The JdbcTemplate class in Spring Boot is used for low-level JDBC operations and offers more control over SQL queries and data access compared to JPA, which is an Object-Relational Mapping (ORM) tool. JdbcTemplate is particularly useful when you need precise control over your SQL queries and want to work with plain SQL. JPA, on the other hand, allows you to work with Java objects and abstracts away the underlying database operations.

To implement password hashing in Spring Security, you should use the PasswordEncoder interface and configure it in the security configuration. This ensures that passwords are securely hashed before being stored in the database and compared during authentication. The other options do not provide a standard or recommended approach for password hashing in Spring Security.

When performing tests with @DataJpaTest, it is essential to ensure that the database is correctly configured. This annotation focuses on testing JPA components and relies on a well-configured database for these tests.

To customize the token endpoint response of an OAuth2 Authorization Server in Spring Boot, you can create a custom OAuth2TokenEndpointConfigurer and configure it in the security configuration. Modifying the source code of the OAuth2 Authorization Server is not recommended for maintainability reasons. Using a filter in the client application is not the standard approach for customizing the token endpoint response.

The @SpringBootTest annotation is primarily used to start the Spring application context, which enables you to test your Spring Boot application in an integrated way. It loads the complete application context and allows you to interact with it during testing.

Custom Auto Configurations are usually defined in a separate "package" to avoid being included by component scanning. By placing your custom Auto Configuration classes in a separate package, you can control which classes are picked up by component scanning and ensure that your custom configurations are only applied when explicitly required.

In Spring Security, the AuthenticationProvider is responsible for creating a user Authentication object from an HttpServletRequest. The AuthenticationProvider is a core component in Spring Security that takes care of authenticating users. It verifies user credentials and loads user-specific details, ultimately creating the Authentication object that represents the authenticated user.

In Spring Boot, you can use the ${env} placeholder in the application properties file to resolve property values from environment variables. This is useful for configuring your Spring Boot application dynamically based on the environment it runs in, such as setting database connection parameters or API keys from environment variables.

The @SpringBootApplication annotation is used to mark the main class of a Spring Boot application. It combines several other annotations, including @Configuration, @ComponentScan, and @EnableAutoConfiguration, making it the entry point for a Spring Boot application. The other options, such as @MainClass and @EntryPoint, are not standard annotations used for this purpose.

In a read-heavy scenario, using a caching mechanism like Redis can significantly improve performance by reducing the load on the database server. By caching frequently accessed data, you can serve read requests from the cache, reducing the database load. Database sharding is more relevant for write-heavy workloads, and increasing write capacity and implementing asynchronous processing are not specific to optimizing read-heavy workloads.