Detailed explanation of Jakarta expression language API: Usage and common questions answers

Jakarta Expression Language (Jexl) is a expression language for dynamic value and operation in Java applications.It is part of JSP technology, which aims to provide a simple and powerful way to handle expression and calculations related to Java type. The use of JEXL is very flexible and can be widely used in Java applications in different fields.Here are some common usage and answers to Jexl. 1. The introduction and configuration of jexl: To use Jexl, we need to add the corresponding dependencies to the construction file of the project.For example, using Maven to build tools, you can add the following dependencies to the pom.xml file: <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-jexl3</artifactId> <version>3.2</version> </dependency> There are many ways to configure the Jexl engine. One of the common methods is to use the `Jexlbuilder` class to create a Jexl engine instance, and the expression can be performed through this instance. 2. Basic expressions find value: JEXL supports basic arithmetic operations, logical operations and comparative operations.For example, the following code shows how to use Jexl to solve a simple arithmetic expression: JexlEngine jexl = new JexlBuilder().create(); JexlExpression expression = jexl.createExpression("2 * (3 + 4)"); Object result = expression.evaluate(null); System.out.println("Result: " + result); // Output: Result: 14 In this example, we created a Jexl engine instance and used the `CreateExpression` method to create an object that expressed an expression of expression.Then, we solve the expression by calling the `Evaluate` method, and finally print the result. 3. Access object attributes and methods: Jexl also supports the attributes and methods of accessing the Java object through an expression.The following is an example: JexlEngine jexl = new JexlBuilder().create(); JexlExpression expression = jexl.createExpression("person.name"); JexlContext context = new MapContext(); Person person = new Person("John"); // Assume person has a "name" property context.set("person", person); Object result = expression.evaluate(context); System.out.println("Name: " + result); // Output: Name: John In this example, we created a `Person` object and set up a context variable called" Person ".Then, we accepted the `name` attribute of the` Person` object, and use the `Evaluate` method to solve the expression. 4. Custom function and variables: Jexl also allows us to define our functions and variables for use in expressions.The following is an example: JexlEngine jexl = new JexlBuilder().create(); JexlExpression expression = jexl.createExpression("myFunc(num)"); JexlContext context = new MapContext(); context.set("num", 5); context.set("myFunc", (JexlScript.Callable) (x) -> x * x); Object result = expression.evaluate(context); System.out.println("Result: " + result); // Output: Result: 25 In this example, we define a function called "MyFunc", which accepts a parameter and returns its square.Then, we set the function to the variables in the context and call it in the expression. 5. Frequently questions and answers: -Q: What types of expressions are supported by Jexl? A: Jexl supports most Java type expressions, including basic types, string, collection, custom objects, etc. -Q: Jexl supports conditional statements? A: Jexl itself does not support conditional statements, but you can use ternary computing symbols in the expression to simulate the behavior of conditional statements. -Q: Jexl supports cycle sentences? A: Jexl itself does not support the cycle statement, but it can use recursive functions or custom functions in the expression to achieve similar functions. -Q: Jexl supports abnormal treatment? A: Jexl itself does not support abnormal processing, but it can cause and capture abnormalities in the Java code. In short, the Jakarta expression language is a powerful and flexible tool that can realize dynamic value and operation requirements in Java applications.It supports basic expressions, access object attributes and methods, custom functions, and variables, which can be used for various business scenarios.