Implementing ObjectWriter and ObjectReader for Custom Serialization and Deserialization

1. Implementing the ObjectWriter Interface for Custom Serialization

1.1 ObjectWriter Definition

package com.alibaba.fastjson2.writer;

public interface ObjectWriter<T> {
    void write(JSONWriter jsonWriter, Object object, Object fieldName, Type fieldType, long features);
}

1.2 ObjectWriter Implementation Example

class InetAddressWriter implements ObjectWriter {
    static final InetAddressWriter INSTANCE = new InetAddressWriter();

    @Override
    public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type fieldType, long features) {
        if (object == null) {
            jsonWriter.writeNull();
            return;
        }

        InetAddress address = (InetAddress) object;
        // Prioritize using the name
        jsonWriter.writeString(address.getHostName());
    }
}

1.3 Registering the Writer

JSON.register(InetAddress.class, InetAddressWriter.INSTANCE);
JSON.register(Inet4Address.class, InetAddressWriter.INSTANCE);
JSON.register(Inet6Address.class, InetAddressWriter.INSTANCE);

1.4 Registering Some Built-in Implementations of ObjectWriters

ObjectWriters provides ofToString/ofToInt/ofToLong methods to make serialization more convenient for users, for example:

 public static class Bean {
    public int id;

    public String toString() {
        return Integer.toString(id);
    }

    public int getId() {
        return id;
    }
}

@Test
public void testToString() {
    ObjectWriter objectWriter = ObjectWriters.ofToString(Bean::toString);

    Bean bean = new Bean();
    bean.id = 101;

    JSONWriter jsonWriter = JSONWriter.of();
    objectWriter.write(jsonWriter, bean);
    assertEquals("\"101\"", jsonWriter.toString());
}

@Test
public void testToInt() {
    ObjectWriter<Bean> objectWriter = ObjectWriters.ofToInt(
            (ToIntFunction<Bean>) Bean::getId
    );

    Bean bean = new Bean();
    bean.id = 101;

    JSONWriter jsonWriter = JSONWriter.of();
    objectWriter.write(jsonWriter, bean);
    assertEquals("101", jsonWriter.toString());
}

2. Implementing ObjectReader for Custom Deserialization

2.1 ObjectReader Definition

package com.alibaba.fastjson2.reader;

public interface ObjectReader<T> {
    T readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features);
}

ObjectReader has many other methods, but they all provide default implementations. The method above must be implemented.

2.2 ObjectReader Implementation Example

import java.time.Instant;
import com.alibaba.fastjson2.reader.ObjectReader;

class InstantReader implements ObjectReader {
    public static final InstantReader INSTANCE = new InstantReader();
    @Override
    public Object readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features) {
        if (jsonReader.nextIfNull()) {
            return null;
        }

        long millis = jsonReader.readInt64Value();
        return Instant.ofEpochMilli(millis);
    }
}

2.3 Registering the Reader

JSON.register(Instant.class, InstantReader.INSTANCE);

2.4 Registering Some Built-in Implementations Provided by ObjectReaders

ObjectReaders provides ofString/ofInt/ofLong methods to make deserialization more convenient for users, for example:

public static class Bean {
    final int code;
    public Bean(String str) {
        code = Integer.parseInt(str);
    }
}

@Test
public void test() {
    JSON.register(Bean.class, ObjectReaders.ofString(Bean::new));
    
    Bean bean = JSON.parseObject("\"123\"", Bean.class);
    assertEquals(123, bean.code);
}