春季Kotlin setter依赖注入

Question

I'm learning about spring data and because I'm also learning about kotlin so I decided to work with kotlin during spring learning. So I want to ask how we can Implement setter dependency injection in kotlin? as in Java we can that as below.

@Component
public class StudentDaoImp {

    public DataSource dataSource;

    public JdbcTemplate jdbcTemplate;

    public DataSource getDataSource() {
        return dataSource;
    }

    @Autowired
    public void setDataSource(DataSource dataSource) {
        this.jdbcTemplate = new JdbcTemplate(dataSource);
    }

    public JdbcTemplate getJdbcTemplate() {
        return jdbcTemplate;
    }

    public void setJdbcTemplate(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }
}

And here is my spring.xml file.

<?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"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-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/aop
    http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">


    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName" value="org.h2.Driver" />
        <property name="url" value="jdbc:h2:~/test" />
    </bean>

    <context:component-scan base-package="com.package.*" />
</beans>

Then I tried it in kotlin.

@Component
class StudentDao {

    @Autowired
    lateinit var dataSource: DataSource

    var jt = JdbcTemplate(dataSource)

}

Then I'm getting the exception.

Caused by: kotlin.UninitializedPropertyAccessException: lateinit property dataSource has not been initialized

I know about this exception because I'm using dataSource before autowired happen. So I have also tried this.

@Autowired
fun setDataSource(dataSource: DataSource) {
    this.jt = JdbcTemplate(dataSource)
}

This is also an error, because JVM already have that signature behind the scene.

So how I can Initialize JdbcTemplate with the dataSource parameter?

Note: I only want Code side example/solution. I know about the XML solution.

Answer 1

like in Java you can't use properties that are injected after instantiation at instantiation time. In Java you'd get a NullPointerException when you would do the equivalent, eg

@Autowired
private Datasource datasource;
private JdbcTemplate jt = new JdbcTemplace(dataSource);

But you can let Spring call a method of your choice after it has injected everything:

lateinit var jt: JdbcTemplate
@PostConstruct
fun initAfterAutowireIsDone() {
    jt = JdbcTemplate(dataSource)
}

There is also the InitializingBean interface that can be used instead of the annotation. The same works in Java.

The other option you have in Kotlin is to use a lazy property when you ensure you don't access it at instantiation time.

val jt: JdbcTemplate by lazy { JdbcTemplate(dataSource) }

Answer 2

While I consider zapl answer correct, I would also suggest you to consider constructor arguments injection:

@Component
class StudentDao @Autowired constructor(
    private val dataSource: DataSource
) {

    private val jt = JdbcTemplate(dataSource)

}

In this way you can obtain non modifiable variables without warning about late initialization.

Also, if you need dataSource variable only to initialize jt , you can remove val key from constructor.