The attribute @JsonDeserialize#as can be used to to deserialize to a specific subtype of declared type of a Java object.

When serializing, Jackson will by default use the specific runtime type, but during deserializing time Jackson cannot determined the actual intended runtime type of a super type without @JsonDeserialize#as.

Example

Java Object

In this example we will assign Number to BigDecimal before serialization.

package com.logicbig.example;

import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import java.math.BigDecimal;

public class CurrencyRate {
    private String pair;

    @JsonDeserialize(as = BigDecimal.class)
    private Number rate;
    .............
}

Main class

public class ExampleMain {
    public static void main(String[] args) throws IOException {
        System.out.println("-- Java object to JSON (serialization) --");
        CurrencyRate cr = new CurrencyRate();
        cr.setPair("USD/JPY");
        cr.setRate(new BigDecimal(109.15).setScale(2, RoundingMode.CEILING));
        System.out.println("Java object: " + cr);

        ObjectMapper om = new ObjectMapper();
        String s2 = om.writeValueAsString(cr);
        System.out.println("JSON string: " + s2);

        System.out.println("-- JSON to Java object (deserialization) --");
        CurrencyRate cr2 = om.readValue(s2, CurrencyRate.class);
        System.out.println("Java Object: " + cr2);
        System.out.println("rate type: "+cr2.getRate().getClass());
    }
}

-- Java object to JSON (serialization) --
Java object: CurrencyRate{pair='USD/JPY', rate=109.16}
JSON string: {"pair":"USD/JPY","rate":109.16}
-- JSON to Java object (deserialization) --
Java Object: CurrencyRate{pair='USD/JPY', rate=109.16}
rate type: class java.math.BigDecimal

Without @JsonDeserialize#as

If we don't use @JsonDeserialize(as = BigDecimal.class) i.e.

public class CurrencyRate {
  private String pair;
  private Number rate;
 .....
}

then output will be:

-- Java object to JSON (serialization) --
Java object: CurrencyRate{pair='USD/JPY', rate=109.16}
JSON string: {"pair":"USD/JPY","rate":109.16}
-- JSON to Java object (deserialization) --
Java Object: CurrencyRate{pair='USD/JPY', rate=109.16}
rate type: class java.lang.Double

Example Project

Dependencies and Technologies Used:

• jackson-databind 2.9.5: General data-binding functionality for Jackson: works on core streaming API.
• JDK 10
• Maven 3.3.9