为了在Assertj中使用Jackson JsonNode，需要使用“JsonNodeAssert”类来测试JsonNode对象。以下是示例代码：

import static org.assertj.core.api.Assertions.assertThat;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;

public class JsonNodeTest {
    
    @Test
    public void testJsonNode() throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        String json = "{\"name\":\"John\",\"age\":30,\"car\":null}";
        JsonNode jsonNode = mapper.readTree(json);

        assertThat(jsonNode).isNotNull();
        assertThat(jsonNode).hasField("name");
        assertThat(jsonNode).hasField("age");
        assertThat(jsonNode).isNull();
    }
}

在代码中，“JsonNodeAssert”类的方法“hasField”和“isNull”可用于测试JsonNode对象的字段和null值。