Common GemFire Converters

Spring Converters are part of the Spring Framework's type conversion system, which allows you to convert between different types in a consistent and flexible manner. They are especially useful in web applications where you need to convert request parameters into Java objects and vice versa.

Why would I need this for GemFire?

In various spring projects I have found the converters can hamper application startup as Spring iterates through the GemFire Region and inspecting the contents. This could be extremely costly and a tremendous impact to application start up as GemFire Regions could hold terabytes of data or more.

The last time I have used these converters spring XML was my favorite. So I have updated and thrown the @Component annotation on it so in theory adding a @ComponentScan(basePackages = "dev.gemfire.spring.converters") to your spring application should do the job.

In the future I will add a demo application highlighting these converters in action.

You can learn more about spring converter over at the spring documentation: https://docs.spring.io/spring-framework/reference/core/validation/convert.html

Just for fun I have included a ChatGPT generated reply to "What are spring converters?"

Key Concepts of Spring Converters (ChatGPT generated)

1. Converter Interface: The core of the Spring conversion system is the Converter<S, T> interface, where S is the source type and T is the target type. Implementing this interface allows you to define custom conversion logic.

   public interface Converter<S, T> {
       T convert(S source);
   }

2. GenericConverter Interface: For more complex conversion scenarios, Spring provides the GenericConverter interface, which allows you to specify multiple source and target types.

   public interface GenericConverter {
       Set<ConvertiblePair> getConvertibleTypes();
       Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
   }

3. ConversionService: The ConversionService interface is a central registry where you can register your custom converters. It provides methods to convert between types and to check if a conversion is possible.

   public interface ConversionService {
       boolean canConvert(Class<?> sourceType, Class<?> targetType);
       <T> T convert(Object source, Class<T> targetType);
   }

4. Default ConversionService: Spring provides a default implementation called DefaultConversionService, which comes with many built-in converters for common types (e.g., String to Integer).

   ConversionService conversionService = new DefaultConversionService();

Creating and Using Custom Converters

Step 1: Implement a Converter

Here's an example of a custom converter that converts a String to a custom User object:

public class StringToUserConverter implements Converter<String, User> {
    @Override
    public User convert(String source) {
        String[] data = source.split(",");
        return new User(data[0], data[1]); // Assuming User has a constructor User(String id, String name)
    }
}

Step 2: Register the Converter with ConversionService

You can register your custom converter with the ConversionService:

ConversionService conversionService = new DefaultConversionService();
((DefaultConversionService) conversionService).addConverter(new StringToUserConverter());

String userString = "123,John Doe";
User user = conversionService.convert(userString, User.class);

Step 3: Use Converters in Spring Applications

Spring MVC can automatically use the registered converters to convert request parameters to method arguments. You can configure the ConversionService in a Spring application context:

@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(new StringToUserConverter());
    }
}

With this configuration, Spring MVC will use the StringToUserConverter to convert String parameters in HTTP requests to User objects in controller method parameters.

@RestController
public class UserController {

    @GetMapping("/user")
    public User getUser(@RequestParam("user") User user) {
        return user;
    }
}

Summary

• Spring Converters are used to convert between different types in a Spring application.
• Custom Converters can be created by implementing the Converter or GenericConverter interface.
• ConversionService is used to register and manage converters.
• Spring MVC Integration allows automatic conversion of request parameters to method arguments using registered converters.