Java-Based Configuration 2 - Dependency Injection (DI) & Wiring

Dependency Injection (DI) allows objects to get their required dependencies (other objects or components they need to work) from an external source, instead of creating those dependencies themselves. An understanding in Spring XML configuration is recommended before the Java based configuration.

Setter injection

In Java based configuration you can inject attributes and dependencies using object’s public setter methods.

Primitive attribute injection

The Person object has a field age, you do so in the configuration class to inject primitive attribute.

@Configuration
public class AppConfig {
    @Bean
    public Person person() {
	    Person obj = new Person();
	    obj.setAge(25);
        return obj;
    }
}

public class Person {  
	int age;
	
    public void setAge(int age) {  
        this.age = age;
    }  
}

Reference attribute injection (Wiring)

In Spring’s Java-based configuration, we can inject the required dependencies into a bean if it depends on other beans. To ensure the code remains loosely coupled, we avoid instantiating objects directly within configuration methods. Instead, we define these dependencies as separate beans and define them as arguments to the configuration methods. The IoC container manages these dependencies, passing them as parameters.

The Person object has a dependent Computer.

@Configuration
public class AppConfig {
    @Bean
    public Person person(Computer com) {
	    Person obj = new Person();
	    obj.setAge(25);
	    obj.setComp(com);
        return obj;
    }
    
	@Bean
	public Computer computer() {
		return new Computer();
	}
}

Excluding a bean from autowiring

Excluding a bean from autowiring by setting the autowireCandidate attribute of the Bean annotation to false.

@Bean(autowireCandidate = false)  
public Desktop desktop() {  
    return new Desktop();  
}