Spring 注入集合



  • Bean 定义继承

    您已经看到了如何在Bean配置文件中使用property标记的ref属性来使用value属性和对象引用来配置基本数据类型。这两种情况都处理将不同值传递给bean的问题。现在,如果您希望传递多个值,比如Java集合类型,比如List、Set、MapProperties,该怎么办呢?为了处理这种情况,Spring提供了四种类型的集合配置元素,如下所示
    • <list> - 这有助于注入,即注入值列表,允许重复。
    • <set> - 这有助于注入一组值,但不能重复。
    • <map> - 这可用于注入名称/值对的集合,其中名称和值可以是任何类型。
    • <props> - 这可用于注入名称-值对的集合,其中名称和值均为字符串。
    您可以使用<list>或<set>连接java.util.Collection或数组的任何实现。您将遇到两种情况(a)传递集合的直接值,以及(b)传递bean的引用作为集合元素之一。
  • 实例

    假设我们拥有一个运行良好的Eclipse IDE,并采取以下步骤来创建一个Spring应用程序:
    1. 创建一个名称为SpringExample的项目,并在创建的项目的src文件夹下创建一个包com.jc2182
    2. 使用“添加外部JAR”选项添加所需的Spring库,如“Spring Hello World示例”一章中所述。
    3. 在com.jc2182包下创建Java类JavaCollection和MainApp。
    4. 在src文件夹下创建Beans配置文件Beans.xml。
    5. 最后一步是创建所有Java文件和Bean配置文件的内容,然后按以下说明运行应用程序。
    以下是JavaCollection.java的内容。
    package com.jc2182;
    import java.util.*;
    
    public class JavaCollection {
       List addressList;
       Set  addressSet;
       Map  addressMap;
       Properties addressProp;
    
       // a setter method to set List
       public void setAddressList(List addressList) {
          this.addressList = addressList;
       }
       
       // prints and returns all the elements of the list.
       public List getAddressList() {
          System.out.println("List Elements :"  + addressList);
          return addressList;
       }
       
       // a setter method to set Set
       public void setAddressSet(Set addressSet) {
          this.addressSet = addressSet;
       }
       
       // prints and returns all the elements of the Set.
       public Set getAddressSet() {
          System.out.println("Set Elements :"  + addressSet);
          return addressSet;
       }
       
       // a setter method to set Map
       public void setAddressMap(Map addressMap) {
          this.addressMap = addressMap;
       }
       
       // prints and returns all the elements of the Map.
       public Map getAddressMap() {
          System.out.println("Map Elements :"  + addressMap);
          return addressMap;
       }
       
       // a setter method to set Property
       public void setAddressProp(Properties addressProp) {
          this.addressProp = addressProp;
       }
       
       // prints and returns all the elements of the Property.
       public Properties getAddressProp() {
          System.out.println("Property Elements :"  + addressProp);
          return addressProp;
       }
    }
    
    以下是MainApp.java文件的内容。
    package com.jc2182;
    
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class MainApp {
       public static void main(String[] args) {
          ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
          JavaCollection jc=(JavaCollection)context.getBean("javaCollection");
    
          jc.getAddressList();
          jc.getAddressSet();
          jc.getAddressMap();
          jc.getAddressProp();
       }
    }
    
    以下是配置文件Beans.xml,它具有所有集合类型的配置
    <?xml version = "1.0" encoding = "UTF-8"?>
    
    <beans xmlns = "http://www.springframework.org/schema/beans"
       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">
    
       <!-- Definition for javaCollection -->
       <bean id = "javaCollection" class = "com.jc2182.JavaCollection">
          
          <!-- results in a setAddressList(java.util.List) call -->
          <property name = "addressList">
             <list>
                <value>CHINA</value>
                <value>Pakistan</value>
                <value>USA</value>
                <value>USA</value>
             </list>
          </property>
    
          <!-- results in a setAddressSet(java.util.Set) call -->
          <property name = "addressSet">
             <set>
                <value>CHINA</value>
                <value>Pakistan</value>
                <value>USA</value>
                <value>USA</value>
             </set>
          </property>
    
          <!-- results in a setAddressMap(java.util.Map) call -->
          <property name = "addressMap">
             <map>
                <entry key = "1" value = "CHINA"/>
                <entry key = "2" value = "Pakistan"/>
                <entry key = "3" value = "USA"/>
                <entry key = "4" value = "USA"/>
             </map>
          </property>
          
          <!-- results in a setAddressProp(java.util.Properties) call -->
          <property name = "addressProp">
             <props>
                <prop key = "one">CHINA</prop>
                <prop key = "one">CHINA</prop>
                <prop key = "two">Pakistan</prop>
                <prop key = "three">USA</prop>
                <prop key = "four">USA</prop>
             </props>
          </property>
       </bean>
    
    </beans>
    
    完成创建源和Bean配置文件后,让我们运行该应用程序。如果您的应用程序一切正常,它将显示以下消息:
    List Elements :[CHINA, Pakistan, USA, USA]
    Set Elements :[CHINA, Pakistan, USA]
    Map Elements :{1=CHINA, 2=Pakistan, 3=USA, 4=USA}
    Property Elements :{two=Pakistan, one=CHINA, three=USA, four=USA}
    
  • 注入 Bean引用

    以下Bean定义将帮助您了解如何将Bean引用作为集合的元素之一进行注入。即使您可以将引用和值混合在一起,如以下代码片段所示
    <?xml version = "1.0" encoding = "UTF-8"?>
    
    <beans xmlns = "http://www.springframework.org/schema/beans"
       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">
    
       <!-- Bean Definition to handle references and values -->
       <bean id = "..." class = "...">
    
          <!-- Passing bean reference  for java.util.List -->
          <property name = "addressList">
             <list>
                <ref bean = "address1"/>
                <ref bean = "address2"/>
                <value>Pakistan</value>
             </list>
          </property>
          
          <!-- Passing bean reference  for java.util.Set -->
          <property name = "addressSet">
             <set>
                <ref bean = "address1"/>
                <ref bean = "address2"/>
                <value>Pakistan</value>
             </set>
          </property>
          
          <!-- Passing bean reference  for java.util.Map -->
          <property name = "addressMap">
             <map>
                <entry key = "one" value = "INDIA"/>
                <entry key = "two" value-ref = "address1"/>
                <entry key = "three" value-ref = "address2"/>
             </map>
          </property>
       </bean>
    
    </beans>
    
    要使用上面的bean定义,您需要以一种应该可以处理引用的方式定义setter方法。
  • 注入NULL和空字符串值

    如果您需要传递一个空字符串作为值,则可以按以下方式传递它:
    <bean id = "..." class = "exampleBean">
       <property name = "email" value = ""/>
    </bean>
    
    前面的示例等效于Java代码:exampleBean.setEmail("");
    如果需要传递NULL值,则可以按以下方式传递它
    <bean id = "..." class = "exampleBean">
       <property name = "email"><null/></property>
    </bean>
    
    前面的示例等效于Java代码:exampleBean.setEmail(null);