使用自定义注释将空字符串转换为null

Question

我想注释字符串字段。例如：

@EmptyToNull
String name

我不想验证该领域。在发送DTO中的字符串的空值时，我想将空白转换为null。也许杰克逊数据处理器有一个解决方案？例如：

@JsonSetter(nulls = Nulls.AS_EMPTY) *changes null to empty*

我需要带有逻辑的注释，但在任何地方都找不到示例。有人帮忙吗？

Answer 1

Deserialization

Extend StringDeserializer

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.deser.std.StringDeserializer;

import java.io.IOException;

public class EmptyToNull extends StringDeserializer {
    @Override
    public String deserialize(JsonParser parser, DeserializationContext ctx) throws IOException {
        String result = super.deserialize(parser, ctx);
        if (result != null && result.isEmpty()) {
            return null;
        }
        return result;
    }
}

Use EmptyToNull deserializer for specific field. It will look like custom annotation

public class MyClass {
    public String title;
    @JsonDeserialize(using = EmptyToNull.class)
    public String name;
}

Also, you can register the deserializer globally for all strings for ObjectMapper

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new SimpleModule().addDeserializer(String.class, new EmptyToNull()));

Alternative solution

Implement logic in constructor

public class MyClass {
    private String title;
    private String name;

    @JsonCreator
    public MyClass(@JsonProperty("title") String title,@JsonProperty("name") String name) {
        this.title = title;
        this.name = name!= null && name.isEmpty() ? null : name;
    }
}

Or create custom setter for field

public class MyClass {
    private String title;
    private String name;

    @JsonSetter
    public void setName(String name) {
        this.name = name != null && name.isEmpty() ? null : name;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

Serialization

Jackson provides out of box annotation

public class MyClass {
    public String title;
    @JsonInclude(JsonInclude.Include.NON_EMPTY)
    public String name;
}