The technical principles of the JAKARTA Expression Language API framework in the Java class library

Jakarta Expression Language (EL) API is an important framework in the Java class library. It provides a powerful and flexible expression language for evaluating and processing string expression during runtime.This article will introduce you to the technical principles of the Jakarta Expression Language API framework, and explain its use through some Java code examples. The Jakarta Expression Language API in the Java class library follows some important technical principles, as shown below: 1. Expressive analysis and compilation: EL API provides a parser that analyzes the string expression as an expression tree that can be executed during runtime.The parser decompose the expression into an expression element and combine it into an expression tree that can be worthy at runtime.EL API also provides a compiler that can pre -compile the expression into the Java bytecode to improve execution efficiency. The following is an example of using EL API parsing and compilation expression: String expressionString = "${firstName} ${lastName}"; ExpressionFactory factory = new ExpressionFactoryImpl(); Expression expression = factory.createExpression(expressionString); 2. Expressive value: EL API provides a running environment for evaluation and execution of expressions.In the expression value phase, EL API can access variables, functions, and properties, and determine its value based on its reference in the expression. The following is an example of using EL API for value -looking expression: String firstName = "John"; String lastName = "Doe"; ELContext context = new ELContext(); context.setVariable("firstName", firstName); context.setVariable("lastName", lastName); Object result = expression.getValue(context); 3. Variable analysis: EL API allows to reference variables in the expression and find its value through variable parsing mechanism.The variable parser is responsible for searching and parsing variables during runtime, so that its value is passed to the expression during the value of the expression value. The following is an example of using EL API to resolve variables: ELResolver resolver = context.getELResolver(); Object value = resolver.getValue(context, null, "firstName"); 4. Function call: EL API also supports calling custom functions in the expression.The function parser is responsible for finding and calling the function at runtime.The function parser needs to register a custom function and provide it. The following is an example of calling the custom function using EL API: FunctionMapper mapper = new FunctionMapperImpl(); ELContext context = new ELContext(); context.setFunctionMapper(mapper); Object result = expression.getValue(context); In short, the Jakarta Expression Language API framework is a powerful expression language framework in the Java class library.By analyzing, compiling, and value for value, it realizes dynamic evaluation and processing of string expression.El API provides a simple and convenient way to handle and operate expressions by flexible variable analysis and function call mechanisms.I hope the introduction and example of this article can understand the technical principles of Jakarta Expression Language API framework.