In this article, I will explain When to Use Constructor Injection and Setter Injection in Spring.

When to use constructor injection?

  1. When the dependencies of a class are required to be set only once, during object creation.
  2. Also, when the class has mandatory dependencies, such that we can not create the object without them.
  3. When the class has a small number of dependencies, making it easy to manage them through constructor arguments.
  4. In case, the class is immutable and its dependencies are set only once.
  5. When we want to test the class, and we want to use mock dependencies.

When to use setter injection?

  1. In fact Setter Injection is a good choice when the dependencies of a class can change during the object’s lifecycle.
  2. If the class has optional dependencies that can be set after the object is created.
  3. When the class has a large number of dependencies. So, it makes it difficult to manage them through the constructor.
  4. When the class is mutable, and its dependencies can change dynamically.
  5. Also, when we want to avoid passing unnecessary dependencies through the constructor.

In general, constructor injection is considered a better choice because it ensures that the dependencies are set during object creation and makes the class immutable. However, setter injection can be useful in cases where we need to change the dependencies dynamically, or when the class has a large number of optional dependencies. Ultimately, the choice between constructor and setter injection depends on the specific use case and requirements.

Further Reading

Spring Framework Practice Problems and Their Solutions

Java Practice Exercise