什么是302错误？
302 Found 是HTTP协议中的一个状态码，可以简单的理解为该资源原本确实存在，但已经被临时改变了位置；302 Found 类似于301，但新的URL应该被视为临时性的替代，而不是永久性的。
官方给出的英语解释为：
The requested resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client SHOULD continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires header field.
The temporary URI SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
If the 302 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
使用response.sendRedirect(url)比使用response.setStatus(response.SC_MOVED_TEMPORARILY)和response.setHeader("Location",url)更好。这是因为：
1、代码更加简洁。
2、使用sendRedirect，Servlet会自动构造一个包含新链接的页面（用于那些不能自动重定向的老式浏览器）。
3、sendRedirect能够处理相对URL，自动把它们转换成绝对URL。
注意这个状态代码有时候可以和301替换使用。例如，如果浏览器错误地请求http://host/~user（缺少了后面的斜杠），有的服务器返回301，有的则返回302。
