深拷贝和浅拷贝

项目里面碰到的一个真实的问题

对应spring 中的原型模型，对象中的属性比较多的时候，我们采用的是复制的方式，怎么高效的复制出来一个对象，这其中又涉及到深拷贝和浅拷贝

最开始我们使用的一种方式是使用Spring bean 中的org.springframework.beans.BeanUtils#instantiate方法通过class 去创建一个新的实例，源码如下：

/**
     * Convenience method to instantiate a class using its no-arg constructor.
     * @param clazz class to instantiate
     * @return the new instance
     * @throws BeanInstantiationException if the bean cannot be instantiated
     * @see Class#newInstance()
     */
    public static <T> T instantiate(Class<T> clazz) throws BeanInstantiationException {
        Assert.notNull(clazz, "Class must not be null");
        if (clazz.isInterface()) {
            throw new BeanInstantiationException(clazz, "Specified class is an interface");
        }
        try {
            return clazz.newInstance();
        }
        catch (InstantiationException ex) {
            throw new BeanInstantiationException(clazz, "Is it an abstract class?", ex);
        }
        catch (IllegalAccessException ex) {
            throw new BeanInstantiationException(clazz, "Is the constructor accessible?", ex);
        }
    }

房里就是通过 clazz.newInstance() 返回一个新的对象，获取到这个对象后，再使用 copyProperties方法将属性拷贝过去，源码如下：

/**
     * Copy the 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 copyProperties(Object source, Object target) throws BeansException {
        copyProperties(source, target, null, (String[]) null);
    }

而实际上内部调用的copyProperties，再来看看：

/**
     * Copy the 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 copyProperties(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 targetPd : targetPds) {
            Method writeMethod = targetPd.getWriteMethod();
            if (writeMethod != null && (ignoreList == null || !ignoreList.contains(targetPd.getName()))) {
                PropertyDescriptor sourcePd = getPropertyDescriptor(source.getClass(), targetPd.getName());
                if (sourcePd != null) {
                    Method readMethod = sourcePd.getReadMethod();
                    if (readMethod != null &&
                            ClassUtils.isAssignable(writeMethod.getParameterTypes()[0], readMethod.getReturnType())) {
                        try {
                            if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {
                                readMethod.setAccessible(true);
                            }
                            Object value = readMethod.invoke(source);
                            if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers())) {
                                writeMethod.setAccessible(true);
                            }
                            writeMethod.invoke(target, value);
                        }
                        catch (Throwable ex) {
                            throw new FatalBeanException(
                                    "Could not copy property '" + targetPd.getName() + "' from source to target", ex);
                        }
                    }
                }
            }
        }
    }

实际上是对属性for循环遍历完成最终的设置

那么我们写了一个util方法去通过这种方式实现bean 拷贝，使用Spring 自带的工具类实现：

public class BeanCopyUtil {

    /**
     * 不同类型对象数据copy
     *
     * @param source
     * @param targetClass
     * @param <T>
     * @return
     */
    public static <T> T copyProperties(Object source, Class<T> targetClass) {
        T target = BeanUtils.instantiate(targetClass);
        BeanUtils.copyProperties(source, target);
        return target;
    }
}

 

但是这种方式只是实现了浅拷贝，如果一个对象中包含的另外的一个dto，在拷贝的时候实际上获得的是指向同一个对象的引用，后面会有结果展示

 

那么怎么实现深拷贝呢，其实很多种方式：

1.用fastjson的序列化方法

WaybillDTO waybillDTOJsonFrom  = JsonUtil.fromJson(JsonUtil.toJson(waybillDTO),WaybillDTO.class);
将对象waybillDTO序列化成json串后 再反序列化出来，这样得到的是一个实现了深拷贝的对象，具体的可以去看 fromJson方法内部的实现，这种方法的缺点是我们知道序列化和反序列化的耗时是比较高的

 

2.通过字节流转换

　通过将对象转换成字节流，分配内存，生成新的对象：

 public static <T extends Serializable> T deepClone(T obj) {
        T cloneObj = null;
        try {
            //写入字节流
            ByteArrayOutputStream out = new ByteArrayOutputStream();
            ObjectOutputStream obs = new ObjectOutputStream(out);
            obs.writeObject(obj);
            obs.close();

            //分配内存，写入原始对象，生成新对象
            ByteArrayInputStream ios = new ByteArrayInputStream(out.toByteArray());
            ObjectInputStream ois = new ObjectInputStream(ios);
            //返回生成的新对象
            cloneObj = (T) ois.readObject();
            ois.close();
        } catch (Exception e) {
            throw new IllegalArgumentException("bean深度拷贝异常", e);
        }
        return cloneObj;
    }

 

 

 

这三种方式的生成的对象对比的结果我们看下，定义对象 WaybillDTO，里面包含了一个Map对象 ：Map<String, String> extendMessage ，那么对这个WaybillDTO进行拷贝后的结果验证一下：

//long start1 = System.currentTimeMillis();
        WaybillDTO waybillDTOClone  = BeanCopyUtil.deepClone(waybillDTO);
      //  System.out.println(System.currentTimeMillis() - start1);
        System.out.println("waybillDTO==waybillDTOClone:" + (waybillDTO==waybillDTOClone));
        System.out.println("waybillDTO.getExtendMessage==waybillDTOClone.getExtendMessage:" + (waybillDTO.getExtendMessage()==waybillDTOClone.getExtendMessage()));


        //long start2 = System.currentTimeMillis();
        WaybillDTO targetWaybillInfo = BeanCopyUtil.copyProperties(waybillDTO, WaybillDTO.class);
       // System.out.println(System.currentTimeMillis() - start2);
        System.out.println("waybillDTO==targetWaybillInfo:" + (waybillDTO==targetWaybillInfo));
        System.out.println("waybillDTO.getExtendMessage==targetWaybillInfo.getExtendMessage:" + (waybillDTO.getExtendMessage()==targetWaybillInfo.getExtendMessage()));

       // long start3 = System.currentTimeMillis();
        WaybillDTO waybillDTOJsonFrom  = JsonUtil.fromJson(JsonUtil.toJson(waybillDTO),WaybillDTO.class);
       // System.out.println(System.currentTimeMillis() - start3);
        System.out.println("waybillDTO==waybillDTOJsonFrom:" + (waybillDTO==waybillDTOJsonFrom));
        System.out.println("waybillDTO.getExtendMessage==waybillDTOClone.getExtendMessage:" + (waybillDTO.getExtendMessage()==waybillDTOJsonFrom.getExtendMessage()));

看下对应是效果，可以看到通过方法2 Spring 的beanUtils 实现的对象拷贝，对内部的Map<String, String> extendMessage对比的结果是true，方法1和方法3都是false，说明是不同的对象

waybillDTO==waybillDTOClone:false
waybillDTO.getExtendMessage==waybillDTOClone.getExtendMessage:false


waybillDTO==targetWaybillInfo:false
waybillDTO.getExtendMessage==targetWaybillInfo.getExtendMessage:true


waybillDTO==waybillDTOJsonFrom:false
waybillDTO.getExtendMessage==waybillDTOClone.getExtendMessage:false

Process finished with exit code 0