()
Forces any content in the buffer to be written to the client.
10 void reset()
Clears any data that exists in the buffer as well as the status code and headers.
11 void resetBuffer()
Clears the content of the underlying buffer in the response without clearing headers or status code.
12 void sendError(int sc)
Sends an error response to the client using the specified status code and clearing the buffer.
13 void sendError(int sc, String msg)
Sends an error response to the client using the specified status.
14 void sendRedirect(String location)
Sends a temporary redirect response to the client using the specified redirect location URL.
15 void setBufferSize(int size)
Sets the preferred buffer size for the body of the response.
16 void setCharacterEncoding(String charset)
Sets the character encoding (MIME charset) of the response being sent to the client, for example, to UTF-8.
17 void setContentLength(int len)
Sets the length of the content body in the response In HTTP servlets, this method sets the HTTP Content-Length header.
18 void setContentType(String type)
Sets the content type of the response being sent to the client, if the response has not been committed yet.
19 void setDateHeader(String name, long date)
Sets a response header with the given name and date-value.
20 void setHeader(String name, String value)
Sets a response header with the given name and value.
21 void setIntHeader(String name, int value)
Sets a response header with the given name and integer value.
22 void setLocale(Locale loc)
Sets the locale of the response, if the response has not been committed yet.
23 void setStatus(int sc)
Sets the status code for this response.

HTTP Header Response 实例:

下面的例子将使用setIntHeader()方法来设置刷新头,模拟数字时钟:

<%@ page import="java.io.*,java.util.*" %>
<html>
<head>
<title>Auto Refresh Header Example</title>
</head>
<body>
<center>
<h2>Auto Refresh Header Example</h2>
<%
   // Set refresh, autoload time as 5 seconds
   response.setIntHeader("Refresh", 5);
   // Get current time
   Calendar calendar = new GregorianCalendar();
   String am_pm;
   int hour = calendar.get(Calendar.HOUR);
   int minute = calendar.get(Calendar.MINUTE);
   int second = calendar.get(Calendar.SECOND);
   if(calendar.get(Calendar.AM_PM) == 0)
      am_pm = "AM";
   else
      am_pm = "PM";
   String CT = hour+":"+ minute +":"+ second +" "+ am_pm;
   out.println("Current Time is: " + CT + "
");
%>
</center>
</body>
</html>

现在把上面的代码在main.jsp,并尝试访问它。每5秒后得出这样会显示当前系统时间。只要运行JSP和等待,看看结果:

Auto Refresh Header 例子

Current Time is: 9:44:50 PM

也可以其他方法可以尝试更上面列出的方法以同样的方式。