Spring Boot中Jackson ObjectMapper应用详解

 

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。

2. org.springframework.boot

3. spring-boot-starter-web

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：

1. import com.fasterxml.jackson.core.JsonProcessingException;

2. import com.fasterxml.jackson.databind.ObjectMapper;

3.  

4. import java.io.IOException;

5.  

6. public final class JsonUtil {

7. private static ObjectMapper mapper = new ObjectMapper();

8.  

9. private JsonUtil() {

10. }

11.  

12. /**

13. * Serialize any Java value as a String.

14. */

15. public static String generate(Object object) throws JsonProcessingException {

16. return mapper.writeValueAsString(object);

17. }

18.  

19. /**

20. * Deserialize JSON content from given JSON content String.

21. */

22. public static T parse(String content, Class valueType) throws IOException {

23. return mapper.readValue(content, valueType);

24. }

25. }

编写一简单POJO测试类：

1. import java.util.Date;

2.  

3. public class Hero {

4.  

5. public static void main(String[] args) throws Exception {

6. System.out.println(JsonUtil.generate(new Hero("Jason", new Date())));

7. }

8.  

9. private String name;

10.  

11. private Date birthday;

12.  

13. public Hero() {

14. }

15.  

16. public Hero(String name, Date birthday) {

17. this.name = name;

18. this.birthday = birthday;

19. }

20.  

21. public String getName() {

22. return name;

23. }

24.  

25. public Date getBirthday() {

26. return birthday;

27. }

28. }

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：

1. public ObjectMapper(JsonFactory jf, DefaultSerializerProvider sp, DefaultDeserializationContext dc) {

2. ...

3. BaseSettings base = DEFAULT_BASE.withClassIntrospector(defaultClassIntrospector());

4. _configOverrides = new ConfigOverrides();

5. _serializationConfig = new SerializationConfig(base, _subtypeResolver, mixins, rootNames, _configOverrides);

6. ...

7. }

1. public SerializationConfig(BaseSettings base, SubtypeResolver str, SimpleMixInResolver mixins, RootNameLookup rootNames,

2. ConfigOverrides configOverrides)

3. {

4. super(base, str, mixins, rootNames, configOverrides);

5. _serFeatures = collectFeatureDefaults(SerializationFeature.class);

6. _filterProvider = null;

7. _defaultPrettyPrinter = DEFAULT_PRETTY_PRINTER;

8. _generatorFeatures = 0;

9. _generatorFeaturesToChange = 0;

10. _formatWriteFeatures = 0;

11. _formatWriteFeaturesToChange = 0;

12. }

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

1. /**

2. * For efficiency, we will serialize Dates as longs, instead of

3. * potentially more readable Strings.

4. */

5. @JacksonStdImpl

6. @SuppressWarnings("serial")

7. public class DateSerializer extends DateTimeSerializerBase {

8. ...

9.  

10. @Override

11. protected long _timestamp(Date value) {

12. return (value == null) ? 0L : value.getTime();

13. }

14.  

15. @Override

16. public void serialize(Date value, JsonGenerator g, SerializerProvider provider) throws IOException {

17. if (_asTimestamp(provider)) {

18. g.writeNumber(_timestamp(value));

19. return;

20. }

21. _serializeAsString(value, g, provider);

22. }

23. }

DateTimeSerializerBase的_asTimestamp方法：

1. protected boolean _asTimestamp(SerializerProvider serializers)

2. {

3. if (_useTimestamp != null) {

4. return _useTimestamp.booleanValue();

5. }

6. if (_customFormat == null) {

7. if (serializers != null) {

8. return serializers.isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);

9. }

10. // 12-Jun-2014, tatu: Is it legal not to have provider? Was NPE:ing earlier so leave a check

11. throw new IllegalArgumentException("Null SerializerProvider passed for "+handledType().getName());

12. }

13. return false;

14. }

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：

1. @JsonFormat(shape = JsonFormat.Shape.STRING)

2. private Date birthday;

还可以定义pattern：

1. @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")

2. 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：

1. public JsonSerializer createContextual(SerializerProvider serializers, BeanProperty property) throws JsonMappingException

2. {

3. ...

4. if (format.hasPattern()) {

5. final Locale loc = format.hasLocale() ? format.getLocale() : serializers.getLocale();

6. SimpleDateFormat df = new SimpleDateFormat(format.getPattern(), loc);

7. TimeZone tz = format.hasTimeZone() ? format.getTimeZone() : serializers.getTimeZone();

8. df.setTimeZone(tz);

9. return withFormat(Boolean.FALSE, df);

10. }

11. ...

12. }

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

1. private List> getDefaultConverters() {

2. List> converters = new ArrayList<>();

3. if (ClassUtils.isPresent("org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport", null)) {

4. converters.addAll(new WebMvcConfigurationSupport() {

5. public List> defaultMessageConverters() {

6. return super.getMessageConverters();

7. }

8. }.defaultMessageConverters());

9. }

10. else {

11. converters.addAll(new RestTemplate().getMessageConverters());

12. }

13. reorderXmlConvertersToEnd(converters);

14. return converters;

15. }

WebMvcConfigurationSupport

1. protected final void addDefaultHttpMessageConverters(List> messageConverters) {

2. StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();

3. stringHttpMessageConverter.setWriteAcceptCharset(false); // see SPR-7316

4.  

5. messageConverters.add(new ByteArrayHttpMessageConverter());

6. messageConverters.add(stringHttpMessageConverter);

7. messageConverters.add(new ResourceHttpMessageConverter());

8. messageConverters.add(new ResourceRegionHttpMessageConverter());

9. messageConverters.add(new SourceHttpMessageConverter<>());

10. messageConverters.add(new AllEncompassingFormHttpMessageConverter());

11.  

12. ...

13.  

14. if (jackson2Present) {

15. Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();

16. if (this.applicationContext != null) {

17. builder.applicationContext(this.applicationContext);

18. }

19. messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));

20. }

21. ...

22. }

默认，Jackson2ObjectMapperBuilder将创建ObjectMapper实例：
Jackson2ObjectMapperBuilder

1. public T build() {

2. ObjectMapper mapper;

3. if (this.createXmlMapper) {

4. mapper = (this.defaultUseWrapper != null ?

5. new XmlObjectMapperInitializer().create(this.defaultUseWrapper) :

6. new XmlObjectMapperInitializer().create());

7. }

8. else {

9. mapper = (this.factory != null ? new ObjectMapper(this.factory) : new ObjectMapper());

10. }

11. configure(mapper);

12. return (T) mapper;

13. }

ObjectMapper以下属性被禁用：

• MapperFeature.DEFAULT_VIEW_INCLUSION
• DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES
• SerializationFeature.WRITE_DATES_AS_TIMESTAMPS

Jackson2ObjectMapperBuilder

1. private void customizeDefaultFeatures(ObjectMapper objectMapper) {

2. if (!this.features.containsKey(MapperFeature.DEFAULT_VIEW_INCLUSION)) {

3. configureFeature(objectMapper, MapperFeature.DEFAULT_VIEW_INCLUSION, false);

4. }

5. if (!this.features.containsKey(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)) {

6. configureFeature(objectMapper, DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

7. }

8. }

JacksonAutoConfiguration

1. static {

2. Map featureDefaults = new HashMap<>();

3. featureDefaults.put(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);

4. FEATURE_DEFAULTS = Collections.unmodifiableMap(featureDefaults);

5. }

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：

1. @DateTimeFormat(pattern = "yyyy-MM-dd")

2. private Date startDate;

两者可以同时使用：

1. @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")

2. @DateTimeFormat(pattern = "yyyy-MM-dd")

3. private Date startDate;

4. 参考：

5. https://blog.csdn.net/weixin_44048532/article/details/86647880