FieldValueResolver.java

/**
 * Copyright (c) 2012-2015 Edgar Espina
 *
 * This file is part of Handlebars.java.
 *
 * 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.github.jknack.handlebars.context;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.Member;
import java.lang.reflect.Modifier;
import java.util.LinkedHashSet;
import java.util.Set;

import com.github.jknack.handlebars.ValueResolver;
import com.github.jknack.handlebars.context.FieldValueResolver.FieldWrapper;

/**
 * A specialization of {@link MemberValueResolver} with lookup and invocation
 * support for {@link Field}.
 * It matches private, protected, package, public and no-static field.
 *
 * @author edgar.espina
 * @since 0.1.1
 */
public class FieldValueResolver extends MemberValueResolver<FieldWrapper> {

  /**
   * Workaround for accessing to the public attribute length of arrays.
   *
   * @author edgar.espina
   * @since 0.12.0
   */
  public interface FieldWrapper extends Member {
    /**
     * Returns the value of the field represented by this {@code Field}, on
     * the specified object. The value is automatically wrapped in an
     * object if it has a primitive type.
     *
     * <p>
     * The underlying field's value is obtained as follows:
     *
     * <p>
     * If the underlying field is a static field, the {@code obj} argument is ignored; it may be
     * null.
     *
     * <p>
     * Otherwise, the underlying field is an instance field. If the specified {@code obj} argument
     * is null, the method throws a {@code NullPointerException}. If the specified object is not an
     * instance of the class or interface declaring the underlying field, the method throws an
     * {@code IllegalArgumentException}.
     *
     * <p>
     * If this {@code Field} object enforces Java language access control, and the underlying field
     * is inaccessible, the method throws an {@code IllegalAccessException}. If the underlying field
     * is static, the class that declared the field is initialized if it has not already been
     * initialized.
     *
     * <p>
     * Otherwise, the value is retrieved from the underlying instance or static field. If the field
     * has a primitive type, the value is wrapped in an object before being returned, otherwise it
     * is returned as is.
     *
     * <p>
     * If the field is hidden in the type of {@code obj}, the field's value is obtained according to
     * the preceding rules.
     *
     * @param obj object from which the represented field's value is
     *        to be extracted
     * @return the value of the represented field in object {@code obj}; primitive values are
     *         wrapped in an appropriate
     *         object before being returned
     *
     * @exception IllegalAccessException if the underlying field
     *            is inaccessible.
     */
    Object get(Object obj) throws IllegalAccessException;
  }

  /**
   * Use a {@link Field} as member.
   *
   * @author edgar.espina
   * @since 0.12.0
   */
  private static class FieldMember extends AccessibleObject implements FieldWrapper {

    /**
     * The field object.
     */
    private Field field;

    /**
     * @param field The field object.
     */
    public FieldMember(final Field field) {
      this.field = field;
    }

    @Override
    public Class<?> getDeclaringClass() {
      return field.getDeclaringClass();
    }

    @Override
    public String getName() {
      return field.getName();
    }

    @Override
    public int getModifiers() {
      return field.getModifiers();
    }

    @Override
    public boolean isSynthetic() {
      return field.isSynthetic();
    }

    @Override
    public Object get(final Object obj) throws IllegalAccessException {
      return field.get(obj);
    }

    @Override
    public String toString() {
      return field.toString();
    }

    @Override
    public boolean isAccessible() {
      return field.isAccessible();
    }

    @Override
    public void setAccessible(final boolean flag) {
      field.setAccessible(flag);
    }
  }

  /**
   * See http://stackoverflow.com/questions/11097658/getting-the-field-length-in-a-java-array-using
   * -reflection.
   *
   * @author edgar.espina
   * @since 0.12.0
   */
  private static final class ArrayLengthMember implements FieldWrapper {

    /**
     * One instance is enough.
     */
    public static final FieldWrapper LENGTH = new ArrayLengthMember();

    /**
     * Not allowed.
     */
    private ArrayLengthMember() {
    }

    @Override
    public Class<?> getDeclaringClass() {
      return null;
    }

    @Override
    public String getName() {
      return "length";
    }

    @Override
    public int getModifiers() {
      return Modifier.PUBLIC;
    }

    @Override
    public boolean isSynthetic() {
      return false;
    }

    @Override
    public Object get(final Object obj) throws IllegalAccessException {
      return Array.getLength(obj);
    }

  }

  /**
   * The default value resolver.
   */
  public static final ValueResolver INSTANCE = new FieldValueResolver();

  @Override
  public boolean matches(final FieldWrapper field, final String name) {
    return !isStatic(field) && field.getName().equals(name);
  }

  @Override
  protected Object invokeMember(final FieldWrapper field, final Object context) {
    try {
      return field.get(context);
    } catch (Exception ex) {
      throw new IllegalStateException(
          "Shouldn't be illegal to access field '" + field.getName()
              + "'", ex);
    }
  }

  @Override
  protected Set<FieldWrapper> members(final Class<?> clazz) {
    Set<FieldWrapper> members = new LinkedHashSet<FieldWrapper>();
    if (clazz.isArray()) {
      members.add(ArrayLengthMember.LENGTH);
    } else {
      Class<?> targetClass = clazz;
      do {
        Field[] fields = targetClass.getDeclaredFields();
        for (Field field : fields) {
          FieldWrapper wrapper = new FieldMember(field);
          if (matches(wrapper, memberName(wrapper))) {
            members.add(wrapper);
          }
        }
        targetClass = targetClass.getSuperclass();
      } while (targetClass != null && targetClass != Object.class);
    }
    return members;
  }

  @Override
  protected String memberName(final FieldWrapper member) {
    return member.getName();
  }
}