1.4Zookeeper和Thymeleaf的使用

什麼是Zookeeper？

Zookeeper 是一個開放源碼的分佈式應用程序協調服務，它包含一個簡單的原語集，分佈式應用程序能夠基於它實現同步服務、配置維護和命名服務等等。
採用下圖描述zookeeper協調服務。

Zookeeper特性

ZooKeeper很是快速且很是簡單。可是，因爲其目標是構建更復雜的服務（如同步）的基礎，所以它提供了一系列保證。這些是：

• 順序一致性 - 客戶端的更新將按發送順序應用。
• 原子性 - 更新成功或失敗。沒有部分結果。
• 單系統映像 - 不管服務器鏈接到哪一個服務器，客戶端都將看到相同的服務視圖。
• 可靠性 - 一旦應用了更新，它將從那時起持續到客戶端覆蓋更新。
• 及時性 - 系統的客戶視圖保證在特定時間範圍內是最新的。

什麼是分佈式鎖

顧名思義，分佈式鎖確定是用在分佈式環境下。在分佈式環境下，使用分佈式鎖的目的也是保證同一時刻只有一個線程來修改共享變量，修改共享緩存……。

前景：
jdk提供的鎖只能保證線程間的安全性，但分佈式環境下，各節點之間的線程同步執行卻得不到保障，分佈式鎖由此誕生。

實現方式有如下幾種：

1. 基於數據庫實現分佈式鎖；
2. 基於緩存（Redis等）實現分佈式鎖；
3. 基於Zookeeper實現分佈式鎖；

本示例利用zookeeper實現分佈式鎖。

實現思路

zookeeper 能夠根據有序節點+watch實現，實現思路，如：
爲每一個線程生成一個有序的臨時節點，爲確保有序性，在排序一次所有節點，獲取所有節點，每一個線程判斷本身是否最小，若是是的話，得到鎖，執行操做，操做完刪除自身節點。若是不是第一個的節點則監聽它的前一個節點，當它的前一個節點被刪除時，則它會得到鎖，以此類推。

使用Zookeeper實現分佈式鎖的優勢

有效的解決單點問題，不可重入問題，非阻塞問題以及鎖沒法釋放的問題。實現起來較爲簡單。

使用Zookeeper實現分佈式鎖的缺點

性能上不如使用緩存實現分佈式鎖。由於須要頻繁的建立刪除節點。而且須要對ZK的原理有所瞭解。

 

參考連接：
三種方案實現分佈式鎖

Spring Boot 使用 Thymeleaf 

<!-- 引入thymeleaf模板引擎-->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-thymeleaf</artifactId>
</dependency>

1、標準表達式語法
它又分爲：
消息
變量
選擇表達式
連接URL
片斷
文字
附加文本
字面替代
算術運算
比較與平等
條件表達式
默認表達式
無操做令牌
數據轉換/格式化
預處理
我就只介紹經常使用的了
${…} 表達式其實是在上下文中包含的變量的地圖上執行的OGNL（Object-Graph Navigation Language）對象。

一、變量
<p>Today is: <span th:text="${today}">13 february 2011</span>.</p>
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意味着 <span>標籤中的內容會被表達式${today}的值所替代，不管模板中它的內容是什麼，之因此在模板中「畫蛇添足「地填充它的內容，徹底是爲了它可以做爲原型在瀏覽器中直接顯示出來。
假設today的值爲2015年8月14日，那麼渲染結果爲：<p>Today is: 2015年8月14日.</p>。可見Thymeleaf的基本變量和JSP同樣，都使用${.}表示獲取變量的值。

二、URL
URL在Web應用模板中佔據着十分重要的地位，須要特別注意的是Thymeleaf對於URL的處理是經過語法@{…}來處理的。Thymeleaf支持絕對路徑URL：
<a th:href="@{http://www.thymeleaf.org}">Thymeleaf</a>
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同時也可以支持相對路徑URL：
另外，若是須要Thymeleaf對URL進行渲染，那麼務必使用th:href，th:src等屬性，下面是一個例子
<!-- Will produce 'http://localhost:8080/gtvg/order/details?orderId=3' (plus rewriting) -->
<a href="details.html"
th:href="@{http://localhost:8080/gtvg/order/details(orderId=${o.id})}">view</a>
<!-- Will produce '/gtvg/order/details?orderId=3' (plus rewriting) -->
<a href="details.html" th:href="@{/order/details(orderId=${o.id})}">view</a>
<!-- Will produce '/gtvg/order/3/details' (plus rewriting) -->
<a href="details.html" th:href="@{/order/{orderId}/details(orderId=${o.id})}">view</a>

幾點說明：
上例中URL最後的(orderId=${o.id})表示將括號內的內容做爲URL參數處理，該語法避免使用字符串拼接，大大提升了可讀性@{...}表達式中能夠經過{orderId}訪問Context中的orderId變量@{/order}是Context相關的相對路徑，在渲染時會自動添加上當前Web應用的Context名字，假設context名字爲app，那麼結果應該是/app/order
三、字符串替換
不少時候可能咱們只須要對一大段文字中的某一處地方進行替換，能夠經過字符串拼接操做完成：

<span th:text="'Welcome to our application, ' + ${user.name} + '!'">
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一種更簡潔的方式是：
<span th:text="|Welcome to our application, ${user.name}!|">
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固然這種形式限制比較多，|…|中只能包含變量表達式${…}，不能包含其餘常量、條件表達式等。

四、運算符
在表達式中可使用各種算術運算符，例如+, -, *, /, %

th:with="isEven=(${prodStat.count} % 2 == 0)"
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邏輯運算符>, <, <=,>=，==,!=均可以使用，惟一須要注意的是使用<,>時須要用它的HTML轉義符：

th:if="${prodStat.count} &gt; 1"
th:text="'Execution mode is ' + ( (${execMode} == 'dev')? 'Development' : 'Production')"
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2、經常使用的表達式
一、for循環
使用 th:each 標籤
<div class="row" >
<div th:each="url,lstat:${links}">
<div class="col-md-2" th:title="${url.description}" title="一我的，信你所信，爲你所現" >
<strong th:text="${url.link_name}">這個冬天不太冷</strong>
<a href="http://www.lrshuai.top" th:href="${url.link}" th:text="${url.link}" >http://www.lrshuai.top</a>
</div>
</div>
</div>

lstat稱做狀態變量，屬性有：
index:當前迭代對象的index（從0開始計算）
count: 當前迭代對象的index(從1開始計算)
size:被迭代對象的大小
current:當前迭代變量
even/odd:布爾值，當前循環是不是偶數/奇數（從0開始計算）
first:布爾值，當前循環是不是第一個
last:布爾值，當前循環是不是最後一個
二、條件求值
If/Unless
demo
<div class="row" >
<div th:each="url,lstat:${links}">
<div class="col-md-2" th:title="${url.description}" th:if="${lstat.index}%4 == 0" >
<strong th:text="${url.link_name}">這個冬天不太冷</strong>
<a href="http://www.lrshuai.top" th:href="${url.link}" th:text="${url.link}">http://www.lrshuai.top</a>
</div>

<div class="col-md-2 col-md-offset-1" th:title="${url.description}" th:unless="${lstat.index}%4==0">
<strong th:text="${url.link_name}">這個冬天不太冷</strong>
<a href="http://www.lrshuai.top" th:href="${url.link}" th:text="${url.link}" >http://www.lrshuai.top</a>
</div>
</div>
</div>

Thymeleaf中使用th:if和th:unless屬性進行條件判斷，上面的例子中，<div>標籤只有在th:if中條件成立時才顯示：

th:unless於th:if剛好相反，只有表達式中的條件不成立，纔會顯示其內容。

Switch
Thymeleaf一樣支持多路選擇Switch結構：
<div th:switch="${user.role}">
<p th:case="'admin'">User is an administrator</p>
<p th:case="#{roles.manager}">User is a manager</p>
</div>

默認屬性default能夠用*表示：
<div th:switch="${user.role}">
<p th:case="'admin'">User is an administrator</p>
<p th:case="#{roles.manager}">User is a manager</p>
<p th:case="*">User is some other thing</p>
</div>

三、內嵌變量
爲了模板更加易用，Thymeleaf還提供了一系列Utility對象（內置於Context中），能夠經過#直接訪問：
dates ： java.util.Date的功能方法類。
calendars : 相似#dates，面向java.util.Calendar
numbers : 格式化數字的功能方法類
strings : 字符串對象的功能類
objects: 對objects的功能類操做。
bools: 對布爾值求值的功能方法。
arrays：對數組的功能類方法。
lists: 對lists功能類方法
sets
maps
說說我經常使用得方法吧，太多了，你也不必定看完
(1)、字符串太多，顯示…
# 這裏的含義是 若是 atc.text 這個變量多餘200個字符，後面顯示...
<p th:text="${#strings.abbreviate(atc.text,200)}">內容內容內容</p>
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(2)、數組判斷是否爲空
<div th:if="${#lists.isEmpty(arrays)} " class="blog-article">
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(3)、request 獲取絕對路徑
<img th:src="${#httpServletRequest.getContextPath()}+${atc.img}" src="/images/logo.jpg">
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經常使用th標籤
標籤 說明 例子
th:id 替換id <input th:id="'xxx' + ${collect.id}"/>
th:text 文本替換 <p th:text="${collect.description}">description</p>
th:utext 支持html的文本替換 <p th:utext="${htmlcontent}">conten</p>
th:object 替換對象 <div th:object="${session.user}">
th:value 屬性賦值 <input th:value="${user.name}" />
th:with 變量賦值運算 <div th:with="isEven=${prodStat.count}%2==0"></div>
th:style 設置樣式 th:style="'display:' + @{(${sitrue} ? 'none' : 'inline-block')} + ''"
th:onclick 點擊事件 th:"'getCollect()'"
th:each 屬性賦值 tr th:each="user,userStat:${users}">
th:if 判斷條件 <a th:if="${userId == collect.userId}" >
th:unless 和th:if判斷相反 <a th:href="@{/login}" th:unless=${session.user != null}>Login</a>
th:href 連接地址 <a th:href="@{/login}" th:unless=${session.user != null}>Login</a> />
th:switch 多路選擇 配合th:case 使用 <div th:switch="${user.role}">
th:case th:switch的一個分支 <p th:case="'admin'">User is an administrator</p>
th:fragment 佈局標籤，定義一個代碼片斷，方便其它地方引用 <div th:fragment="alert">
th:include 佈局標籤，替換內容到引入的文件 <head th:include="layout :: htmlhead" th:with="title='xx'"></head> />
th:replace 佈局標籤，替換整個標籤到引入的文件 <div th:replace="fragments/header :: title"></div>
th:selected selected選擇框 選中 th:selected="(${xxx.id} == ${configObj.dd})"
th:src 圖片類地址引入 <img class="img-responsive" alt="App Logo" th:src="@{/img/logo.png}" />
th:inline 定義js腳本可使用變量 <script type="text/javascript" th:inline="javascript">
th:action 表單提交的地址 <form action="subscribe.html" th:action="@{/subscribe}">
th:remove 刪除某個屬性 <tr th:remove="all">1.all:刪除包含標籤和全部的孩子。2.body:不包含標記刪除,但刪除其全部的孩子。3.tag:包含標記的刪除,但不刪除它的孩子。4.all-but-first:刪除全部包含標籤的孩子,除了第一個。5.none:什麼也不作。這個值是有用的動態評估。
th:attr 設置標籤屬性，多個屬性能夠用逗號分隔 好比<p th:attr="src=@{/image/aa.jpg},title=${title}">內容</p>，這樣若是${title}=‘這個是title’ 則結果就是<p src="/image/aa.jpg" title="這個是title">內容</p>
html 有的，它幾乎都有相對應的標籤

下面是一組的API
日期: #dates
/*
* ======================================================================
* See javadoc API for class org.thymeleaf.expression.Dates
* ======================================================================
*/

/*
* Format date with the standard locale format
* Also works with arrays, lists or sets
*/
${#dates.format(date)}
${#dates.arrayFormat(datesArray)}
${#dates.listFormat(datesList)}
${#dates.setFormat(datesSet)}

/*
* Format date with the ISO8601 format
* Also works with arrays, lists or sets
*/
${#dates.formatISO(date)}
${#dates.arrayFormatISO(datesArray)}
${#dates.listFormatISO(datesList)}
${#dates.setFormatISO(datesSet)}

/*
* Format date with the specified pattern
* Also works with arrays, lists or sets
*/
${#dates.format(date, 'dd/MMM/yyyy HH:mm')}
${#dates.arrayFormat(datesArray, 'dd/MMM/yyyy HH:mm')}
${#dates.listFormat(datesList, 'dd/MMM/yyyy HH:mm')}
${#dates.setFormat(datesSet, 'dd/MMM/yyyy HH:mm')}

/*
* Obtain date properties
* Also works with arrays, lists or sets
*/
${#dates.day(date)} // also arrayDay(...), listDay(...), etc.
${#dates.month(date)} // also arrayMonth(...), listMonth(...), etc.
${#dates.monthName(date)} // also arrayMonthName(...), listMonthName(...), etc.
${#dates.monthNameShort(date)} // also arrayMonthNameShort(...), listMonthNameShort(...), etc.
${#dates.year(date)} // also arrayYear(...), listYear(...), etc.
${#dates.dayOfWeek(date)} // also arrayDayOfWeek(...), listDayOfWeek(...), etc.
${#dates.dayOfWeekName(date)} // also arrayDayOfWeekName(...), listDayOfWeekName(...), etc.
${#dates.dayOfWeekNameShort(date)} // also arrayDayOfWeekNameShort(...), listDayOfWeekNameShort(...), etc.
${#dates.hour(date)} // also arrayHour(...), listHour(...), etc.
${#dates.minute(date)} // also arrayMinute(...), listMinute(...), etc.
${#dates.second(date)} // also arraySecond(...), listSecond(...), etc.
${#dates.millisecond(date)} // also arrayMillisecond(...), listMillisecond(...), etc.

/*
* Create date (java.util.Date) objects from its components
*/
${#dates.create(year,month,day)}
${#dates.create(year,month,day,hour,minute)}
${#dates.create(year,month,day,hour,minute,second)}
${#dates.create(year,month,day,hour,minute,second,millisecond)}

/*
* Create a date (java.util.Date) object for the current date and time
*/
${#dates.createNow()}

${#dates.createNowForTimeZone()}

/*
* Create a date (java.util.Date) object for the current date (time set to 00:00)
*/
${#dates.createToday()}

${#dates.createTodayForTimeZone()}

數字:#numbers
/*
* ======================================================================
* See javadoc API for class org.thymeleaf.expression.Numbers
* ======================================================================
*/

/*
* ==========================
* Formatting integer numbers
* ==========================
*/

/*
* Set minimum integer digits.
* Also works with arrays, lists or sets
*/
${#numbers.formatInteger(num,3)}
${#numbers.arrayFormatInteger(numArray,3)}
${#numbers.listFormatInteger(numList,3)}
${#numbers.setFormatInteger(numSet,3)}

/*
* Set minimum integer digits and thousands separator:
* 'POINT', 'COMMA', 'WHITESPACE', 'NONE' or 'DEFAULT' (by locale).
* Also works with arrays, lists or sets
*/
${#numbers.formatInteger(num,3,'POINT')}
${#numbers.arrayFormatInteger(numArray,3,'POINT')}
${#numbers.listFormatInteger(numList,3,'POINT')}
${#numbers.setFormatInteger(numSet,3,'POINT')}

/*
* ==========================
* Formatting decimal numbers
* ==========================
*/

/*
* Set minimum integer digits and (exact) decimal digits.
* Also works with arrays, lists or sets
*/
${#numbers.formatDecimal(num,3,2)}
${#numbers.arrayFormatDecimal(numArray,3,2)}
${#numbers.listFormatDecimal(numList,3,2)}
${#numbers.setFormatDecimal(numSet,3,2)}

/*
* Set minimum integer digits and (exact) decimal digits, and also decimal separator.
* Also works with arrays, lists or sets
*/
${#numbers.formatDecimal(num,3,2,'COMMA')}
${#numbers.arrayFormatDecimal(numArray,3,2,'COMMA')}
${#numbers.listFormatDecimal(numList,3,2,'COMMA')}
${#numbers.setFormatDecimal(numSet,3,2,'COMMA')}

/*
* Set minimum integer digits and (exact) decimal digits, and also thousands and
* decimal separator.
* Also works with arrays, lists or sets
*/
${#numbers.formatDecimal(num,3,'POINT',2,'COMMA')}
${#numbers.arrayFormatDecimal(numArray,3,'POINT',2,'COMMA')}
${#numbers.listFormatDecimal(numList,3,'POINT',2,'COMMA')}
${#numbers.setFormatDecimal(numSet,3,'POINT',2,'COMMA')}

/*
* =====================
* Formatting currencies
* =====================
*/

${#numbers.formatCurrency(num)}
${#numbers.arrayFormatCurrency(numArray)}
${#numbers.listFormatCurrency(numList)}
${#numbers.setFormatCurrency(numSet)}

/*
* ======================
* Formatting percentages
* ======================
*/

${#numbers.formatPercent(num)}
${#numbers.arrayFormatPercent(numArray)}
${#numbers.listFormatPercent(numList)}
${#numbers.setFormatPercent(numSet)}

/*
* Set minimum integer digits and (exact) decimal digits.
*/
${#numbers.formatPercent(num, 3, 2)}
${#numbers.arrayFormatPercent(numArray, 3, 2)}
${#numbers.listFormatPercent(numList, 3, 2)}
${#numbers.setFormatPercent(numSet, 3, 2)}

/*
* ===============
* Utility methods
* ===============
*/

/*
* Create a sequence (array) of integer numbers going
* from x to y
*/
${#numbers.sequence(from,to)}
${#numbers.sequence(from,to,step)}

字符串:#strings
/*
* Null-safe toString()
*/
${#strings.toString(obj)} // also array*, list* and set*

/*
* Check whether a String is empty (or null). Performs a trim() operation before check
* Also works with arrays, lists or sets
*/
${#strings.isEmpty(name)}
${#strings.arrayIsEmpty(nameArr)}
${#strings.listIsEmpty(nameList)}
${#strings.setIsEmpty(nameSet)}

/*
* Perform an 'isEmpty()' check on a string and return it if false, defaulting to
* another specified string if true.
* Also works with arrays, lists or sets
*/
${#strings.defaultString(text,default)}
${#strings.arrayDefaultString(textArr,default)}
${#strings.listDefaultString(textList,default)}
${#strings.setDefaultString(textSet,default)}

/*
* Check whether a fragment is contained in a String
* Also works with arrays, lists or sets
*/
${#strings.contains(name,'ez')} // also array*, list* and set*
${#strings.containsIgnoreCase(name,'ez')} // also array*, list* and set*

/*
* Check whether a String starts or ends with a fragment
* Also works with arrays, lists or sets
*/
${#strings.startsWith(name,'Don')} // also array*, list* and set*
${#strings.endsWith(name,endingFragment)} // also array*, list* and set*

/*
* Substring-related operations
* Also works with arrays, lists or sets
*/
${#strings.indexOf(name,frag)} // also array*, list* and set*
${#strings.substring(name,3,5)} // also array*, list* and set*
${#strings.substringAfter(name,prefix)} // also array*, list* and set*
${#strings.substringBefore(name,suffix)} // also array*, list* and set*
${#strings.replace(name,'las','ler')} // also array*, list* and set*

/*
* Append and prepend
* Also works with arrays, lists or sets
*/
${#strings.prepend(str,prefix)} // also array*, list* and set*
${#strings.append(str,suffix)} // also array*, list* and set*

/*
* Change case
* Also works with arrays, lists or sets
*/
${#strings.toUpperCase(name)} // also array*, list* and set*
${#strings.toLowerCase(name)} // also array*, list* and set*

/*
* Split and join
*/
${#strings.arrayJoin(namesArray,',')}
${#strings.listJoin(namesList,',')}
${#strings.setJoin(namesSet,',')}
${#strings.arraySplit(namesStr,',')} // returns String[]
${#strings.listSplit(namesStr,',')} // returns List<String>
${#strings.setSplit(namesStr,',')} // returns Set<String>

/*
* Trim
* Also works with arrays, lists or sets
*/
${#strings.trim(str)} // also array*, list* and set*

/*
* Compute length
* Also works with arrays, lists or sets
*/
${#strings.length(str)} // also array*, list* and set*

/*
* Abbreviate text making it have a maximum size of n. If text is bigger, it
* will be clipped and finished in "..."
* Also works with arrays, lists or sets
*/
${#strings.abbreviate(str,10)} // also array*, list* and set*

/*
* Convert the first character to upper-case (and vice-versa)
*/
${#strings.capitalize(str)} // also array*, list* and set*
${#strings.unCapitalize(str)} // also array*, list* and set*

/*
* Convert the first character of every word to upper-case
*/
${#strings.capitalizeWords(str)} // also array*, list* and set*
${#strings.capitalizeWords(str,delimiters)} // also array*, list* and set*

/*
* Escape the string
*/
${#strings.escapeXml(str)} // also array*, list* and set*
${#strings.escapeJava(str)} // also array*, list* and set*
${#strings.escapeJavaScript(str)} // also array*, list* and set*
${#strings.unescapeJava(str)} // also array*, list* and set*
${#strings.unescapeJavaScript(str)} // also array*, list* and set*

/*
* Null-safe comparison and concatenation
*/
${#strings.equals(first, second)}
${#strings.equalsIgnoreCase(first, second)}
${#strings.concat(values...)}
${#strings.concatReplaceNulls(nullValue, values...)}

/*
* Random
*/
${#strings.randomAlphanumeric(count)}

布爾:#bools
/*
* Evaluate a condition in the same way that it would be evaluated in a th:if tag
* (see conditional evaluation chapter afterwards).
* Also works with arrays, lists or sets
*/
${#bools.isTrue(obj)}
${#bools.arrayIsTrue(objArray)}
${#bools.listIsTrue(objList)}
${#bools.setIsTrue(objSet)}

/*
* Evaluate with negation
* Also works with arrays, lists or sets
*/
${#bools.isFalse(cond)}
${#bools.arrayIsFalse(condArray)}
${#bools.listIsFalse(condList)}
${#bools.setIsFalse(condSet)}

/*
* Evaluate and apply AND operator
* Receive an array, a list or a set as parameter
*/
${#bools.arrayAnd(condArray)}
${#bools.listAnd(condList)}
${#bools.setAnd(condSet)}

/*
* Evaluate and apply OR operator
* Receive an array, a list or a set as parameter
*/
${#bools.arrayOr(condArray)}
${#bools.listOr(condList)}
${#bools.setOr(condSet)}

數組 :#arrays
/*
* Converts to array, trying to infer array component class.
* Note that if resulting array is empty, or if the elements
* of the target object are not all of the same class,
* this method will return Object[].
*/
${#arrays.toArray(object)}

/*
* Convert to arrays of the specified component class.
*/
${#arrays.toStringArray(object)}
${#arrays.toIntegerArray(object)}
${#arrays.toLongArray(object)}
${#arrays.toDoubleArray(object)}
${#arrays.toFloatArray(object)}
${#arrays.toBooleanArray(object)}

/*
* Compute length
*/
${#arrays.length(array)}

/*
* Check whether array is empty
*/
${#arrays.isEmpty(array)}

/* * Check if element or elements are contained in array */${#arrays.contains(array, element)}${#arrays.containsAll(array, elements)}