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How do you choose between constructor, setter, and field injection in Java?

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Dependency injection (DI) is a design pattern that allows you to decouple your classes from their dependencies, making them easier to test and reuse. In Java, there are three common ways to implement DI: constructor, setter, and field injection. Each of them has its pros and cons, and choosing the right one depends on your use case and preferences. In this article, we will compare these three types of injection and give you some tips on how to decide which one to use.

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Darsh Gupta

Senior software Engineer at Precisely | Certified Cloud Professional

1 Constructor injection

Constructor injection is the most widely used and recommended type of DI in Java. It involves passing the dependencies as parameters to the constructor of the class that needs them. This way, you can ensure that the class is always in a valid state, with all its dependencies initialized and ready to use. Constructor injection also makes your code more readable, as you can clearly see what dependencies a class has and where they come from. Additionally, constructor injection enables you to use final fields, which can prevent accidental changes and improve performance.

However, constructor injection also has some drawbacks. It can make your constructors too long and complex, especially if you have many dependencies or nested dependencies. It can also make your code harder to mock and stub, as you need to create and pass the mock objects to the constructor every time you want to test a class. Furthermore, constructor injection does not support optional or dynamic dependencies, as you cannot change or remove them once the class is created.

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2 Setter injection

Setter injection is another common type of DI in Java. It involves providing setter methods for the dependencies of a class, and calling them after the class is instantiated. This way, you can have more flexibility and control over when and how to inject the dependencies. Setter injection also allows you to have optional or dynamic dependencies, as you can change or remove them at any time. Moreover, setter injection can make your code easier to mock and stub, as you can use the setter methods to inject the mock objects without creating a new instance of the class.

However, setter injection also has some disadvantages. It can make your class mutable and vulnerable to inconsistent states, as the dependencies can be changed or nullified after the class is created. Setter injection also makes your code less readable, as you cannot tell what dependencies a class has and where they come from by looking at the constructor. Additionally, setter injection does not support final fields, which can lead to accidental changes and performance issues.

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3 Field injection

Field injection is the least recommended type of DI in Java. It involves using annotations, such as @Autowired or @Inject, to mark the fields that need to be injected with dependencies. This way, you can avoid writing constructors or setters for the dependencies, and let a framework or a container handle the injection for you. Field injection can make your code concise and simple, as you only need to declare the fields and annotate them.

However, field injection also has many drawbacks. It can make your class tightly coupled to the framework or the container that does the injection, making it harder to test and reuse. Field injection also makes your class opaque and brittle, as you cannot see or control what dependencies a class has and where they come from. Furthermore, field injection does not support final fields, optional or dynamic dependencies, or constructor validation, which can cause many problems and bugs.

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Darsh Gupta

Senior software Engineer at Precisely | Certified Cloud Professional
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Although field injection is the least recommended type of DI in my experience, this is the most used one in Spring context. Since in spring we usually create stateless beans, having field injection is acceptable with injection taken care of by Spring itself.

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4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you choose between constructor, setter, and field injection in Java?

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1 Constructor injection

2 Setter injection

3 Field injection

4 Here’s what else to consider

Software Design

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How do you choose between constructor, setter, and field injection in Java?

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1 Constructor injection

2 Setter injection

3 Field injection

4 Here’s what else to consider

Software Design

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