Introduction to the JAKARTA Expression Language API framework in the Java Class Library aries)

JAKARTA Expression Language API framework technical principles in the Java class library The Jakarta Expression Language (Jel) API framework in the Java class library is a powerful tool for evaluating and processing expressions in Java applications.This article will introduce Jel's technical principles, focusing on its application in the Java class library. Jakarta Expression Language (Jel) API is part of the Java family, which aims to provide flexible and powerful expression computing and processing functions for Java developers.Jel API contains two core parts: expression syntax and expression parser. First, let us understand Jel's expression grammar.Jel uses a simple and flexible syntax to represent the expression.It supports a variety of different data types, such as integer, floating -point numbers, Boolean values, string, etc., and provides various operators and functions to process these data.For example, the following is a simple Jel expression: 1 + 2 * (3 - 4) In this expression, Jel calculates the results by using arithmetic operators and brackets. Next is Jel's expression parser.The expression parser is Jel's core component, which is responsible for analyzing and evaluating the expression.It uses expression as input and converts it into internal data structure so that it can be calculated.Jel's expression parser uses recursively decorating algorithms, decomposes the expression into a series of syntax units, and builds a syntax tree to represent the entire expression.For example, for the previously mentioned Jel expression, the expression parser will create the following syntax tree: + / \ 1 * / \ 2 - / \ 3 4 By using the grammar tree, Jel can perform each operation in the correct order and calculate the final result. The following is a simple Java code example, demonstrating how to use Jel API in the Java class library: import javax.el.ExpressionFactory; import javax.el.ValueExpression; import javax.el.ELContext; import javax.el.StandardELContext; public class JELExample { public static void main(String[] args) { // Create an expression factory ExpressionFactory factory = ExpressionFactory.newInstance(); // Create an expression ValueExpression expression = factory.createValueExpression( new StandardELContext(), "${1 + 2 * (3 - 4)}", Object.class); // Set the expression context ELContext context = new StandardELContext(); context.setVariable("x", factory.createValueExpression( new StandardELContext(), "${5}", Object.class)); // Evaluate expression Object result = expression.getValue(context); System.out.println ("Result: + Result); // Output: Result: -1 } } In this example, we created a expression using Jel API, and then set the expression context to set the value of the variable `x` to 5.Finally, we evaluate the expression by calling the `GetValue ()" method and obtain the results.In this case, the result of the expression will be -1. In summary, Jakarta Expression Language (Jel) API framework is a powerful expression calculation and processing tool in the Java class library.It analyzes and evaluates the expression through simple syntax and expression parser.By understanding Jel's technical principles, developers can better use this framework to provide their Java applications with more flexible and powerful expression processing functions.