Dependency injection (DI) achieves decoupling through the dependencies of external control objects, improving code testability, maintainability and flexibility. 1. DI is a design pattern, and the core is to create it by external incoming dependencies rather than objects themselves; 2. Common injection methods include constructor injection (most commonly used), Setter injection (applicable to optional dependencies), and field injection (not recommended); 3. DI can be implemented manually, such as passing dependencies through constructors; 4. Use the Spring framework to simplify dependency management, and automatically handle dependencies through @Component and @Autowired annotations; 5. Pay attention to avoiding complex constructors and bean conflicts, not all classes require framework management. Mastering these key points can help you apply DI in Java more efficiently.

Implementing dependency injection (DI) in Java applications is the core of decoupling the dependencies between components and separating the creation and use of objects. The benefit of doing this is to improve the testability, maintainability and flexibility of the code. If you are just starting to get involved in DI, you may find it a bit abstract, but in fact, you can easily get started by just mastering a few key points.

What is dependency injection? Why do you need it?

Dependency injection is a design pattern. Its core idea is to control the dependencies of the object from the outside, rather than to create or find dependencies by the object itself . This may sound a bit confusing, for a simple example:

For example, you have an EmailService class that is used by UserService . If you do not use DI, UserService may directly new an EmailService instance. The problem with this is that if you want to change the service sent by SMS in the future, you have to modify UserService code.

Through dependency injection, you can pass EmailService as a parameter to UserService , or the framework will automatically inject it for you, thus achieving loose coupling.

There are three common dependency injection methods:

• Constructor injection
• Setter Injection
• Field injection (not recommended)

Constructor injection is most commonly used and is easiest to test; Setter injection is suitable for optional dependencies; field injection is simple to write, but it is not conducive to testing and maintenance, so it is recommended to use it less.

How to implement a simple DI manually?

If you don't want to introduce frameworks like Spring or Dagger from the beginning, you can also manually implement a simple dependency injection logic. Take constructor injection as an example:

 class EmailService {
    public void sendEmail(String to, String message) {
        System.out.println("Sending email to " to ": " message);
    }
}

class UserService {
    private EmailService emailService;

    public UserService(EmailService emailService) {
        this.emailService = emailService;
    }

    public void registerUser(String email) {
        // do something
        emailService.sendEmail(email, "Welcome!");
    }
}

Then in the main program:

 EmailService emailService = new EmailService();
UserService userService = new UserService(emailService);
userService.registerUser("user@example.com");

Although "manual", this approach clearly demonstrates the nature of DI: dependencies are not created internally, but are passed from outside . This is helpful for understanding the principles of DI.

Dependency injection using Spring framework

Once the project becomes larger, manual management of dependencies will become cumbersome, and this is the time to simplify this process with the help of the Spring framework. Spring is one of the most mainstream DI containers in the Java ecosystem.

To implement DI using Spring, there are two main steps:

1. Add annotations on the class to indicate which classes need to be managed by Spring
2. Automatically inject dependencies using @Autowired annotation

Continue with the above example:

 @Component
class EmailService {
    public void sendEmail(String to, String message) {
        System.out.println("Sending email to " to ": " message);
    }
}

@Component
class UserService {
    @Autowired
    private EmailService emailService;

    public void registerUser(String email) {
        emailService.sendEmail(email, "Welcome!");
    }
}

After Spring starts, these classes will be automatically scanned and the dependencies between them will be handled. You just need to focus on business logic.

A few points to note:

• Don't do too many complicated operations in the constructor, as it can easily lead to initialization failure.
• Try to avoid multiple bean conflicts, which can be specified explicitly through @Primary or @Qualifier
• If you try to inject it in a non-Spring-managed class, it will not take effect

Tips: Don't over-reliance on frameworks

Although frameworks like Spring are very useful, DI frameworks are not necessarily required in some small projects or tool classes. Manually implementing dependency injection is sometimes more intuitive and easier to debug.

In addition, some developers are accustomed to adding @Component or @Service to each class, but in fact, not all classes need to be managed by containers. Only classes that do need to be used by other components should be handed over to Spring.

To summarize:

• Identify which classes need to be injected and which are just ordinary auxiliary classes
• Constructor injection is the clearest way
• Framework is a tool, don't use it for the sake of use

Basically that's it. If you master these points well, it will not be too difficult to apply DI in Java.

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