EL expression and JSTL expression

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1. EL overview

1. What is EL

EL (Expression Language) is to make JSP easier to write; it provides a way to simplify expressions in JSP, making JSP code more simplified.

2. EL function

EL and JSTL are used together to replace the writing of embedded Java code in JSP pages;

3. EL function

• retrieve data;
• Perform calculations;
• Get the objects commonly used in web development;

4. EL syntax

${EL 表达式}

2. EL access to data

1. How EL gets data

• When the EL expression is executed, the pageContext.findAttribute() method will be called;
• Find the corresponding objects from the scope of page, request, session, and application respectively;
• If found, return the corresponding object, if not found, return "" (not null, it is an empty string);
• The data obtained by EL must be in four scopes;

2. Example

Three, EL access to data in arrays and collections

1. EL get array data

2. Get the data of the List collection

3. Get the data of the Map collection

Four, EL execution calculation

1. Arithmetic operations

2. Relational operations

3. Logic Operations

4. Ternary Operation

Five, EL access to common objects in web development

1. What are the commonly used objects

EL expression defines 11 common objects in web development;

name	meaning
pageContext	Equivalent to pageContext in JSP built-in object
pageScope	Get the data of the name under the specified domain
requestScope	Get the data of the name under the specified domain
sessionScope	Get the data of the name under the specified domain
applicationScope	Get the data of the name under the specified domain
param	Receive request parameters in the page (receive a parameter with a name corresponding to a value)
paramValues	Receive request parameters in the page (receive a parameter with a name corresponding to multiple values)
header	Get the request header on the page (get a key corresponds to a value header)
headerValues	Get the request header on the page (get a key corresponds to multiple value headers)
cookie	The name and value of the access cookie (${cookie.key.name} ${cookie.key.value})
initParam	Get the value of the global initialization parameter

2. Example

Six, JSTL overview

1. What is JSTL

JSTL (Java server pages standarded tag library, namely JSP standard tag library) is a standard specification made by JCP (Java community Proces), it mainly provides Java Web developers with a standard and common tag library; developers can use these tags Replace the Java code on the JSP page, thereby improving the readability of the program and reducing the difficulty of program maintenance.

2. JSTL Tag Library

c tag (core tag library)
fmt tag
xml tag
sql tag
jstl function library

Seven, JSTL introduction

1. Introduce the jar package of JSTL
2. New JSP page
3. Introduce tag library
4. Use JSTL

8. JSTL if tag (judgment)

1. Example

2. Attributes of if tag

• test attribute: condition;
• var attribute: assign the condition in test to a variable, and define the variable in var;
• scope attribute: scope of action;

Nine, JSTL's foreach tag (loop)