Introduce Converter in junit-platform-commons

junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/DefaultArgumentsAccessor.java

@@ -21,6 +21,7 @@
 import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.params.converter.DefaultArgumentConverter;
+import org.junit.platform.commons.support.conversion.TypeDescriptor;
 import org.junit.platform.commons.util.ClassUtils;
 import org.junit.platform.commons.util.Preconditions;
 
@@ -47,7 +48,7 @@ public static DefaultArgumentsAccessor create(ExtensionContext context, int invo
 
 		BiFunction<@Nullable Object, Class<?>, @Nullable Object> converter = (source,
 				targetType) -> new DefaultArgumentConverter(context) //
-						.convert(source, targetType, classLoader);
+						.convert(source, TypeDescriptor.forClass(targetType), classLoader);
 		return new DefaultArgumentsAccessor(converter, invocationIndex, arguments);
 	}
 

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/DefaultArgumentConverter.java

@@ -30,6 +30,7 @@
 import org.junit.jupiter.params.support.FieldContext;
 import org.junit.platform.commons.support.conversion.ConversionException;
 import org.junit.platform.commons.support.conversion.ConversionSupport;
+import org.junit.platform.commons.support.conversion.TypeDescriptor;
 import org.junit.platform.commons.util.ReflectionUtils;
 
 /**
@@ -43,7 +44,7 @@
  * {@link File}, {@link BigDecimal}, {@link BigInteger}, {@link Currency},
  * {@link Locale}, {@link URI}, {@link URL}, {@link UUID}, etc.
  *
- * <p>If the source and target types are identical the source object will not
+ * <p>If the source and target types are identical, the source object will not
  * be modified.
  *
  * @since 5.0
@@ -82,48 +83,42 @@ public DefaultArgumentConverter(ExtensionContext context) {
 
 	@Override
 	public final @Nullable Object convert(@Nullable Object source, ParameterContext context) {
-		Class<?> targetType = context.getParameter().getType();
 		ClassLoader classLoader = getClassLoader(context.getDeclaringExecutable().getDeclaringClass());
-		return convert(source, targetType, classLoader);
+		return convert(source, TypeDescriptor.forParameter(context.getParameter()), classLoader);
 	}
 
 	@Override
 	public final @Nullable Object convert(@Nullable Object source, FieldContext context)
 			throws ArgumentConversionException {
-
-		Class<?> targetType = context.getField().getType();
 		ClassLoader classLoader = getClassLoader(context.getField().getDeclaringClass());
-		return convert(source, targetType, classLoader);
+		return convert(source, TypeDescriptor.forField(context.getField()), classLoader);
 	}
 
-	public final @Nullable Object convert(@Nullable Object source, Class<?> targetType, ClassLoader classLoader) {
+	public final @Nullable Object convert(@Nullable Object source, TypeDescriptor targetType, ClassLoader classLoader) {
 		if (source == null) {
 			if (targetType.isPrimitive()) {
 				throw new ArgumentConversionException(
-					"Cannot convert null to primitive value of type " + targetType.getTypeName());
+					"Cannot convert null to primitive value of type " + targetType.getType().getTypeName());
 			}
 			return null;
 		}
 
-		if (ReflectionUtils.isAssignableTo(source, targetType)) {
+		if (ReflectionUtils.isAssignableTo(source, targetType.getType())) {
 			return source;
 		}
 
-		if (source instanceof String string) {
-			if (targetType == Locale.class && getLocaleConversionFormat() == LocaleConversionFormat.BCP_47) {
-				return Locale.forLanguageTag(string);
-			}
-
-			try {
-				return convert(string, targetType, classLoader);
-			}
-			catch (ConversionException ex) {
-				throw new ArgumentConversionException(ex.getMessage(), ex);
-			}
+		if (source instanceof String //
+				&& targetType.getType() == Locale.class //
+				&& getLocaleConversionFormat() == LocaleConversionFormat.BCP_47) {
+			return Locale.forLanguageTag((String) source);
 		}
 
-		throw new ArgumentConversionException("No built-in converter for source type %s and target type %s".formatted(
-			source.getClass().getTypeName(), targetType.getTypeName()));
+		try {
+			return delegateConversion(source, targetType, classLoader);
+		}
+		catch (ConversionException ex) {
+			throw new ArgumentConversionException(ex.getMessage(), ex);
+		}
 	}
 
 	private LocaleConversionFormat getLocaleConversionFormat() {
@@ -132,7 +127,7 @@ private LocaleConversionFormat getLocaleConversionFormat() {
 	}
 
 	@Nullable
-	Object convert(@Nullable String source, Class<?> targetType, ClassLoader classLoader) {
+	Object delegateConversion(@Nullable Object source, TypeDescriptor targetType, ClassLoader classLoader) {
 		return ConversionSupport.convert(source, targetType, classLoader);
 	}
 

junit-platform-commons/src/main/java/module-info.java

@@ -58,5 +58,6 @@
 			org.junit.platform.suite.engine,
 			org.junit.platform.testkit,
 			org.junit.vintage.engine;
+	uses org.junit.platform.commons.support.conversion.Converter;
 	uses org.junit.platform.commons.support.scanning.ClasspathScanner;
 }

junit-platform-commons/src/main/java/org/junit/platform/commons/support/conversion/ConversionContext.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.platform.commons.support.conversion;
+
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
+
+import org.apiguardian.api.API;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * {@code ConversionContext} encapsulates the <em>context</em> in which the
+ * current conversion is being executed.
+ *
+ * <p>{@link Converter Converters} are provided an instance of
+ * {@code ConversionContext} to perform their work.
+ *
+ * @param sourceType
+ * @param targetType
+ * @param classLoader
+ *
+ * @since 6.0
+ * @see Converter
+ */
+@API(status = EXPERIMENTAL, since = "6.0")
+public record ConversionContext(TypeDescriptor sourceType, TypeDescriptor targetType, ClassLoader classLoader) {
+
+	/**
+	 *
+	 * @param source
+	 * @param targetType
+	 * @param classLoader
+	 */
+	public ConversionContext(@Nullable Object source, TypeDescriptor targetType, ClassLoader classLoader) {
+		this(TypeDescriptor.forInstance(source), targetType, classLoader);
+	}
+
+}

junit-platform-commons/src/main/java/org/junit/platform/commons/support/conversion/ConversionSupport.java

@@ -10,15 +10,15 @@
 
 package org.junit.platform.commons.support.conversion;
 
+import static org.apiguardian.api.API.Status.DEPRECATED;
 import static org.apiguardian.api.API.Status.EXPERIMENTAL;
-import static org.junit.platform.commons.util.ReflectionUtils.getWrapperType;
 
-import java.util.List;
-import java.util.Optional;
+import java.util.ServiceLoader;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
 
 import org.apiguardian.api.API;
 import org.jspecify.annotations.Nullable;
-import org.junit.platform.commons.util.ClassLoaderUtils;
 
 /**
  * {@code ConversionSupport} provides static utility methods for converting a
@@ -29,17 +29,6 @@
 @API(status = EXPERIMENTAL, since = "1.11")
 public final class ConversionSupport {
 
-	private static final List<StringToObjectConverter> stringToObjectConverters = List.of( //
-		new StringToBooleanConverter(), //
-		new StringToCharacterConverter(), //
-		new StringToNumberConverter(), //
-		new StringToClassConverter(), //
-		new StringToEnumConverter(), //
-		new StringToJavaTimeConverter(), //
-		new StringToCommonJavaTypesConverter(), //
-		new FallbackStringToObjectConverter() //
-	);
-
 	private ConversionSupport() {
 		/* no-op */
 	}
@@ -48,43 +37,6 @@ private ConversionSupport() {
 	 * Convert the supplied source {@code String} into an instance of the specified
 	 * target type.
 	 *
-	 * <p>If the target type is {@code String}, the source {@code String} will not
-	 * be modified.
-	 *
-	 * <p>Some forms of conversion require a {@link ClassLoader}. If none is
-	 * provided, the {@linkplain ClassLoaderUtils#getDefaultClassLoader() default
-	 * ClassLoader} will be used.
-	 *
-	 * <p>This method is able to convert strings into primitive types and their
-	 * corresponding wrapper types ({@link Boolean}, {@link Character}, {@link Byte},
-	 * {@link Short}, {@link Integer}, {@link Long}, {@link Float}, and
-	 * {@link Double}), enum constants, date and time types from the
-	 * {@code java.time} package, as well as common Java types such as {@link Class},
-	 * {@link java.io.File}, {@link java.nio.file.Path}, {@link java.nio.charset.Charset},
-	 * {@link java.math.BigDecimal}, {@link java.math.BigInteger},
-	 * {@link java.util.Currency}, {@link java.util.Locale}, {@link java.util.UUID},
-	 * {@link java.net.URI}, and {@link java.net.URL}.
-	 *
-	 * <p>If the target type is not covered by any of the above, a convention-based
-	 * conversion strategy will be used to convert the source {@code String} into the
-	 * given target type by invoking a static factory method or factory constructor
-	 * defined in the target type. The search algorithm used in this strategy is
-	 * outlined below.
-	 *
-	 * <h4>Search Algorithm</h4>
-	 *
-	 * <ol>
-	 * <li>Search for a single, non-private static factory method in the target
-	 * type that converts from a String to the target type. Use the factory method
-	 * if present.</li>
-	 * <li>Search for a single, non-private constructor in the target type that
-	 * accepts a String. Use the constructor if present.</li>
-	 * </ol>
-	 *
-	 * <p>If multiple suitable factory methods are discovered they will be ignored.
-	 * If neither a single factory method nor a single constructor is found, the
-	 * convention-based conversion strategy will not apply.
-	 *
 	 * @param source the source {@code String} to convert; may be {@code null}
 	 * but only if the target type is a reference type
 	 * @param targetType the target type the source should be converted into;
@@ -96,49 +48,49 @@ private ConversionSupport() {
 	 * type is a reference type
 	 *
 	 * @since 1.11
+	 * @see DefaultConverter
+	 * @deprecated Use {@link #convert(Object, TypeDescriptor, ClassLoader)} instead.
 	 */
-	@SuppressWarnings("unchecked")
+	@Deprecated
+	@API(status = DEPRECATED, since = "6.0")
 	public static <T> @Nullable T convert(@Nullable String source, Class<T> targetType,
 			@Nullable ClassLoader classLoader) {
-		if (source == null) {
-			if (targetType.isPrimitive()) {
-				throw new ConversionException(
-					"Cannot convert null to primitive value of type " + targetType.getTypeName());
-			}
-			return null;
-		}
-
-		if (String.class.equals(targetType)) {
-			return (T) source;
-		}
+		return convert(source, TypeDescriptor.forClass(targetType), classLoader);
+	}
 
-		Class<?> targetTypeToUse = toWrapperType(targetType);
-		Optional<StringToObjectConverter> converter = stringToObjectConverters.stream().filter(
-			candidate -> candidate.canConvertTo(targetTypeToUse)).findFirst();
-		if (converter.isPresent()) {
-			try {
-				ClassLoader classLoaderToUse = classLoader != null ? classLoader
-						: ClassLoaderUtils.getDefaultClassLoader();
-				return (T) converter.get().convert(source, targetTypeToUse, classLoaderToUse);
-			}
-			catch (Exception ex) {
-				if (ex instanceof ConversionException conversionException) {
-					// simply rethrow it
-					throw conversionException;
-				}
-				// else
-				throw new ConversionException(
-					"Failed to convert String \"%s\" to type %s".formatted(source, targetType.getTypeName()), ex);
-			}
-		}
+	/**
+	 * Convert the supplied source object into an instance of the specified
+	 * target type.
+	 *
+	 * @param source the source object to convert; may be {@code null}
+	 * but only if the target type is a reference type
+	 * @param targetType the target type the source should be converted into;
+	 * never {@code null}
+	 * @param classLoader the {@code ClassLoader} to use; may be {@code null} to
+	 * use the default {@code ClassLoader}
+	 * @param <T> the type of the target
+	 * @return the converted object; may be {@code null} but only if the target
+	 * type is a reference type
+	 *
+	 * @since 6.0
+	 */
+	@API(status = EXPERIMENTAL, since = "6.0")
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	public static <T> @Nullable T convert(@Nullable Object source, TypeDescriptor targetType,
+			@Nullable ClassLoader classLoader) {
+		ConversionContext context = new ConversionContext(source, targetType, classLoader);
+		ServiceLoader<Converter> serviceLoader = ServiceLoader.load(Converter.class, context.classLoader());
 
-		throw new ConversionException(
-			"No built-in converter for source type java.lang.String and target type " + targetType.getTypeName());
-	}
+		Converter converter = Stream.concat( //
+			StreamSupport.stream(serviceLoader.spliterator(), false), //
+			Stream.of(DefaultConverter.INSTANCE)) //
+				.filter(candidate -> candidate.canConvert(context)) //
+				.findFirst() //
+				.orElseThrow(() -> new ConversionException(
+					"No registered or built-in converter for source '%s' and target type %s".formatted( //
+						source, targetType.getTypeName())));
 
-	private static Class<?> toWrapperType(Class<?> targetType) {
-		Class<?> wrapperType = getWrapperType(targetType);
-		return wrapperType != null ? wrapperType : targetType;
+		return (T) converter.convert(source, context);
 	}
 
 }

junit-platform-commons/src/main/java/org/junit/platform/commons/support/conversion/Converter.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.platform.commons.support.conversion;
+
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
+
+import org.apiguardian.api.API;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * {@code Converter} is an abstraction that allows an input object to
+ * be converted to an instance of a different class.
+ *
+ * <p>Implementations are loaded via the {@link java.util.ServiceLoader} and must
+ * follow the service provider requirements. They should not make any assumptions
+ * regarding when they are instantiated or how often they are called. Since
+ * instances may potentially be cached and called from different threads, they
+ * should be thread-safe.
+ *
+ * <p>Extend {@link TypedConverter} if your implementation always converts
+ * from a given source type into a given target type and does not need access to
+ * the {@link ClassLoader} to perform the conversion.
+ *
+ * @param <S>
+ * @param <T>
+ *
+ * @since 6.0
+ * @see ConversionSupport
+ * @see TypedConverter
+ */
+@API(status = EXPERIMENTAL, since = "6.0")
+public interface Converter<S, T> {
+
+	/**
+	 * Determine if the supplied conversion context is supported.
+	 *
+	 * @param context the context for the conversion; never {@code null}
+	 * @return {@code true} if the conversion is supported
+	 */
+	boolean canConvert(ConversionContext context);
+
+	/**
+	 * Convert the supplied source object according to the supplied conversion context.
+	 * <p>This method will only be invoked if {@link #canConvert(ConversionContext)}
+	 * returned {@code true} for the same context.
+	 *
+	 * @param source the source object to convert; may be {@code null}
+	 * but only if the target type is a reference type
+	 * @param context the context for the conversion; never {@code null}
+	 * @return the converted object; may be {@code null} but only if the target
+	 * type is a reference type
+	 * @throws ConversionException if an error occurs during the conversion
+	 */
+	@Nullable
+	T convert(@Nullable S source, ConversionContext context) throws ConversionException;
+
+}