Using autowired dependency in constructor in spring

Question

I have class NetworkUsageService which uses NetworkUsageMapper to access a database. Mapper class is autowired to the service class.

I need to access the database during the construcion of service class, so I do something like this:

private int someField;    

@Autowired
private NetworkUsageMapper networkUsageMapper;


public NetworkUsageService() {
    someField = networkUsageMapper.getSomeResultFromDB();
}

However, this doesn't seem to work as I get exception while creating the service bean. Is there a way to use an autowired dependency during the construction of an object?

EDIT: this is my beans configuration as requested:

<context:component-scan base-package="mypackage" />
<mybatis:scan base-package="mypackage.mappers" />

<mvc:annotation-driven />

<context:property-placeholder location="classpath:/jdbc.properties"/>

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="dataSource" />
    <property name="mapperLocations" value="classpath:mappers/*.xml" />
    <property name="typeAliasesPackage" value="mypackage.transfer" />
</bean>

<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
    <property name="dataSource" ref="dataSource" />
</bean>

<bean  id="dataSource"
       class="org.springframework.jdbc.datasource.DriverManagerDataSource">

    <property name="driverClassName" value="${jdbc.driverClassName}" />
    <property name="url" value="${jdbc.url}" />
    <property name="username" value="${jdbc.username}" />
    <property name="password" value="${jdbc.password}" />
</bean>

@KeerthiRamanathan: <mybatis:scan base-package="mypackage.mappers" /> does it for me — Wolf
– Wolf, Commented Dec 2, 2013 at 15:38

Janoz · Accepted Answer · 2013-12-02 15:25:29Z

12

Instead of doing a lot of work in the constructor (and potentially introduce problems because the application tries to go to the database when it isn't fully initialized) it's probably better to do this initialization in a different method and annotate this with @PostConstruct . This way you know for sure your application is completely wired before networkusermapper is called.

answered Dec 2, 2013 at 15:25

Janoz

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2 Comments

Wolf Over a year ago

thank you for the answer, this sounds like something I need, I will try that, but can you be sure that dependencies are autowired before @PostConstruct methods are invoked?

Janoz Over a year ago

Yes, that is part of the contract. The method annotated with @PostConstruct is called after all the beans are (auto)wired.

Chris Thompson · Accepted Answer · 2013-12-02 15:07:35Z

1

That's correct. The bean is first constructed, then the dependencies are injected. If you need it in your constructor, do this

@Autowired
public NetworkUsageService(NetworkUsageMapper mapper){
// do stuff
}

answered Dec 2, 2013 at 15:07

Chris Thompson

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2 Comments

Wolf Over a year ago

thank you, just for clarification - if I do as you supposed and leave the autowired dependency as is in my code, does it mean that there will be 2 instances of NetworkUsageMapper constructed?

Chris Thompson Over a year ago

I'm not actually sure how Spring handles that. I would remove your existing annotation to eliminate ambiguity. That said, you can annotate your NetworkUsageMapper class with the @Singleton annotation and that will guarantee that only one instance is created by spring and used for all dependency injection. Otherwise, multiple instances could be created if more than one class uses the dependency.

Keerthivasan · Accepted Answer · 2013-12-02 15:33:40Z

1

The other way is to make the setterMethod of NetworkUsageMapper annotated with @Required annotation

@Required
public void setNetworkUsageMapper(NetworkUsageMapper mapper){
this.networkUsageMapper = mapper;
this.someField = this.networkUsageMapper.getSomeResultFromDB();
}

This method would be invoked immediately compulsorily after construction of the object. This is really useful when you have a long list of mandatory properties for the object to work properly. Instead of having the constructor with long parameter list, we can set all the properties using setters with @Required annotation. It seems to be very handy for me.

edited Dec 2, 2013 at 15:33

answered Dec 2, 2013 at 15:11

Keerthivasan

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3 Comments

Wolf Over a year ago

Thank you for your answer, but as you wrote : "This method would be invoked immediately compulsorily after construction of the object." - I need to use the dependency during the construction of the object, not after it's construction

Wolf Over a year ago

i need to populate a field of service object with database data before I use it for anything else

Keerthivasan Over a year ago

I have updated the answer setting the field of the service object. Hope this helps

Mak · Accepted Answer · 2013-12-02 15:18:07Z

0

If you can add the exception stack-trace along with your spring configuration that will help to answer more accurately. I am guessing it is because you might have more than one bean instances for NetworkUsageMapper class or its missing.

If its missing just add one :). To avoid exception you can use required attribute:

@Autowired(required=false) private NetworkUsageMapper networkUsageMapper;
If more than one then you need to use qualifier for example :

then

@Autowired
@Qualifier("networkUsageMapper1")
private NetworkUsageMapper networkUsageMapper;

Hope it helps :Mak

answered Dec 2, 2013 at 15:18

Mak

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