Question

我的目标是将一个对象的字段复制到另一个对象中，但只复制那些不为空的对象。我不想明确指定它。更通用的解决方案非常有用且易于维护，即在REST API中实现PATCH，您只允许提供特定字段。

我看到了这个类似的帖子，我试图从这里实现一些想法：Helper in order to copy non null properties from object to another ? (Java)

但是在程序执行后，对象不会以任何方式改变。

所以这是我创建的示例类，例如：

class Person {
    String name;
    int age;
    Pet friend;

    public Person() {
    }

    public Person(String name, int age, Pet friend) {
        this.name = name;
        this.age = age;
        this.friend = friend;
    }

    // getters and setters here
}

class Pet {
    String name;
    int age;

    public Pet(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // getters and setters here
}

这是我重写的copyProperty方法：

import org.apache.commons.beanutils.BeanUtilsBean;
import java.lang.reflect.InvocationTargetException;

public class MyBeansUtil extends BeanUtilsBean {

@Override
public void copyProperty(Object dest, String name, Object value)
        throws IllegalAccessException, InvocationTargetException {
    if(value == null) return;
    super.copyProperty(dest, name, value);
}
}

...这里是我试图在一些例子上测试它的地方：

public class SandBox {
    public static void main(String[] args) {
        Person db = new Person("John", 36, new Pet("Lucy", 3));
        Person db2 = new Person("John", 36, new Pet("Lucy", 2));
        Person db3 = new Person("John", 36, new Pet("Lucy", 4));

        Person in = new Person();
        in.age = 17;
        in.name = "Paul";
        in.friend = new Pet(null, 35);

        Person in2 = new Person();
        in2.name = "Damian";

        Person in3 = new Person();
        in3.friend = new Pet("Lup", 25);

        try {
            BeanUtilsBean notNull  =new MyBeansUtil();
            notNull.copyProperties(db, in);
            notNull.copyProperties(db2, in2);
            notNull.copyProperties(db3, in3);

        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}

不幸的是，原始对象db，db1，db2保持原样。我在这里做错了吗？

Answer 1

我最终使用了Spring BeanUtils库。这是我的工作方法：

import org.springframework.beans.BeanWrapper;
import org.springframework.beans.BeanWrapperImpl;

import java.lang.reflect.Field;
import java.util.Collection;

public class MyBeansUtil<T> {
    public T copyNonNullProperties(T target, T in) {
        if (in == null || target == null || target.getClass() != in.getClass()) return null;

        final BeanWrapper src = new BeanWrapperImpl(in);
        final BeanWrapper trg = new BeanWrapperImpl(target);

        for (final Field property : target.getClass().getDeclaredFields()) {
            Object providedObject = src.getPropertyValue(property.getName());
            if (providedObject != null && !(providedObject instanceof Collection<?>)) {
                trg.setPropertyValue(
                        property.getName(),
                        providedObject);
            }
        }
        return target;
    }
}

它运行正常，但请注意它会忽略集合的字段。这是故意的，我单独处理它们。

Answer 2

使用BeanUtils和java8，我们可以实现以下目标：

BeanUtils.copyProperties(Object_source, Object_target, getNullPropertyNames(Object_source));

private String[] getNullPropertyNames(Object source) {
        final BeanWrapper wrappedSource = new BeanWrapperImpl(source);
        return Stream.of(wrappedSource.getPropertyDescriptors()).map(FeatureDescriptor::getName)
                .filter(propertyName -> wrappedSource.getPropertyValue(propertyName) == null).toArray(String[]::new);
    }

Answer 3

您可以创建自己的方法来复制属性，同时忽略空值。

public static String[] getNullPropertyNames (Object source) {
    final BeanWrapper src = new BeanWrapperImpl(source);
    java.beans.PropertyDescriptor[] pds = src.getPropertyDescriptors();

    Set<String> emptyNames = new HashSet<String>();
    for(java.beans.PropertyDescriptor pd : pds) {
        Object srcValue = src.getPropertyValue(pd.getName());
        if (srcValue == null) emptyNames.add(pd.getName());
    }
    String[] result = new String[emptyNames.size()];
    return emptyNames.toArray(result);
}

// then use Spring BeanUtils to copy and ignore null
public static void myCopyProperties(Object src, Object target) {
    BeanUtils.copyProperties(src, target, getNullPropertyNames(src))
}

Answer 4

使用ProprtyUtils，我们可以使用以下方法实现此目标：

    private void copyNonNullProperties(Object destination,
            Object source) {
        try {
            PropertyUtils.describe(source).entrySet().stream()
                    .filter(source -> source.getValue() != null)
                    .filter(source -> !source.getKey().equals("class"))
                    .forEach(source -> {
                        try {
                            PropertyUtils.setProperty(destination, source.getKey(), source.getValue());
                        } catch (Exception e22) {
                            log.error("Error setting properties : {}", e22.getMessage());
                        }
                    });

        } catch (Exception e1) {
            log.error("Error setting properties : {}", e1.getMessage());
        }

    }

使用BeanUtils或类似方法将非null属性从一个对象复制到另一个对象

4 个答案: