{"id":3000,"date":"2026-06-19T12:24:57","date_gmt":"2026-06-19T04:24:57","guid":{"rendered":"http:\/\/www.treningi4you.com\/blog\/?p=3000"},"modified":"2026-06-19T12:24:57","modified_gmt":"2026-06-19T04:24:57","slug":"what-is-dependency-injection-in-spring-4999-94b0c4","status":"publish","type":"post","link":"http:\/\/www.treningi4you.com\/blog\/2026\/06\/19\/what-is-dependency-injection-in-spring-4999-94b0c4\/","title":{"rendered":"What is dependency injection in Spring?"},"content":{"rendered":"

Dependency injection (DI) is a fundamental concept in Spring, a powerful and widely – used Java framework. As a Spring supplier, I’ve had the privilege of working closely with this technology and helping numerous clients leverage its capabilities. In this blog, I’ll delve into what dependency injection is in Spring, its significance, and how it can benefit your projects. Spring<\/a><\/p>\n

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Understanding Dependency Injection<\/h3>\n

At its core, dependency injection is a design pattern that allows objects to receive dependencies (objects that they rely on) from an external source rather than creating them internally. To understand this better, let’s consider a simple example without dependency injection.<\/p>\n

Suppose we have a Car<\/code> class that depends on an Engine<\/code> class. Without dependency injection, the Car<\/code> class would create its own Engine<\/code> instance within its constructor. Here is a simple Java code snippet to illustrate this:<\/p>\n

class Engine {\n    public void start() {\n        System.out.println("Engine started");\n    }\n}\n\nclass Car {\n    private Engine engine;\n\n    public Car() {\n        this.engine = new Engine();\n    }\n\n    public void startCar() {\n        engine.start();\n    }\n}\n<\/code><\/pre>\n
In this example, the Car<\/code> class is tightly coupled to the Engine<\/code> class. If we want to change the Engine<\/code> implementation, we have to modify the Car<\/code> class. This lack of flexibility can lead to difficulties in testing and maintaining the code.<\/p>\n
Now, let’s see how dependency injection can solve this problem. With dependency injection, the Car<\/code> class no longer creates its own Engine<\/code> instance. Instead, the Engine<\/code> is passed to the Car<\/code> class from an external source. Here is the code after applying dependency injection:<\/p>\n
class Engine {\n    public void start() {\n        System.out.println("Engine started");\n    }\n}\n\nclass Car {\n    private Engine engine;\n\n    public Car(Engine engine) {\n        this.engine = engine;\n    }\n\n    public void startCar() {\n        engine.start();\n    }\n}\n<\/code><\/pre>\n
In this new implementation, the Car<\/code> class receives the Engine<\/code> as a parameter in its constructor. This makes the Car<\/code> class more flexible and easier to test. We can easily swap the Engine<\/code> implementation without modifying the Car<\/code> class.<\/p>\n
Types of Dependency Injection in Spring<\/h3>\n
Spring supports several types of dependency injection, each with its own advantages.<\/p>\n
Constructor Injection<\/h4>\n
Constructor injection is the most common type of dependency injection in Spring. As we saw in the previous example, the dependencies are passed to the class through its constructor. This ensures that the object is fully initialized when it is created.<\/p>\n
import org.springframework.beans.factory.annotation.Autowired;\nimport org.springframework.stereotype.Component;\n\n@Component\nclass Engine {\n    public void start() {\n        System.out.println("Engine started");\n    }\n}\n\n@Component\nclass Car {\n    private Engine engine;\n\n    @Autowired\n    public Car(Engine engine) {\n        this.engine = engine;\n    }\n\n    public void startCar() {\n        engine.start();\n    }\n}\n<\/code><\/pre>\n
In this Spring – based example, the @Autowired<\/code> annotation is used to tell Spring to inject the Engine<\/code> instance into the Car<\/code> class. Spring will automatically find the appropriate Engine<\/code> bean and inject it.<\/p>\n
Setter Injection<\/h4>\n
Setter injection allows dependencies to be set after the object is created. This is useful when the dependency is optional or when you want to change the dependency at runtime.<\/p>\n
import org.springframework.beans.factory.annotation.Autowired;\nimport org.springframework.stereotype.Component;\n\n@Component\nclass Engine {\n    public void start() {\n        System.out.println("Engine started");\n    }\n}\n\n@Component\nclass Car {\n    private Engine engine;\n\n    @Autowired\n    public void setEngine(Engine engine) {\n        this.engine = engine;\n    }\n\n    public void startCar() {\n        engine.start();\n    }\n}\n<\/code><\/pre>\n
In this example, the setEngine<\/code> method is used to inject the Engine<\/code> dependency. Spring will call this method to set the Engine<\/code> instance.<\/p>\n
Field Injection<\/h4>\n
Field injection is the simplest form of dependency injection. It involves directly injecting the dependency into a field of the class.<\/p>\n
import org.springframework.beans.factory.annotation.Autowired;\nimport org.springframework.stereotype.Component;\n\n@Component\nclass Engine {\n    public void start() {\n        System.out.println("Engine started");\n    }\n}\n\n@Component\nclass Car {\n    @Autowired\n    private Engine engine;\n\n    public void startCar() {\n        engine.start();\n    }\n}\n<\/code><\/pre>\n
However, field injection has some drawbacks. It can make the code harder to test because the dependencies are not explicitly passed in.<\/p>\n
Significance of Dependency Injection in Spring<\/h3>\n
Loose Coupling<\/h4>\n
One of the main benefits of dependency injection is that it promotes loose coupling between classes. When a class receives its dependencies from an external source, it doesn’t need to know how those dependencies are created or implemented. This makes the code more modular and easier to maintain.<\/p>\n
Testability<\/h4>\n
Dependency injection makes it easier to write unit tests. In unit testing, we often want to isolate the class being tested from its dependencies. With dependency injection, we can easily replace the real dependencies with mock objects. For example, in the Car<\/code> and Engine<\/code> example, we can create a mock Engine<\/code> object and inject it into the Car<\/code> class for testing purposes.<\/p>\n
Flexibility<\/h4>\n
Dependency injection allows for greater flexibility in the application. We can easily change the implementation of a dependency without affecting the classes that depend on it. For instance, if we want to use a different type of Engine<\/code> in the Car<\/code> class, we can simply provide a different Engine<\/code> implementation to the Car<\/code> class.<\/p>\n
How Our Spring Services Can Help<\/h3>\n
As a Spring supplier, we offer a wide range of services related to dependency injection in Spring. Our team of experts has extensive experience in implementing Spring applications with effective dependency injection.<\/p>\n
We can help you design and develop Spring – based applications that are highly modular and easy to maintain. Our services include:<\/p>\n