Spring Injection Types

Spring supports three types of dependency injections:

Constructor injection

@Component
public class SecondBeanImpl implements SecondBean {

    private FirstBean firstBean;

    @Autowired
    public SecondBeanImpl(FirstBean firstBean) {
        this.firstBean = firstBean;
    }
}

That is similar to:

FirstBean firstBean = new FirstBeanImpl();
SecondBean secondBean = new SecondBeanImpl(firstBean);

This type of dependency injection instantiates and initializes the object.
In this approach, beans are immutable and dependencies are not null. However, if you define many parameters in the constructor, your code is not clean.
From Spring 4.3 the @Autowired annotation is not required if the class has a single constructor.

Setter injection

@Component
public class SecondBeanImpl implements SecondBean {

    private FirstBean firstBean;

    @Autowired
    public setFirstBean(FirstBean firstBean) {
        this.firstBean = firstBean;
    }
}

That is similar to:

FirstBean firstBean = new FirstBeanImpl();
SecondBean secondBean = new SecondBeanImpl();
secondBean.setFirstBean(firstBean);

In this approach, beans are not immutable (the setter could be called later), and not mandatory dependencies can lead to NullPointerExceptions.

Field injection

@Component
public class SecondBeanImpl implements SecondBean {

    @Autowired
    private FirstBean firstBean;
}

This approach may look cleaner but hides the dependencies and makes testing difficult. While constructor and setter injections use proxies, field injection uses reflection which could affect the performance. Could be used in test classes.