10 Spring Boot Performance Optimization Techniques That Actually Make a Difference

1. Fix N+1 Queries Before Optimizing Java Code

Consider an API that returns customers along with their orders.

@GetMapping("/customers")
public List<CustomerResponse> getCustomers() {
    return customerRepository.findAll()
            .stream()
            .map(customer -> new CustomerResponse(
                    customer.getId(),
                    customer.getName(),
                    customer.getOrders().size()
            ))
            .toList();
}

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Suppose the application retrieves 100 customers. If accessing getOrders() triggers a separate database query for every customer, the application executes:

• 1 query to retrieve customers.
• 100 additional queries to retrieve their orders.

That’s 101 queries for a single API request.

This is the N+1 query problem.

One solution is to fetch the required relationship using a fetch join.

@Query("""
    SELECT DISTINCT c
    FROM Customer c
    LEFT JOIN FETCH c.orders
    """)
List<Customer> findAllWithOrders();