Jakarta Expression Language API Framework Guide

Jakarta Expression Language API Framework Guide Introduction: Jakarta Expression Language (EL) API is a standard expression language for JSP and Javaseerver Faces.It provides a way to evaluate and operate the JavaBean attributes, array and sets such as JavaBean.This article will introduce the basic concepts and usage methods of Jakarta Expression Language API, and provide corresponding Java code examples. 1. Basic concept: 1. Expression: Expression is a statement containing variables and computing symbols for calculating results.In Jakarta El, the expression is surrounded by double flower brackets ({{}}), such as {{Expression}}. 2. Variables and attributes: Variables are a place where the object represents the object, which can be an object of JavaBean, array or collection.In the expression, you can use "." To access the properties and methods of the variable. Logical operators (&&, || ,!). 4. Value of expression: Jakarta El uses the Elresolver object to parse the expression and obtain the corresponding value.ElResolver can find more complicated expressions by connecting multiple parsers. 2. How to use: 1. Dependent configuration: First of all, you need to add Jakarta Expression Language API to the project dependency configuration file. For example, the Maven project can add the following code to the POM.XML file: <dependency> <groupId>jakarta.el</groupId> <artifactId>jakarta.el-api</artifactId> <version>3.0.3</version> </dependency> 2. Create an expression value device: Use javax.el.expressionFactory class to create an expression value device.For example: ExpressionFactory factory = ExpressionFactory.newInstance(); 3. Compilation and value -looking expression: Use expressions to value device to compile and value value.For example: String expressionString = "{{expression}}"; ValueExpression expression = factory.createValueExpression(context, expressionString, Object.class); Object result = expression.getValue(context); The Context here is an object of the Javax.el.elContext type, which can be created by the implementation class of javax.el.elcontext.For example, the instance of javax.servlet.jsp.jspContext can be used as context on the jsp page. 4. Get the expression result: The result of obtaining the expression by calling the GetValue method is assigned to the Java variable.For example: String resultString = (String) result; Third, sample code: Here are a simple Java code example to demonstrate how to use Jakarta Expression Language API: import javax.el.ExpressionFactory; import javax.el.ValueExpression; import javax.servlet.jsp.JspContext; import jakarta.el.ELContext; import jakarta.el.ELResolver; public class JakartaELExample { public static void main(String[] args) { // Create an expression value device ExpressionFactory factory = ExpressionFactory.newInstance(); // Create Elcontext objects JspContext context = new JspContext(); // Compile and value value expression String expressionString = "{{expression}}"; ValueExpression expression = factory.createValueExpression(context, expressionString, Object.class); Object result = expression.getValue(context); // Get the expression result String resultString = (String) result; // Output expression results System.out.println("Expression result: " + resultString); } } In the above example, we first created an ExpressionFactory object, and then created a JSPContext object to act as ElContext.Next, we compiled and value the ExpressionFactory object, and obtained the result of the expression by calling the getValue method.Finally, we converted the results of the expression to the String type and output it to the console. In summary, this article introduces the basic concepts and usage methods of the Jakarta Expression Language API framework, and provides the corresponding Java code example.Through these examples, you can learn how to use Jakarta El in the Java Web application to evaluate and operate expression.I wish you a happy use of the Jakarta El framework!