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Custom Conversions
The following example of a Spring Converter
implementation converts from a String
to a custom Email
value object:
@ReadingConverter
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
andString
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.
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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> { … }