Add CharSequenceToObjectConverter support

Update `ApplicationConversionService` with support for converting
`CharSequence` source types by using existing `String` based converters.

The addition is primarily to allow `ConfigTreePropertySource` values
to be converted correctly.

Closes gh-24171

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/ApplicationConversionService.java

@@ -122,6 +122,13 @@ public class ApplicationConversionService extends FormattingConversionService {
 		registry.addConverter(new InputStreamSourceToByteArrayConverter());
 		registry.addConverterFactory(new LenientStringToEnumConverterFactory());
 		registry.addConverterFactory(new LenientBooleanToEnumConverterFactory());
+		if (registry instanceof ConversionService) {
+			addApplicationConverters(registry, (ConversionService) registry);
+		}
+	}
+
+	private static void addApplicationConverters(ConverterRegistry registry, ConversionService conversionService) {
+		registry.addConverter(new CharSequenceToObjectConverter(conversionService));
 	}
 
 	/**

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/CharSequenceToObjectConverter.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.convert;
+
+import java.util.Collections;
+import java.util.Set;
+
+import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.ConditionalGenericConverter;
+
+/**
+ * {@link ConditionalGenericConverter} to convert {@link CharSequence} type by delegating
+ * to existing {@link String} converters.
+ *
+ * @author Phillip Webb
+ */
+class CharSequenceToObjectConverter implements ConditionalGenericConverter {
+
+	private static final TypeDescriptor STRING = TypeDescriptor.valueOf(String.class);
+
+	private static final Set<ConvertiblePair> TYPES;
+
+	private final ThreadLocal<Boolean> disable = new ThreadLocal<>();
+
+	static {
+		TYPES = Collections.singleton(new ConvertiblePair(CharSequence.class, Object.class));
+	}
+
+	private final ConversionService conversionService;
+
+	CharSequenceToObjectConverter(ConversionService conversionService) {
+		this.conversionService = conversionService;
+	}
+
+	@Override
+	public Set<ConvertiblePair> getConvertibleTypes() {
+		return TYPES;
+	}
+
+	@Override
+	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
+		if (sourceType.getType() == String.class || this.disable.get() == Boolean.TRUE) {
+			return false;
+		}
+		this.disable.set(Boolean.TRUE);
+		try {
+			return !this.conversionService.canConvert(sourceType, targetType)
+					&& this.conversionService.canConvert(STRING, targetType);
+		}
+		finally {
+			this.disable.set(null);
+		}
+	}
+
+	@Override
+	public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+		return this.conversionService.convert(source.toString(), STRING, targetType);
+	}
+
+}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/convert/CharSequenceToObjectConverterTests.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.convert;
+
+import java.util.stream.Stream;
+
+import org.junit.jupiter.params.provider.Arguments;
+
+import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.converter.Converter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link CharSequenceToObjectConverter}
+ *
+ * @author Phillip Webb
+ */
+class CharSequenceToObjectConverterTests {
+
+	@ConversionServiceTest
+	void convertWhenCanConvertViaToString(ConversionService conversionService) {
+		assertThat(conversionService.convert(new StringBuilder("1"), Integer.class)).isEqualTo(1);
+	}
+
+	@ConversionServiceTest
+	void convertWhenCanConvertDirectlySkipsStringConversion(ConversionService conversionService) {
+		assertThat(conversionService.convert(new String("1"), Long.class)).isEqualTo(1);
+		System.out.println(conversionService.getClass());
+		if (!ConversionServiceArguments.isApplicationConversionService(conversionService)) {
+			assertThat(conversionService.convert(new StringBuilder("1"), Long.class)).isEqualTo(2);
+		}
+	}
+
+	static Stream<? extends Arguments> conversionServices() {
+		return ConversionServiceArguments.with((conversionService) -> {
+			conversionService.addConverter(new StringToIntegerConverter());
+			conversionService.addConverter(new StringToLongConverter());
+			conversionService.addConverter(new CharSequenceToLongConverter());
+			conversionService.addConverter(new CharSequenceToObjectConverter(conversionService));
+		});
+	}
+
+	static class StringToIntegerConverter implements Converter<String, Integer> {
+
+		@Override
+		public Integer convert(String source) {
+			return Integer.valueOf(source);
+		}
+
+	}
+
+	static class StringToLongConverter implements Converter<String, Long> {
+
+		@Override
+		public Long convert(String source) {
+			return Long.valueOf(source);
+		}
+
+	}
+
+	static class CharSequenceToLongConverter implements Converter<CharSequence, Long> {
+
+		@Override
+		public Long convert(CharSequence source) {
+			return Long.valueOf(source.toString()) + 1;
+		}
+
+	}
+
+}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/convert/ConversionServiceArguments.java

@@ -57,6 +57,13 @@ final class ConversionServiceArguments {
 						"Application conversion service")));
 	}
 
+	static boolean isApplicationConversionService(ConversionService conversionService) {
+		if (conversionService instanceof NamedConversionService) {
+			return isApplicationConversionService(((NamedConversionService) conversionService).delegate);
+		}
+		return conversionService instanceof ApplicationConversionService;
+	}
+
 	static class NamedConversionService implements ConversionService {
 
 		private final ConversionService delegate;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/env/ConfigTreePropertySourceTests.java

@@ -79,7 +79,7 @@ class ConfigTreePropertySourceTests {
 	@Test
 	void getPropertyNamesFromFlatReturnsPropertyNames() throws Exception {
 		ConfigTreePropertySource propertySource = getFlatPropertySource();
-		assertThat(propertySource.getPropertyNames()).containsExactly("a", "b", "c");
+		assertThat(propertySource.getPropertyNames()).containsExactly("a", "b", "c", "one");
 	}
 
 	@Test
@@ -154,6 +154,7 @@ class ConfigTreePropertySourceTests {
 		assertThat(environment.getProperty("b")).isEqualTo("B");
 		assertThat(environment.getProperty("c", InputStreamSource.class).getInputStream()).hasContent("C");
 		assertThat(environment.getProperty("c", byte[].class)).contains('C');
+		assertThat(environment.getProperty("one", Integer.class)).isEqualTo(1);
 	}
 
 	@Test
@@ -227,6 +228,7 @@ class ConfigTreePropertySourceTests {
 		addProperty("a", "A");
 		addProperty("b", "B");
 		addProperty("c", "C");
+		addProperty("one", "1");
 		return new ConfigTreePropertySource("test", this.directory);
 	}