Jackson常用配置示例

对输出缩进
Fail on Unknown Properties
允许序列化空对象
将空字符串反序列化为 null POJO
空字符串与 Java simple objects
序列化时的时间格式化
原文链接

Jackson 提供了 SerializationFeature、 DeserializationFeature 来自定义 ObjectMapper 的行为。

对输出缩进

public class MyObject {private int intVal;private String stringVal;private List<String> list;.............
}

默认配置下的 JSON 输出是没有缩进的：

MyObject myObject = new MyObject(3, "test String",
List.of("one", "two"));
ObjectMapper om = new ObjectMapper();
String s = om.writeValueAsString(myObject);
System.out.println(s);

{"intVal":3,"stringVal":"test String","list":["one","two"]}

可以用 om.enable(..)、om.configure(.., true) 开启缩进或其他功能：

MyObject myObject = new MyObject(3, "test String",List.of("one", "two"));
ObjectMapper om = new ObjectMapper();om.enable(SerializationFeature.INDENT_OUTPUT);
// om.configure(SerializationFeature.INDENT_OUTPUT, true);String s = om.writeValueAsString(myObject);
System.out.println(s);

{"intVal" : 3,"stringVal" : "test String","list" : [ "one", "two" ]
}

Fail on Unknown Properties

反序列化时，若遇到未知的属性（没有对应的属性，也没有setter 或 handler来处理这样的属性）会抛出异常，

public class MyObject2 {private int intVal;public MyObject2(int intVal) {this.intVal = intVal;}public MyObject2() {}...
}

默认会报错：

String jsonSting = "{\"intVal\":3,\"strVal\":\"3\"}";
ObjectMapper om = new ObjectMapper();
System.out.println(jsonSting);
MyObject2 myObject = om.readValue(jsonSting, MyObject2.class);
System.out.println(myObject);

{"intVal":3,"strVal":"3"}
Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "strVal" (class org.example.c2.MyObject2), not marked as ignorable (one known property: "intVal"])at [Source: (String)"{"intVal":3,"strVal":"3"}"; line: 1, column: 23] (through reference chain: org.example.c2.MyObject2["strVal"])at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:61)at com.fasterxml.jackson.databind.DeserializationContext.handleUnknownProperty(DeserializationContext.java:987)at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:1974)at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1701)at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1679)at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:330)at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:187)at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4593)at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3548)at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3516)at org.example.c2.DisableFailOnUnknownProps.disableFailOnUnknownProps2(DisableFailOnUnknownProps.java:49)at org.example.c2.DisableFailOnUnknownProps.main(DisableFailOnUnknownProps.java:19)

禁用 DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES

String jsonSting = "{\"intVal\":3,\"strVal\":\"3\"}";ObjectMapper om = new ObjectMapper();
om.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);System.out.println(jsonSting);
MyObject2 myObject = om.readValue(jsonSting, MyObject2.class);
System.out.println(myObject);

{"intVal":3,"strVal":"3"}
MyObject2{intVal=3}

允许序列化空对象

MyEmptyObject 没有可序列化的属性，Jackson 默认的字段属性发现规则：所有被 public 修饰的字段、所有被 public 修饰的getter。没有可序列化的属性时，会抛出异常。

public class MyEmptyObject {private int i = 10;
}

默认：

MyEmptyObject myObject = new MyEmptyObject();
ObjectMapper om = new ObjectMapper();
String s = om.writeValueAsString(myObject);
System.out.println(s);

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.example.c2.MyEmptyObject and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1276)at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:400)at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71)at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33)at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)at com.fasterxml.jackson.databind.ObjectMapper._writeValueAndClose(ObjectMapper.java:4487)at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3742)at org.example.c2.DisableFailOnEmptyBeans.defaultOutput(DisableFailOnEmptyBeans.java:24)at org.example.c2.DisableFailOnEmptyBeans.main(DisableFailOnEmptyBeans.java:17)Process finished with exit code 1

禁用 SerializationFeature.FAIL_ON_EMPTY_BEANS

MyEmptyObject myObject = new MyEmptyObject();
ObjectMapper om = new ObjectMapper();
om.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
String s = om.writeValueAsString(myObject);
System.out.println(s);

{}

将空字符串反序列化为 null POJO

默认情况下，空字符串不能反序列化为 null 对象，会抛异常：

String s = "{\"myObjKey\":\"\"}";
System.out.println("input:\n" + s);
ObjectMapper om = new ObjectMapper();
Map<String, MyObject> myObject = om.readValue(s, new TypeReference<Map<String, MyObject>>() { });
System.out.println(myObject);

input:
{"myObjKey":""}
Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot coerce empty String ("") to `org.example.c2.MyObject` value (but could if coercion was enabled using `CoercionConfig`)at [Source: (String)"{"myObjKey":""}"; line: 1, column: 13] (through reference chain: java.util.LinkedHashMap["myObjKey"])at com.fasterxml.jackson.databind.exc.InvalidFormatException.from(InvalidFormatException.java:67)at com.fasterxml.jackson.databind.DeserializationContext.reportBadCoercion(DeserializationContext.java:1666)at com.fasterxml.jackson.databind.deser.std.StdDeserializer._checkCoercionFail(StdDeserializer.java:1432)at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromEmptyString(StdDeserializer.java:325)at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromString(StdDeserializer.java:270)at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1495)at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:207)at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:197)at com.fasterxml.jackson.databind.deser.std.MapDeserializer._readAndBindStringKeyMap(MapDeserializer.java:609)at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:437)at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:32)at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4593)at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3548)at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3531)at org.example.c2.EnableEmptyStringAsNullPojo.defaultOutput(EnableEmptyStringAsNullPojo.java:25)at org.example.c2.EnableEmptyStringAsNullPojo.main(EnableEmptyStringAsNullPojo.java:17)Process finished with exit code 1

启用 DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT，此设置只能用于 POJO 或一些结构化的值（比如 java.util.Maps、java.util.Collection），但不能用于标量值（scalar values ，比如 boolean 和 number），是否可以强制使用它们取决于 MapperFeature.ALLOW_COERCION_OF_SCALARS。

String s = "{\"myObjKey\":\"\"}";
System.out.println("input:\n" + s);
ObjectMapper om = new ObjectMapper();
om.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
Map<String, MyObject> myObject = om.readValue(s, new TypeReference<Map<String, MyObject>>() { });
System.out.println(myObject);

input:
{"myObjKey":""}
{myObjKey=null}

空字符串与 Java simple objects

Java simple objects 不受 DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT 的控制。

BigDecimal

String s = "{\"myObjKey\":\"\"}";
System.out.println("input:\n " + s);
ObjectMapper om = new ObjectMapper();
Map<String, BigDecimal> myObject = om.readValue(s,
new TypeReference<Map<String, BigDecimal>>() { });
System.out.println(myObject);

input:{"myObjKey":""}
{myObjKey=null}

String

String s = "{\"myObjKey\":\"\"}";
System.out.println("input:\n " + s);ObjectMapper om = new ObjectMapper();
Map<String, String> myObject = om.readValue(s,
new TypeReference<Map<String, String>>() { });
System.out.println(myObject);

input:{"myObjKey":""}
{myObjKey=}

序列化时的时间格式化

默认情况下，时间会以时间戳的形式序列化：

Date date = new Date();
ObjectMapper om = new ObjectMapper();
String s = om.writeValueAsString(Map.of("myDate", date));
System.out.println(s);

{"myDate":1618323884853}

禁用 SerializationFeature.WRITE_DATES_AS_TIMESTAMPS

Date date = new Date();
ObjectMapper om = new ObjectMapper();
om.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
String s = om.writeValueAsString(Map.of("myDate", date));
System.out.println(s);

{"myDate":"2021-04-13T14:24:45.061+00:00"}

原文链接

Jackson JSON - Jackson Commonly used Configuration Examples