EL expression

The basic concept

• EL (Expression Language) expressions provide a way to simplify expressions in JSPS, making it easy to access and output various kinds of data.

The main function

• Access data stored in pageContext, Request, Session, and Application scoped objects in sequence.
• Gets the request parameter value.
• Access the properties of the Bean object.
• Access the data in the collection.
• Output simple operation results.

Access the data of built-in objects

access

• <%=request.getAttribute(“varName”)%>
• ${varName} ${varName}

Execute the process

Access the data for the request parameters

• Request.getparameter (name) = request.getParameter(name); request.getParameterValues(name);
• The data param of the request parameter is accessed in EL using the following method: the parameter is received with a single value. ParamValues: The accepted parameter has multiple values.

<! ${param.name} <! ${paramvalues.hobby [--> ${paramvalues.hobby [--> ${paramvalues.hobby [0]}
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Access the properties of the Bean object

access

• Method 1: Object name. Attribute name, for example: {object name. Attribute name}, for example: object name. Attribute name, for example: {user.name}
• Method 2: Object name [” Attribute name”], for example: {object name [” Attribute name”]}, for example: {user[“name”]}

The main difference

• Use [] instead of alphanumeric characters when accessing property names that contain special characters such as. Or, etc. The way of
• You can use [] to dynamically value the value as follows:

<% 
    request.setAttribute("prop"."age"); % > <! ${user[prop]} ${user[prop]}Copy the code

Access the data in the collection

<! --> ${student[0].name}
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Commonly used built-in objects

Commonly used operators

A commonly used arithmetic operator

Commonly used relational operators

A common logical operator

Conditional operator

${conditional expression? Statement 1: Statement 2}Copy the code

Validation operator

${empty expression} returns a Boolean value to determine whether the expression is "empty". Null values, collections or arrays without elements, and strings of zero length are considered null.Copy the code

JSTL tags

The basic concept

• JSTL(JSP Standard Tag Library) is called the JSP Standard Tag Library.
• Developers can use these tags to replace Java code on JSP pages, making programs more readable and less difficult to maintain.

use

• Download JSTL jars and added to the project, download address is: tomcat.apache.org/download-ta…
• Import the JSTL tag library using the taglib specification in the JSP page by:

<! The -- prefix attribute is used to specify the library prefix --> <! The uri attribute is used to specify the library's identity --> <% @taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"% >Copy the code

Common core tags

The output label

<c:out></c:out> The label used to output the specified contentCopy the code

Set up the label

<c:set></c:set> The label used to set the value of the property rangeCopy the code

Remove the label

<c:remove></c:remove> Is used to remove the label of the specified dataCopy the code

Single condition judgment tag

<c:if test ="EL conditional expression ">Copy the code

Multi-conditional judgment tag

<c:choose > <c:when test ="EL expression "> </c:when>... <c:otherwise> Execute </c:otherwise> </c:choose > when the above conditions are not metCopy the code

Loop label

<c:forEach var=" loop "items=" collection "> </c:forEach>Copy the code

Common function labels

<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
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Common Formatting labels

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
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Custom labels

• If the above tags do not meet the requirements, the programmer can also customize the tags as follows:
• Write a tag class that inherits the SimpleTagSupport or TagSupport class and overwrites the doTag or doStartTag methods.

public class HelloTag extends SimpleTagSupport { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public void doTag() throws JspException, IOException { JspWriter out = this.getJspContext().getOut(); Out. println(" Custom tag argument: "+ name); }}Copy the code

• Define the tag library file (TLD tag library file) and configure the tag description file to web-INF:

<tag>
    <name>helloTag</name> 
    <tag-class>com.lagou.demo02.HelloTag</tag-class> 
    <body-content>empty</body-content> 
    <attribute> 
        <name>name</name> 
        <required>true</required> 
    </attribute> 
</tag>
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• Add the taglib directive to the JSP to introduce the tag library using:

<%@ taglib prefix="hello" uri="http://lagou.com" %>
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