Modelmapper:当源对象为null时，如何应用自定义映射? [英] Modelmapper: How to apply custom mapping when source object is null?

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

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():

输出控制台:NullExampleMain.MyTarget(fieldA = null，fieldB = null)

Output Console: NullExampleMain.MyTarget(fieldA=null, fieldB=null)

因此它已初始化.

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