Spring Boot支持与三种JSON mapping库集成：Gson、Jackson和JSON-B。Jackson是首选和默认的。

Jackson是spring-boot-starter-json的一部分，spring-boot-starter-web中包含spring-boot-starter-json。也就是说，当项目中引入spring-boot-starter-web后会自动引入spring-boot-starter-json。

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>

ObjectMapper

ObjectMapper是jackson-databind包中的一个类，提供读写JSON的功能，可以方便的进行对象和JSON转换：

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.IOException;
public final class JsonUtil {
private static ObjectMapper mapper = new ObjectMapper();
private JsonUtil() {
}
/**
* Serialize any Java value as a String.
*/
public static String generate(Object object) throws JsonProcessingException {
return mapper.writeValueAsString(object);
}
/**
* Deserialize JSON content from given JSON content String.
*/
public static <T> T parse(String content, Class<T> valueType) throws IOException {
return mapper.readValue(content, valueType);
}
}

编写一简单POJO测试类：

import java.util.Date;
public class Hero {
public static void main(String[] args) throws Exception {
System.out.println(JsonUtil.generate(new Hero("Jason", new Date())));
}
private String name;
private Date birthday;
public Hero() {
}
public Hero(String name, Date birthday) {
this.name = name;
this.birthday = birthday;
}
public String getName() {
return name;
}
public Date getBirthday() {
return birthday;
}
}

运行后输出结果如下：

{"name":"Jason","birthday":1540909420353}

可以看到日期转换为长整型。

ObjectMapper默认序列化配置启用了SerializationFeature.WRITE_DATES_AS_TIMESTAMPS，日期将转换为Timestamp。可查看如下源码：

public ObjectMapper(JsonFactory jf, DefaultSerializerProvider sp, DefaultDeserializationContext dc) {
...
BaseSettings base = DEFAULT_BASE.withClassIntrospector(defaultClassIntrospector());
_configOverrides = new ConfigOverrides();
_serializationConfig = new SerializationConfig(base, _subtypeResolver, mixins, rootNames, _configOverrides);
...
}

public SerializationConfig(BaseSettings base, SubtypeResolver str, SimpleMixInResolver mixins, RootNameLookup rootNames,
ConfigOverrides configOverrides)
{
super(base, str, mixins, rootNames, configOverrides);
_serFeatures = collectFeatureDefaults(SerializationFeature.class);
_filterProvider = null;
_defaultPrettyPrinter = DEFAULT_PRETTY_PRINTER;
_generatorFeatures = 0;
_generatorFeaturesToChange = 0;
_formatWriteFeatures = 0;
_formatWriteFeaturesToChange = 0;
}

默认情况下，Date类型序列化将调用DateSerializer的_timestamp 方法：

/**
* For efficiency, we will serialize Dates as longs, instead of
* potentially more readable Strings.
*/
@JacksonStdImpl
@SuppressWarnings("serial")
public class DateSerializer extends DateTimeSerializerBase<Date> {
...
@Override
protected long _timestamp(Date value) {
return (value == null) ? 0L : value.getTime();
}
@Override
public void serialize(Date value, JsonGenerator g, SerializerProvider provider) throws IOException {
if (_asTimestamp(provider)) {
g.writeNumber(_timestamp(value));
return;
}
_serializeAsString(value, g, provider);
}
}

DateTimeSerializerBase的_asTimestamp方法：

protected boolean _asTimestamp(SerializerProvider serializers)
{
if (_useTimestamp != null) {
return _useTimestamp.booleanValue();
}
if (_customFormat == null) {
if (serializers != null) {
return serializers.isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
}
// 12-Jun-2014, tatu: Is it legal not to have provider? Was NPE:ing earlier so leave a check
throw new IllegalArgumentException("Null SerializerProvider passed for "+handledType().getName());
}
return false;
}

禁用WRITE_DATES_AS_TIMESTAMPS

若要将日期序列化为字符串，可禁用SerializationFeature.WRITE_DATES_AS_TIMESTAMPS：

mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);

这时序列化将调用StdDateFormat的format()方法，使用ISO-8601兼容格式"yyyy-MM-dd'T'HH:mm:ss.SSSZ"，输出内容如下：

{"name":"Jason","birthday":"2018-10-31T03:07:34.485+0000"}

StdDateFormat反序列化支持ISO-8601兼容格式和RFC-1123("EEE, dd MMM yyyy HH:mm:ss zzz")格式。

@JsonFormat

使用@JsonFormat注解，代替全局设置，是一种更灵活的方法：

@JsonFormat(shape = JsonFormat.Shape.STRING)
private Date birthday;

还可以定义pattern：

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
private Date birthday;

当自定义pattern后，将创建新的SimpleDateFormat实例来序列化日期，参见DateTimeSerializerBase的createContextual()方法：

public JsonSerializer<?> createContextual(SerializerProvider serializers, BeanProperty property) throws JsonMappingException
{
...
if (format.hasPattern()) {
final Locale loc = format.hasLocale() ? format.getLocale() : serializers.getLocale();
SimpleDateFormat df = new SimpleDateFormat(format.getPattern(), loc);
TimeZone tz = format.hasTimeZone() ? format.getTimeZone() : serializers.getTimeZone();
df.setTimeZone(tz);
return withFormat(Boolean.FALSE, df);
}
...
}

Spring Boot与Jackson ObjectMapper

Spring Boot使用HttpMessageConverters处理HTTP交换中的内容转换。当classpath中存在Jackson时，Jackson2ObjectMapperBuilder将是默认的Converter，源码请查看HttpMessageConverters和WebMvcConfigurationSupport：
HttpMessageConverters

private List<HttpMessageConverter<?>> getDefaultConverters() {
List<HttpMessageConverter<?>> converters = new ArrayList<>();
if (ClassUtils.isPresent("org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport", null)) {
converters.addAll(new WebMvcConfigurationSupport() {
public List<HttpMessageConverter<?>> defaultMessageConverters() {
return super.getMessageConverters();
}
}.defaultMessageConverters());
}
else {
converters.addAll(new RestTemplate().getMessageConverters());
}
reorderXmlConvertersToEnd(converters);
return converters;
}

WebMvcConfigurationSupport

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
stringHttpMessageConverter.setWriteAcceptCharset(false); // see SPR-7316
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(stringHttpMessageConverter);
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new ResourceRegionHttpMessageConverter());
messageConverters.add(new SourceHttpMessageConverter<>());
messageConverters.add(new AllEncompassingFormHttpMessageConverter());
...
if (jackson2Present) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
}
...
}

默认，Jackson2ObjectMapperBuilder将创建ObjectMapper实例：
Jackson2ObjectMapperBuilder

public <T extends ObjectMapper> T build() {
ObjectMapper mapper;
if (this.createXmlMapper) {
mapper = (this.defaultUseWrapper != null ?
new XmlObjectMapperInitializer().create(this.defaultUseWrapper) :
new XmlObjectMapperInitializer().create());
}
else {
mapper = (this.factory != null ? new ObjectMapper(this.factory) : new ObjectMapper());
}
configure(mapper);
return (T) mapper;
}

ObjectMapper以下属性被禁用：

MapperFeature.DEFAULT_VIEW_INCLUSION
DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES
SerializationFeature.WRITE_DATES_AS_TIMESTAMPS

Jackson2ObjectMapperBuilder

private void customizeDefaultFeatures(ObjectMapper objectMapper) {
if (!this.features.containsKey(MapperFeature.DEFAULT_VIEW_INCLUSION)) {
configureFeature(objectMapper, MapperFeature.DEFAULT_VIEW_INCLUSION, false);
}
if (!this.features.containsKey(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)) {
configureFeature(objectMapper, DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
}
}

JacksonAutoConfiguration

static {
Map<Object, Boolean> featureDefaults = new HashMap<>();
featureDefaults.put(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
FEATURE_DEFAULTS = Collections.unmodifiableMap(featureDefaults);
}

自定义Jackson ObjectMapper配置

针对ObjectMapper的六种Feature，Spring Boot都提供了相应的配置，列表如下：

Feature(Enum)	Spring Boot Property	Values
com.fasterxml.jackson.databind.DeserializationFeature	spring.jackson.deserialization.feature_name	true, false
com.fasterxml.jackson.core.JsonGenerator.Feature	spring.jackson.generator.feature_name	true, false
com.fasterxml.jackson.databind.MapperFeature	spring.jackson.mapper.feature_name	true, false
com.fasterxml.jackson.core.JsonParser.Feature	spring.jackson.parser.feature_name	true, false
com.fasterxml.jackson.databind.SerializationFeature	spring.jackson.serialization.feature_name	true, false
com.fasterxml.jackson.annotation.JsonInclude.Include	spring.jackson.default-property-inclusion	always, non_null, non_absent, non_default, non_empty

例如，为启用美化打印，设置spring.jackson.serialization.indent_output=true，相当于启用SerializationFeature.INDENT_OUTPUT，配置中忽略feature_name大小写。

其他的Jackson配置属性：

spring.jackson.date-format= # Date format string or a fully-qualified date format class name. For instance, yyyy-MM-dd HH:mm:ss.
spring.jackson.joda-date-time-format= # Joda date time format string. If not configured, "date-format" is used as a fallback if it is configured with a format string.
spring.jackson.locale= # Locale used for formatting.
spring.jackson.property-naming-strategy= # One of the constants on Jackson's PropertyNamingStrategy. Can also be a fully-qualified class name of a PropertyNamingStrategy subclass.
spring.jackson.time-zone= # Time zone used when formatting dates. For instance, "America/Los_Angeles" or "GMT+10".

其中spring.jackson.date-format默认值为com.fasterxml.jackson.databind.util.StdDateFormat。

@DateTimeFormat和@JsonFormat

在REST编程中，当提交application/json的POST/PUT请求时，JSON会通过Jackson进行转换。当提交GET请求时，如参数中包含日期，后台代码需要使用注解@DateTimeFormat：

@DateTimeFormat(pattern = "yyyy-MM-dd")
private Date startDate;

两者可以同时使用：

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
@DateTimeFormat(pattern = "yyyy-MM-dd")
private Date startDate;
参考：
https://blog.csdn.net/weixin_44048532/article/details/86647880

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