You can enable transaction management in a Spring Boot application by using the @EnableTransactionManagement annotation. This annotation tells Spring to enable transactional behavior for annotated methods. While you can also annotate methods or classes with @Transactional to specify transactional behavior, you need to enable transaction management globally with @EnableTransactionManagement. Spring Boot does not enable transaction management by default, and configuring transactions in application.properties is not a common approach for enabling transaction management.

When integrating with external APIs in a Spring Boot application, it's crucial to implement a global exception handler (Option 4) to manage failures. This handler should return standardized error responses with clear error messages and appropriate HTTP status codes, ensuring informative messages are relayed back to the user. Implementing a retry mechanism (Option 1) is a good practice, but it should be combined with proper error handling. Options 2 and 3 are not recommended as they either log errors internally or provide generic and uninformative error messages to users.

Option 1 is correct. Enabling garbage collection logging with JVM flags allows you to analyze memory usage patterns and identify memory leaks or excessive memory consumption, which are common causes of OutOfMemoryErrors. Increasing the number of application instances may exacerbate the issue if it's related to memory consumption. Disabling Spring beans or reducing threads may not be suitable solutions for addressing OutOfMemoryErrors.

In a Spring Boot application, the @Cacheable annotation is used to enable conditional caching of method return values based on the evaluation of a SpEL (Spring Expression Language) expression. By using this annotation, you can specify when a method's result should be cached, which is helpful for optimizing performance in certain scenarios.

In reactive programming with Spring Boot, you can handle errors in a reactive stream by using the onError operator and other error-handling operators provided by Project Reactor. These operators allow you to define how to react to errors within the stream, whether it's by logging, retrying, switching to a fallback stream, or propagating the error to the subscriber. This enables robust error handling and recovery strategies in reactive applications.

In Spring Boot unit testing with Mockito, you use the @InjectMocks annotation to inject mock objects into the object being tested. This allows you to control the behavior of dependencies during testing.

When developing a Spring Boot application with a large team, it's best to use Spring Boot's external configuration mechanisms like application.properties or application.yml files. These can be organized into separate folders or packages for each module, making it easier to manage and segregate configuration properties. Storing all properties in a single file or embedding them in the source code is not a scalable or maintainable approach. A centralized configuration service can be complex and less flexible for individual modules.

The @ConditionalOnClass annotation, when used in the context of Auto Configuration, defines a required class for Auto Configuration to be enabled. If the specified class is present on the classpath, the associated configuration will be applied. This allows developers to conditionally configure components based on the availability of certain classes. It does not ignore, disable, or indicate conditional bean creation.

In Spring Boot, the value attribute of the @ExceptionHandler annotation is used to specify the types of exceptions that a particular method should handle. By specifying the exception types, you can ensure that the method is invoked only when those specific exceptions are thrown, allowing for more fine-grained exception handling in your application.

In Mockito, the when method is used to stub a return value for a method call. This allows you to define the behavior of a mocked object when a specific method is called during testing.