SPRING IN ACTION 第4版笔记-第二章WIRING BEANS-006-当构造函数有集合时的注入

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  • 2024-01-10
  • 分类：综合

本文内容纲要：SPRING IN ACTION 第4版笔记-第二章WIRING BEANS-006-当构造函数有集合时的注入

一.当构造函数有集合时，只能用，不能用C-NAMESPACE

二、

1 package soundsystem.collections;
 2 
 3 import java.util.List;
 4 
 5 import soundsystem.CompactDisc;
 6 
 7 public class BlankDisc implements CompactDisc {
 8 
 9   private String title;
10   private String artist;
11   private List<String> tracks;
12 
13   public BlankDisc(String title, String artist, List<String> tracks) {
14     this.title = title;
15     this.artist = artist;
16     this.tracks = tracks;
17   }
18 
19   public void play() {
20     System.out.println("Playing " + title + " by " + artist);
21     for (String track : tracks) {
22       System.out.println("-Track: " + track);
23     }
24   }
25 
26 }

(1)可以注入null,但运行会nullpointerException

1 <bean id="compactDisc" class="soundsystem.BlankDisc">
2 <constructor-arg value="Sgt. Pepper's Lonely Hearts Club Band" />
3 <constructor-arg value="The Beatles" />
4 <constructor-arg><null/></constructor-arg>
5 </bean>

(2)注入list

<bean id="compactDisc" class="soundsystem.BlankDisc">
<constructor-arg value="Sgt. Pepper's Lonely Hearts Club Band" />
<constructor-arg value="The Beatles" />
<constructor-arg>
<list>
<value>Sgt. Pepper's Lonely Hearts Club Band</value>
<value>With a Little Help from My Friends</value>
<value>Lucy in the Sky with Diamonds</value>
<value>Getting Better</value>
<value>Fixing a Hole</value>
<!-- ...other tracks omitted for brevity... -->
</list>
</constructor-arg>

(2)如果构造函数的集合参数的元素是对象，则用

public Discography(String artist, List<CompactDisc> cds) { ... }
<bean id="beatlesDiscography"
class="soundsystem.Discography">
<constructor-arg value="The Beatles" />
<constructor-arg>
<list>
<ref bean="sgtPeppers" />
<ref bean="whiteAlbum" />
<ref bean="hardDaysNight" />
<ref bean="revolver" />
...
</list>
</constructor-arg>
</bean>

(3)也可以注入set

It makes sense to use when wiring a constructor argument of type

java.util.List . Even so, you could also use the element in the same way:

<bean id="compactDisc" class="soundsystem.BlankDisc">
<constructor-arg value="Sgt. Pepper's Lonely Hearts Club Band" />
<constructor-arg value="The Beatles" />
<constructor-arg>
<set>
<value>Sgt. Pepper's Lonely Hearts Club Band</value>
<value>With a Little Help from My Friends</value>
<value>Lucy in the Sky with Diamonds</value>
<value>Getting Better</value>
<value>Fixing a Hole</value>
<!-- ...other tracks omitted for brevity... -->
</set>
</constructor-arg>
</bean>

There’s little difference between and . The main difference is that when

Spring creates the collection to be wired, it will create it as either a java.util.Set or

a java.util.List . If it’s a Set , then any duplicate values will be discarded and the

ordering may not be honored. But in either case, either a or a can be

wired into a List , a Set , or even an array.

本文内容总结：SPRING IN ACTION 第4版笔记-第二章WIRING BEANS-006-当构造函数有集合时的注入

原文链接：https://www.cnblogs.com/shamgod/p/5233785.html

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