在AWS DynamoDB JAVA SDK中，null值会被转换为true。如果您希望将null值转换为其他值或不进行转换，可以使用DynamoDBMapperConfig对象来配置转换行为。

下面是一个示例代码，演示如何使用DynamoDBMapperConfig来自定义null值的转换行为：

import com.amazonaws.services.dynamodbv2.document.DynamoDB;
import com.amazonaws.services.dynamodbv2.document.DynamoDBMapper;
import com.amazonaws.services.dynamodbv2.document.Item;
import com.amazonaws.services.dynamodbv2.document.Table;
import com.amazonaws.services.dynamodbv2.model.PutItemResult;
import com.amazonaws.services.dynamodbv2.document.spec.PutItemSpec;
import com.amazonaws.services.dynamodbv2.document.utils.ValueMap;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDB;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClientBuilder;

public class DynamoDBExample {
    public static void main(String[] args) {
        // Create an instance of AmazonDynamoDB
        AmazonDynamoDB client = AmazonDynamoDBClientBuilder.standard().build();
        
        // Create a DynamoDBMapperConfig object
        DynamoDBMapperConfig mapperConfig = DynamoDBMapperConfig.builder()
                .withSaveBehavior(DynamoDBMapperConfig.SaveBehavior.CLOBBER)
                .build();
        
        // Create a DynamoDBMapper object with the custom mapperConfig
        DynamoDBMapper mapper = new DynamoDBMapper(client, mapperConfig);
        
        // Create an instance of the Item class
        Item item = new Item()
                .withPrimaryKey("id", 1)
                .withNull("nullValue", true);
        
        // Save the item to DynamoDB
        mapper.save(item);
        
        // Retrieve the item from DynamoDB
        Table table = new DynamoDB(client).getTable("YourTableName");
        Item retrievedItem = table.getItem("id", 1);
        
        // Print the retrieved item
        System.out.println(retrievedItem.toJSONPretty());
    }
}

在上面的示例中，我们创建了一个DynamoDBMapperConfig对象，并通过withSaveBehavior方法将其配置为DynamoDBMapperConfig.SaveBehavior.CLOBBER。这将使null值被转换为true。

请替换YourTableName为您的表名，并根据您的实际需求进行其他自定义配置。