Add String to File conversion support

Add `String` -> `File` support to the `ApplicationConversionService`
that can support both simple filename as well as file URLs.

This allows Spring Boot application to work in a similar way to vanilla
Spring applications where Spring's `FileEditor` provides similar
support.

Closes gh-16931

Author: philwebb

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

@@ -115,6 +115,7 @@ public static void addApplicationConverters(ConverterRegistry registry) {
 		registry.addConverter(new DurationToNumberConverter());
 		registry.addConverter(new StringToDataSizeConverter());
 		registry.addConverter(new NumberToDataSizeConverter());
+		registry.addConverter(new StringToFileConverter());
 		registry.addConverterFactory(new LenientStringToEnumConverterFactory());
 		registry.addConverterFactory(new LenientBooleanToEnumConverterFactory());
 	}

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

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2012-2019 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.io.File;
+import java.io.IOException;
+
+import org.springframework.core.convert.converter.Converter;
+import org.springframework.core.io.DefaultResourceLoader;
+import org.springframework.core.io.Resource;
+import org.springframework.core.io.ResourceLoader;
+import org.springframework.util.ResourceUtils;
+
+/**
+ * {@link Converter} to convert from a {@link String} to a {@link File}. Supports basic
+ * file conversion as well as file URLs.
+ *
+ * @author Phillip Webb
+ */
+class StringToFileConverter implements Converter<String, File> {
+
+	private static final ResourceLoader resourceLoader = new DefaultResourceLoader();
+
+	@Override
+	public File convert(String source) {
+		if (ResourceUtils.isUrl(source)) {
+			return getFile(resourceLoader.getResource(source));
+		}
+		File file = new File(source);
+		if (file.isAbsolute()) {
+			return file;
+		}
+		Resource resource = resourceLoader.getResource(source);
+		if (resource.exists()) {
+			return getFile(resource);
+		}
+		return file;
+	}
+
+	private File getFile(Resource resource) {
+		try {
+			return resource.getFile();
+		}
+		catch (IOException ex) {
+			throw new IllegalStateException("Could not retrieve file for " + resource + ": " + ex.getMessage());
+		}
+	}
+
+}

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

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2012-2019 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.io.File;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.io.TempDir;
+import org.junit.jupiter.params.provider.Arguments;
+
+import org.springframework.core.convert.ConversionService;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link StringToFileConverter}.
+ *
+ * @author Phillip Webb
+ */
+class StringToFileConverterTests {
+
+	@TempDir
+	File temp;
+
+	@ConversionServiceTest
+	void convertWhenSimpleFileReturnsFile(ConversionService conversionService) {
+		assertThat(convert(conversionService, this.temp.getAbsolutePath() + "/test"))
+				.isEqualTo(new File(this.temp, "test").getAbsoluteFile());
+	}
+
+	@ConversionServiceTest
+	void convertWhenFilePrefixedReturnsFile(ConversionService conversionService) {
+		assertThat(convert(conversionService, "file:" + this.temp.getAbsolutePath() + "/test").getAbsoluteFile())
+				.isEqualTo(new File(this.temp, "test").getAbsoluteFile());
+	}
+
+	private File convert(ConversionService conversionService, String source) {
+		return conversionService.convert(source, File.class);
+	}
+
+	static Stream<? extends Arguments> conversionServices() {
+		return ConversionServiceArguments
+				.with((conversionService) -> conversionService.addConverter(new StringToFileConverter()));
+	}
+
+}