使用fastasterxml .jackson将JSON解析为Java记录

Question

根据设计，Java记录不能从另一个对象继承(参见为什么Java记录不支持继承?)所以我想知道什么是实现以下目标的最好方法。

给定我的JSON数据包含有一些共同数据+唯一数据的对象。例如，类型、宽度和高度是所有形状，但根据类型的不同，它们可以有额外的字段:

{
"name": "testDrawing",
"shapes": [
{
"type": "shapeA",
"width": 100,
"height": 200,
"label": "test"
},
{
"type": "shapeB",
"width": 100,
"height": 200,
"length": 300
},
{
"type": "shapeC",
"width": 100,
"height": 200,
"url": "www.test.be",
"color": "#FF2233"
}
]
}

在"traditional"在Java中，你可以用

BaseShape with width and height
ShapeA extends BaseShape with label
ShapeB extends BaseShape with length
ShapeC extends BaseShape with URL and color

但是我有点固执，真的很想用唱片。

我的解决方案现在看起来像这样:

没有BaseShape

• 公共字段在所有记录中重复

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record Drawing(
@JsonProperty("name")
String name,
@JsonProperty("shapes")
@JsonDeserialize(using = TestDeserializer.class)
List<Object> shapes // I don't like the Objects here... 
) {
}
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeA (
@JsonProperty("type") String type,
@JsonProperty("width") Integer width,
@JsonProperty("height") Integer height,
@JsonProperty("label") String label
) {
}
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeB(
@JsonProperty("type") String type,
@JsonProperty("width") Integer width,
@JsonProperty("height") Integer height,
@JsonProperty("length") Integer length
) {
}
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeC(
@JsonProperty("type") String type,
@JsonProperty("width") Integer width,
@JsonProperty("height") Integer height,
@JsonProperty("url") String url,
@JsonProperty("color") String color
) {
}

我不喜欢重复的代码，这是一个不好的做法…但最后我可以用这个helper类来加载它:

public class TestDeserializer extends JsonDeserializer {
ObjectMapper mapper = new ObjectMapper();
@Override
public List<Object> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
List<Object> rt = new ArrayList<>();
JsonNode node = jsonParser.getCodec().readTree(jsonParser);
if (node instanceof ArrayNode array) {
for (Iterator<JsonNode> it = array.elements(); it.hasNext(); ) {
JsonNode childNode = it.next();
rt.add(getShape(childNode));
}
} else {
rt.add(getShape(node));
}
return rt;
}
private Object getShape(JsonNode node) {
var type = node.get("type").asText();
switch (type) {
case "shapeA":
return mapper.convertValue(node, ShapeA.class);
case "shapeB":
return mapper.convertValue(node, ShapeB.class);
case "shapeC":
return mapper.convertValue(node, ShapeC.class);
default:
throw new IllegalArgumentException("Shape could not be parsed");
}
}
}

这个测试证明工作正常:

@Test
void fromJsonToJson() throws IOException, JSONException {
File f = new File(this.getClass().getResource("/test.json").getFile());
String jsonFromFile = Files.readString(f.toPath());
ObjectMapper mapper = new ObjectMapper();
Drawing drawing = mapper.readValue(jsonFromFile, Drawing.class);
String jsonFromObject = mapper.writeValueAsString(drawing);
System.out.println("Original:n" + jsonFromFile.replace("n", "").replace(" ", ""));
System.out.println("Generated:n" + jsonFromObject);
assertAll(
//() -> assertEquals(jsonFromFile, jsonFromObject),
() -> assertEquals("testDrawing", drawing.name()),
() -> assertTrue(drawing.shapes().get(0) instanceof ShapeA),
() -> assertTrue(drawing.shapes().get(1) instanceof ShapeB),
() -> assertTrue(drawing.shapes().get(2) instanceof ShapeC)
);
}

用Jackson库和Java Records实现这一目标的最好方法是什么?

额外的旁注:我还需要能够以与原始格式相同的格式写回JSON。

Answer 1

记录不能继承，因为它们打算作为一个可靠的契约，但它们可以实现一个接口。因此，您可以在Jackson 2.12或以上版本的JasonSubTypes中执行类似的操作:

模型

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record Drawing(
String name,
List<BaseShape> shapes
) { }
// added benefit of interface here is it reminds you to have the default fields
@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
@JsonSubTypes({
@JsonSubTypes.Type(ShapeA.class),
@JsonSubTypes.Type(ShapeB.class),
@JsonSubTypes.Type(ShapeC.class)
})
public interface BaseShape {
Integer width();
Integer height();
}
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeA (
Integer width,
Integer height,
String label
) implements BaseShape { }
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeB(
Integer width,
Integer height,
Integer length
) implements BaseShape { }
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public record ShapeC(
Integer width,
Integer height,
String url,
String color
) implements BaseShape { }

测试类

@Slf4j
class DemoTest {
private ObjectMapper objectMapper = ObjectMapperBuilder.getObjectMapper();
@Test
void test() throws JsonProcessingException {
final String testString = objectMapper
.writerWithDefaultPrettyPrinter()
.writeValueAsString(
new Drawing(
"happy",
List.of(
new ShapeA(1, 1, "happyShape"),
new ShapeB(2, 2, 3),
new ShapeC(2, 2, "www.shape.com/shape", "blue"
)
)
)
);
log.info("From model to string {}", testString);
Drawing drawing = objectMapper.readValue(testString, Drawing.class);
log.info(
"Captured types {}",
drawing
.shapes()
.stream()
.map(s -> s.getClass().getName())
.collect(Collectors.toSet())
);
log.info(
"From string back to model then again to string {}",
objectMapper
.writerWithDefaultPrettyPrinter()
.writeValueAsString(drawing)
);
}
}

下面是测试日志的输出:

17:06:41.293 [Test worker] INFO com.demo.DemoTest - From model to string {
"name" : "happy",
"shapes" : [ {
"width" : 1,
"height" : 1,
"label" : "happyShape"
}, {
"width" : 2,
"height" : 2,
"length" : 3
}, {
"width" : 2,
"height" : 2,
"url" : "www.shape.com/shape",
"color" : "blue"
} ]
}
17:06:41.353 [Test worker] INFO com.demo.DemoTest - Captured types [com.demo.DemoTest$ShapeB, com.demo.DemoTest$ShapeA, com.demo.DemoTest$ShapeC]
17:06:41.354 [Test worker] INFO com.demo.DemoTest - From string back to model then again to string {
"name" : "happy",
"shapes" : [ {
"width" : 1,
"height" : 1,
"label" : "happyShape"
}, {
"width" : 2,
"height" : 2,
"length" : 3
}, {
"width" : 2,
"height" : 2,
"url" : "www.shape.com/shape",
"color" : "blue"
} ]
}

请注意，您可以将type字段添加为@JsonSubTypes.Type注释的name属性，但只要记录中的字段永远不会完全相同，这就可以使用或不使用discriminator。

你可以在这里阅读更多关于JsonSubtypes的信息.