8.Spring EL与ExpressionParser

Spring EL与ExpressionParser

介绍

Spring表达式语言(SpEL)支持多种功能，并且可以测试这个特殊的“ExpressionParser”接口的表达式功能。

下面是两个代码片段，展示了使用 Spring EL 的基本用法

使用SpEL来计算评估文字字符串表达式

ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'put spel expression here'");
String msg = exp.getValue(String.class); 

使用SpEL来计算评估 bean 属性 – “item.name”

Item item = new Item("yiibai", 100);
StandardEvaluationContext itemContext = new StandardEvaluationContext(item);//display the value of item.name property
Expression exp = parser.parseExpression("name");
String msg = exp.getValue(itemContext, String.class);

举几个例子来测试使用SpEL

import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;public class App {public static void main(String[] args) {ExpressionParser parser = new SpelExpressionParser();//literal expressions Expression exp = parser.parseExpression("'Hello World'");String msg1 = exp.getValue(String.class);System.out.println(msg1);//method invocationExpression exp2 = parser.parseExpression("'Hello World'.length()");  int msg2 = (Integer) exp2.getValue();System.out.println(msg2);//Mathematical operatorsExpression exp3 = parser.parseExpression("100 * 2");  int msg3 = (Integer) exp3.getValue();System.out.println(msg3);//create an item objectItem item = new Item("yiibai", 100);//test EL with item objectStandardEvaluationContext itemContext = new StandardEvaluationContext(item);//display the value of item.name propertyExpression exp4 = parser.parseExpression("name");String msg4 = exp4.getValue(itemContext, String.class);System.out.println(msg4);//test if item.name == 'yiibai'Expression exp5 = parser.parseExpression("name == 'yiibai'");boolean msg5 = exp5.getValue(itemContext, Boolean.class);System.out.println(msg5);}
}

public class Item {private String name;private int qty;public Item(String name, int qty) {super();this.name = name;this.qty = qty;}//...
}

输出结果

Hello World