Constructor-based or setter-based DI?
Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use constructors for mandatory dependencies and setter methods or configuration methods for optional dependencies.
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The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null.
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Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. …
(From Spring Reference Documentation https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#beans-dependencies
Although field injection is discouraged in production code, field injection is actually quite natural in test code.…
(From Spring Reference Documentation https://docs.spring.io/spring/docs/current/spring-framework-reference/testing.html#testcontext-fixture-di)
Every time you use fieldinjection, a unit test dies
From Josh Long, Spring Developer Advocate
The first rule of thumb is when building web apps is to keep Spring controllers as light as possible. We can think of them as converters between HTTP traffic and our system.
(From “Learning Spring Boot 2.0 - Second Edition” by Greg L. Turnquist https://www.packtpub.com/application-development/learning-spring-boot-20-second-edition)
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http://dolszewski.com/spring/how-to-bind-requestparam-to-object/