Custom Conversions

The following example of a Spring Converter implementation converts from a String to a custom Email value object:

public class EmailReadConverter implements Converter<String, Email> {

  public Email convert(String source) {
    return Email.valueOf(source);

If you write a Converter whose source and target type are native types, we cannot determine whether we should consider it as a reading or a writing converter. Registering the converter instance as both might lead to unwanted results. For example, a Converter<String, Long> is ambiguous, although it probably does not make sense to try to convert all String instances into Long instances when writing. To let you force the infrastructure to register a converter for only one way, we provide @ReadingConverter and @WritingConverter annotations to be used in the converter implementation.

Converters are subject to explicit registration as instances are not picked up from a classpath or container scan to avoid unwanted registration with a conversion service and the side effects resulting from such a registration. Converters are registered with CustomConversions as the central facility that allows registration and querying for registered converters based on source- and target type.

CustomConversions ships with a pre-defined set of converter registrations:

  • JSR-310 Converters for conversion between java.time, java.util.Date and String types.

Default converters for local temporal types (e.g. LocalDateTime to java.util.Date) rely on system-default timezone settings to convert between those types. You can override the default converter, by registering your own converter.

Converter Disambiguation

Generally, we inspect the Converter implementations for the source and target types they convert from and to. Depending on whether one of those is a type the underlying data access API can handle natively, we register the converter instance as a reading or a writing converter. The following examples show a writing- and a read converter (note the difference is in the order of the qualifiers on Converter):

// Write converter as only the target type is one that can be handled natively
class MyConverter implements Converter<Person, String> { … }

// Read converter as only the source type is one that can be handled natively
class MyConverter implements Converter<String, Person> { … }