学习内容源于 官方文档 https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#expressions
学习摘要：
SpEL: Spring Express Language, a java expression language.
Other known expression Language: OGNL, MVEL, JBoss EL ...
SpEL 支持的功能：
- Literal expressions: "'Hello world'"
- Boolean and relational operators: "false", "true", " 1 > 5"
- Regular expressions
- Class expressions
- Accessing properties, arrays, lists, and maps: "a.b" , "new arr[4]","arr[0]", "{1,2,3}", "m['key']" "{k1:v1,k2:v2}"
- Method invocation: "'abc'.substring(1,3)"
- Relational operators : "2 == 2" , "'xyz' instanceof T<Integer>" "'5.00' matches '^-?\\d+(\\.\\d{2})?$'")
- Assignment: " v1='hello world' "
- Calling constructors : "new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German')"
- Bean references: "@something"
- Array construction:
- Inline lists : "{1,2,3}"
- Inline maps: "{k1:v1,k2:v2}"
- Ternary operator : "true?trueExp:falseExp"
- Variables : #varibleName, #this, #root
- User-defined functions
- Collection projection
- Collection selection
- Templated expressions
- "name?:'Unknow'"
- "object1?.property1"
- "members.?[country=='China']"
- "map.?[value < 10]"
- "collection.^[selectionExpression]"
- "collection.$[selectionExpression]"
- "collection.![projectionExpression]"
- #{}
- "This value is #{1+2}"
遇到的类(org.springframework.expression)：