Spring Field Formatting
Overview
Spring Field Formatting is a pivotal component of the Spring Framework, allowing seamless data conversion and rendering across various contexts, particularly in client environments. This guide provides an in-depth look into the mechanics, interfaces, and practical implementations of Spring Field Formatting, elucidating its significance in modern web and desktop applications.
Overview of the ConversionService API
The Spring Framework’s core.convert is a robust type conversion system that provides the ConversionService API. This system aids in binding bean property values, and it’s crucial for both the Spring Expression Language (SpEL) and DataBinder when binding field values.
For instance, in environments like web or desktop applications, typical type conversion requirements involve converting between Strings and other data types for various processes such as client postbacks and view rendering. Furthermore, these strings often necessitate localization.
While the core.convert Converter SPI is versatile, it doesn’t directly cater to the formatting requirements of these environments. Enter the Formatter SPI, which Spring offers to specifically address these formatting challenges.
The Formatter SPI offers a structured way to implement field formatting logic. It’s an interface derived from two foundational interfaces: Printer and Parser.
package org.springframework.format;
public interface Formatter<T> extends Printer<T>, Parser<T> {}
Printer
Printer<T> is responsible for printing the field value, based on the locale.
public interface Printer<T> {
String print(T fieldValue, Locale locale);
}
Parser
Parser<T>, on the other hand, parses a client’s value given the locale.
import java.text.ParseException;
public interface Parser<T> {
T parse(String clientValue, Locale locale) throws ParseException;
}
When implementing your own Formatter, it’s paramount to ensure thread-safety. It’s also worth noting that the format subpackages come with numerous pre-implemented Formatter classes for convenience.
Spring allows field formatting to be configured either by field type or through annotations. When you need to bind an annotation to a Formatter, the AnnotationFormatterFactory interface comes into play.
package org.springframework.format;
public interface AnnotationFormatterFactory<A extends Annotation> {
Set<Class<?>> getFieldTypes();
Printer<?> getPrinter(A annotation, Class<?> fieldType);
Parser<?> getParser(A annotation, Class<?> fieldType);
}
This interface provides a robust way to tie annotations with formatters, thereby enhancing customization and flexibility in field formatting. For instance, the @NumberFormat annotation can be bound to a specific formatter to define a number’s style or pattern.
Annotation-driven Formatting is a mechanism in Spring that allows developers to specify how a particular field should be formatted or parsed by simply annotating it. This eliminates the need for manual formatting and parsing in the application code. Here are some examples that demonstrate this concept:
Suppose you have a model class with a Date field, and you want to ensure that this field is always formatted as “yyyy-MM-dd” when rendered in a view:
import org.springframework.format.annotation.DateTimeFormat;
import java.util.Date;
public class Event {
@DateTimeFormat(pattern = "yyyy-MM-dd")
private Date eventDate;
// getters and setters
}
With the above annotation, the eventDate will automatically be formatted as “yyyy-MM-dd” when presented in views.
Suppose you have a field representing a monetary amount and you want to ensure it’s formatted as currency:
import org.springframework.format.annotation.NumberFormat;
import org.springframework.format.annotation.NumberFormat.Style;
public class Product {
@NumberFormat(style = Style.CURRENCY)
private BigDecimal price;
// getters and setters
}
The price will automatically be formatted in the currency style, such as “$100.00”.
Let’s imagine you have a custom annotation that requires a specific formatter. You would first create the annotation:
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface CustomFormat {
String value() default "";
}
Then, you would create a custom formatter and an AnnotationFormatterFactory for this annotation:
import org.springframework.format.AnnotationFormatterFactory;
import org.springframework.format.Parser;
import org.springframework.format.Printer;
public class CustomFormatAnnotationFormatterFactory implements AnnotationFormatterFactory<CustomFormat> {
@Override
public Set<Class<?>> getFieldTypes() {
return Collections.singleton(String.class);
}
@Override
public Printer<?> getPrinter(CustomFormat annotation, Class<?> fieldType) {
return new CustomPrinter(annotation.value());
}
@Override
public Parser<?> getParser(CustomFormat annotation, Class<?> fieldType) {
return new CustomParser(annotation.value());
}
}
In your model, you can then use this annotation:
public class UserDetails {
@CustomFormat("custom-pattern")
private String customField;
// getters and setters
}
These are just a few examples of how “Annotation-driven Formatting” can be applied in Spring applications. The main advantage is that the formatting logic is decoupled from the application logic, ensuring cleaner and more maintainable code.
The FormatterRegistry SPI (Service Provider Interface) plays a central role in registering formatters and converters.
package org.springframework.format;
public interface FormatterRegistry extends ConverterRegistry {
void addPrinter(Printer<?> printer);
void addParser(Parser<?> parser);
//... other methods
}
This registry facilitates centralized configuration of formatting rules, promoting consistency and reduced redundancy. For instance, by using a shared FormatterRegistry, date fields formatting can be standardized across the application.
The FormatterRegistry SPI in Spring provides an API to register custom formatters and converters. This allows developers to define how specific types should be converted and formatted in the Spring application.
Here’s a look at some examples:
Let’s say you have a custom formatter for converting strings to uppercase:
public class UppercaseFormatter implements Formatter<String> {
@Override
public String print(String object, Locale locale) {
return object;
}
@Override
public String parse(String text, Locale locale) throws ParseException {
return text.toUpperCase();
}
}
To register this formatter, you would use the FormatterRegistry:
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Override
public void addFormatters(FormatterRegistry registry) {
registry.addFormatter(new UppercaseFormatter());
}
}
2. Registering a Converter
Imagine you have a converter that transforms a string into a custom Profile object:
public class StringToProfileConverter implements Converter<String, Profile> {
@Override
public Profile convert(String source) {
// Split the source based on comma and create a profile object
String[] parts = source.split(",");
return new Profile(parts[0], parts[1]);
}
}
public class Profile {
private String name;
private String email;
// Constructor, getters, setters
}
Registering this converter would look like this:
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Override
public void addFormatters(FormatterRegistry registry) {
registry.addConverter(new StringToProfileConverter());
}
}
If you have multiple formatters and converters and want to register them collectively, you can use the FormatterRegistrar:
public class MyFormatterRegistrar implements FormatterRegistrar {
@Override
public void registerFormatters(FormatterRegistry registry) {
registry.addFormatter(new UppercaseFormatter());
registry.addConverter(new StringToProfileConverter());
// ... other formatters and converters
}
}
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Override
public void addFormatters(FormatterRegistry registry) {
MyFormatterRegistrar registrar = new MyFormatterRegistrar();
registrar.registerFormatters(registry);
}
}
These examples demonstrate how to work with the FormatterRegistry SPI to manage formatters and converters within a Spring application. Using this approach ensures type safety and clean data transformation throughout the application.
In Conclusion
The Spring Framework’s field formatting capabilities, rooted in the ConversionService API and the Formatter SPI, empower developers to efficiently convert and format data across various contexts. This guide has illuminated the intricacies of Spring Field Formatting, underscoring its relevance in modern application development.
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