Why Constructor injection is better

Dependency injection is effective in these situations:

1. You need to inject configuration data into one or more components.

2. You need to inject the same dependency into multiple components.

3. You need to inject different implementations of the same dependency.

4. You need to inject the same implementation in different configurations.

5. You need some of the services provided by the container.

Dependency injection is not effective if:

1. You will never need a different implementation.

2. You will never need a different configuration.

A class that takes a required dependency as a constructor argument can only be instantiated if that argument is provided (you should have a guard clause to make sure the argument is not null.) A constructor therefore enforces the dependency requirement whether or not you’re using Spring, making it container-agnostic.

If you use setter injection, the setter may or may not be called, so the instance may never be provided with its dependency. The only way to force the setter to be called is using @Required or @Autowired , which is specific to Spring and is therefore not container-agnostic.

So to keep your code independent of Spring, use constructor arguments for injection

By using Constructor Injection, you assert the requirement for the dependency in a container-agnostic manner

This mean that you can enforce requirements for all injected fields without using any container specific solution.

Setter injection example

With setter injection special spring annotation @Required is required.

@Required

Marks a method (typically a JavaBean setter method) as being ‘required‘: that is, the setter method must be configured to be dependency-injected with a value.

Usage

import org.springframework.beans.factory.annotation.Required;

import javax.inject.Inject;
import javax.inject.Named;

@Named
public class Foo {

private Bar bar;

@Inject
@Required
public void setBar(Bar bar) {
this.bar = bar;
}
}

Constructor injection example

All required fields are defined in constructor, pure Java solution.

Usage

import javax.inject.Inject;
import javax.inject.Named;

@Named
public class Foo {

private Bar bar;

@Inject
public Foo(Bar bar) {
this.bar = bar;
}

}

Main Differences :

1. Partial dependency: can be injected using setter injection but it is not possible by constructor. Suppose there are 3 properties in a class, having 3 arg constructor and setters methods. In such case, if you want to pass information for only one property, it is possible by setter method only.

2.Overriding: Setter injection overrides the constructor injection. If we use both constructor and setter injection, IOC container will use the setter injection.

3.Changes: We can easily change the value by setter injection. It doesn’t create a new bean instance always like constructor. So setter injection is flexible than constructor injection

To make it simple, let us say that we can use constructor based dependency injection for mandatory dependencies and setter based injection for optional dependencies. It is a rule of thumb!!

Let’s say for example.

If you want to instantiate a class you always do it with its constructor. So if you are using constructor based injection, the only way to instantiate the class is through that constructor. If you pass the dependency through constructor it becomes evident that it is a mandatory dependency.

On the other hand, if you have a setter method in a POJO class, you may or may not set value for your class variable using that setter method. It is completely based on your need. i.e. it is optional. So if you pass the dependency through setter method of a class it implicitly means that it is an optional dependency. Hope this is clear!!

Constructor injection is used when the class cannot function without the dependent class.

Property injection is used when the class can function without the dependent class.

As a concrete example, consider a ServiceRepository which depends on IService to do its work. Since ServiceRepository cannot function usefully without IService, it makes sense to have it injected via the constructor.

The same ServiceRepository class may use a Logger to do tracing. The ILogger can be injected via Property injection.

Other common examples of Property injection are ICache (another aspect in AOP terminology) or IBaseProperty (a property in the base class).

By using Constructor Injection, you assert the requirement for the dependency in a container-agnostic manner

We need the assurance from the IoC container that, before using any bean, the injection of necessary beans must be done.

In setter injection strategy, we trust the IoC container that it will first create the bean first but will do the injection right before using the bean using the setter methods. And the injection is done according to your configuration. If you somehow misses to specify any beans to inject in the configuration, the injection will not be done for those beans and your dependent bean will not function accordingly when it will be in use!

But in constructor injection strategy, container imposes (or must impose) to provide the dependencies properly while constructing the bean. This was addressed as ” container-agnostic manner”, as we are required to provide dependencies while creating the bean, thus making the visibility of dependency, independent of any IoC container.

Q: And how to prevent container from creating bean by constructor with null values instead of missing beans?

You have no option to really miss any <constructor-arg> (in case of Spring), because you are imposed by IoC container to provide all the constructor arguments needed to match a provided constructor for creating the bean. If you provide null in your <constructor-arg> intentionally. Then there is nothing IoC container can do or need to do with it!

When Dependency injection is not good ?

Basically, dependency injection makes some (usually but not always valid) assumptions about the nature of your objects. If those are wrong, DI may not be the best solution:

• First, most basically, DI assumes that tight coupling of object implementations is ALWAYS bad. This is the essence of the Dependency Inversion Principle: “a dependency should never be made upon a concretion; only upon an abstraction”.

  This closes the dependent object to change based on a change to the concrete implementation; a class depending upon ConsoleWriter specifically will need to change if output needs to go to a file instead, but if the class were dependent only on an IWriter exposing a Write() method, we can replace the ConsoleWriter currently being used with a FileWriter and our dependent class wouldn’t know the difference (Liskhov Substitution Principle).

  However, a design can NEVER be closed to all types of change; if the design of the IWriter interface itself changes, to add a parameter to Write(), an extra code object (the IWriter interface) must now be changed, on top of the implementation object/method and its usage(s). If changes in the actual interface are more likely than changes to the implementation of said interface, loose coupling (and DI-ing loosely-coupled dependencies) can cause more problems than it solves.

• Second, and corollary, DI assumes that the dependent class is NEVER a good place to create a dependency. This goes to the Single Responsibility Principle; if you have code which creates a dependency and also uses it, then there are two reasons the dependent class may have to change (a change to the usage OR the implementation), violating SRP.

  However, again, adding layers of indirection for DI can be a solution to a problem that doesn’t exist; if it is logical to encapsulate logic in a dependency, but that logic is the only such implementation of a dependency, then it is more painful to code the loosely-coupled resolution of the dependency (injection, service location, factory) than it would be to just use new and forget about it.

• Lastly, DI by its nature centralizes knowledge of all dependencies AND their implementations. This increases the number of references that the assembly which performs the injection must have, and in most cases does NOT reduce the number of references required by actual dependent classes’ assemblies.

  SOMETHING, SOMEWHERE, must have knowledge of the dependent, the dependency interface, and the dependency implementation in order to “connect the dots” and satisfy that dependency. DI tends to place all that knowledge at a very high level, either in an IoC container, or in the code that creates “main” objects such as the main form or Controller which must hydrate (or provide factory methods for) the dependencies. This can put a lot of necessarily tightly-coupled code and a lot of assembly references at high levels of your app, which only needs this knowledge in order to “hide” it from the actual dependent classes (which from a very basic perspective is the best place to have this knowledge; where it’s used).

  It also normally doesn’t remove said references from lower down in code; a dependent must still reference the library containing the interface for its dependency, which is in one of three places:

  • all in a single “Interfaces” assembly that becomes very application-centric,

  • each one alongside the primary implementation(s), removing the advantage of not having to recompile dependents when dependencies change, or

  • one or two apiece in highly-cohesive assemblies, which bloats the assembly count, dramatically increases “full build” times and decreases application performance.

  All of this, again to solve a problem in places where there may be none.