Spring @Autowired註解、@Resource註解和@Service註解
什麼是註解
傳統的Spring做法是使用.xml檔案來對bean進行注入或者是配置aop、事物,這麼做有兩個缺點:
1、如果所有的內容都配置在.xml檔案中,那麼.xml檔案將會十分龐大;如果按需求分開.xml檔案,那麼.xml檔案又會非常多。總之這將導致配置檔案的可讀性與可維護性變得很低
2、在開發中在.java檔案和.xml檔案之間不斷切換,是一件麻煩的事,同時這種思維上的不連貫也會降低開發的效率
為了解決這兩個問題,Spring引入了註解,通過"@XXX"的方式,讓註解與Java Bean緊密結合,既大大減少了配置檔案的體積,又增加了Java Bean的可讀性與內聚性。
本篇文章,講講最重要的三個Spring註解,也就是@Autowired、@Resource和@Service,希望能通過有限的篇幅說清楚這三個註解的用法。
不使用註解
先看一個不使用註解的Spring示例,在這個示例的基礎上,改成註解版本的,這樣也能看出使用與不使用註解之間的區別,先定義一個老虎:
public class Tiger { private String tigerName = "TigerKing"; public String toString() { return "TigerName:" + tigerName; } } 再定義一個猴子: public class Monkey { private String monkeyName = "MonkeyKing"; public String toString() { return "MonkeyName:" + monkeyName; } } 定義一個動物園: public class Zoo { private Tiger tiger; private Monkey monkey; public void setTiger(Tiger tiger) { this.tiger = tiger; } public void setMonkey(Monkey monkey) { this.monkey = monkey; } public Tiger getTiger() { return tiger; } public Monkey getMonkey() { return monkey; } public String toString() { return tiger + "\n" + monkey; } } spring的配置檔案這麼寫: <?xml version="1.0" encoding="UTF-8"?> <beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.2.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.2.xsd" default-autowire="byType"> <bean id="zoo" class="com.xrq.bean.Zoo" > <property name="tiger" ref="tiger" /> <property name="monkey" ref="monkey" /> </bean> <bean id="tiger" class="com.xrq.domain.Tiger" /> <bean id="monkey" class="com.xrq.domain.Monkey" /> </beans>
都很熟悉,權當複習一遍了。
@Autowired
@Autowired顧名思義,就是自動裝配,其作用是為了消除程式碼Java程式碼裡面的getter/setter與bean屬性中的property。當然,getter看個人需求,如果私有屬性需要對外提供的話,應當予以保留。
因此,引入@Autowired註解,先看一下spring配置檔案怎麼寫:
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 3 xmlns="http://www.springframework.org/schema/beans" 4 xmlns:context="http://www.springframework.org/schema/context" 5 xsi:schemaLocation="http://www.springframework.org/schema/beans 6 http://www.springframework.org/schema/beans/spring-beans-4.2.xsd 7 http://www.springframework.org/schema/context 8 http://www.springframework.org/schema/context/spring-context-4.2.xsd"> 9 10 <context:component-scan base-package="com.xrq" /> 11 12 <bean id="zoo" class="com.xrq.bean.Zoo" /> 13 <bean id="tiger" class="com.xrq.domain.Tiger" /> 14 <bean id="monkey" class="com.xrq.domain.Monkey" /> 15 16 </beans> 注意第10行,使用必須告訴spring一下我要使用註解了,告訴的方式有很多,<context:component-scan base-package="xxx" />是一種最簡單的,spring會自動掃描xxx路徑下的註解。 看到第12行,原來zoo裡面應當注入兩個屬性tiger、monkey,現在不需要注入了。再看下,Zoo.java也很方便,把getter/setter都可以去掉: public class Zoo { @Autowired private Tiger tiger; @Autowired private Monkey monkey; public String toString() { return tiger + "\n" + monkey; } }
這裡@Autowired註解的意思就是,當Spring發現@Autowired註解時,將自動在程式碼上下文中找到和其匹配(預設是型別匹配)的Bean,並自動注入到相應的地方去。
有一個細節性的問題是,假如bean裡面有兩個property,Zoo.java裡面又去掉了屬性的getter/setter並使用@Autowired註解標註這兩個屬性那會怎麼樣?答案是Spring會按照xml優先的原則去Zoo.java中尋找這兩個屬性的getter/setter,導致的結果就是初始化bean報錯。
OK,假設此時我把.xml檔案的13行、14行兩行給去掉,再執行,會丟擲異常:
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'Zoo': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.xrq.domain.Tiger com.xrq.bean.Zoo.ttiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.xrq.domain.Tiger] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1214)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:543)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:305)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:301)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:196)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:835)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at com.xrq.test.MyTest.main(MyTest.java:13)
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.xrq.domain.Tiger com.xrq.bean.Zoo.ttiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.xrq.domain.Tiger] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:571)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331)
... 13 more
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.xrq.domain.Tiger] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:1373)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1119)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1014)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:543)
... 15 more
因為,@Autowired註解要去尋找的是一個Bean,Tiger和 Monkey的Bean定義都給去掉了,自然就不是一個Bean了,Spring容器找不到也很好理解。那麼,如果屬性找不到我不想讓Spring容器拋 出異常,而就是顯示null,可以嗎?可以的,其實異常資訊裡面也給出了提示了,就是將@Autowired註解的required屬性設定為false 即可:
public class Zoo
{
@Autowired(required = false)
private Tiger tiger;
@Autowired(required = false)
private Monkey monkey;
public String toString()
{
return tiger + "\n" + monkey;
}
}
此時,找不到tiger、monkey兩個屬性,Spring容器不再丟擲異而是認為這兩個屬性為null。
@Autowired介面注入
上面的比較簡單,我們只是簡單注入一個Java類,那麼如果有一個介面,有多個實現,Bean裡引用的是介面名,又該怎麼做呢?比如有一個Car介面:
public interface Car
{
public String carName();
}
兩個實現類BMW和Benz:
@Service
public class BMW implements Car
{
public String carName()
{
return "BMW car";
}
}
@Service
public class Benz implements Car
{
public String carName()
{
return "Benz car";
}
}
寫一個CarFactory,引用Car:
@Service
public class CarFactory
{
@Autowired
private Car car;
public String toString()
{
return car.carName();
}
}
不用說,一定是報錯的,Car介面有兩個實現類,Spring並不知道應當引用哪個實現類。這種情況通常有兩個解決辦法:
1、刪除其中一個實現類,Spring會自動去base-package下尋找Car介面的實現類,發現Car介面只有一個實現類,便會直接引用這個實現類
2、實現類就是有多個該怎麼辦?此時可以使用@Qualifier註解:
@Service
public class CarFactory
{
@Autowired
@Qualifier("BMW")
private Car car;
public String toString()
{
return car.carName();
}
}
注意@Qualifier註解括號裡面的應當是Car介面實現類的類名,我之前試的時候一直以為是bean的名字,所以寫了"bMW",結果一直報錯。
@Resource
把@Resource註解放在@Autowired下面說,是因為它們作用非常相似,這個就簡單說了,例子過後點明一下@Resource和@Autowired的區別。先看一下@Resource,直接寫Zoo.java了:
@Service
public class Zoo
{
@Resource(name = "tiger")
private Tiger tiger;
@Resource(type = Monkey.class)
private Monkey monkey;
public String toString()
{
return tiger + "\n" + monkey;
}
}
這是詳細一些的用法,說一下@Resource的裝配順序:
1、@Resource後面沒有任何內容,預設通過name屬性去匹配bean,找不到再按type去匹配
2、指定了name或者type則根據指定的型別去匹配bean
3、指定了name和type則根據指定的name和type去匹配bean,任何一個不匹配都將報錯
然後,區分一下@Autowired和@Resource兩個註解的區別:
1、@Autowired預設按照byType方式進行bean匹配,@Resource預設按照byName方式進行bean匹配
2、@Autowired是Spring的註解,@Resource是J2EE的註解,這個看一下匯入註解的時候這兩個註解的包名就一清二楚了
Spring屬於第三方的,J2EE是Java自己的東西,因此,建議使用@Resource註解,以減少程式碼和Spring之間的耦合。
@Service
上面這個例子,還可以繼續簡化,因為spring的配置檔案裡面還有12行~14行三個bean,下一步的簡化是把這三個bean也給去掉,使得spring配置檔案裡面只有一個自動掃描的標籤,增強Java程式碼的內聚性並進一步減少配置檔案。
要繼續簡化,可以使用@Service。先看一下配置檔案,當然是全部刪除了:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://www.springframework.org/schema/beans"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.2.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-4.2.xsd">
<context:component-scan base-package="com.xrq" />
</beans>
是不是感覺很爽?起碼我覺得是的。OK,下面以Zoo.java為例,其餘的Monkey.java和Tiger.java都一樣:
@Service
public class Zoo
{
@Autowired
private Tiger ttiger;
@Autowired
private Monkey mmonkey;
public String toString()
{
return ttiger + "\n" + mmonkey;
}
}
這樣,Zoo.java在Spring容器中存在的形式就是"zoo",即可以通過ApplicationContext的getBean("zoo")方法來得到Zoo.java。@Service註解,其實做了兩件事情:
1、宣告Zoo.java是一個bean,這點很重要,因為Zoo.java是一個bean,其他的類才可以使用@Autowired將Zoo作為一個成員變數自動注入
2、Zoo.java在bean中的id是"zoo",即類名且首字母小寫
如果,我不想用這種形式怎麼辦,就想讓Zoo.java在Spring容器中的名字叫做"Zoo",可以的:
@Service
@Scope("prototype")
public class Zoo
{
@Autowired
private Monkey monkey;
@Autowired
private Tiger tiger;
public String toString()
{
return "MonkeyName:" + monkey + "\nTigerName:" + tiger;
}
}
這樣,就可以通過ApplicationContext的getBean("zoo")方法來得到Zoo.java了。
這裡我還多加了一個@Scope註解,應該 很好理解。因為Spring預設產生的bean是單例的,假如我不想使用單例怎麼辦,xml檔案裡面可以在bean裡面配置scope屬性。註解也是一 樣,配置@Scope即可,預設是"singleton"即單例,"prototype"表示原型即每次都會new一個新的出來。
最後:@Compent註解(和@Service註解類似):
package com.java.pojo;
import javax.annotation.Resource;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component("p")
public class Product {
private int id;
private String name="product 1";
@Autowired
private Category category;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Category getCategory() {
return category;
}
public void setCategory(Category category) {
this.category = category;
}
}
package com.java.pojo;
import org.springframework.stereotype.Component;
@Component("c")
public class Category {
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
private int id;
private String name="category 1";
}
<?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:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx-3.0.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<context:component-scan base-package="com.java.pojo"/>
</beans>
測試:
package com.java.test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.how2java.pojo.Product;
public class TestSpring {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext(new String[] { "applicationContext.xml" });
Product p = (Product) context.getBean("p");
System.out.println(p.getName());
System.out.println(p.getCategory().getName());
}
}