在此示例中,您将学习如何创建集合对象的投影。使用投影,我们可以创建一个仅具有原始集合中特定属性的新集合。
举例来说,Book我们不希望返回对象的集合,而只希望获得书名。为此,我们可以使用Spring EL投影运算符。此运算符的符号用法是![]。
让我们开始创建Spring配置文件:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:util="http://www.springframework.org/schema/util"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
<util:list id="books">
<bean
p:title="Essential C# 4.0" p:author="Michaelis" p:pages="450"/>
<bean
p:title="User Stories Applied" p:author="Mike Cohen" p:pages="268"/>
<bean
p:title="Learning Android" p:author="Marco Gargenta" p:pages="245"/>
<bean
p:title="The Ruby Programming Language"
p:author="David Flanagan & Yukihiro Matsumoto" p:pages="250"/>
<bean
p:title="Einstein" p:author="Walter Isaacson" p:pages="1000"/>
</util:list>
<bean id="library">
<property name="bookTitles" value="#{books.![title]}"/>
</bean>
</beans>
这是BookandLibrary类的定义。为了简化代码段,删除了getter和setter方法。
package org.nhooo.example.spring.el;
public class Book {
private Long id;
private String title;
private String author;
private String type;
private int pages;
// Getters & Setters
}
package org.nhooo.example.spring.el;
import java.util.List;
public class Library {
private List<Book> books;
private List<String> bookTitles;
// Getters & Setters
}
现在,让我们谈谈上面的Spring配置。配置是通过使用来创建“书籍”集合的开始<util:elements>。使用投影运算符的部分是配置的这一部分:
<bean id="library">
<property name="bookTitles" value="#{books.![title]}"/>
</bean>
bean上面的元素创建一个library类型为的bean org.nhooo.example.spring.model.Library。我们为bean的bookTitles属性分配值,这些值是+ books +集合的投影,其中我们只接受书名。![projectionExpression]是投影运算符的语法。
下面的代码段将演示并运行我们的配置文件并打印出结果:
package org.nhooo.example.spring.el;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class SpELProjectionExample {
public static void main(String[] args) {
ApplicationContext context =
new ClassPathXmlApplicationContext("spel-projection.xml");
Library library = context.getBean("library", Library.class);
for (String title : library.getBookTitles()) {
System.out.println("title = " + title);
}
}
}
结果如下:
title = Essential C# 4.0
title = User Stories Applied
title = Learning Android
title = The Ruby Programming Language
title = Einstein