Spring Type Conversion & Formatter

1. Why Type Conversion Matters in Spring

Spring often needs to convert data types between String ↔ Object, especially in:

• @RequestParam, @PathVariable, @ModelAttribute
• YAML (@Value)
• View rendering (Thymeleaf)
• Form input/output

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2. Converter Interface

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

• Generic, bidirectional type conversion (e.g. String ↔ Integer, IpPort ↔ String)
• Used programmatically or injected into Spring's type system

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3. ConversionService

conversionService.convert("10", Integer.class); // returns 10

• Central API for runtime type conversion
• Main implementation: DefaultConversionService

✅ ISP (Interface Segregation Principle)

Clients should not depend on methods they do not use.

• DefaultConversionService implements both:
  • ConversionService: For type conversion
  • ConverterRegistry: For registering converters

This design separates usage from registration, following ISP.

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4. Registering Converters via WebMvcConfigurer

@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(new StringToIpPortConverter());
        registry.addConverter(new IpPortToStringConverter());
    }
}

These converters are then automatically applied in @RequestParam, @PathVariable, etc.

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5. Formatter Interface

public interface Formatter<T> extends Printer<T>, Parser<T> {
    String print(T object, Locale locale);
    T parse(String text, Locale locale);
}

• Specialization of Converter for String ↔ Object with Locale support
• Used for formatting numbers, dates, etc.
• Example: "1,000" ↔ 1000

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6. Registering Formatters: FormattingConversionService

⚠️ Note: ConversionService cannot accept formatters.

✅ Use FormattingConversionService (or its subclass DefaultFormattingConversionService) to register formatters.

conversionService.addFormatter(new MyNumberFormatter());

• Internally, it adapts Formatter to Converter using the Adapter Pattern.
• This allows uniform access to all converters/formatters through the ConversionService interface.

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7. Built-in Formatter Annotations

Annotation Description

@NumberFormat	Controls number formatting (e.g., #,###)
@DateTimeFormat	Controls date/time formatting

Example:

@NumberFormat(pattern = "###,###")
private Integer number;

@DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")
private LocalDateTime localDateTime;

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8. Thymeleaf & Conversion

• ${object} → plain output
• ${{object}} → applies ConversionService for formatting

Works for both converters and formatters.

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9. ❗ Not Applied: HttpMessageConverter

Spring's ConversionService is not used in:

• JSON (de)serialization via HttpMessageConverter (uses Jackson/Gson)
• To customize JSON formats (e.g., for numbers or dates), configure the JSON library directly.

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✅ Final Takeaway

Concept	Use Case
Converter	General-purpose conversion between any two types
Formatter	Locale-aware String ↔ Object formatting
ConversionService	Unified API to perform conversions
FormattingConversionService	Registers both converters and formatters (uses adapter pattern)
WebMvcConfigurer	Hook to register your converters/formatters in Spring MVC
@NumberFormat / @DateTimeFormat	Field-level formatting in models
Not Used In	JSON serialization (handled by Jackson, not ConversionService)