Provide a public API for resolving the type of a field

See gh-549

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/AbstractFieldsSnippet.java

@@ -38,6 +38,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Andreas Evers
  * @author Andy Wilkinson
+ * @author Mathias Düsterhöft
  */
 public abstract class AbstractFieldsSnippet extends TemplatedSnippet {
 
@@ -160,7 +161,7 @@ public abstract class AbstractFieldsSnippet extends TemplatedSnippet {
 			content = verifyContent(
 					this.subsectionExtractor.extractSubsection(content, contentType));
 		}
-		ContentHandler contentHandler = getContentHandler(content, contentType);
+		ContentHandler contentHandler = ContentHandler.forContent(content, contentType);
 
 		validateFieldDocumentation(contentHandler);
 
@@ -168,7 +169,7 @@ public abstract class AbstractFieldsSnippet extends TemplatedSnippet {
 		for (FieldDescriptor descriptor : this.fieldDescriptors) {
 			if (!descriptor.isIgnored()) {
 				try {
-					Object type = contentHandler.determineFieldType(descriptor);
+					Object type = contentHandler.resolveFieldType(descriptor);
 					descriptorsToDocument.add(copyWithType(descriptor, type));
 				}
 				catch (FieldDoesNotExistException ex) {
@@ -200,36 +201,6 @@ public abstract class AbstractFieldsSnippet extends TemplatedSnippet {
 		return content;
 	}
 
-	private ContentHandler getContentHandler(byte[] content, MediaType contentType) {
-		ContentHandler contentHandler = createJsonContentHandler(content);
-		if (contentHandler == null) {
-			contentHandler = createXmlContentHandler(content);
-			if (contentHandler == null) {
-				throw new PayloadHandlingException("Cannot handle " + contentType
-						+ " content as it could not be parsed as JSON or XML");
-			}
-		}
-		return contentHandler;
-	}
-
-	private ContentHandler createJsonContentHandler(byte[] content) {
-		try {
-			return new JsonContentHandler(content);
-		}
-		catch (Exception ex) {
-			return null;
-		}
-	}
-
-	private ContentHandler createXmlContentHandler(byte[] content) {
-		try {
-			return new XmlContentHandler(content);
-		}
-		catch (Exception ex) {
-			return null;
-		}
-	}
-
 	private void validateFieldDocumentation(ContentHandler payloadHandler) {
 		List<FieldDescriptor> missingFields = payloadHandler
 				.findMissingFields(this.fieldDescriptors);

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/ContentHandler.java

@@ -18,12 +18,15 @@ package org.springframework.restdocs.payload;
 
 import java.util.List;
 
+import org.springframework.http.MediaType;
+
 /**
  * A handler for the content of a request or response.
  *
  * @author Andy Wilkinson
+ * @author Mathias Düsterhöft
  */
-interface ContentHandler {
+interface ContentHandler extends FieldTypeResolver {
 
 	/**
 	 * Finds the fields that are missing from the handler's payload. A field is missing if
@@ -48,11 +51,26 @@ interface ContentHandler {
 	String getUndocumentedContent(List<FieldDescriptor> fieldDescriptors);
 
 	/**
-	 * Returns the type of the field that is described by the given
-	 * {@code fieldDescriptor} based on the content of the payload.
-	 * @param fieldDescriptor the field descriptor
-	 * @return the type of the field
+	 * Create a {@link ContentHandler} for the given content type and payload.
+	 * @param content the payload
+	 * @param contentType the content type
+	 * @return the ContentHandler
+	 * @throws PayloadHandlingException if no known ContentHandler can handle the content
 	 */
-	Object determineFieldType(FieldDescriptor fieldDescriptor);
+	static ContentHandler forContent(byte[] content, MediaType contentType) {
+
+		try {
+			return new JsonContentHandler(content);
+		}
+		catch (Exception je) {
+			try {
+				return new XmlContentHandler(content);
+			}
+			catch (Exception xe) {
+				throw new PayloadHandlingException("Cannot handle " + contentType
+						+ " content as it could not be parsed as JSON or XML");
+			}
+		}
+	}
 
 }

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/FieldTypeResolver.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2014-2018 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
+ *
+ *      http://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.restdocs.payload;
+
+import org.springframework.http.MediaType;
+
+/**
+ * Public abstraction for external access to field type determination for xml and json
+ * payloads.
+ *
+ * @author Mathias Düsterhöft
+ * @since 2.0.3
+ */
+public interface FieldTypeResolver {
+
+	/**
+	 * Create a FieldTypeResolver for the given content and contentType.
+	 * @param content the payload that the {@link FieldTypeResolver} should handle
+	 * @param contentType the content type of the payload
+	 * @return the {@link FieldTypeResolver}
+	 */
+	static FieldTypeResolver forContent(byte[] content, MediaType contentType) {
+		return ContentHandler.forContent(content, contentType);
+	}
+
+	/**
+	 * Returns the type of the field that is described by the given
+	 * {@code fieldDescriptor} based on the content of the payload.
+	 * @param fieldDescriptor the field descriptor
+	 * @return the type of the field
+	 */
+	Object resolveFieldType(FieldDescriptor fieldDescriptor);
+
+}

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/JsonContentHandler.java

@@ -32,8 +32,9 @@ import org.springframework.restdocs.payload.JsonFieldProcessor.ExtractedField;
  * A {@link ContentHandler} for JSON content.
  *
  * @author Andy Wilkinson
+ * @author Mathias Düsterhöft
  */
-class JsonContentHandler implements ContentHandler {
+class JsonContentHandler implements ContentHandler, FieldTypeResolver {
 
 	private final JsonFieldProcessor fieldProcessor = new JsonFieldProcessor();
 
@@ -145,7 +146,7 @@ class JsonContentHandler implements ContentHandler {
 	}
 
 	@Override
-	public Object determineFieldType(FieldDescriptor fieldDescriptor) {
+	public Object resolveFieldType(FieldDescriptor fieldDescriptor) {
 		if (fieldDescriptor.getType() == null) {
 			return this.fieldTypesDiscoverer
 					.discoverFieldTypes(fieldDescriptor.getPath(), readContent())

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/XmlContentHandler.java

@@ -190,7 +190,7 @@ class XmlContentHandler implements ContentHandler {
 	}
 
 	@Override
-	public Object determineFieldType(FieldDescriptor fieldDescriptor) {
+	public Object resolveFieldType(FieldDescriptor fieldDescriptor) {
 		if (fieldDescriptor.getType() != null) {
 			return fieldDescriptor.getType();
 		}