SPRING IN ACTION 第4版笔记-第三章ADVANCING WIRING-008-SpEL介绍

一、

1.SpEL expressions are framed with  #{ ... }

2.SpEl的作用

Sp EL has a lot of tricks up its sleeves, including the following:
 The ability to reference beans by their ID s
 Invoking methods and accessing properties on objects
 Mathematical, relational, and logical operations on values
 Regular expression matching
 Collection manipulation
As you’ll see later in this book, S p EL can also be used for purposes other than depen-
dency injection. Spring Security, for example, supports defining security constraints
using S p EL expressions. And if you’re using Thymeleaf templates as the views in your
Spring MVC application, those templates can use S p EL expressions to reference
model data.

3.基础例子

(1)解析原始类型数据 #{1}

(2)解析java类的静态方法#{T(System).currentTimeMillis()}，"T()"会把被包括的内容解析为java类

(3)解析bean：#{sgtPeppers.artist}，取出id为“sgtPeppers”的artist属性

(4)解析properties:#{systemProperties['disc.title']}

二、进一步了解SpEl

1.EXPRESSING LITERAL VALUES

(1)浮点型：#{3.14159}

(2)科学计数法：#{9.87E4}  PS:=98,700

(3)字符串：#{'Hello'}

(4)布尔型：#{false}

2.REFERENCING BEANS ,  PROPERTIES ,  AND METHODS

(1)引用bean：#{sgtPeppers}   (in this case, a bean whose  ID is  sgtPeppers )

(2)引用bean的属性：#{sgtPeppers.artist} 引用id为"sgtPeppers"的bean的artist属性

(3)调用bean的方法：#{artistSelector.selectArtist()}

(4)链式调用：#{artistSelector.selectArtist().toUpperCase()}

(5)防止空指针：#{artistSelector.selectArtist()?.toUpperCase()}

3.WORKING WITH TYPES IN EXPRESSIONS

The key to working with class-scoped methods and constants in S p EL is to use the T()
operator. For example, to express Java’s Math class in S p EL , you need to use the T()
operator like this:
T(java.lang.Math)
The result of the T() operator, as shown here, is a Class object that represents java.lang.Math .

(1)T(java.lang.Math).PI

(2)T(java.lang.Math).random()　　 random value between 0 and 1:

(1)计算一个代表圆的bean的周长：#{2 * T(java.lang.Math).PI * circle.radius}

(2)计算面积：#{T(java.lang.Math).PI * circle.radius ^ 2}

(3)用"+"连接字符串:#{disc.title + ' by ' + disc.artist}

(4)符号逻辑和文本逻辑操作符是等效的：#{counter.total == 100}   或者   #{counter.total eq 100}

(5)三目运算符：#{scoreboard.score > 1000 ? "Winner!" : "Loser"}

     设置默认值：#{disc.title ?: 'Rattle and Hum'}

4.EVALUATING REGULAR EXPRESSIONS

(1)验证邮箱(不全) ：#{admin.email matches '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.com'}

5.EVALUATING COLLECTIONS

(1)#{jukebox.songs[4].title}

(2)随机播放歌曲：#{jukebox.songs[T(java.lang.Math).random() *jukebox.songs.size()].title}

(3)"[]"也可用来从字符串中截取字符：#{'This is a test'[3]} ，返回's'

(4)筛选：#{jukebox.songs.?[artist eq 'Aerosmith']}，会返回所有Aerosmith的歌曲的新集合

(5)SpEL also offers two other selection operations: .^[] for selecting the first matching entry and .$[] for selecting the last matching entry.

返回第一个符合条件的条目：#{jukebox.songs.^[artist eq 'Aerosmith']}

(6)投影：SpEL offers a projection operator ( .![] ) to project properties from the elements in the collection onto a new collection.

抽取所有title:#{jukebox.songs.![title]}

组合操作，获取Aerosmith的所有歌名：#{jukebox.songs.?[artist eq 'Aerosmith'].![title]}