Spring boot与Jackson ObjectMapper

【Spring boot与Jackson ObjectMapper】莫问天涯路几重，轻衫侧帽且从容。这篇文章主要讲述Spring boot与Jackson ObjectMapper相关的知识，希望能为你提供帮助。
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>

ObjectMapperObjectMapper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(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，日期将转换为长整型。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，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 =https://www.songbingjia.com/android/= 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; }

待续....