随便记点

0830

response的运行流程:

1.客户端发出http请求

2.tomcat解析请求的资源

3.将请求的信息封装城一个request对象,同时创建一个response对象

4.创建servlet对象

5.调用service方法,当service调用结束后,方法返回.会从tomcat内核的response缓存区获取设置的内容(其中PrintWriter流、ServletOutputStream流的writer方法会将内容写入response缓冲区。)

6.tomcat服务器会从response获取内容,将响应的内容封装起来,返回一个http响应(相应行,响应头,响应体),传递到客户端,游览器将对内容进行解析.

BUG问题

1.该问题是无法利用new Date对cookies设置值.能打印,转为字符串也行,代码真没问题但是就是不行.

解决方法:利用system.cruentTimeMillis获取秒数,然后利用new Date()的构造函数解决.

但是我总感觉不是代码出现了问题,而是…自己试试我也想知道

修改之前:

/*
 * 使用Cookie记录客户端上次访问的时间
 * 
 * 思路：
 *     访问时获取想要的Cookie 如果没有获取到说明是第一次访问，把当前时间存到一个Cookie中响应给客户端    
 *     如果获取到了，从cookie中取时间（上次访问时间），再把当前时间存入Cookie中
 * 
 * */
public class CookieDemo4Servlet extends HttpServlet {
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	   this.doPost(request, response);
	}

	
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
        response.setContentType("text/html;charset=utf-8");
		PrintWriter out = response.getWriter();
		
		Cookie[] cs = request.getCookies();
		
		Cookie c = CookieUtils.findCookieByName("lasttime", cs);
		
		/*if(cs!=null){
			
			for(Cookie cc:cs){
				
				if(cc.getName().equals("lasttime")){
					c = cc;
					break;
				}
			}
		}*/
		
		if(c!=null){
			out.print("<h1>上次访问时间为："+c.getValue()+"</h1>");
		}else{  //第一次访问
			out.print("<h1>欢迎第一次访问</h1>");
		}
		
		Cookie cookie = new Cookie("lasttime",new Date()+"");
		cookie.setMaxAge(10000);
		response.addCookie(cookie);
		
		
	}

}

修改之后:

/*
 * 使用Cookie记录客户端上次访问的时间
 * 
 * 思路：
 *     访问时获取想要的Cookie 如果没有获取到说明是第一次访问，把当前时间存到一个Cookie中响应给客户端    
 *     如果获取到了，从cookie中取时间（上次访问时间），再把当前时间存入Cookie中
 * 
 * */
@WebServlet("/CookieDemo4Servlet")
public class CookieDemo4Servlet extends HttpServlet {
	
	@Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	   this.doPost(request, response);
	}

	
	@Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
        response.setContentType("text/html;charset=utf-8");
		PrintWriter out = response.getWriter();
		
		Cookie[] cs = request.getCookies();
		
		Cookie c = CookieUtils.findCookieByName("lasttime", cs);
		
		/*if(cs!=null){
			
			for(Cookie cc:cs){
				
				if(cc.getName().equals("lasttime")){
					c = cc;
					break;
				}
			}
		}*/
		
		if(c!=null){
            long cookieValue = Long.parseLong (c.getValue ( ));       //得到用户的上次访问时间
            Date date = new Date (cookieValue);                      //将long转换为日期
			out.print("<h1>上次访问时间为："+date+"</h1>");
		}else{  //第一次访问
			out.print("<h1>欢迎第一次访问</h1>");
		}

		Cookie cookie = new Cookie("lasttime",System.currentTimeMillis () + "");
		cookie.setMaxAge(60*20*24);
		cookie.setPath ("/");
		response.addCookie(cookie);

    }

}

涉及到的cookie工具类

public class CookieUtils {
	
	public static Cookie findCookieByName(String name,Cookie[] cs){
		
		if(cs!=null){
			for(Cookie c:cs){
				if(name.equals(c.getName())){
					return c;
				}
			}
		}
		return null;
	}
}

Java8新特性LocalDateTime类(顺便教你读源码)

/**
 * A date-time without a time-zone in the ISO-8601 calendar system,
 * such as {@code 2007-12-03T10:15:30}.
 * <p>
 * {@code LocalDateTime} is an immutable date-time object that represents a date-time,
 * often viewed as year-month-day-hour-minute-second. Other date and time fields,
 * such as day-of-year, day-of-week and week-of-year, can also be accessed.
 * Time is represented to nanosecond precision.
 * For example, the value "2nd October 2007 at 13:45.30.123456789" can be
 * stored in a {@code LocalDateTime}.
 * <p>
 * This class does not store or represent a time-zone.
 * Instead, it is a description of the date, as used for birthdays, combined with
 * the local time as seen on a wall clock.
 * It cannot represent an instant on the time-line without additional information
 * such as an offset or time-zone.
 * <p>
 * The ISO-8601 calendar system is the modern civil calendar system used today
 * in most of the world. It is equivalent to the proleptic Gregorian calendar
 * system, in which today's rules for leap years are applied for all time.
 * For most applications written today, the ISO-8601 rules are entirely suitable.
 * However, any application that makes use of historical dates, and requires them
 * to be accurate will find the ISO-8601 approach unsuitable.
 *
 * <p>
 * This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code LocalDateTime} may have unpredictable results and should be avoided.
 * The {@code equals} method should be used for comparisons.
 *
 * @implSpec
 * This class is immutable and thread-safe.
 *
 * @since 1.8
 */

1.读源码之前看看该类的注释,英文不好没有关系(有道翻译),咱们看重点

{@code LocalDateTime} is an immutable date-time object that represents a date-time是一个不可变的日期时间对象，表示日期时间

Time is represented to nanosecond precision.时间以纳秒精度表示

This class is immutable and thread-safe.这个类是不可变的，线程安全的

咱们可以大概总结一下就是,这个新特性的类是不可变的日期对象,时间精度到达纳秒级别,而且最重要的是线程安全.

2.不管是idea还是eclipse都可以打开一个类的方法列表,咱们先来看一下.第一眼从词汇中就可以读出来,无法就是各种日期的构造函数,及获取时间属性字段的方法.到了这一步就算是对该类有了初步的认识.

3.读源码不是读全部,是挑重点读,读重点,加猜想.你不可能读全部内容,猜读和精读结合然后返回的去验证所猜所想,大胆的去读去猜就完事啦.说的跟屁话一样,实际上真的就是屁话.我看懂的话还需要你来指点江山.先继续往下走,你就明白啦

进入正题3.1

读实现 读继承关系,看看该类继承何处.localDateTime继承一堆***,我就看懂一个serializable序列化,说明该类可以被序列化.那么它肯定有一个序列化id号.

没错下面果然有serversionID,其他的可能继承与跟date,time有关的接口或者抽象类,真是是什么咱不用去看和咱没有太打关系.用到才读,不用不读.记住啦没.

除了序列化id号还有其他关于时间,日期的成员变量说明该类与Local’Date,LocalTime类存在依赖关系.间接的说明了该类的出现可能是对这些做了优化,不然你写它干什么. 已经存在的事情如果还需要再次做一遍那就是因为它做的不够好.

4.找一个方法来读吧.看下面这个方法从字面上可以知道应该是关于日期格式的问题,英文好还是很重要的.这是对日期格式的转化.咱们看看的内部方法

5.看不懂呀,是我也看不懂,看看上的注解.

    * @param <T> the type of the parsed date-time
     * @param text  the text to parse, not null
     * @param query  the query defining the type to parse to, not null
     * @return the parsed date-time, not null
     * @throws DateTimeParseException if unable to parse the requested result

第一个参数应该是传入一个date-time类型的参数,对把

第二个参数是文本格式,对,肯定是,因为这个方法不就是日期格式转换吗

它会返回一个data-time类型的结果.

会抛出异常

总上可知,该类就是一个日期格式转换类,传入日期,制定格式,返回相应日期类型.

文件下载

1.告诉游览器的文件类型response.setContentType(文件的MIME类型)\

2.告诉游览器的下载方式

response.setHeader(“Content-Disposition”,“attachment;filename=文件名称”)

关于base64编码Encode和Decode编码

转载地址:https://magiclen.org/java-base64/

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解决各游览器文件下载名乱码问题