发布于 2015-08-17 14:57:42 | 269 次阅读 | 评论: 0 | 来源: 网络整理
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| Expires | This header specifies the time at which the content should be considered out-of-date and thus no longer be cached.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. |
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. |
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 + "n");
%>
</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。