Java Constructor

Java Default Constructor in Spring Boot

Spring Boot, by default, uses reflection to create beans. If a class lacks a constructor, Spring attempts to use a no-argument (default) constructor. Absence of a default constructor prevents Spring from instantiating the bean.

Problem: SpringApplication fails if a class lacks a default constructor and no other constructor is explicitly specified in the configuration.

Solution: Provide a no-argument constructor.

Example:

public class MyClass {

    private String name;

    // Default constructor REQUIRED by Spring
    public MyClass() {} 

    public MyClass(String name) {
        this.name = name;
    }

    // ... other methods ...
}

Alternative (Constructor Injection):

If you intend to always use a constructor with parameters, declare it explicitly in your Spring configuration using @Bean or @Component with constructor injection. Spring will then use this constructor.

@Component
public class MyClass {
    private final String name;

    public MyClass(String name) {
        this.name = name;
    }
    // ... other methods
}

@Configuration
public class AppConfig {
    @Bean
    public MyClass myClass() {
        return new MyClass("My Name");
    }
}

Key takeaway: Always ensure your Spring beans have either a public no-argument constructor or are explicitly configured with constructor injection in your Spring configuration to avoid instantiation errors. Using constructor injection promotes better dependency management and testability.