如何将两个Java对象合并成一个？

Question

我有两个Java对象实例，想要将它们的值合并到一个实例中。当两个对象实例都包含某个字段的值时，我需要选择哪个对象实例优先。我还不想用null值覆盖其他值。

示例：

MyClass source = new MyClass("source", 1, null);
MyClass target = new MyClass("target", 2, "b");
merge(source, target);
// target now has values ("source", 1, "b")

我正在使用Java 8和Spring boot 1.4.1。

编辑：看起来我没有表达清楚，所以我增加了更多描述。此外，我已经提供了自己的解决方案。目的是为了在其他人遇到相同问题时能够提供这个解决方案。我在这里发现了其他帖子问同样或类似的问题，但是它们没有像我下面发布的完整解决方案。

- Chip

---

这是一个不明确的问题。在什么意义上进行合并？你可以合并列表，至少如果它们是排序的，但我怀疑你有其他意思吗？ - Ole V.V.

我有对象A和对象B。它们都有字段String X，int Y，List Z。我想将对象A中所有非空字段复制到对象B中。这样我就可以将两个来源的输入合并成一个最终的对象进行持久化。 - Chip

Answer 1

Spring的spring-beans库有一个org.springframework.beans.BeanUtils类，提供了一个copyProperties方法来将源对象实例复制到目标对象实例。然而，它只能为对象的第一级字段做到这点。以下是我的解决方案，基于BeanUtils.copyProperties，递归执行复制每个子对象，包括集合和映射。

package my.utility;

import org.springframework.beans.BeanWrapper;
import org.springframework.beans.BeansException;
import org.springframework.beans.FatalBeanException;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Map;

/**
 * Created by cdebergh on 1/6/17.
 */
public class BeanUtils extends org.springframework.beans.BeanUtils {

    /**
     * Copy the not null property values of the given source bean into the target bean.
     * <p>Note: The source and target classes do not have to match or even be derived
     * from each other, as long as the properties match. Any bean properties that the
     * source bean exposes but the target bean does not will silently be ignored.
     * <p>This is just a convenience method. For more complex transfer needs,
     * consider using a full BeanWrapper.
     * @param source the source bean
     * @param target the target bean
     * @throws BeansException if the copying failed
     * @see BeanWrapper
     */
    public static void copyPropertiesNotNull(Object source, Object target) throws BeansException {
        copyPropertiesNotNull(source, target, null, (String[]) null);
    }

    private static void setAccessible(Method method) {
        if (!Modifier.isPublic(method.getDeclaringClass().getModifiers())) {
            method.setAccessible(true);
        }
    }

    /**
     * Copy the not null property values of the given source bean into the given target bean.
     * <p>Note: The source and target classes do not have to match or even be derived
     * from each other, as long as the properties match. Any bean properties that the
     * source bean exposes but the target bean does not will silently be ignored.
     * @param source the source bean
     * @param target the target bean
     * @param editable the class (or interface) to restrict property setting to
     * @param ignoreProperties array of property names to ignore
     * @throws BeansException if the copying failed
     * @see BeanWrapper
     */
    private static void copyPropertiesNotNull(Object source, Object target, Class<?> editable, String... ignoreProperties)
            throws BeansException {

        Assert.notNull(source, "Source must not be null");
        Assert.notNull(target, "Target must not be null");

        Class<?> actualEditable = target.getClass();
        if (editable != null) {
            if (!editable.isInstance(target)) {
                throw new IllegalArgumentException("Target class [" + target.getClass().getName() +
                        "] not assignable to Editable class [" + editable.getName() + "]");
            }
            actualEditable = editable;
        }
        PropertyDescriptor[] targetPds = getPropertyDescriptors(actualEditable);
        List<String> ignoreList = (ignoreProperties != null ? Arrays.asList(ignoreProperties) : null);

        for (PropertyDescriptor targetPropertyDescriptor : targetPds) {
            Method targetWriteMethod = targetPropertyDescriptor.getWriteMethod();
            if (targetWriteMethod != null
                    && (ignoreList == null || !ignoreList.contains(targetPropertyDescriptor.getName()))) {
                PropertyDescriptor sourcePropertyDescriptor =
                        getPropertyDescriptor(source.getClass(), targetPropertyDescriptor.getName());
                if (sourcePropertyDescriptor != null) {
                    Method sourceReadMethod = sourcePropertyDescriptor.getReadMethod();
                    if (sourceReadMethod != null &&
                            ClassUtils.isAssignable(
                                    targetWriteMethod.getParameterTypes()[0], sourceReadMethod.getReturnType())) {
                        try {
                            Method targetReadMethod = targetPropertyDescriptor.getReadMethod();
                            setAccessible(sourceReadMethod);
                            setAccessible(targetWriteMethod);
                            Object sourceValue = sourceReadMethod.invoke(source);

                            if (sourceValue != null && targetReadMethod != null) {
                                setAccessible(targetReadMethod);
                                Object targetValue = targetReadMethod.invoke(target);
                                if (targetValue == null) {
                                    targetWriteMethod.invoke(target, sourceValue);
                                } else if(targetValue instanceof Collection<?>) {
                                    ((Collection) targetValue).addAll((Collection) sourceValue);
                                } else if (targetValue instanceof Map<?,?>) {
                                    ((Map) targetValue).putAll((Map) sourceValue);
                                } else {
                                    copyPropertiesNotNull(sourceValue, targetValue, editable, ignoreProperties);
                                }
                            }
                        }
                        catch (Throwable ex) {
                            throw new FatalBeanException(
                                    "Could not copy property '" + targetPropertyDescriptor.getName() +
                                    "' from source to target", ex);
                        }
                    }
                }
            }
        }
    }
}

- Chip