探究 Java 类库中 Jakarta Expression Language API 框架的技术原理 (Exploring the Technical Principles of Jakarta Expression Language API Framework in Java Class Libraries)

Explore the technical principles of Jakarta Expression Language API framework in the Java class library Overview: Jakarta Expression Language (EL) API is a framework for parsing and evaluating expressions in Java applications.It is part of the JavaserVer Pages (JSP) specification and is widely used in the Jakarta EE project.This article will explore the technical principles of the Jakarta El API framework and provide some Java code examples to help readers better understand. 1. What is Jakarta Expression Language (EL) API framework? Jakarta El API is a framework for processing and computing expressions.It provides a set of classes and methods to analyze and interpret expressions to achieve dynamic computing and data binding.Jakarta El API is similar to the expression value of the script language (such as JavaScript) or other programming languages, but its main purpose is to use it with JSP and other technologies to realize dynamically generating web pages. Second, the component of Jakarta El API 1. EL expression: EL expression is a simple language for dynamic calculation values.It can be used to obtain and set object attributes, call methods, and perform arithmetic and logical operations.EL expression is embedded in the JSP file or via the code for value in the JSP file in the "$ {}" format. 2. EL engine: EL engine is the core part of the Jakarta El API.It is responsible for analyzing EL expressions and performing corresponding calculations.The EL engine provides a set of core parser and voucher to handle different types of expressions. 3. EL context: EL context is a key concept of the EL engine.It provides access to the environment and variable information required for expression operations.EL context can include variables, functions, class definitions, and references.When using the Jakarta El API, you can customize EL context to meet specific needs. 3. How to use Jakarta El API? Here are some examples of examples using Jakarta El API: 1. Analyze and value EL expression: import javax.el.ExpressionFactory; import javax.el.ValueExpression; import javax.el.ELContext; import javax.el.ELResolver; // Create an EL expression factory ExpressionFactory factory = ExpressionFactory.newInstance(); // Create an EL context ELContext context = new SomeELContext(); // Analyze EL expression ValueExpression expression = factory.createValueExpression(context, "${user.name}", String.class); // Value EL expression String result = (String) expression.getValue(context); 2. Custom EL context: import javax.el.ELContext; import javax.el.ELResolver; import javax.el.FunctionMapper; import javax.el.BeanELResolver; import javax.el.CompositeELResolver; public class MyELContext extends ELContext { private CompositeELResolver resolver; public MyELContext() { resolver = new CompositeELResolver(); resolver.add(new MyBeanELResolver()); resolver.add(new BeanELResolver()); } @Override public ELResolver getELResolver() { return resolver; } @Override public FunctionMapper getFunctionMapper() { // Custom function mapping return new MyFunctionMapper(); } // Other methods to implement ... } // Custom EL parser public class MyBeanELResolver extends BeanELResolver { // Implement custom EL parsing logic ... } // Custom EL function mapping public class MyFunctionMapper extends FunctionMapper { // Implement a custom function mapping logic ... } Through the above example, we can see that the working principle of the Jakarta El API is: first create an EL context, add custom parser and mapper; then use EL expression factory to analyze and value EL expressions, and finally get the calculation resultsEssence in conclusion: This article explores the technical principles of Jakarta Expression Language (EL) API framework in the Java class library.Through the Jakarta El API, we can easily analyze and calculate expressions in Java applications to achieve dynamic computing and data binding.Readers can practice according to the example code, and customize EL context according to specific needs to achieve higher -level functions.