Jakarta Expression Language API's application cases in the Java library

Jakarta Expression Language (EL) API is part of the Java EE specifications, which are used to evaluate and process expressions in JSP, JSF and other related technologies.This article will introduce the application cases of Jakarta Expression Language API in the Java library. Case: Calculate the area of the circle import javax.el.ExpressionFactory; import javax.el.ValueExpression; import javax.el.ELContext; import javax.el.ELResolver; import javax.el.FunctionMapper; import javax.el.VariableMapper; public class CircleAreaCalculator { public static void main(String[] args) { // Create the Jakarta EL expression factory ExpressionFactory factory = ExpressionFactory.newInstance(); // Create EL context ELContext context = new SimpleELContext(); // Define radius variables and expressions double radius = 5.0; String expression = "${Math.PI * (radius * radius)}"; // Create value expression ValueExpression valueExpression = factory.createValueExpression(context, expression, double.class); // Set variable maper VariableMapper variableMapper = context.getVariableMapper(); variableMapper.setVariable("radius", factory.createValueExpression(radius, double.class)); // Calculate the expression results double area = (double) valueExpression.getValue(context); // Output results System.out.println ("The area of the circle is" + Area); } } In this case, we will use the Jakarta El API to calculate the area of a circle. First, we need to introduce the related classes of the Jakarta El API.Then, we created a Jakarta El expression factory `expressionFactory`.Next, we create an EL context `elcontext`, we use instances of the` SimplelContext` class to implement it. Then, we define a variable `radius` and an expression` $ {math.pi * (Radius * Radius)} `. The PI constant of variables` Radius` and Math classes is used to calculate the area of the circle. We use the Jakarta El API's `CreatevalueExpression 'method to create a value expression` valueExpression`, which will calculate the expression we define. Next, we set the variable maper `variableMapper`, and set the value of the variable` radius` through the `setvariable` method, and use the factory's` CreateValueExpression 'method to express it as the Jakarta El expression. Finally, we calculate the results of the expression by calling the `GetValue` method of value expression, and assign the result to the variable` area`.Finally, the circular area obtained from the output calculation. By using the Jakarta Exposition Language API, we can easily calculate and process various expressions in the Java class library to make our code more flexible and scalable.