Java Spring各种依赖注入注解的区别

Spring对于Bean的依赖注入,支持多种注解方式:

1、@Inject

@Resource
javax.annotation
JSR250 (Common Annotations for Java)

@Inject
javax.inject
JSR330 (Dependency Injection for Java)

@Autowired
org.springframework.bean.factory
Spring
  • javax.inject
  • JSR330 (Dependency Injection for Java)

直观上看起来,@Autowired是Spring提供的注解,其他几个都是JDK本身内建的注解,Spring对这些注解也进行了支持。但是使用起来这三者到底有什么区别呢?笔者经过方法的测试,发现一些有意思的特性。

这是JSR330中的规范,通过AutowiredAnnotationBeanPostProcessor类实现的依赖注入。

区别总结如下:

一、@Autowired有个required属性,可以配置为false,这种情况下如果没有找到对应的bean是不会抛异常的。@Inject和@Resource没有提供对应的配置,所以必须找到否则会抛异常。

二、
@Autowired和@Inject基本是一样的,因为两者都是使用AutowiredAnnotationBeanPostProcessor来处理依赖注入。但是@Resource是个例外,它使用的是CommonAnnotationBeanPostProcessor来处理依赖注入。当然,两者都是BeanPostProcessor。

@Autowired和@Inject
- 默认 autowired by type
- 可以 通过@Qualifier 显式指定 autowired by qualifier name。
- 如果 autowired by type 失败(找不到或者找到多个实现),则退化为autowired by field name

@Resource
- 默认 autowired by field name
- 如果 autowired by field name失败,会退化为 autowired by type
- 可以 通过@Qualifier 显式指定 autowired by qualifier name
- 如果 autowired by qualifier name失败,会退化为 autowired by field name。但是这时候如果 autowired by field name失败,就不会再退化为autowired by type了。

TIPS Qualified name VS Bean name

在Spring设计中,Qualified name并不等同于Bean
name,后者必须是唯一的,但是前者类似于tag或者group的作用,对特定的bean进行分类。可以达到getByTag(group)的效果。对于XML配置的bean,可以通过id属性指定bean
name(如果没有指定,默认使用类名首字母小写),通过标签指定qualifier
name:

<bean id="lamborghini" class="me.arganzheng.study.spring.autowired.Lamborghini">
    <qualifier value="luxury"/>
    <!-- inject any dependencies required by this bean -->
</bean>

如果是通过注解方式,那么可以通过@Qualifier注解指定qualifier
name,通过@Named或者@Component(@Service,@Repository等)的value值指定bean
name:

@Component("lamborghini")
@Qualifier("luxury")
public class Lamborghini implements Car {

}

或者

@Component
@Named("lamborghini")
@Qualifier("luxury")
public class Lamborghini implements Car {

}

同样,如果没有指定bean
name,那么Spring会默认是用类名首字母小写(Lamborghini=>lamborghini)。

三、
通过Anotation注入依赖的方式在XML注入方式之前进行。如果对同一个bean的依赖同时使用了两种注入方式,那么XML的优先。但是不同担心通过Anotation注入的依赖没法注入XML中配置的bean,依赖注入是在bean的注册之后进行的。

四、目前的autowired by
type方式(笔者用的是3.2.3.RELEASE版本),Spring的AutowiredAnnotationBeanPostProcessor实现都是有”bug”的,也就是说@Autowired和@Inject都是有坑的(称之为坑,不称之为bug是因为貌似是故意的。。)。这是来源于线上的一个bug,也是这边文章的写作原因。现场如下:

application-context.xml中有如下定义:

<?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:context="http://www.springframework.org/schema/context"
    xmlns:util="http://www.springframework.org/schema/util"
    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/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.5.xsd">

    <context:annotation-config />

    <context:component-scan base-package="me.arganzheng.study" />

    <util:constant id="en"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN" />
    <util:constant id="ja"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.JP" />
    <util:constant id="ind"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.IND" />
    <util:constant id="pt"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.PT" />
    <util:constant id="th"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.TH" />
    <util:constant id="ar"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.AR" />
    <util:constant id="en-rIn"
        static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN_RIN" />

    <util:map id="languageChangesMap" key-type="java.lang.String"
        value-type="java.lang.String">
        <entry key="pt" value="pt" />
        <entry key="br" value="pt" />
        <entry key="jp" value="ja" />
        <entry key="ja" value="ja" />
        <entry key="ind" value="ind" />
        <entry key="id" value="ind" />
        <entry key="en-rin" value="en-rIn" />
        <entry key="in" value="en-rIn" />
        <entry key="en" value="en" />
        <entry key="gb" value="en" />
        <entry key="th" value="th" />
        <entry key="ar" value="ar" />
        <entry key="eg" value="ar" />
    </util:map>

</beans>

其中static-field应用的常量定义在如下类中:

package me.arganzheng.study.spring.autowired;

public interface Constants {

    public interface Language {
        public static final String EN = "CommonConstants.LANG_ENGLISH";
        public static final String JP = "CommonConstants.LANG_JAPANESE";
        public static final String IND = "CommonConstants.LANG_INDONESIAN";
        public static final String PT = "CommonConstants.LANG_PORTUGUESE";
        public static final String TH = "CommonConstants.LANG_THAI";
        public static final String EN_RIN = "CommonConstants.LANG_ENGLISH_INDIA";
        public static final String AR = "CommonConstants.LANG_Arabic";
    }
}

然后如果我们在代码中如下声明依赖:

public class AutowiredTest extends BaseSpringTestCase {

    @Autowired
    private Map<String, String> languageChangesMap;

    @Test
    public void testAutowired() {
        notNull(languageChangesMap);
        System.out.println(languageChangesMap.getClass().getSimpleName());
        System.out.println(languageChangesMap);
    }
}

Guess what,诡异的事情发生了!

运行结果如下:

LinkedHashMap
{en=CommonConstants.LANG_ENGLISH, ja=CommonConstants.LANG_JAPANESE, ind=CommonConstants.LANG_INDONESIAN, pt=CommonConstants.LANG_PORTUGUESE, th=CommonConstants.LANG_THAI, ar=CommonConstants.LANG_Arabic, en-rIn=CommonConstants.LANG_ENGLISH_INDIA}

也就是说Map

严重: Caught exception while allowing TestExecutionListener [org.springframework.test.context.support.DependencyInjectionTestExecutionListener@5c51ee0a] to prepare test instance [me.arganzheng.study.spring.autowired.AutowiredTest@6e301e0]
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'me.arganzheng.study.spring.autowired.AutowiredTest': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.Map me.arganzheng.study.spring.autowired.AutowiredTest.languageChangesMap; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [java.lang.String] found for dependency [map with value type java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    ...
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [java.lang.String] found for dependency [map with value type java.lang.String]: 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:986)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486)
    ... 28 more

debug了一下,发现确实是Spring的一个bug。在DefaultListableBeanFactory的这个方法出问题了:

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
            Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
        ...     

        else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
            Class<?> keyType = descriptor.getMapKeyType();
            if (keyType == null || !String.class.isAssignableFrom(keyType)) {
                if (descriptor.isRequired()) {
                    throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +
                            "] must be assignable to [java.lang.String]");
                }
                return null;
            }
            Class<?> valueType = descriptor.getMapValueType();
            if (valueType == null) {
                if (descriptor.isRequired()) {
                    throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");
                }
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, descriptor);
            if (matchingBeans.isEmpty()) {
                if (descriptor.isRequired()) {
                    raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);
                }
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            return matchingBeans;
        }
        ...
    }

关键在这一句:Map

严重: Caught exception while allowing TestExecutionListener [org.springframework.test.context.support.DependencyInjectionTestExecutionListener@9476189] to prepare test instance [me.arganzheng.study.spring.autowired.AutowiredTest@2d546e21]
...
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [java.lang.String] found for dependency [map with value type java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true), @org.springframework.beans.factory.annotation.Qualifier(value=languageChangesMap)}
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486)
    ... 28 more

debug了一下,发现跟没有指定qualifie name是一样的执行路径。不是指定了bean
name了吗?为什么还是autowired by
type呢?仔细查看了一下才发现。DefaultListableBeanFactory的doResolveDependency方法对首先对类型做了区别:

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
            Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

        Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
        if (value != null) {
            if (value instanceof String) {
                String strVal = resolveEmbeddedValue((String) value);
                BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
                value = evaluateBeanDefinitionString(strVal, bd);
            }
            TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
            return (descriptor.getField() != null ?
                    converter.convertIfNecessary(value, type, descriptor.getField()) :
                    converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
        }

        if (type.isArray()) {
            Class<?> componentType = type.getComponentType();
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, descriptor);
            if (matchingBeans.isEmpty()) {
                if (descriptor.isRequired()) {
                    raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);
                }
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
            return converter.convertIfNecessary(matchingBeans.values(), type);
        }
        else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
            Class<?> elementType = descriptor.getCollectionType();
            if (elementType == null) {
                if (descriptor.isRequired()) {
                    throw new FatalBeanException("No element type declared for collection [" + type.getName() + "]");
                }
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, descriptor);
            if (matchingBeans.isEmpty()) {
                if (descriptor.isRequired()) {
                    raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);
                }
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
            return converter.convertIfNecessary(matchingBeans.values(), type);
        }
        else if (Map.class.isAssignableFrom(type) && type.isInterface()) {
            Class<?> keyType = descriptor.getMapKeyType();
            if (keyType == null || !String.class.isAssignableFrom(keyType)) {
                if (descriptor.isRequired()) {
                    throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +
                            "] must be assignable to [java.lang.String]");
                }
                return null;
            }
            Class<?> valueType = descriptor.getMapValueType();
            if (valueType == null) {
                if (descriptor.isRequired()) {
                    throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");
                }
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, descriptor);
            if (matchingBeans.isEmpty()) {
                if (descriptor.isRequired()) {
                    raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);
                }
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            return matchingBeans;
        }
        else {
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
            if (matchingBeans.isEmpty()) {
                if (descriptor.isRequired()) {
                    raiseNoSuchBeanDefinitionException(type, "", descriptor);
                }
                return null;
            }
            if (matchingBeans.size() > 1) {
                String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);
                if (primaryBeanName == null) {
                    throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
                }
                if (autowiredBeanNames != null) {
                    autowiredBeanNames.add(primaryBeanName);
                }
                return matchingBeans.get(primaryBeanName);
            }
            // We have exactly one match.
            Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
            if (autowiredBeanNames != null) {
                autowiredBeanNames.add(entry.getKey());
            }
            return entry.getValue();
        }
    }

如果是Array,Collection或者Map,则根据集合类中元素的类型来进行autowired
by
type(Map使用value的类型)。为什么这么特殊处理呢?原来,Spring是为了达到这样的目的:让你可以一次注入所有符合类型的实现,也就是说可以这样子注入:

@Autowired
private List<Car> cars;

如果你的car有多个实现,那么都会注入进来,不会再报

org.springframework.beans.factory.NoSuchBeanDefinitionException: 
No unique bean of type [me.arganzheng.study.spring.autowired.Car] is defined: 
expected single matching bean but found 2: [audi, toyota].

然而,上面的情况如果你用@Resource则不会有这个问题:

public class AutowiredTest extends BaseSpringTestCase {

    @Resource
    @Qualifier("languageChangesMap")
    private Map<String, String> languageChangesMap;

    @Test
    public void testAutowired() {
        assertNotNull(languageChangesMap);
        System.out.println(languageChangesMap.getClass().getSimpleName());
        System.out.println(languageChangesMap);
    }
}

正常运行:

LinkedHashMap
{pt=pt, br=pt, jp=ja, ja=ja, ind=ind, id=ind, en-rin=en-rIn, in=en-rIn, en=en, gb=en, th=th, ar=ar, eg=ar}

当然,你如果不指定@Qualifier(“languageChangesMap”),同时field
name不是languageChangesMap,那么还是一样报错的。

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [java.lang.String] found for dependency [map with value type java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@javax.annotation.Resource(shareable=true, mappedName=, description=, name=, type=class java.lang.Object, authenticationType=CONTAINER, lookup=)}
at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:438)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:416)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:550)
at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:150)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:303)
... 26 more

而且,@Resource也可以实现上面的List接收所有实现:

public class AutowiredTest extends BaseSpringTestCase {

    @Resource
    @Qualifier("languageChangesMap")
    private Map<String, String> languageChangesMap;

    @Resource
    private List<Car> cars;

    @Test
    public void testAutowired() {
        assertNotNull(languageChangesMap);
        System.out.println(languageChangesMap.getClass().getSimpleName());
        System.out.println(languageChangesMap);

        assertNotNull(cars);
        System.out.println(cars.getClass().getSimpleName());
        System.out.println(cars);
    }
}

运行的妥妥的:

LinkedHashMap
{pt=pt, br=pt, jp=ja, ja=ja, ind=ind, id=ind, en-rin=en-rIn, in=en-rIn, en=en, gb=en, th=th, ar=ar, eg=ar}
ArrayList
[me.arganzheng.study.spring.autowired.Audi@579584da, me.arganzheng.study.spring.autowired.Toyota@19453122]

这是因为@Resource注解使用的是CommonAnnotationBeanPostProcessor处理器,跟AutowiredAnnotationBeanPostProcessor不是同一个作者[澳门新葡亰网站注册,/偷笑]。这里就不分析了,感兴趣的同学可以自己看代码研究一下。

最终结论如下:

1、@Autowired和@Inject

autowired by type 可以 通过@Qualifier 显式指定 autowired by qualifier name(非集合类。注意:不是autowired by bean name!)
如果 autowired by type 失败(找不到或者找到多个实现),则退化为autowired by field name(非集合类)

2、@Resource

默认 autowired by field name
如果 autowired by field name失败,会退化为 autowired by type
可以 通过@Qualifier 显式指定 autowired by qualifier name
如果 autowired by qualifier name失败,会退化为 autowired by field name。但是这时候如果 autowired by field name失败,就不会再退化为autowired by type了

测试工程保存在GitHub上,是标准的maven工程,感兴趣的同学可以clone到本地运行测试一下。

@Inject使用

补充

有同事指出Spring官方文档上有这么一句话跟我的结有点冲突:

However, although you can use this convention to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with the bean name fallback, always have narrowing semantics within the set of type matches; they do not semantically express a reference to a unique bean id.

也就是说@Autowired即使加了@Qualifier注解,其实也是autowired by
type。@Qualifier只是一个限定词,过滤条件而已。重新跟进了一下代码,发现确实是这样子的。Spring设计的这个
@Qualifier name 并不等同于 bean
name。他有点类似于一个tag。不过如果这个tag是唯一的化,那么其实效果上等同于bean
name。实现上,Spring是先getByType,得到list
candicates,然后再根据qualifier name进行过滤。

再定义一个兰博基尼,这里使用@Qualifier指定:

package me.arganzheng.study.spring.autowired;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
@Qualifier("luxury")
public class Lamborghini implements Car {

}

再定义一个劳斯莱斯,这里故意用@Named指定:

package me.arganzheng.study.spring.autowired;

import javax.inject.Named;

import org.springframework.stereotype.Component;

@Component
@Named("luxury")
public class RollsRoyce implements Car {

}

测试一下注入定义的豪华车:

package me.arganzheng.study.spring.autowired;

import static junit.framework.Assert.assertNotNull;

import java.util.List;

import me.arganzheng.study.BaseSpringTestCase;

import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;

/**
 * 
 * @author zhengzhibin
 * 
 */
public class AutowiredTest extends BaseSpringTestCase {

    @Autowired
    @Qualifier("luxury")
    private List<Car> luxuryCars;

    @Test
    public void testAutowired() {

        assertNotNull(luxuryCars);
        System.out.println(luxuryCars.getClass().getSimpleName());
        System.out.println(luxuryCars);
    }

}

运行结果如下:

ArrayList
[me.arganzheng.study.spring.autowired.Lamborghini@66b875e1, me.arganzheng.study.spring.autowired.RollsRoyce@58433b76]

补充:Autowiring modes

Spring支持四种autowire模式,当使用XML配置方式时,你可以通过autowire属性指定。

no. (Default) No autowiring. Bean references must be defined via a ref element. Changing the default setting is not recommended for larger deployments, because specifying collaborators explicitly gives greater control and clarity. To some extent, it documents the structure of a system.
byName. Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named master, and uses it to set the property.
byType. Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set.
constructor. Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised.

如果使用@Autowired、@Inject或者@Resource注解的时候,则稍微复杂一些,会有一个失败退化过程,并且引入了Qualifier。不过基本原理是一样。

如下是@Inject的使用,不加@Named注解,需要配置与变量名一致即可。

@Inject
@Named("mongo")
private Mongo mongo;

2、@Autowired

  • org.springframework.bean.factory
  • Spring

@Autowired是Spring提供的注解,通过AutowiredAnnotationBeanPostProcessor类实现的依赖注入,与@inject二者具有可互换性。

@Autowired的使用

@Autowired有个属性为required,可以配置为false,如果配置为false之后,当没有找到相应bean的时候,系统不会抛错。

@Autowired
private MongoTemplate mongoTemplate;

3、@Resource

  • javax.annotation
  • JSR250 (Common Annotations for Java)

这是JSR250规范的实现,@Resource通过CommonAnnotationBeanPostProcessor类实现依赖注入。