Spring Boot 3.2.5 New Features: Multithreaded Bean Init, @Fallback, RestClient & More

1. Multithreaded Bean Initialization

Since Spring Framework 6.2, singleton beans can be instantiated concurrently using CompletableFuture.runAsync. Define a custom executor bean named bootstrapExecutor to control the thread pool.

@Bean
public Executor bootstrapExecutor() {
    ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
    taskExecutor.setCorePoolSize(5);
    taskExecutor.setMaxPoolSize(5);
    taskExecutor.initialize();
    return taskExecutor;
}

2. New @Fallback Annotation

The @Fallback annotation acts as an alternative to @Primary when multiple beans of the same type exist. It marks a bean as a fallback candidate, selected only if no primary bean matches.

interface CommonDAO {}
class UserDAO implements CommonDAO {}
class PersonDAO implements CommonDAO {}

@Configuration
public class AppConfig {
    @Bean
    public UserDAO userDAO() {
        return new UserDAO();
    }

    @Bean
    @Fallback
    public PersonDAO personDAO() {
        return new PersonDAO();
    }

    @Bean
    public CommonService commonService(CommonDAO dao) {
        return new CommonService(dao);
    }
}

In this example, CommonService receives UserDAO unless it is absent, in which case PersonDAO (marked with @Fallback) is injected.

3. Defining Two Beans with the Same Name

Spring 6.2 now allows beans with identical names; the later definition overrides the earlier one.

@Configuration
public class AppConfig {
    @Bean(name = "a")
    Person personA() {
        return new Person(1);
    }

    @Bean(name = "a")
    Person personB() {
        return new Person(2);
    }
}

4. RestClient Improvements

The RestClient API adds a create(URI) overload that parses the supplied URI as the base URL, simplifying client calls.

@GetMapping("/client")
public String client() {
    return RestClient.create("http://localhost:8080")
        .get()
        .uri(URI.create("/params/index"))
        .retrieve()
        .body(String.class);
}

5. Binding Header Data to Records

From Spring 6.2 onward, @RequestHeader values can be bound directly to fields of a record, enabling concise parameter objects.

@GetMapping("/{path}")
public void get(Params params, @PathVariable long path,
                @RequestParam String foo, @RequestHeader String accept) {
    System.out.printf("path: %d%n", path);
    System.out.printf("foo: %s%n", foo);
    System.out.printf("accept: %s%n", accept);
    System.out.printf("params: %s%n", params);
}

public static record Params(@PathVariable long path,
                           @RequestParam String foo,
                           @RequestHeader String accept) {}

6. Task Scheduler New Metadata

The /actuator/scheduledtasks endpoint now reports additional fields for each scheduled task:

Last execution time

Last execution status

Next execution time

7. Other Backend Improvements

Updated UndertowHttpHandlerAdapter for scheduling.

Enhanced Javadoc for @Contract with new return‑value information.

AOT now ignores cascading and container‑element constraints during bean validation.

Lazy‑proxy bean resolution avoids duplicate singleton parsing.

Introduced @DisabledInAotMode with custom reasons.

Optimized ResponseBodyEmitter with optimistic locking.

Improved Cookie support in WebClient and WebTestClient.

Added @CheckReturnValue annotation.

Virtual‑thread friendly options for ThreadPoolTaskExecutor and ThreadPoolTaskScheduler.

Various refactorings and bug fixes across the Spring ecosystem.