Spring Boot支持與三種JSON mapping庫集成：Gson、Jackson和JSON-B。Jackson是首選和默認的。

Jackson是spring-boot-starter-json的一部分，spring-boot-starter-web中包含spring-boot-starter-json。也就是說，當項目中引入spring-boot-starter-web後會自動引入spring-boot-starter-json。

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

ObjectMapper

ObjectMapper是jackson-databind包中的一個類，提供讀寫JSON的功能，可以方便的進行對象和JSON轉換：

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);
    }
}

編寫一簡單POJO測試類：

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;
    }
}

運行後輸出結果如下：

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

可以看到日期轉換爲長整型。

ObjectMapper默認序列化配置啓用了SerializationFeature.WRITE_DATES_AS_TIMESTAMPS，日期將轉換爲Timestamp。可查看如下源碼：

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;
}

默認情況下，Date類型序列化將調用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

若要將日期序列化爲字符串，可禁用SerializationFeature.WRITE_DATES_AS_TIMESTAMPS：

mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);

這時序列化將調用StdDateFormat的format()方法，使用ISO-8601兼容格式"yyyy-MM-dd'T'HH:mm:ss.SSSZ"，輸出內容如下：

{"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註解，代替全局設置，是一種更靈活的方法：

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

還可以定義pattern：

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

當自定義pattern後，將創建新的SimpleDateFormat實例來序列化日期，參見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與Jackson ObjectMapper

Spring Boot使用HttpMessageConverters處理HTTP交換中的內容轉換。當classpath中存在Jackson時，Jackson2ObjectMapperBuilder將是默認的Converter，源碼請查看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()));
    }
    ...
}

默認，Jackson2ObjectMapperBuilder將創建ObjectMapper實例：
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以下屬性被禁用：

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);
}

自定義Jackson ObjectMapper配置

針對ObjectMapper的六種Feature，Spring Boot都提供了相應的配置，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，爲啓用美化打印，設置spring.jackson.serialization.indent_output=true，相當於啓用SerializationFeature.INDENT_OUTPUT，配置中忽略feature_name大小寫。

其他的Jackson配置屬性：

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默認值爲com.fasterxml.jackson.databind.util.StdDateFormat。

@DateTimeFormat和@JsonFormat

在REST編程中，當提交application/json的POST/PUT請求時，JSON會通過Jackson進行轉換。當提交GET請求時，如參數中包含日期，後臺代碼需要使用註解@DateTimeFormat：

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

兩者可以同時使用：

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

Spring Boot中Jackson ObjectMapper應用詳解

ObjectMapper

禁用WRITE_DATES_AS_TIMESTAMPS

@JsonFormat

Spring Boot與Jackson ObjectMapper

自定義Jackson ObjectMapper配置

@DateTimeFormat和@JsonFormat

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