您已经看到了如何配置基本数据类型使用value属性和使用<property>标签的ref属性在你的bean配置文件中的对象引用。这两种情况下处理过单值到一个bean。
现在什么样，如果你想通过多元价值，如Java Collection类型List, Set, Map 及 Properties。要处理这种情况，Spring提供了四种类型的如下集合的配置元素：
| 元素 | 描述 |
|---|---|
| <list> | 这有助于注入值列表List配线，使重复。 |
| <set> | 这有助于在配线的一组值，但不能重复。 |
| <map> | 这可用于注入的名称 – 值对，其中名称和值可以是任何类型的集合。 |
| <props> | 这可以用来注入的名称 – 值对，其中名称和值都是字符串的集合。 |
可以使用<list> 或<set> 来连接任何实现java.util.Collection或数组。
会遇到两种情况（a）将收集的直接的值及（b）传递一个bean的引用作为集合的元素之一。
例子:
我们使用Eclipse IDE，然后按照下面的步骤来创建一个Spring应用程序：
| 步骤 | 描述 |
|---|---|
| 1 | Create a project with a name SpringExample and create a package com.yiibai 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.yiibai 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. |
这里是JavaCollection.java文件的内容：
package
com
.
yiibai
;
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
.
yiibai
;
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文件里面有配置的集合的所有类型：
<?
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.yiibai.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
=
"two"
>
Pakistan
</prop>
<prop
key
=
"three"
>
USA
</prop>
<prop
key
=
"four"
>
USA
</prop>
</props>
</property>
</bean>
</beans>
创建源代码和bean配置文件完成后，让我们运行应用程序。如果应用程序一切顺利，这将打印以下信息：
List Elements :[INDIA, Pakistan, USA, USA]
Set Elements :[INDIA, Pakistan, USA]
Map Elements :{1=INDIA, 2=Pakistan, 3=USA, 4=USA}
Property Elements :{two=Pakistan, one=INDIA, three=USA, four=USA}
注入Bean引用：
下面bean定义将帮助您了解如何注入bean的引用作为集合的元素之一。甚至可以混合引用和值都在一起，如下图所示：
<?
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>
使用上面的bean定义，需要定义这样一种方式，他们应该能够处理的参考，以及setter方法。
注入null和空字符串的值
如果需要传递一个空字符串作为值，如下所示：
<bean
id
=
"..."
class
=
"exampleBean"
>
<property
name
=
"email"
value
=
""
/>
</bean>
前面的例子等同于Java代码： exampleBean.setEmail("")
如果需要传递一个null值，如下所示：
<bean
id
=
"..."
class
=
"exampleBean"
>
<property
name
=
"email"
>
<null/>
</property>
</bean>
前面的例子等同于Java代码：exampleBean.setEmail(null)
原文发布时间为：2018-10-22
本文作者：小白教程
