Spring 装配Bean详解

一、简介

​ Spring容器负责创建应用程序中的bean并通过DI来协调这些对象之间的关系。Spring具有非常大的灵活性，它提供了三种主要的装配机制：

• 在XML中进行显示配置；
• 在Java中进行显示配置；
• 隐式的bean发现机制和自动装配。

二、在XML中进行显示配置

1. 声明一个简单的bean

1. 创建CD接口

public interface CompactDisc {void play();
}

2. 创建NowAndThen类

public class NowAndThen implements CompactDisc{private String title = "Now And Then.";private String artist = "The Beatles";@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);}}

3. 在bean.xml文件中配置

<!--  通过class属性来声明一个简单的bean，此时因为没有指定ID，bean的ID将会是com.shiftycat.soundsystem.NowAndThen#0  -->
<bean class="com.shiftycat.soundsystem.NowAndThen"/>
<!--  使用id属性可以用来指定ID  -->
<bean id="CompactDisc" class="com.shiftycat.soundsystem.NowAndThen"/>

4. 进行测试

@Test
public void compactDiscTest() {ClassPathXmlApplicationContext classPathXmlApplicationContext =new ClassPathXmlApplicationContext("bean.xml");CompactDisc compactDisc = classPathXmlApplicationContext.getBean(CompactDisc.class);compactDisc.play();
}

5. 测试结果

Playing Now And Then. by The Beatles

2. 借助构造器注入初始化bean

​ 构造器注入有两种方案：

• constructor-arg元素
• 使用Spring 3.0所引入的c-命名空间

1. 创建CD接口

public interface CompactDisc {void play();
}

2. 创建NowAndThen类

public class NowAndThen implements CompactDisc{private String title = "Now And Then.";private String artist = "The Beatles";@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);}}

3. 创建MediaPlayer接口

public interface MediaPlayer {void play();
}

4. 创建CDPlayer类

public class CDPlayer implements MediaPlayer{private CompactDisc cd;// 构造器注入public CDPlayer(CompactDisc cd) {this.cd = cd;}@Overridepublic void play() {System.out.println("Loading...");cd.play();}
}

5. 在bean.xml文件中配置

<bean id="CompactDisc" class="com.shiftycat.soundsystem.NowAndThen"/>\
<!--  第一种配置方案：`constructor-arg`元素  -->
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer"><constructor-arg ref="CompactDisc"/>
</bean>
<!--  第二种配置方案：使用Spring 3.0所引入的c-命名空间  -->
<!--  c:构造器参数名-ref=“要注入的bean的ID”  -->
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer" c:cd-ref="CompactDisc"/>
<!--  或者将参数的名称替换为"_0"或者"_"  -->
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer" c:_0-ref="CompactDisc" />
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer" c:_-ref="CompactDisc" />

6. 进行测试

@Test
public void cdPlayerTest() {ClassPathXmlApplicationContext classPathXmlApplicationContext =new ClassPathXmlApplicationContext("bean.xml");CDPlayer cdPlayer = classPathXmlApplicationContext.getBean(CDPlayer.class);cdPlayer.play();}

7. 测试结果

Loading...
Playing Now And Then. by The Beatles

将字面量注入到构造器中

1. 创建BlackDisc类

public class BlackDisc implements CompactDisc{private String title;private String artist;public BlackDisc(String title, String artist) {this.title = title;this.artist = artist;}@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);}
}

2. 在bean.xml中进行配置

<bean id="blackDisc" class="com.shiftycat.soundsystem.BlackDisc"><constructor-arg value="Now And Then." /><constructor-arg value="The Beatles" />
</bean><bean id="blackDisc" class="com.shiftycat.soundsystem.BlackDisc"c:title="Now And Then."c:artist="The Beatles"
/><bean id="blackDisc" class="com.shiftycat.soundsystem.BlackDisc"c:_0="Now And Then."c:_1="The Beatles"
/>

3. 进行测试

@Test
public void blackDiscTest() {ClassPathXmlApplicationContext classPathXmlApplicationContext =new ClassPathXmlApplicationContext("bean.xml");BlackDisc blackDisc = classPathXmlApplicationContext.getBean(BlackDisc.class);blackDisc.play();
}

4. 测试结果

Playing Now And Then. by The Beatles

P.S. 装配集合

public class BlackDisc implements CompactDisc {private String title;private String artist;private List<String> tracks;public BlackDisc(String title, String artist, List<String> tracks) {this.title = title;this.artist = artist;this.tracks = tracks;}@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);for (String track : tracks) {System.out.println("-Track: " + track);}}
}

<bean id="blackDisc" class="com.shiftycat.soundsystem.BlackDisc"><constructor-arg value="Now And Then." /><constructor-arg value="The Beatles" /><constructor-arg><list><value>I know it's true</value><value>It's all because of you</value><value>And if I make it through</value><value>It's all because of you</value><value>And now and then</value></list></constructor-arg>
</bean>

Playing Now And Then. by The Beatles
-Track: I know it's true
-Track: It's all because of you
-Track: And if I make it through
-Track: It's all because of you
-Track: And now and then

3. 设置属性初始化bean

一般而言，对于强依赖使用构造器注入，对可选性的依赖使用属性注入。

1. 在CDPlayer类中设置setter方法

public class CDPlayer implements MediaPlayer{private CompactDisc cd;public void setCompactDisc(CompactDisc cd) {this.cd = cd;}@Overridepublic void play() {System.out.println("Loading...");cd.play();}
}

2. 在bean.xml文件中进行配置

<!--  可以使用property进行配置-->
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer"><property name="compactDisc" ref="compactDisc"/>
</bean>
<!--  或者使用p-命名空间性配置-->
<bean id="cdPlayer" class="com.shiftycat.soundsystem.CDPlayer"p:compactDisc-ref="compactDisc"/>

3. 进行测试

@Test
public void cdPlayerTest() {ClassPathXmlApplicationContext classPathXmlApplicationContext =new ClassPathXmlApplicationContext("bean.xml");CDPlayer cdPlayer = classPathXmlApplicationContext.getBean(CDPlayer.class);cdPlayer.play();
}

4. 测试结果

Loading...
Playing Now And Then. by The Beatles

将字面量注入到属性中

1. 在WhiteDisc类中设置setter方法

public class WhiteDisc implements CompactDisc{private String title;private String artist;private List<String> tracks;public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}public String getArtist() {return artist;}public void setArtist(String artist) {this.artist = artist;}public List<String> getTracks() {return tracks;}public void setTracks(List<String> tracks) {this.tracks = tracks;}@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);for (String track : tracks) {System.out.println("-Track: " + track);}}
}

2. 在bean.xml文件中进行配置

<bean id="whileDisc" class="com.shiftycat.soundsystem.WhiteDisc"><property name="artist" value="Now And Then."/><property name="title" value="The Beatles" /><property name="tracks"><list><value>I know it's true</value><value>It's all because of you</value><value>And if I make it through</value><value>It's all because of you</value><value>And now and then</value></list></property>
</bean>
<!--或者-->
<bean id="whileDisc" class="com.shiftycat.soundsystem.WhiteDisc"p:artist="Now And Then."p:title="The Beatles"><property name="tracks"><list><value>I know it's true</value><value>It's all because of you</value><value>And if I make it through</value><value>It's all because of you</value><value>And now and then</value></list></property>
</bean>
<!--或者-->
<bean id="whileDisc" class="com.shiftycat.soundsystem.WhiteDisc"p:artist="Now And Then."p:title="The Beatles"p:tracks-ref="tracks"
>
</bean><util:list id="tracks"><value>I know it's true</value><value>It's all because of you</value><value>And if I make it through</value><value>It's all because of you</value><value>And now and then</value>
</util:list>

3. 进行测试

@Test
public void whiteDiscTest() {ClassPathXmlApplicationContext classPathXmlApplicationContext =new ClassPathXmlApplicationContext("bean.xml");WhiteDisc whiteDisc = classPathXmlApplicationContext.getBean(WhiteDisc.class);whiteDisc.play();
}

4. 测试结果

Playing The Beatles by Now And Then.
-Track: I know it's true
-Track: It's all because of you
-Track: And if I make it through
-Track: It's all because of you
-Track: And now and then

三、在Java中进行显式配置

1. 声明简单的bean

1. 创建配置类

@Configuration
public class CDPlayerConfig {}

2. 声明简单的bean

@Configuration
public class CDPlayerConfig {@Beanpublic CompactDisc nowAndThen() {return new nowAndThen();}}

2. 借助JavaConfig实现注入

@Configuration
//@ComponentScan(basePackages = "com.shiftycat.soundsystem") //在Spring中启动组件扫描，默认扫描该类所在包及这个包下的所有子包
public class CDPlayerConfig {@Beanpublic CompactDisc nowAndThen() {return new nowAndThen();}// 方法一: 最简单的方法是引用创建bean的方法@Beanpublic CDPlayer cdPlayer() {return new CDPlayer(nowAndThen());}// 方法二: 请求一个CompactDisc作为参数，将CompactDisc注入到CDPlayer的构造器中，// 而且不用明确引用CompactDisc的@Bean方法。@Beanpublic CDPlayer cdPlayer(CompactDisc compactDisc) {return new CDPlayer(compactDisc);}// 方法三: 通过setter方法注入CompactDisc@Beanpublic CDPlayer cdPlayer(CompactDisc compactDisc) {CDPlayer  cdPlayer = new CDPlayer();cdPlayer.setCd(compactDisc);return cdPlayer;}
}

四、隐式的bean发现机制和自动装配

1. 自动化装配bean

​ Spring通过两个方面实现自动化装配：

• 组件扫描（component scanning）：Spring会自动发现应用上下文所创建的bean。
• 自动装配（autowiring）：Spring自动满足bean之间的依赖。

2. 步骤

1. 创建可被发现的bean

public interface CompactDisc {void play();
}public class nowAndThen implements CompactDisc{private String title = "Now And Then.";private String artist = "The Beatles";@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);}
}public interface MediaPlayer {void play();
}

2. 开启组件扫描

@Configuration
@ComponentScan(basePackages = "com.shiftycat.soundsystem") //在Spring中启动组件扫描，默认扫描该类所在包及这个包下的所有子包
public class CDPlayerConfig {}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans.xsd><context:component-scan base-package="com.shiftycat.soundsystem" />
</beans>

@Autowired
private CompactDisc compactDisc;@Test
public void compactDiscTest() {compactDisc.play();
}

Playing Now And Then. by The Beatles

3. 通过为bean添加注解实现自动装配

不管是构造器、Setter方法还是其他的方法，Spring都会尝试满足方法参数上所声明的依赖。假如有且只有一个bean匹配以来需求的话，那么这个bean将会被装配进来。

@Component
public class CDPlayer implements MediaPlayer{private CompactDisc cd;@Autowiredpublic CDPlayer(CompactDisc cd) {this.cd = cd;}@Autowiredpublic void setCd(CompactDisc cd) {this.cd = cd;}@Overridepublic void play() {System.out.println("Loading...");cd.play();}
}

@Autowired
private CDPlayer cdPlayer;
@Test
public void cdPlayerTest() {cdPlayer.play();
}

Loading...
Playing Now And Then. by The Beatles

五、导入和混合配置

1. 在JavaConfig中引用XML配置

​ 假如现存由两个config文件和一个XML文件，如何实现在一个JavaConfig文件中引用两个config文件并且在其中引入XML配置文件。

1. 配置文件1

public interface CompactDisc {void play();
}@Component
public class NowAndThen implements CompactDisc {private String title = "Now And Then.";private String artist = "The Beatles";@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);}
}@Configuration
public class CDConfig {@Beanpublic CompactDisc compactDisc() {return new NowAndThen();}
}

2. 配置文件2

public interface MediaPlayer {void play();
}@Component
public class CDPlayer implements MediaPlayer {private CompactDisc cd;public CDPlayer(CompactDisc cd) {this.cd = cd;}@Overridepublic void play() {System.out.println("Loading...");cd.play();}
}@Configuration
//@Import(CDConfig.class)
public class CDPlayerConfig {@Beanpublic CDPlayer cdPlayer(CompactDisc compactDisc) {return new CDPlayer(compactDisc);}
}

3. 配置文件3

public class BlackDisc implements CompactDisc {private String title;private String artist;private List<String> tracks;public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}public String getArtist() {return artist;}public void setArtist(String artist) {this.artist = artist;}public List<String> getTracks() {return tracks;}public void setTracks(List<String> tracks) {this.tracks = tracks;}@Overridepublic void play() {System.out.println("Playing " + title + " by " + artist);for (String track : tracks) {System.out.println("-Track: " + track);}}
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:c="http://www.springframework.org/schema/c"xmlns:p="http://www.springframework.org/schema/p"xmlns:util="http://www.springframework.org/schema/util"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans.xsdhttp://www.springframework.org/schema/utilhttp://www.springframework.org/schema/util/spring-util.xsd"><bean id="blackDisc" class="com.shiftycat.importAndBlend.BlackDisc"p:artist="Now And Then."p:title="The Beatles"p:tracks-ref="tracks"></bean><util:list id="tracks"><value>I know it's true</value><value>It's all because of you</value><value>And if I make it through</value><value>It's all because of you</value><value>And now and then</value></util:list></beans>

4. 主要的Config文件

@Configuration
@Import({CDConfig.class, CDPlayerConfig.class})
@ImportResource("classpath:cd-config.xml")
public class SoundSystemConfig {}

5. 进行测试

@Test
public void JavaConfigTest() {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(SoundSystemConfig.class);context.refresh();System.out.println("BlackDisc: ");CompactDisc blackDisc = context.getBean(BlackDisc.class);blackDisc.play();System.out.println("\nnowAndThen: ");CompactDisc nowAndThen = context.getBean(NowAndThen.class);nowAndThen.play();System.out.println("\nCDPlayer: ");CDPlayer player = context.getBean(CDPlayer.class);player.play();
}

6. 测试结果

BlackDisc: 
Playing The Beatles by Now And Then.
-Track: I know it's true
-Track: It's all because of you
-Track: And if I make it through
-Track: It's all because of you
-Track: And now and thennowAndThen: 
Playing Now And Then. by The BeatlesCDPlayer: 
Loading...
Playing Now And Then. by The Beatles

2. 在XML配置中引用JavaConfig

​ 与在JavaConfig中引用XML配置一样，我们可以使用更高层次的配置文件，这个文件不声明任何的bean，只是负责将两个或更多的配置组合起来。

<?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/beanshttp://www.springframework.org/schema/beans/spring-beans.xsd"><!--  用来导入JavaConfig类配置到Xml文件中  --><bean class="com.shiftycat.importAndBlend.CDConfig" /><!--  用来导入xml文件配置  --><import resource="cd-config.xml"/></beans>

《Spring实战(第4版)》