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Spring Boot中Jackson ObjectMapper应用详解

2019-05-07 18:06 796 查看

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：

[code]{"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：

[code]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：

4000

[code]{"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://www.geek-share.com/detail/2759404365.html

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