Spring学习笔记--Spring表达式语言SpEL

￡神魔★判官ぃ 2022-01-09 12:23 242阅读 0赞

Spring3引入了Spring表达式语言（Spring Expression Language,SpEL）。SpEL是一种强大的、简洁的装配Bean的方式，它通过运行期执行的表达式将值装配到Bean的属性或构造器参数中。  
字面值  
我们可以在<property>元素的value属性中使用\#\{\}界定符将值装配到Bean的属性中。

来源：https://www.cnblogs.com/moonlightpoet/p/5541010.html

浮点型数字一样可以出现在SpEL表达式中。

表达式中的数字也可以实用科学计数法。

这里将capacity属性设为了10000.0。  
String类型的字面值可以使用单引号或者双引号作为字符串界定符。

或者

还可以使用布尔值true和false。

引用Bean  
新建Poem类。

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package com.moonlit.myspring;
    
    public class Poem {
        private static String[] LINES = {
                "床前明月光，",
                "疑是地上霜。",
                "举头望明月，",
                "低头思故乡。",
        };
        public void perform() {
            for (String line : LINES)
                System.out.println(line);
        }
    }

![复制代码][copycode.gif]

新建Poet类。

![复制代码][copycode.gif]

package com.moonlit.myspring;
    
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class Poet {
        private Poem poem;
        public void recite() {
            System.out.println("the poet begin to recite...");
            poem.perform();
        }
        public static void main(String[] args) {
            ApplicationContext context = new ClassPathXmlApplicationContext(
                    "spring-idol.xml");
            Poet poet = (Poet) context.getBean("poet");
            poet.recite();
        }
        public Poem getPoem() {
            return poem;
        }
        public void setPoem(Poem poem) {
            this.poem = poem;
        }
    }

![复制代码][copycode.gif]

并在xml文件中声明他们对应的Bean。

poet通过SpEL获得了poem这个Bean。  
此时运行Poet程序得到结果如下：

the poet begin to recite...
    床前明月光，
    疑是地上霜。
    举头望明月，
    低头思故乡。

还可以通过SpEL获得bean的对象。

通过这句话就获得了poet的poem。  
还可以通过SpEL获得bean的对象。

通过这句话通过poet.getPoem()获得poet的poem。  
在SpEL中避免抛出空指针异常（NullPointException）的方法是使用null-safe存取器：

这里我们使用 ?. 运算符代替点（.）来访问toUpperCase()方法。在访问邮编方法之前，该运算符会确保左边项的值不为null。所以，如果selectorSong返回null，SpEL就不再尝试调用toUpperCase()方法。  
操作类  
在SpEL中，使用T()运算符会调用类作用域的方法和常量。例如，在SpEL中使用Java的Math类，我们可以像下面的示例这样使用T()运算符：  
T(java.lang.Math)  
T()运算符的结果会返回一个java.lang.Math类对象。  
装配PI或者一个随机值的配置方法如下：

SpEL上还可以执行bean值和数值之间的多种运算。  
这里我定义一个Circle类：

![复制代码][copycode.gif]

package com.moonlit.myspring;
    
    public class Circle {
        private double radius;
        public double getRadius() {
            return radius;
        }
        public void setRadius(double radius) {
            this.radius = radius;
        }
    }

![复制代码][copycode.gif]

它的Bean声明如下：

使用如下语句进行SpEL的数值运算示例：

“+”还可以进行字符串的连接。  
比较值

操作一个表达式的值：  
eq(==),lt(<),le(<=),gt(>),ge(>=)。  
逻辑表达式：  
and,or,not或!。  
条件运算符：使用三元运算符

一个常见的三元运算符的使用场景是检查一个值是否为null。

虽然以上配置可以正常工作，但这里kenny.song的引用重复了两次。SpEL提供了三元运算符的变体来简化表达式：

在以上示例中，如果kenny.song不为null，那么表达式的求值结果是kenny.song，否则就是"Greensleeves"。当我们以这种方式使用时，“?:”通常被称为elvis运算符。这个名字的来历是，使用这个运算符来表示微笑表情（?:) ，头左转90度看）时，问号看起来像猫王（Elvis Presley）的头发。  
SpEL支持正则表达式匹配

Hero类用于演示上述的大部分效果：

![ExpandedBlockStart.gif][]

package com.moonlit.myspring;
    
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class Hero {
        private int count;
        private double frequency;
        private double capacity;
        private String name;
        private boolean enabled;
        private Poem poem;
        private double multiplier;
        private double randomNumber;
        private double total;
        private boolean hasCapacity;
        public void perform() {
            System.out.println("count = " + count);
            System.out.println("frequency = " + frequency);
            System.out.println("capacity = " + capacity);
            System.out.println("name = " + name);
            System.out.println("enabled = " + enabled);
            System.out.println("hero begin to recite...");
            poem.perform();
            System.out.println("pi = " + multiplier);
            System.out.println("random number = " + randomNumber);
            System.out.println("total = " + total);
            System.out.println("hasCapacity = " + hasCapacity);
        }
        public static void main(String[] args) {
            ApplicationContext context = new ClassPathXmlApplicationContext(
                    "spring-idol.xml");
            Hero hero = (Hero) context.getBean("hero");
            hero.perform();
        }
        public int getCount() {
            return count;
        }
        public void setCount(int count) {
            this.count = count;
        }
        public double getFrequency() {
            return frequency;
        }
        public void setFrequency(double frequency) {
            this.frequency = frequency;
        }
        public double getCapacity() {
            return capacity;
        }
        public void setCapacity(double capacity) {
            this.capacity = capacity;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public boolean isEnabled() {
            return enabled;
        }
        public void setEnabled(boolean enabled) {
            this.enabled = enabled;
        }
        public Poem getPoem() {
            return poem;
        }
        public void setPoem(Poem poem) {
            this.poem = poem;
        }
        public double getMultiplier() {
            return multiplier;
        }
        public void setMultiplier(double multiplier) {
            this.multiplier = multiplier;
        }
        public double getRandomNumber() {
            return randomNumber;
        }
        public void setRandomNumber(double randomNumber) {
            this.randomNumber = randomNumber;
        }
        public double getTotal() {
            return total;
        }
        public void setTotal(double total) {
            this.total = total;
        }
        public boolean isHasCapacity() {
            return hasCapacity;
        }
        public void setHasCapacity(boolean hasCapacity) {
            this.hasCapacity = hasCapacity;
        }
        
    }

![复制代码][copycode.gif]

package com.moonlit.myspring;
    
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class Hero {
        private int count;
        private double frequency;
        private double capacity;
        private String name;
        private boolean enabled;
        private Poem poem;
        private double multiplier;
        private double randomNumber;
        private double total;
        private boolean hasCapacity;
        public void perform() {
            System.out.println("count = " + count);
            System.out.println("frequency = " + frequency);
            System.out.println("capacity = " + capacity);
            System.out.println("name = " + name);
            System.out.println("enabled = " + enabled);
            System.out.println("hero begin to recite...");
            poem.perform();
            System.out.println("pi = " + multiplier);
            System.out.println("random number = " + randomNumber);
            System.out.println("total = " + total);
            System.out.println("hasCapacity = " + hasCapacity);
        }
        public static void main(String[] args) {
            ApplicationContext context = new ClassPathXmlApplicationContext(
                    "spring-idol.xml");
            Hero hero = (Hero) context.getBean("hero");
            hero.perform();
        }
        public int getCount() {
            return count;
        }
        public void setCount(int count) {
            this.count = count;
        }
        public double getFrequency() {
            return frequency;
        }
        public void setFrequency(double frequency) {
            this.frequency = frequency;
        }
        public double getCapacity() {
            return capacity;
        }
        public void setCapacity(double capacity) {
            this.capacity = capacity;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public boolean isEnabled() {
            return enabled;
        }
        public void setEnabled(boolean enabled) {
            this.enabled = enabled;
        }
        public Poem getPoem() {
            return poem;
        }
        public void setPoem(Poem poem) {
            this.poem = poem;
        }
        public double getMultiplier() {
            return multiplier;
        }
        public void setMultiplier(double multiplier) {
            this.multiplier = multiplier;
        }
        public double getRandomNumber() {
            return randomNumber;
        }
        public void setRandomNumber(double randomNumber) {
            this.randomNumber = randomNumber;
        }
        public double getTotal() {
            return total;
        }
        public void setTotal(double total) {
            this.total = total;
        }
        public boolean isHasCapacity() {
            return hasCapacity;
        }
        public void setHasCapacity(boolean hasCapacity) {
            this.hasCapacity = hasCapacity;
        }
        
    }

![复制代码][copycode.gif]

spring-idol.xml：

![ExpandedBlockStart.gif][]

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:p="http://www.springframework.org/schema/p"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd"
        default-init-method="defaultBorn"
        default-destroy-method="defaultDead" >
        
      <bean id="poem" class="com.moonlit.myspring.Poem" />
      <bean id="poet" class="com.moonlit.myspring.Poet">
        <property name="poem" value="#{poem}" />
      </bean>
      <bean id="circle" class="com.moonlit.myspring.Circle">
        <property name="radius" value="2.1" />
      </bean>
      <bean id="hero" class="com.moonlit.myspring.Hero">
        <property name="count" value="#{5}" />
        <property name="frequency" value="#{89.7}" />
        <property name="capacity" value="#{1e4}" />
        
        <property name="name" value='#{"moonlit"}' />
        <property name="enabled" value="#{true}" />
        
        <property name="poem" value="#{poet.getPoem()}" />
        <property name="multiplier" value="#{T(java.lang.Math).PI}" />
        <property name="randomNumber" value="#{T(java.lang.Math).random()}" />
        <property name="total" value="#{circle.radius + 100.0}" />
        <property name="hasCapacity" value="#{circle.radius lt 3.3}" />
      </bean>
    </beans>

![复制代码][copycode.gif]

转载于:https://www.cnblogs.com/CaptainFM/p/10728690.html

[copycode.gif]: /images/20220109/83bb93800e85461fb2af2ab66e214f87.png
[ExpandedBlockStart.gif]: /images/20220109/fd41476e991a4dc0bc71c25a9e69a03b.png