Spring 简明教程

Spring - Injecting Collection

您已经看到如何使用 Bean 配置文件中的 <property> 标记的 value 属性配置原始数据类型，以及使用 ref 属性配置对象引用。这两种情况都涉及将奇异值传递给 Bean。

You have seen how to configure primitive data type using value attribute and object references using ref attribute of the <property> tag in your Bean configuration file. Both the cases deal with passing singular value to a bean.

现在，如果您想传递复数值，如 Java 集合类型（例如列表、集合、映射和属性），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−

Now what if you want to pass plural values like Java Collection types such as List, Set, Map, and Properties. To handle the situation, Spring offers four types of collection configuration elements which are as follows −

Sr.No	Element & Description
1	<list> This helps in wiring ie injecting a list of values, allowing duplicates.
2	<set> This helps in wiring a set of values but without any duplicates.
3	<map> This can be used to inject a collection of name-value pairs where name and value can be of any type.
4	<props> This can be used to inject a collection of name-value pairs where the name and value are both Strings.

您可以使用 <list> 或 <set> 来接线任何 java.util.Collection 的实现或 array 。

You can use either <list> or <set> to wire any implementation of java.util.Collection or an array.

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

You will come across two situations (a) Passing direct values of the collection and (b) Passing a reference of a bean as one of the collection elements.

Example

让我们准备一个可用的 Eclipse IDE，然后执行以下步骤来创建一个 Spring 应用程序−

Let us have a working Eclipse IDE in place and take the following steps to create a Spring application −

Steps	Description
1	Create a project with a name SpringExample and create a package com.tutorialspoint under the src folder in the created project.
2	Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes JavaCollection, and MainApp under the com.tutorialspoint package.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

以下是 JavaCollection.java 文件的内容−

Here is the content of JavaCollection.java file −

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−

Following is the content of the MainApp.java file −

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 ，其中包含对所有类型集合的配置−

Following is the configuration file Beans.xml which has configuration for all the type of collections −

<?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>

完成源文件和 Bean 配置文件创建后，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−

Once you are done creating the source and bean configuration files, let us run the application. If everything is fine with your application, it will print the following message −

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}

Injecting Bean References

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。您甚至可以将引用和值混合在一起，如下面的代码片段所示−

The following Bean definition will help you understand how to inject bean references as one of the collection’s element. Even you can mix references and values all together as shown in the following code snippet −

<?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>

要使用上述 Bean 定义，您需要以能够处理引用的方式定义您的 setter 方法。

To use the above bean definition, you need to define your setter methods in such a way that they should be able to handle references as well.

Injecting null and empty string values

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−

If you need to pass an empty string as a value, then you can pass it as follows −

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

前面的示例等效于 Java 代码：exampleBean.setEmail("")

The preceding example is equivalent to the Java code: exampleBean.setEmail("")

如果您需要传递一个 NULL 值，那么您可以按如下方式传递−

If you need to pass a NULL value, then you can pass it as follows −

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

前面的示例等价于 Java 代码：exampleBean.setEmail(null)

The preceding example is equivalent to the Java code: exampleBean.setEmail(null)