1 EL expression

EL expression provides a simplified expression in JSP method, can easily access a variety of data and output, the main functions are:

  • Access the data stored in the pageContext, Request, Session, and Application-scoped objects in sequence
  • Get request parameters
  • Access the properties of the Bean object
  • Access the data in the collection
  • Output simple operation results

1.1 Accessing data of built-in objects

The syntax for traditional access is: <%= request.getAttribute(“varName”)%>

${varName} ${varName}

1.2 Accessing data for request parameters

Access request parameter data need to use the traditional way: request. The getParameter (name) or request. GetParameterValue (name)

To access request parameters using an EL expression, use the following commands: Param accepts only one value for parameters. ParamValues accepts multiple values for parameters

${param.name}
${paramValues.hobby[0]}
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1.3 Accessing properties of Bean objects

There are two ways to access a Bean object in an EL expression:

  • Object name. Property name, for example {object name. Attribute name}, for example, object name. Attribute names, such as {user.name}
  • Object name [” attribute name “], for example [” attribute name “]} {object name, such as object name (” attribute name “), such as {user} [” name “]

The difference between these two methods is that when the attribute name to be accessed contains some special characters, such as:. For non-subtitle or numeric symbols, use [] instead of. The way of

In addition, the [] mode can also be used to dynamically value the value, as follows:

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

1.4 Accessing the data in the collection

<! --> ${student[0].name}
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Built-in objects commonly used in 1.5 EL expressions

category identifier describe
JSP pageContext PageContext processes the current page
scope pageScope Map classes associated with page scoped property names and values
requestSCope Map class associated with request scope attribute names and values
sessionScope Map classes associated with session scoped property names and values
applicationScope Map classes associated with the names and values of the program scope properties
Request parameters param Map class that stores the values of the request parameters by name
paramValue A Map class that stores all the values of the request parameters as a String array
Request header header Map class that stores the main value of the request header by name
headerValue A Map class that stores all the values of the request header as a String array class
Cookie cookie Map class that stores the cookies that come with the request by name
Initialization parameter initParam Map class that stores Web application context initialization parameters by name

1.6 Common Operators

Common arithmetic operators are + addition, -subtraction, * multiplication, div or/division, and % or mod mod, which are the same as Java arithmetic operators

Common relational operators are: == equals,! = Not equal to, < less than, > greater than, <= less than or equal to, and >= greater than or equal to

Commonly used logical operators are: && and operation, | | or operation and! The operation

The conditional operator is: ${conditional expression? Statement 1: Statement 2}, similar to the ternary operator in Java

Validation operator: ${empty expression} returns a Boolean value that determines whether the expression is null or null, a collection or array without elements, and a string of length 0

2 JSTL tags

JSTL is a LIBRARY of JSP identification tags that developers can use to replace Java code on JSP pages, making programs more readable and easier to maintain.

Before using JSTL, you need to import the JSTL JAR package and import the JSTL tag library in the JSP page with the taglib specification as follows:

<% taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
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Here are some common tags used in JSTL:

The output label

<c:out></c:out>Prints the specified contentCopy the code

Set up the label

<c:set></c:set>Sets the standard property range valueCopy the code

Remove the label

<c:remove></c:remove>Delete specified dataCopy the code

Single condition judgment tag

<c:if test="EL conditional expression">Conditional execution</c:if>
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Multi-conditional judgment tag

<c:chose>
  <c:when test="EL conditional expression">Conditional execution</c:when>...<c:otherwise>If all the preceding conditions are not met, the command is executed</c:otherwise>
</c:chose>
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Loop label

<c:forEach var="Cyclic variable" items="Set">.</c:forEach>
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Common function tags

The < % taglib prefix = "fn" uri = "http://java.sun.com/jsp/jstl/functions" > % import functionCopy the code

Common formatting labels

The < % taglib prefix = "FMT" uri = "http://java.sun.com/jsp/jstl/fmt" > % import functionCopy the code

Of course, in addition to the above tags, we can also customize the tag, the steps are 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(a){
    return name;
  }
  public void setName(String s){
    name=s;
  }
  @Override
  public void doTag(a) throws JspEXception,IOException{
    JspWriter out=this.getJspContext().getOut();
    out.println("The parameters of the custom tag are :"+name); }}Copy the code

  • Define the tag library file and configure the tag description file in web-INF

    <tag>
  <name>helloTag</name>
  <tag-class>com.bjx.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 JSP to introduce the tag library to use

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