Class Cookie
java.lang.Object
  |
  +--javax.servlet.http.Cookie

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public class Cookie
extends java.lang.Object
implements java.lang.Cloneable

作用：Creates a cookie, a small amount of information sent by a servlet to a Web browser, saved by the browser, 
      and later sent back to the server. A cookie's value can uniquely identify a client, so cookies are commonly 
      used for session management. 

构成：A cookie has a name, a single value, and optional attributes such as a comment, path and domain qualifiers,
      a maximum age, and a version number. Some Web browsers have bugs in how they handle the optional attributes,
      so use them sparingly to improve the interoperability of your servlets. 

用法：The servlet sends cookies to the browser by using the HttpServletResponse.addCookie(javax.servlet.http.Cookie) method,
       which adds fields to HTTP response headers to send cookies to the browser, one at a time. The browser is expected to 
       support 20 cookies for each Web server, 300 cookies total, and may limit cookie size to 4 KB each. 

      The browser returns cookies to the servlet by adding fields to HTTP request headers. Cookies can be retrieved from a 
      request by using the HttpServletRequest.getCookies() method. Several cookies might have the same name but different 
      path attributes. 

 注： 

     This class supports both the Version 0 (by Netscape) and Version 1 (by RFC 2109) cookie specifications. By default, 
      cookies are created using Version 0 to ensure the best interoperability. 



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Cookie构造器： 
Cookie(java.lang.String name, java.lang.String value)  //Constructs a cookie with a specified name and value. 

主要方法：
 java.lang.Object clone() //重载 java.lang.Object.clone 方法来产生 a copy of this cookie. 
 java.lang.String getComment() //Returns the comment describing the purpose of this cookie, 
				//or null if the cookie has no comment. 
 java.lang.String getDomain()  //Returns the domain name set for this cookie. 
 int getMaxAge()  //Returns the maximum age of the cookie, specified in seconds, 
		  //By default, -1 indicating the cookie will persist until browser shutdown. 
 java.lang.String getName()   //Returns the name of the cookie. 
 java.lang.String getPath()   //Returns the path on the server to which the browser returns this cookie. 
 boolean getSecure()   //Returns true if the browser is sending cookies only over a secure protocol, 
			//or false if the browser can send cookies using any protocol. 
 java.lang.String getValue()  //Returns the value of the cookie. 
 int getVersion()  //Returns the version of the protocol this cookie complies with. 
 void setComment(java.lang.String purpose)  //Specifies a comment that describes a cookie's purpose. 
 void setDomain(java.lang.String pattern)   //Specifies the domain within which this cookie should be presented. 
 void setMaxAge(int expiry)   //Sets the maximum age of the cookie in seconds. 
 void setPath(java.lang.String uri)   //Specifies a path for the cookie to which the client should return the cookie. 
 void setSecure(boolean flag)   //Indicates to the browser whether the cookie should only be sent using a secure protocol,
				// such as HTTPS or SSL. 
 void setValue(java.lang.String newValue)   //Assigns a new value to a cookie after the cookie is created. 
 void setVersion(int v)   //Sets the version of the cookie protocol this cookie complies with. 

