Refactor spring-boot JSON parsers

Refactor JSON parser wrappers to remove duplicate code portions in the
parseMap() and parseList() methods by adding an AbstractJsonParser.

See gh-12428

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/AbstractJsonParser.java

+/*
+ * Copyright 2012-2017 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.boot.json;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.function.Function;
+
+/**
+ * Base class for parsers wrapped or implemented in this package.
+ *
+ * @author Anton Telechev
+ */
+abstract class AbstractJsonParser implements JsonParser {
+
+	/** Start symbol of a JSON map. **/
+	private static final String START_MAP = "{";
+
+	/** Start symbol of a JSON list. **/
+	private static final String START_LIST = "[";
+
+	/**
+	 * Parses the specified JSON string and returns the extracted contents as a Map of
+	 * String to Object.
+	 *
+	 * @param json the JSON string to parse.
+	 * @param parser the parser function.
+	 * @return Map<String, Object> parsed contents
+	 * @throws IllegalArgumentException if the json String cannot be parsed as a
+	 * Map<String, Object>
+	 */
+	Map<String, Object> parseMap(String json,
+			Function<String, Map<String, Object>> parser) {
+		assert parser != null;
+
+		return trimIfStartsWith(json, START_MAP).map(parser::apply)
+				.orElseThrow(AbstractJsonParser::cannotParseJson);
+	}
+
+	/**
+	 * Parses the specified JSON string and returns the extracted contents as a List of Objects.
+	 *
+	 * @param json the JSON string to parse.
+	 * @param parser the parser function.
+	 * @return List&lt;Object&gt; parsed contents
+	 * @throws IllegalArgumentException if the json String cannot be parsed as a
+	 * List&lt;Object&gt;
+	 */
+	List<Object> parseList(String json, Function<String, List<Object>> parser) {
+		assert parser != null;
+
+		return trimIfStartsWith(json, START_LIST).map(parser::apply)
+				.orElseThrow(AbstractJsonParser::cannotParseJson);
+	}
+
+	private static IllegalArgumentException cannotParseJson() {
+		return cannotParseJson(null);
+	}
+
+	static IllegalArgumentException cannotParseJson(Exception cause) {
+		return new IllegalArgumentException("Cannot parse JSON", cause);
+	}
+
+	private static Optional<String> trimIfStartsWith(String json, String expectedPrefix) {
+		assert expectedPrefix != null;
+
+		if (json != null) {
+			final String trimmed = json.trim();
+			if (trimmed.startsWith(expectedPrefix)) {
+				return Optional.of(trimmed);
+			}
+		}
+		return Optional.empty();
+	}
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/BasicJsonParser.java

@@ -35,28 +35,16 @@ import org.springframework.util.StringUtils;
  * @since 1.2.0
  * @see JsonParserFactory
  */
-public class BasicJsonParser implements JsonParser {
+public class BasicJsonParser extends AbstractJsonParser {
 
 	@Override
 	public Map<String, Object> parseMap(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("{")) {
-				return parseMapInternal(json);
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseMap(json, this::parseMapInternal);
 	}
 
 	@Override
 	public List<Object> parseList(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("[")) {
-				return parseListInternal(json);
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseList(json, this::parseListInternal);
 	}
 
 	private List<Object> parseListInternal(String json) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/GsonJsonParser.java

@@ -31,7 +31,7 @@ import com.google.gson.reflect.TypeToken;
  * @since 1.2.0
  * @see JsonParserFactory
  */
-public class GsonJsonParser implements JsonParser {
+public class GsonJsonParser extends AbstractJsonParser {
 
 	private static final TypeToken<?> MAP_TYPE = new MapTypeToken();
 
@@ -41,24 +41,14 @@ public class GsonJsonParser implements JsonParser {
 
 	@Override
 	public Map<String, Object> parseMap(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("{")) {
-				return this.gson.fromJson(json, MAP_TYPE.getType());
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseMap(json,
+				(trimmed) -> this.gson.fromJson(trimmed, MAP_TYPE.getType()));
 	}
 
 	@Override
 	public List<Object> parseList(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("[")) {
-				return this.gson.fromJson(json, LIST_TYPE.getType());
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseList(json,
+				(trimmed) -> this.gson.fromJson(trimmed, LIST_TYPE.getType()));
 	}
 
 	private static final class MapTypeToken extends TypeToken<Map<String, Object>> {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/JacksonJsonParser.java

@@ -42,7 +42,7 @@ public class JacksonJsonParser implements JsonParser {
 			return getObjectMapper().readValue(json, MAP_TYPE);
 		}
 		catch (Exception ex) {
-			throw new IllegalArgumentException("Cannot parse JSON", ex);
+			throw AbstractJsonParser.cannotParseJson(ex);
 		}
 	}
 
@@ -52,7 +52,7 @@ public class JacksonJsonParser implements JsonParser {
 			return getObjectMapper().readValue(json, LIST_TYPE);
 		}
 		catch (Exception ex) {
-			throw new IllegalArgumentException("Cannot parse JSON", ex);
+			throw AbstractJsonParser.cannotParseJson(ex);
 		}
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/JsonSimpleJsonParser.java

@@ -39,7 +39,7 @@ public class JsonSimpleJsonParser implements JsonParser {
 			return (Map<String, Object>) new JSONParser().parse(json);
 		}
 		catch (ParseException ex) {
-			throw new IllegalArgumentException("Cannot parse JSON", ex);
+			throw AbstractJsonParser.cannotParseJson(ex);
 		}
 	}
 
@@ -50,7 +50,7 @@ public class JsonSimpleJsonParser implements JsonParser {
 			return (List<Object>) new JSONParser().parse(json);
 		}
 		catch (ParseException ex) {
-			throw new IllegalArgumentException("Cannot parse JSON", ex);
+			throw AbstractJsonParser.cannotParseJson(ex);
 		}
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/YamlJsonParser.java

@@ -28,30 +28,18 @@ import org.yaml.snakeyaml.Yaml;
  * @author Jean de Klerk
  * @see JsonParserFactory
  */
-public class YamlJsonParser implements JsonParser {
+public class YamlJsonParser extends AbstractJsonParser {
 
 	@Override
 	@SuppressWarnings("unchecked")
 	public Map<String, Object> parseMap(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("{")) {
-				return new Yaml().loadAs(json, Map.class);
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseMap(json, (trimmed) -> new Yaml().loadAs(trimmed, Map.class));
 	}
 
 	@Override
 	@SuppressWarnings("unchecked")
 	public List<Object> parseList(String json) {
-		if (json != null) {
-			json = json.trim();
-			if (json.startsWith("[")) {
-				return new Yaml().loadAs(json, List.class);
-			}
-		}
-		throw new IllegalArgumentException("Cannot parse JSON");
+		return parseList(json, (trimmed) -> new Yaml().loadAs(trimmed, List.class));
 	}
 
 }