gson/src/main/java/com/google/gson/ObjectNavigator.java

/*
 * Copyright (C) 2008 Google Inc.
 *
 * 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 com.google.gson;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;
import java.lang.reflect.Type;

/**
 * Provides ability to apply a visitor to an object and all of its fields recursively.
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 */
final class ObjectNavigator {

  public interface Visitor {
    /**
     * This is called before the object navigator starts visiting the current object
     */
    void startVisitingObject(Object node);

    /**
     * This is called to visit the current object if it is an array
     */
    void visitArray(Object array, Type componentType);

    /**
     * This is called to visit an object field of the current object
     */
    void visitObjectField(Field f, Type typeOfF, Object obj);

    /**
     * This is called to visit an array field of the current object
     */
    void visitArrayField(Field f, Type typeOfF, Object obj);

    /**
     * This is called to visit an object using a custom handler
     * @return true if a custom handler exists, false otherwise
     */
    public boolean visitUsingCustomHandler(Object obj, Type objType);

    /**
     * This is called to visit a field of the current object using a custom handler
     */
    public boolean visitFieldUsingCustomHandler(Field f, Type actualTypeOfField, Object parent);
    
    /**
     * Retrieve the current target
     */
    Object getTarget();
  }

  private final ExclusionStrategy exclusionStrategy;
  private final MemoryRefStack<Object> ancestors;
  private final Object obj;
  private final Type objType;

  /**
   * @param obj The object being navigated
   * @param objType The (fully genericized) type of the object being navigated
   * @param exclusionStrategy the concrete strategy object to be used to
   *        filter out fields of an object.
   */
  ObjectNavigator(Object obj, Type objType, ExclusionStrategy exclusionStrategy,
      MemoryRefStack<Object> ancestors) {
    Preconditions.checkNotNull(exclusionStrategy);
    Preconditions.checkNotNull(ancestors);

    this.obj = obj;
    this.objType = objType;
    this.exclusionStrategy = exclusionStrategy;
    this.ancestors = ancestors;
  }

  /**
   * Navigate all the fields of the specified object.
   * If a field is null, it does not get visited.
   */
  public void accept(Visitor visitor) {
    boolean visitedWithCustomHandler = visitor.visitUsingCustomHandler(obj, objType);
    if (!visitedWithCustomHandler) {
      Object objectToVisit = (obj == null) ? visitor.getTarget() : obj;
      if (objectToVisit == null) {
        return;
      }
      TypeInfo objTypeInfo = new TypeInfo(objType);
      if (exclusionStrategy.shouldSkipClass(objTypeInfo.getRawClass())) {
        return;
      }
  
      if (ancestors.contains(objectToVisit)) {
        throw new IllegalStateException("Circular reference found: " + objectToVisit);
      }
      ancestors.push(objectToVisit);
  
      try {
        if (objTypeInfo.isArray()) {
          visitor.visitArray(objectToVisit, objType);
        } else {
          visitor.startVisitingObject(objectToVisit);
          // For all classes in the inheritance hierarchy (including the current class),
          // visit all fields
          for (Class<?> curr = objTypeInfo.getRawClass();
          curr != null && !curr.equals(Object.class); curr = curr.getSuperclass()) {
            if (!curr.isSynthetic()) {
              navigateClassFields(objectToVisit, curr, visitor);
            }
          }
        }
      } finally {
        ancestors.pop();
      }
    }
  }

  private void navigateClassFields(Object obj, Class<?> clazz, Visitor visitor) {
    Field[] fields = clazz.getDeclaredFields();
    AccessibleObject.setAccessible(fields, true);
    for (Field f : fields) {
      if (exclusionStrategy.shouldSkipField(f)) {
        continue; // skip
      } else {
        TypeInfo fieldTypeInfo = TypeInfoFactory.getTypeInfoForField(f, objType);
        Type actualTypeOfField = fieldTypeInfo.getActualType();
        boolean visitedWithCustomHandler = 
          visitor.visitFieldUsingCustomHandler(f, actualTypeOfField, obj);
        if (!visitedWithCustomHandler) {
          if (fieldTypeInfo.isArray()) {
            visitor.visitArrayField(f, actualTypeOfField, obj);
          } else {
            visitor.visitObjectField(f, actualTypeOfField, obj);
          }
        }
      }
    }
  }
}