BeanInstantiationException in Spring Boot

In this post, we'll dive deep into what the BeanInstantiationException exception is, how to reproduce it, its common causes, and how to solve them.

What is BeanInstantiationException?

BeanInstantiationException is thrown when Spring's application context is unable to instantiate a bean. This can occur for a number of reasons ranging from issues with the bean's constructor to configuration errors.

How to Reproduce BeanInstantiationException

To see this exception in action, let's consider a few scenarios:

1. Constructor Throws an Exception

import org.springframework.stereotype.Service;

@Service
public class MyService {
    public MyService() {
        throw new RuntimeException("Constructor failed!");
    }
}

Running the above Spring Boot application will result in a BeanInstantiationException due to the exception thrown in the constructor.

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.springboot.MyService]: Constructor threw exception

2. Abstract Class or Interface

If you mistakenly annotate an abstract class or an interface with @Component or @Service, Spring will fail to create a bean instance.

import org.springframework.stereotype.Service;

@Service
public abstract class AbstractService {
    // some methods...
}

3. No Default Constructor

A class without a default constructor, where Spring doesn't have any way to provide the required arguments, can also cause this issue.

package com.example.demo;

import org.springframework.stereotype.Component;

@Component
public class MyComponent {
    private final String name;

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

Common Causes and Solutions

Now that we've seen how to reproduce the issue, let's look at the causes behind these scenarios and their respective solutions:

1. Constructor Throws an Exception

Cause: The bean's constructor threw an unchecked exception.

Solution: Ensure the constructor does not throw any unchecked exceptions. Move such logic outside of the constructor to a @PostConstruct method or handle them with appropriate checks.

2. Abstract Class or Interface

Cause: Spring cannot instantiate an abstract class or an interface.

Solution: Do not annotate interfaces or abstract classes with @Component, @Service, etc. Use composition or ensure concrete implementations are available.

3. No Default Constructor

Cause: Spring requires a no-argument constructor to instantiate the bean.

Solution: Provide a default constructor. If using library classes, consider extending the class or using configuration to provide necessary arguments.

Conclusion

BeanInstantiationException can seem intimidating at first, especially for newcomers to Spring Boot. However, with a proper understanding of its causes, it becomes easier to debug and solve. Always refer to the exception stack trace for a clearer picture of what went wrong!