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How do I use a custom serializer with Jackson?

Barbara Streisand
Barbara StreisandOriginal
2024-11-26 15:42:11509browse

How do I use a custom serializer with Jackson?

How do I use a custom Serializer with Jackson?

Context

To customize the serialization of a Java class using Jackson, one can define a custom serializer that extends the JsonSerializer class.

Implementation

1. Create a Custom Serializer for Item:

public class ItemSerializer extends JsonSerializer<Item> {

    @Override
    public void serialize(Item value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException, JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeNumberField("id", value.id);
        jgen.writeNumberField("itemNr", value.itemNr);
        jgen.writeNumberField("createdBy", value.createdBy.id);
        jgen.writeEndObject();
    }

}

2. Create a Custom Serialization Module and Register the Serializer:

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", new Version(1,0,0,null));
simpleModule.addSerializer(new ItemSerializer());
mapper.registerModule(simpleModule);

3. Serialize the Item Object:

StringWriter writer = new StringWriter();
mapper.writeValue(writer, myItem);

Issue Resolution

The error encountered when registering the custom serializer (java.lang.IllegalArgumentException: JsonSerializer of type ... does not define valid handledType()) indicates that the serializer does not specify the type it handles.

To resolve this issue, ensure that the JsonSerializer implementation correctly defines its handledType() method.

Alternative: Using Annotations

An alternative approach is to annotate a field with @JsonSerialize(using = CustomSerializer.class) to use a specific serializer.

Example: Customizing Date Serialization

To customize date serialization, create a custom serializer that extends SerializerBase:

public class CustomDateSerializer extends SerializerBase<Date> {

    public CustomDateSerializer() {
        super(Date.class, true);
    }

    @Override
    public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)
        throws IOException, JsonProcessingException {
        SimpleDateFormat formatter = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'ZZZ (z)");
        String format = formatter.format(value);
        jgen.writeString(format);
    }

}

And then add the annotation to the field:

@JsonSerialize(using = CustomDateSerializer.class)
private Date createdAt;

This allows for detailed control over the serialization and deserialization process.

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