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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang. The code is tested using the latest revision of the JDK for supported LTS releases: 8, 11, 17 and 21 currently. See https://github.com/apache/commons-lang/blob/master/.github/workflows/maven.yml Please ensure your build environment is up-to-date and kindly report any build issues.

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/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.lang3.builder;

import static org.apache.commons.lang3.reflect.FieldUtils.readField;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

import org.apache.commons.lang3.ClassUtils;
import org.apache.commons.lang3.reflect.FieldUtils;

/**
 * 
 * Assists in implementing {@link Diffable#diff(Object)} methods.
 * 
 * 
 * All non-static, non-transient fields (including inherited fields)
 * of the objects to diff are discovered using reflection and compared
 * for differences.
 * 
 *
 * 
 * To use this class, write code as follows:
 * 
 *
 *  * public class Person implements Diffable<Person> {
 *   String name;
 *   int age;
 *   boolean smoker;
 *   ...
 *
 *   public DiffResult diff(Person obj) {
 *     // No need for null check, as NullPointerException correct if obj is null
 *     return new ReflectionDiffBuilder(this, obj, ToStringStyle.SHORT_PREFIX_STYLE)
 *       .build();
 *   }
 * }
 * 
 *
 * 
 * The {@code ToStringStyle} passed to the constructor is embedded in the
 * returned {@code DiffResult} and influences the style of the
 * {@code DiffResult.toString()} method. This style choice can be overridden by
 * calling {@link DiffResult#toString(ToStringStyle)}.
 * 
 * @see Diffable
 * @see Diff
 * @see DiffResult
 * @see ToStringStyle
 * @since 3.6
 */
public class ReflectionDiffBuilder implements Builder {

    private final Object left;
    private final Object right;
    private final DiffBuilder diffBuilder;

    /**
     * 
     * Constructs a builder for the specified objects with the specified style.
     * 
     *
     * 
     * If {@code lhs == rhs} or {@code lhs.equals(rhs)} then the builder will
     * not evaluate any calls to {@code append(...)} and will return an empty
     * {@link DiffResult} when {@link #build()} is executed.
     * 
     * @param 
     *            type of the objects to diff
     * @param lhs
     *            {@code this} object
     * @param rhs
     *            the object to diff against
     * @param style
     *            the style will use when outputting the objects, {@code null}
     *            uses the default
     * @throws IllegalArgumentException
     *             if {@code lhs} or {@code rhs} is {@code null}
     */
    public  ReflectionDiffBuilder(final T lhs, final T rhs, final ToStringStyle style) {
        this.left = lhs;
        this.right = rhs;
        diffBuilder = new DiffBuilder(lhs, rhs, style);
    }

    @Override
    public DiffResult build() {
        if (left.equals(right)) {
            return diffBuilder.build();
        }

        appendFields(left.getClass());
        return diffBuilder.build();
    }

    private void appendFields(final Class clazz) {
        for (final Field field : FieldUtils.getAllFields(clazz)) {
            if (accept(field)) {
                try {
                    diffBuilder.append(field.getName(), readField(field, left, true),
                            readField(field, right, true));
                } catch (final IllegalAccessException ex) {
                    //this can't happen. Would get a Security exception instead
                    //throw a runtime exception in case the impossible happens.
                    throw new InternalError("Unexpected IllegalAccessException: " + ex.getMessage());
                }
            }
        }
    }

    private boolean accept(final Field field) {
        if (field.getName().indexOf(ClassUtils.INNER_CLASS_SEPARATOR_CHAR) != -1) {
            return false;
        }
        if (Modifier.isTransient(field.getModifiers())) {
            return false;
        }
        return !Modifier.isStatic(field.getModifiers());
    }

}