Jackson 反序列化其他未知属性

2019-11-16 687 words 2 minutes

Contents

1. 概述

在本教程中，我们将了解 Jackson 2.x 的解组过程，特别是如何处理具有未知属性的 JSON 内容。

2. 使用附加/未知字段解组 JSON

JSON 输入有各种形状和大小，大多数时候，我们需要将其映射到具有一定数量字段的预定义 Java 对象。目标是简单地忽略任何无法映射到现有 Java 字段的 JSON 属性。例如，假设我们需要将 JSON 解组为以下 Java 实体：

public class MyDto {
    private String stringValue;
    private int intValue;
    private boolean booleanValue;
    // standard constructor, getters and setters 
}

2.1. 未知字段上的UnrecognizedPropertyException

尝试将具有未知属性的 JSON 解组到这个简单的 Java 实体将导致com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException：

@Test(expected = UnrecognizedPropertyException.class)
public void givenJsonHasUnknownValues_whenDeserializing_thenException()
  throws JsonParseException, JsonMappingException, IOException {
    String jsonAsString = 
        "{"stringValue":"a"," +
        ""intValue":1," +
        ""booleanValue":true," +
        ""stringValue2":"something"}";
    ObjectMapper mapper = new ObjectMapper();
    MyDto readValue = mapper.readValue(jsonAsString, MyDto.class);
    assertNotNull(readValue);
}

这将失败，但出现以下异常：

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: 
Unrecognized field "stringValue2" (class com.blogdemo.jackson.ignore.MyDto), 
not marked as ignorable (3 known properties: "stringValue", "booleanValue", "intValue"])

2.2. 使用ObjectMapper处理未知字段

我们现在可以配置完整的ObjectMapper以忽略 JSON 中的未知属性：

new ObjectMapper()
  .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)

然后我们应该能够将这种 JSON 读入预定义的 Java 实体：

@Test
public void givenJsonHasUnknownValuesButJacksonIsIgnoringUnknowns_whenDeserializing_thenCorrect()
  throws JsonParseException, JsonMappingException, IOException {
 
    String jsonAsString = 
        "{"stringValue":"a"," +
        ""intValue":1," +
        ""booleanValue":true," +
        ""stringValue2":"something"}";
    ObjectMapper mapper = new ObjectMapper()
      .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    MyDto readValue = mapper.readValue(jsonAsString, MyDto.class);
    assertNotNull(readValue);
    assertThat(readValue.getStringValue(), equalTo("a"));
    assertThat(readValue.isBooleanValue(), equalTo(true));
    assertThat(readValue.getIntValue(), equalTo(1));
}

2.3. 在类级别处理未知字段

我们还可以将单个类标记为接受未知字段，而不是整个 Jackson ObjectMapper：

@JsonIgnoreProperties(ignoreUnknown = true)
public class MyDtoIgnoreUnknown { ... }

现在我们应该能够测试与以前相同的行为。未知字段被简单地忽略，只映射已知字段：

@Test
public void givenJsonHasUnknownValuesButIgnoredOnClass_whenDeserializing_thenCorrect() 
  throws JsonParseException, JsonMappingException, IOException {
 
    String jsonAsString =
        "{"stringValue":"a"," +
        ""intValue":1," +
        ""booleanValue":true," +
        ""stringValue2":"something"}";
    ObjectMapper mapper = new ObjectMapper();
    MyDtoIgnoreUnknown readValue = mapper
      .readValue(jsonAsString, MyDtoIgnoreUnknown.class);
    assertNotNull(readValue);
    assertThat(readValue.getStringValue(), equalTo("a"));
    assertThat(readValue.isBooleanValue(), equalTo(true));
    assertThat(readValue.getIntValue(), equalTo(1));
}

3. 解组不完整的 JSON

与其他未知字段类似，解组不完整的 JSON，即不包含 Java 类中所有字段的 JSON，对 Jackson 来说不是问题：

@Test
public void givenNotAllFieldsHaveValuesInJson_whenDeserializingAJsonToAClass_thenCorrect() 
  throws JsonParseException, JsonMappingException, IOException {
    String jsonAsString = "{"stringValue":"a","booleanValue":true}";
    ObjectMapper mapper = new ObjectMapper();
    MyDto readValue = mapper.readValue(jsonAsString, MyDto.class);
    assertNotNull(readValue);
    assertThat(readValue.getStringValue(), equalTo("a"));
    assertThat(readValue.isBooleanValue(), equalTo(true));
}