referer
IntroThe Referer HTTP request header contains an absolute or partial address of the page that makes the request. The Referer header allows a server to identify a page where people are visiting it from. This data can be used for analytics, logging, optimized caching, and more.1
Referrer-PolicyThe Referrer-Policy HTTP header controls how much referrer information (sent with the Referer header) should be included with requests. Aside from the HTTP header, you can set this policy in HTML.
Directives ...
Java Thread State
Threads can be in one of six states:
New
Runnable
Blocked
Waiting
Timed Waiting
Terminated
New ThreadsWhen you create a thread with the new operator—for example, new Thread(r)— the thread is not yet running. This means that it is in the new state.
Runnable ThreadsOnce you invoke the start method, the thread is in the runnable state. A runnable thread may or may not actually be running. It is up to the operating system to give the thread time to run.
static void yield():causes the currentl ...
对外键的自我理解
1、简介外键是表中的一组属性，引用另外一张表的主键。
外键的作用是标识被引用表的某一行，一般要求外键等于主表某行的候选键，否则没有值。
更多可参考外键
2、个人对它的粗浅理解外键是在数据库层面维护数据库数据一致性和完整性的一种手段，你也可以在应用层保证数据库的一致性和完整性。
Permutations And Combinations
PermutationsHow many different ordered arrangements of the letters a, b, and c are possible? By direct enumeration we see that there are 6, namely, abc, acb, bac, bca, cab, and cba. Each arrangement is known as a permutation. Thus, there are 6 possible permutations of a set of 3 objects. This result could also have been obtained from the basic principle, since the first object in the permutation can be any of the 3, the second object in the permutation can then be chosen from any of the remainin ...
The Basic Principle of Counting
The basic principle of counting will be fundamental to all our work. Loosely put, it states that if one experiment can result in any of m possible outcomes and if another experiment can result in any of n possible outcomes, then there are mn possible outcomes of the two experiments.
The basic principle of countingSuppose that two experiments are to be performed. Then if experiment 1 can result in any one of m possible outcomes and if, for each outcome of experiment1, there are n possible outcome ...
操作系统的定义和作用（角色）简述
1. 定义操作系统是为用户和应用程序管理计算机资源的软件层。
2. 操作系统扮演的角色为了能在一个操作系统上运行多个应用程序，操作系统必须扮演三个角色：
2.1. Referee（裁判）操作系统必须管理在同一台物理机运行的，由多个应用程序共享的资源。在面对这些不同的应用程序，它必须执行一个裁判的职责。
需要考虑的问题：
2.1.1. 资源分配操作系统必须将所有同时发生的活动分开，适当地为每个活动分配资源。
如果操作系统确保每个程序都获得自己的计算机资源切片，则特定应用程序可能会锁定，但其他程序可以不受阻碍地继续运行。 此外，用户可以要求操作系统强制退出循环程序。
2.1.2. 隔离一个应用程序中的错误不应中断其他应用程序，甚至操作系统本身，这称为故障隔离。
故障隔离需要将应用程序的行为限制在低于底层硬件的全部能力。否则，任何从网上下载的应用程序，或嵌入网页的任何脚本，都可以完全控制机器。任何应用程序都可以将间谍软件安装到操作系统中以记录您键入的每个按键，或记录您访问的每个网站的密码。 如果没有操作系统提供的故障隔离，任何程序中的任何错误都可能无法挽回地损坏磁盘。容易出错或恶意的应用程序 ...
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