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Spring Type Conversion

Post Date: 01 Oct 2023


Spring provides a robust type conversion system through its core.convert package, offering a versatile mechanism for converting data types within your applications. This system leverages an SPI (Service Provider Interface) for implementing type conversion logic and a user-friendly API for executing these conversions during runtime. This article will explore the Spring type conversion system in-depth, covering the SPI, various converter implementations, and best practices for utilizing this feature effectively.

Converter SPI

At the heart of Spring’s type conversion system is the Converter SPI (Service Provider Interface), which is both simple and strongly typed. The Converter<S, T> interface defines a single method:

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

To create your custom converter, you need to implement this interface, specifying the source (S) and target (T) types you intend to convert between. When implementing a converter, ensure that the convert method can handle any non-null source value and throw an IllegalArgumentException if the conversion fails. Thread safety should also be a consideration for your implementation.

Built-in Converters

Spring offers a set of pre-implemented converters in the core.convert.support package to handle common conversion scenarios. For example, consider the StringToInteger converter:

final class StringToInteger implements Converter<String, Integer> {
    public Integer convert(String source) {
        return Integer.valueOf(source);

These built-in converters cover conversions between strings and numbers and other frequently used types, saving you the effort of writing converters for common cases.

Using ConverterFactory

When you need to centralize conversion logic for an entire class hierarchy or perform complex conversions, you can implement the ConverterFactory interface. Here’s how it works:

public interface ConverterFactory<S, R> {
    <T extends R> Converter<S, T> getConverter(Class<T> targetType);

In this interface, S represents the source type, and R is the base type for the target classes. You implement the getConverter method, which returns a Converter<S, T> for the desired target type T.

As an example, consider the StringToEnumConverterFactory, which converts strings to enum values:

final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {
    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new StringToEnumConverter(targetType);

    private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {
        private Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;

        public T convert(String source) {
            return (T) Enum.valueOf(this.enumType, source.trim());

This converter factory allows you to handle conversions from strings to enum types effectively.

Using GenericConverter

For even more sophisticated conversion scenarios, Spring offers the GenericConverter interface. This interface is less strongly typed than Converter and supports converting between multiple source and target types. Additionally, it provides context for conversion, enabling you to make decisions based on field annotations or generic information in the field signature.

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

To implement a GenericConverter, you specify the supported source→target type pairs using the getConvertibleTypes method and provide your conversion logic in the convert method. The TypeDescriptor objects give you access to source and target fields, allowing you to make context-aware conversion decisions.

Conditional Conversion

In some cases, you may want a converter to run only when specific conditions are met. Spring provides the ConditionalGenericConverter interface, which combines GenericConverter and ConditionalConverter. This allows you to define custom matching criteria for when a converter should be applied:

public interface ConditionalConverter {
    boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);

public interface ConditionalGenericConverter extends GenericConverter, ConditionalConverter {

A typical use case for conditional conversion is running a converter only when a particular annotation is present on the target field or when a specific method is defined in the target class.

ConversionService API

Spring’s ConversionService defines a unified API for executing type conversions at runtime. It provides methods for checking if a conversion is possible and for performing the conversion itself. The ConversionService can be used behind a facade interface for convenient type conversion:

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

Most ConversionService implementations also implement ConverterRegistry, which allows you to register custom converters. These converters are then used by the ConversionService to carry out type conversion operations.

Configuring a ConversionService

To use a ConversionService in your Spring application, you typically configure it during application startup and share it across multiple threads. You can register a default ConversionService with Spring using the following bean definition:

<bean id="conversionService" class="org.springframework.context.support.ConversionServiceFactoryBean"/>

A default ConversionService can handle conversions between strings, numbers, enums, collections, maps, and other common types. To add your custom converters or override default ones, set the converters property in the bean definition.

<bean id="conversionService" class="org.springframework.context.support.ConversionServiceFactoryBean">
    <property name="converters">
            <bean class="example.MyCustomConverter"/>

This configuration allows you to extend the conversion capabilities to suit your application’s specific needs.

Using a ConversionService Programmatically

To utilize a ConversionService programmatically, you can inject a reference to it into your beans and invoke it directly. Here’s an example:

public class MyService {
    private final ConversionService conversionService;

    public MyService(ConversionService conversionService) {
        this.conversionService = conversionService;

    public void doIt() {
        // Perform type conversion using the conversionService
        // ...

You can use the convert method, specifying the target type, for most use cases. However, for more complex type conversions, such as converting a list of integers to a list of strings, you may need to use TypeDescriptor to specify the source and target types explicitly.


Spring’s type conversion system is a powerful and flexible feature that simplifies data type conversions within your applications. By understanding the Converter SPI, built-in converters, ConverterFactory, GenericConverter, and conditional conversion, you can effectively handle a wide range of conversion scenarios. Additionally, configuring a ConversionService and using it programmatically allows you to seamlessly integrate type conversions into your Spring applications. Whether you’re working with strings, numbers, enums, collections, or custom types, Spring’s type conversion system provides the tools you need to manage data types efficiently.

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