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El expression and JSTL

2022-06-12 07:55:00 【Watermelon 77】

What is? javaBean:
It is java Components commonly used in development , It's really just a java class , Its function is to encapsulate data .
Writing JavaBean Five specifications need to be met ：
1. This java class , By public modification
2. This java Class should provide a public parameterless constructor
3. To provide private properties
4. To provide public for private properties set perhaps get Method
5. To achieve Serializable Interface

Be careful ：
1.EL Case sensitive

2. Use EL front , You must first store the object in the scope . Saving objects to scope does not copy objects , Scope values hold references to objects , So it has little impact on the performance of the server .

Use EL expression

1. Defining variables

2 Put variables in request in

3 Use EL expression

Scope Range Scope

${username} And$ {requestScope.username} It means the same thing

Use EL Access scope

1 Request scope ：${requestScope.news}

2. Conversation scope ：${sessionScope.username}

3. Program scope ：${applicationScope.count}

4. Page scope ：${pageScope.userNum}

Use JSTL expression

JSTL yes JSp Short for standard label library , It provides a label that can replace... To some extent java Code , for example <c:forEach/> Tags can achieve java The circular function in language .

1. download jstl.jar and standard.jar package

2 Copy these two packages to web-inf\lib Under the table of contents

3. stay JSp Add instructions to the page , for example ：

<%@taglib uri=“http://java.sun.com/jsp/jstl/core” prefix=“c”%>

<c:out value="value"default=“default” escapXml=“ture” default=“0”> Used to change the meaning of tags in the database ;escapXml="ture " Can change one's mind false Don't change your mind .default Set text defaults

One . Relating to display operations

<c:out value=" ${ news.title }"/> The value of the output property The tags given in the database can show ;

<c:set var=“uid” value=“admin” scope=“request”/> Storing variables

hold uid The variables of are stored in request in

<c:remove var=“uid” value=“admin” scope=“request”/> Remove Variable hold request Medium uid Remove the variable of

Two . Implement program logic related

<c:forEach var=“ object ” items=” Save in request In the array " varStatus=“status”> Loop variables <c:forEach />

<c:forEach> Tags have some of the following properties ：
var： The name of the iteration parameter . The name of the variable that can be used in the iteration body , Used to represent each iteration variable . The type is String.
items： The set to iterate over . The types it supports will be explained below .
varStatus： The name of the iteration variable , Used to represent the state of the iteration , You can access the iteration's own information .
begin： If you specify items, So iteration starts from items[begin] Start iterating ; If not specified items, Then from begin Start the iteration . It's of type integer .
end： If you specify items, Then in items[end] End iteration ; If not specified items, Then in end End iteration . It's also of type integer .
step： Step length of iteration .
<c:forEach> Labeled items Property support Java All standard collection types provided by the platform . Besides , You can use this operation to iterate over arrays （ Including basic type array ） The elements in . The collection types it supports and the elements of the iteration are as follows ：
java.util.Collection： call iterator() To get the elements . java.util.Map： adopt java.util.Map.Entry Examples obtained .
java.util.Iterator： Iterator elements .
java.util.Enumeration： Enumeration elements .
Object Instance array ： Array elements .
Basic type value array ： Wrapped array elements .
Delimited by commas String： Split substring .
javax.servlet.jsp.jstl.sql.Result：SQL Query the row obtained .
Whether it's iterating over integers or sets , <c:forEach> Of varStatus Attributes play the same role . and var Same property ,varStatus Used to create scoped variables （ The amount of change only works in the current label body ）. however , from varStatus The variable named by the attribute does not store the current index value or the current element , But to give javax.servlet.jsp.jstl.core.LoopTagStatus Class . This class contains a series of features , They describe the current state of the iteration , The meanings of these attributes are as follows ：
current： For the current iteration （ In the collection ） term .
index： The current iteration starts from 0 The initial iteration index .
count： The current iteration starts from 1 The number of iterations started .
first： Used to indicate whether the current iteration is the first iteration , The attribute is boolean type .
last： Used to indicate whether the current iteration is the last one , The attribute is boolean type .
begin：begin The value of the property .
end：end The value of the property
step：step The value of the property

utilize varStatus attribute ：

There is a pure way to desplay chekbox with two collums in jsp page.
checkboxLists yes checkBoxTO Set , contain key,value,checke attribute .
Java Code
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1 <c:forEach items=" ${checkboxList}" begin="0" step="2" varStatus="status"> 2 <tr> 3 <td><c:out value="$ {status.index}"/><input type=“checkbox” value=’<c:out value=" ${checkboxList[status.index].key}"/>' 4 <c:if test="$ {checkboxList[status.index].checked eq true}">checked=“checked”</c:if>
5 /><c:out value=" ${checkboxList[status.index].value}"/> 6 </td> 7 8 <td><c:if test="$ {!empty checkboxList[status.index+1]}">
9 <c:out value=" ${status.index+1}"/><input type="checkbox" value='<c:out value="$ {checkboxList[status.index+1].key}"/>’
10 <c:if test=" ${checkboxList[status.index+1].checked eq true}">checked="checked"</c:if> 11 /><c:out value="$ {checkboxList[status.index+1].value}"></c:out>
12 </c:if>
13
14
15 </c:forEach>
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${status.count} The values in are derived from 1 Began to increase

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<c:forEach var=“Books” items=" ${list }" varStatus="status"> <tr> <td><input type="checkbox" name="bookId" value="$ {status.count}" />
${Books.bookname }</td> <td>$ {Books.b_price }
${Books.stock}</td> <td class="thumb"><img src=$ {Books.image } />

</c:forEach>
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