Master the core concept of the Jakarta Expression Language API framework

Master the core concept of the Jakarta Expression Language (EL) API framework Overview: Jakarta Expression Language (EL) API is part of the Java Enterprise Edition (Java Ee), which is used to evaluate and process expressions in Java applications.The API provides a simple and flexible way to handle expression, which aims to enable developers to dynamically operate and operate the JavaBean attributes, array, collection and maps. Core idea: 1. Expression: The expression is a string written by EL language for evaluating and operating data in Java applications.It can contain variables, operators, and method calls.Through the EL API, the expression can be embedded in the code and evaluated at runtime. 2. EL Resolver: EL parser is responsible for evaluating the expression and obtaining the corresponding value.It can analyze the variables, attributes, method calls, etc. in the expression, and return the correct result.EL parser also provides access to JavaBean attributes, arrays, collection and maps. 3. EL Context: EL context is part of the environment of EL expressions.It provides support for variables and functions, and defines the rules for analysis and value of expression.EL context can also create, manage and destroy the EL parser. 4. EL Function (EL Function): EL function allows developers to define their functions in order to use in EL expressions.These functions can be static or non -static for performing various operations.By registering these functions, developers can expand EL functions. Example code: Here are a simple Java code example to demonstrate how to use the Jakarta El API to evaluate the expression and get the results. import javax.el.ExpressionFactory; import javax.el.StandardELContext; import javax.el.ValueExpression; public class Evaluator { public static void main(String[] args) { // Factory that creates an EL parser ExpressionFactory factory = ExpressionFactory.newInstance(); // Create EL context StandardELContext context = new StandardELContext(); // Define a variable context.getELResolver().setValue(context, null, "name", "John"); // Create an expression ValueExpression expression = factory.createValueExpression(context, "${name}", String.class); // Evaluate expression and get results String result = (String) expression.getValue(context); // Output results System.out.println("Name: " + result); } } In the above example, we created an EL parser with ExpressionFactory and used StandardelContext to create an EL context.We also define a variable called "name" and use the expression `$ {name}` to get the value of the variable.Finally, we evaluate the expression and store the results in the string variables and print out the results. Summarize: Understanding and mastering the core concept of the Jakarta Expression Language (EL) API framework is the key to evaluating and processing expressions in Java applications.EL parser, EL context, and EL function are the core components of the API, and provide powerful and flexible functions to operate and process data.By using the example code, developers can better understand how to use the Jakarta El API to evaluate the expression and get the results.