What problems may arise when injecting dependencies through fields in a class?

Dependency injection through class fields (field injection) in Java can lead to several problems:

• Testing difficulty: fields injected directly are often private, making it difficult to replace them with mocks or stubs during unit testing.
• Lack of explicit dependency in constructor: it is hard to understand what the class depends on, as dependencies are not visible in its interface.
• Immutability issues: fields injected via fields are usually not final, which can lead to dependencies changing after object creation.
• Initialization order: when using field injection, it is difficult to control the order of dependency initialization, which can lead to errors.

Therefore, it is often recommended to use constructor injection, which makes dependencies explicit and facilitates testing.