Spring - 注入集合

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您已经了解了如何使用Bean 配置文件中 <property> 标记的ref属性和值属性来配置原始数据类型和对象引用。这两种情况都涉及将奇异值传递给 bean。

现在，如果您想传递复数值，例如 Java 集合类型（例如 List、Set、Map 和 Properties），该怎么办？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：

您可以使用 <list> 或 <set> 来连接 java.util.Collection 或array的任何实现。

您将遇到两种情况（a）传递集合的直接值和（b）传递 bean 的引用作为集合元素之一。

例子

让我们准备好一个可用的 Eclipse IDE，并按照以下步骤创建一个 Spring 应用程序 -

这是JavaCollection.java文件的内容-

package com.tutorialspoint;
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.tutorialspoint;

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，其中包含所有类型集合的配置 -

<?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.tutorialspoint.JavaCollection">
      
      <!-- results in a setAddressList(java.util.List) call -->
      <property name = "addressList">
         <list>
            <value>INDIA</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>INDIA</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 = "INDIA"/>
            <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">INDIA</prop>
            <prop key = "one">INDIA</prop>
            <prop key = "two">Pakistan</prop>
            <prop key = "three">USA</prop>
            <prop key = "four">USA</prop>
         </props>
      </property>
   </bean>

</beans>

创建完源文件和 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，它将打印以下消息 -

List Elements :[INDIA, Pakistan, USA, USA] 
Set Elements :[INDIA, Pakistan, USA] 
ap Elements :{1 = INDIA, 2 = Pakistan, 3 = USA, 4 = USA} 
Property Elements :{two = Pakistan, one = INDIA, three = USA, four = USA} 

注入 Bean 引用

以下 Bean 定义将帮助您了解如何将 bean 引用注入为集合的元素之一。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，您需要定义 setter 方法，使其也能够处理引用。

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，那么您可以按如下方式传递它 -

<bean id = "..." class = "exampleBean">
   <property name = "email" value = ""/>
</bean>

前面的示例相当于 Java 代码： exampleBean.setEmail("")

如果您需要传递 NULL 值，则可以按如下方式传递它 -

<bean id = "..." class = "exampleBean">
   <property name = "email"><null/></property>
</bean>

前面的示例相当于 Java 代码： exampleBean.setEmail(null)