Dependency injection (DI) is a design pattern that enables loose coupling of code by externally managing dependencies of objects. Its core lies in injecting object dependencies from the outside rather than creating them internally, thereby improving flexibility and maintainability. For example, in the UserService, pass into the UserRepository instance through the constructor, that is, the constructor injection. Spring framework supports multiple injection methods through IoC containers: 1. Constructor injection, suitable for forced dependencies; 2. Setter injection, suitable for optional dependencies; 3. Field injection (@Autowired), using annotations directly in fields. Advantages of DI include: decoupling, enhanced testability, flexible configuration, and easy maintenance. In practical applications, you need to pay attention to: avoid abuse of field injection, rationally select scopes, and deal with circular dependency problems. Mastering DI helps develop high-quality, easy-to-scaling Java applications.

Dependency Injection (DI) is a very common concept in Java frameworks, especially in frameworks like Spring. Simply put, it is a design pattern used to implement loosely coupled code structures, allowing dependencies between objects to be managed externally, rather than hard-coded inside the class.

For example, if you have a Service class that requires a Repository object, the traditional practice may be to directly new a Repository instance in the Service. But the problem with this is: if you want to change another implementation in the future, you have to change the code and recompile it; and it is not convenient to replace the mock object during testing. If DI is used, this Repository object will be "injected" by the framework or container at runtime, rather than created by itself.

What is dependency injection?

The core idea of ??dependency injection is: leave the object's dependencies to the outside to handle , rather than create or search by yourself. This makes the code more flexible and easier to maintain and test.

For example, suppose you have a UserService that needs to use UserRepository :

 public class UserService {
    private UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public void doSomething() {
        userRepository.save();
    }
}

Here, there is no new UserRepository inside UserService , but is passed in through the constructor. This is one way of dependency injection - constructor injection.

How does Spring use dependency injection?

The Spring framework has a powerful dependency injection mechanism built in. It manages the life cycle and dependencies of an object through an IoC container (control inversion container).

Spring supports several common injection methods:

• Constructor injection : Dependencies are passed through constructor methods
• Setter injection : Set dependencies through the setter method
• Field Injection (@Autowired) : Apply annotations directly on the field

for example:

 @Service
public class UserService {

    @Autowired
    private UserRepository userRepository;

    // ...
}

Spring automatically scans these annotations at startup and injects the required objects into it. You don't need to manually new objects, everything is done by the framework for you.

Why use dependency injection? What are the benefits?

1. Decoupling : Classes no longer care about the specific implementation of dependencies, but only care about interfaces.
2. More testable : You can easily replace mock objects for unit testing.
3. Flexible configuration : Replacing the implementation only requires changing the configuration, no code changes.
4. Easy to maintain : When demand changes, the range of changes is small.

Let me give you a practical scenario: If you are using MySQL database now and then want to change to PostgreSQL, just change the implementation class of Repository, and you basically don't need to move other codes.

What should you pay attention to when using dependency injection?

Although dependency injection brings flexibility, there are some things to note:

• Don't abuse field injection (@Autowired is written directly on the field), it hides dependencies and is not conducive to understanding.
• Constructor injection is more suitable for forced dependencies (must have), and setter is more suitable for optional dependencies.
• Pay attention to the scope of beans (such as singleton, prototype) to avoid concurrency problems or memory leaks.
• Circular dependencies should be handled with caution. Although Spring can solve some of them, it is best to avoid them in terms of design.

Basically that's it. Dependency injection is not a profound thing, but it does make the code clearer and easier to scale. Mastering the use of DI is very helpful in developing high-quality Java applications.

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