Spring表達式（Spring Expression Language，SpringEL）官方文檔

SpringEL是一個強大的表達式語言，支持在運行時查詢和操作對象圖。

官方地址：https://docs.spring.io/spring-framework/docs/5.3.18/reference/html/core.html#expressions

需要引入依賴：

<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-expression</artifactId>
  <version>5.3.18</version>
</dependency>

引用：

import org.springframework.expression.EvaluationContext;
import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.common.TemplateParserContext;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.SimpleEvaluationContext;

文檔部分例子：

ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'"); 
String message = (String) exp.getValue();


ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.concat('!')"); 
String message = (String) exp.getValue();


// invokes 'getBytes().length'
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.bytes.length"); 
int length = (Integer) exp.getValue();


ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("new String('hello world').toUpperCase()"); 
String message = exp.getValue(String.class);


ExpressionParser parser = new SpelExpressionParser();
// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue();
double avogadrosNumber = (Double) parser.parseExpression("6.0221415E+23").getValue();
// evals to 2147483647
int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();
boolean trueValue = (Boolean) parser.parseExpression("true").getValue();
Object nullValue = parser.parseExpression("null").getValue();

// evals to 1856
int year = (Integer) parser.parseExpression("birthdate.year + 1900").getValue(context);
String city = (String) parser.parseExpression("placeOfBirth.city").getValue(context);


// evaluates to a Java list containing the four numbers
List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context);
List listOfLists = (List) parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context);


// evaluates to a Java map containing the two entries
Map inventorInfo = (Map) parser.parseExpression("{name:'Nikola',dob:'10-July-1856'}").getValue(context);
Map mapOfMaps = (Map) parser.parseExpression("{name:{first:'Nikola',last:'Tesla'},dob:{day:10,month:'July',year:1856}}").getValue(context);


int[] numbers1 = (int[]) parser.parseExpression("new int[4]").getValue(context);
// Array with initializer
int[] numbers2 = (int[]) parser.parseExpression("new int[]{1,2,3}").getValue(context);
// Multi dimensional array
int[][] numbers3 = (int[][]) parser.parseExpression("new int[4][5]").getValue(context);


// string literal, evaluates to "bc"
String bc = parser.parseExpression("'abc'.substring(1, 3)").getValue(String.class);
// evaluates to true
boolean isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(societyContext, Boolean.class);


// evaluates to true
boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class);
// evaluates to false
boolean falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean.class);
// evaluates to true
boolean trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean.class);


// -- AND --
// evaluates to false
boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class);
// evaluates to true
String expression = "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
// -- OR --
// evaluates to true
boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class);
// evaluates to true
String expression = "isMember('Nikola Tesla') or isMember('Albert Einstein')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
// -- NOT --
// evaluates to false
boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class);
// -- AND and NOT --
String expression = "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')";
boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);


// Addition
int two = parser.parseExpression("1 + 1").getValue(Integer.class);  // 2
String testString = parser.parseExpression("'test' + ' ' + 'string'").getValue(String.class);  // 'test string'
// Subtraction
int four = parser.parseExpression("1 - -3").getValue(Integer.class);  // 4
double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class);  // -9000
// Multiplication
int six = parser.parseExpression("-2 * -3").getValue(Integer.class);  // 6
double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class);  // 24.0
// Division
int minusTwo = parser.parseExpression("6 / -3").getValue(Integer.class);  // -2
double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class);  // 1.0
// Modulus
int three = parser.parseExpression("7 % 4").getValue(Integer.class);  // 3
int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class);  // 1
// Operator precedence
int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class);  // -21


String randomPhrase = parser.parseExpression(
        "random number is #{T(java.lang.Math).random()}",
        new TemplateParserContext()).getValue(String.class);
// evaluates to "random number is 0.7038186818312008"



ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
context.setVariable("reverseString", StringUtils.class.getDeclaredMethod("reverseString", String.class));
String helloWorldReversed = parser.parseExpression("#reverseString('hello')").getValue(context, String.class);

public abstract class StringUtils {
    public static String reverseString(String input) {
        StringBuilder backwards = new StringBuilder(input.length());
        for (int i = 0; i < input.length(); i++) {
            backwards.append(input.charAt(input.length() - 1 - i));
        }
        return backwards.toString();
    }
}