Modelmapper:当源对象为null时，如何应用自定义映射?

问题描述

假设我拥有类 MySource :

public class MySource {
    public String fieldA;
    public String fieldB;

    public MySource(String A, String B) {
       this.fieldA = A;
       this.fieldB = B;
    }
}

，我想将其翻译为对象 MyTarget :

and I would like to translate it to object MyTarget:

public class MyTarget {
    public String fieldA;
    public String fieldB;
}

使用默认的ModelMapper设置，我可以通过以下方式实现它:

using default ModelMapper settings I can achieve it in the following way:

ModelMapper modelMapper = new ModelMapper();
MySource src = new MySource("A field", "B field");
MyTarget trg = modelMapper.map(src, MyTarget.class); //success! fields are copied

但是，可能会发生 MySource 对象为 null 的情况.在这种情况下，MyTarget也将为 null :

It can happen however, that MySource object will be null. In this case, MyTarget will be also null:

    ModelMapper modelMapper = new ModelMapper();
    MySource src = null;
    MyTarget trg = modelMapper.map(src, MyTarget.class); //trg = null

我想以这种方式指定自定义映射，即(伪代码):

I would like to specify custom mapping in such a way, that (pseudo code):

MySource src！= null吗?[执行默认映射]:[返回新的MyTarget()]

有人知道如何编写合适的转换器来实现这一目标吗?

Does anybody have idea how to write an appropriate converter to achieve that?

推荐答案

无法直接使用ModelMapper，因为 ModelMapper map(Source，Destination)方法检查source是否为null，在这种情况下，它将引发异常...

It is not possible using ModelMapper directly because ModelMapper map(Source, Destination) method checks if source is null, in that case it throws an exception...

看看ModelMapper Map方法的实现:

Take a look to ModelMapper Map method implementation:

public <D> D map(Object source, Class<D> destinationType) {
    Assert.notNull(source, "source"); -> //IllegalArgument Exception
    Assert.notNull(destinationType, "destinationType");
    return mapInternal(source, null, destinationType, null);
  }

解决方案

我提议扩展ModelMapper类并重写 map(Object source，Class< D> destinationType)

public class MyCustomizedMapper extends ModelMapper{

    @Override
    public <D> D map(Object source, Class<D> destinationType) {
        Object tmpSource = source;
        if(source == null){
            tmpSource = new Object();
        }

        return super.map(tmpSource, destinationType);
    }
}

它检查source是否为null，在这种情况下，它会初始化，然后调用super map(Object source，Class< D> destinationType).

It checks if source is null, in that case it initializes then it calls super map(Object source, Class<D> destinationType).

最后，您可以像这样使用自定义的映射器:

Finally, you could use your customized mapper like this:

public static void main(String args[]){
    //Your customized mapper
    ModelMapper modelMapper = new MyCustomizedMapper();

    MySource src = null;
    MyTarget trg = modelMapper.map(src, MyTarget.class); //trg = null
    System.out.println(trg);
}

输出将为 new MyTarget():

因此它已初始化.

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