Expose a way to add custom path functions with minimal impact

json-path/src/main/java/com/jayway/jsonpath/PathFunctionFactory.java

@@ -0,0 +1,80 @@
+package com.jayway.jsonpath;
+
+import com.jayway.jsonpath.internal.function.PathFunction;
+
+import java.util.Collections;
+import java.util.Map;
+
+public class PathFunctionFactory {
+
+	private static boolean initialized = false;
+
+	/**
+	 * by default, we add th common path functions
+	 */
+	private static Map<String, Class> functions = com.jayway.jsonpath.internal.function.PathFunctionFactory.getFunctions();
+
+	/**
+	 * Add a single custom path function
+	 * Should only be done when your application is being initialized
+	 *
+	 * @param name  The name of the function
+	 * @param clazz Class of a function
+	 * @throws InvalidPathException
+	 */
+	public static void add(String name, Class clazz) throws InvalidPathException {
+		assertNotInitialized();
+		if (functions.containsKey(name)) {
+			throw new InvalidPathException("Path function with name: " + name + " already exists");
+		}
+
+		functions.put(name, clazz);
+	}
+
+	/**
+	 * Add a map custom path function
+	 * Should only be done when your application is being initialized
+	 *
+	 * @param functionMap The name of the function as key, Class of a function as Value
+	 * @throws InvalidPathException
+	 */
+	public static void add(Map<String, Class> functionMap) throws InvalidPathException {
+		for (Map.Entry<String, Class> entry : functionMap.entrySet()) {
+			add(entry.getKey(), entry.getValue());
+		}
+	}
+
+	/**
+	 * Should only your application initialized to
+	 */
+	public static void init() {
+		if (initialized) {
+			return;
+		}
+		functions.putAll(com.jayway.jsonpath.internal.function.PathFunctionFactory.getFunctions());
+		functions = Collections.unmodifiableMap(functions);
+		initialized = true;
+	}
+
+	public static PathFunction newFunction(String name) throws InvalidPathException {
+		// Lazy init
+		init();
+
+		Class functionClazz = functions.get(name);
+		if (functionClazz == null) {
+			throw new InvalidPathException("Function with name: " + name + " does not exist.");
+		} else {
+			try {
+				return (PathFunction) functionClazz.newInstance();
+			} catch (Exception e) {
+				throw new InvalidPathException("Function of name: " + name + " cannot be created", e);
+			}
+		}
+	}
+
+	private static void assertNotInitialized() {
+		if (initialized) {
+			throw new InvalidPathException("Can not change path function factory after it is initialized");
+		}
+	}
+}