如何使用ModelMapper仅将源对象字段值映射到目标对象空/Null字段？

现有Foo和Bar两个结构相似的类，使用ModelMapper进行字段映射时，默认会覆盖目标对象的所有字段值，但我们需要实现仅填充目标对象的空/Null字段、保留非空值的逻辑。

相关代码 #

Foo类 #

class Foo {
    public Foo(int fooId, String name, int age) {
        this.fooId = fooId;
        this.name = name;
        this.age = age;
    }

    private int fooId;
    private String name;
    private int age;

    // Getters, Setters, toString方法已省略
}

Bar类 #

class Bar {
    public Bar(int fooId, String name, int age) {
        this.fooId = fooId;
        this.name = name;
        this.age = age;
    }

    private int fooId;
    private String name;
    private int age;

    // Getters, Setters, toString方法已省略
}

测试代码 #

public class Mapper {
    public static void main(String args[]) throws IOException {
        Foo foo = new Foo(1, "ABC", 30);            
        Bar bar = new Bar(2, null, 25);
        ModelMapper modelMapper = new ModelMapper();
        modelMapper.map(foo, bar);      
        System.out.println(bar.toString());
    }
}

结果对比 #

• 实际输出：Bar [fooId=1, name=ABC, age=30]
• 预期输出：Bar [fooId=2, name=ABC, age=25]

利用ModelMapper的**条件映射（Condition）**功能，自定义判断规则：仅当目标字段为null时才执行赋值操作。

全局配置方案（所有字段生效） #

public class Mapper {
    public static void main(String args[]) throws IOException {
        Foo foo = new Foo(1, "ABC", 30);            
        Bar bar = new Bar(2, null, 25);
        
        ModelMapper modelMapper = new ModelMapper();
        // 设置全局映射条件：目标字段为null时才覆盖
        modelMapper.getConfiguration()
                   .setPropertyCondition(context -> context.getDestination() == null);
        
        modelMapper.map(foo, bar);      
        System.out.println(bar.toString());
    }
}

精细化配置方案（指定字段生效） #

如果只需要对特定字段应用该规则，可通过TypeMap单独配置：

public class Mapper {
    public static void main(String args[]) throws IOException {
        Foo foo = new Foo(1, "ABC", 30);            
        Bar bar = new Bar(2, null, 25);
        
        ModelMapper modelMapper = new ModelMapper();
        // 创建Foo到Bar的类型映射
        TypeMap<Foo, Bar> typeMap = modelMapper.createTypeMap(Foo.class, Bar.class);
        
        // 仅对name字段应用"目标为null才覆盖"的规则
        typeMap.addMappings(mapper -> 
            mapper.when(context -> context.getDestination() == null)
                  .map(Foo::getName, Bar::setName));
        
        modelMapper.map(foo, bar);      
        System.out.println(bar.toString());
    }
}

运行上述代码后，输出结果将符合预期：Bar [fooId=2, name=ABC, age=25]

内容的提问来源于stack exchange，提问作者Hullar