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@JsonInclude是一种json序列化规则，有下面这些值，某些值的具体作用我也没测过，有需要的大家可以自己研究一下。
/**
* Enumeration used with {@link JsonInclude}
* to define which properties
* of Java Beans are to be included in serialization.
*/
public enum Include
{
/**
* Value that indicates that property is to be always included,
* independent of value of the property.
*/
ALWAYS,
/**
* Value that indicates that only properties with non-null
* values are to be included.
*/
NON_NULL,
/**
* Value that indicates that properties are included unless their value
* is:
*<ul>
* <li>null</li>
* <li>"absent" value of a referential type (like Java 8 `Optional`, or
* {link java.utl.concurrent.atomic.AtomicReference}); that is, something
* that would not deference to a non-null value.
* </ul>
* This option is mostly used to work with "Optional"s (Java 8, Guava).
*
* @since 2.6
*/
NON_ABSENT,
/**
* Value that indicates that only properties with null value,
* or what is considered empty, are not to be included.
* Definition of emptiness is data type specific; see below
* for details on actual handling.
*<p>
* Default emptiness for all types includes:
*<ul>
* <li><code>Null</code> values.</li>
* <li>"Absent" values (see {@link #NON_ABSENT})</li>
*</ul>
* so that as baseline, "empty" set includes values that would be
* excluded by both {@link #NON_NULL} and {@link #NON_ABSENT}.
*<br>
* Beyond this base, following types have additional empty values:
*<ul>
* <li>For {@link java.util.Collection}s and {@link java.util.Map}s,
* method <code>isEmpty()</code> is called;
* </li>
* <li>For Java arrays, empty arrays are ones with length of 0
* </li>
* <li>For Java {@link java.lang.String}s, <code>length()</code> is called,
* and return value of 0 indicates empty String
* </li>
* </ul>
* and for other types, null values are excluded but other exclusions (if any).
*<p>
* Note that this default handling can be overridden by custom
* <code>JsonSerializer</code> implementation: if method <code>isEmpty()</code>
* is overridden, it will be called to see if non-null values are
* considered empty (null is always considered empty).
*<p>
* Compatibility note: Jackson 2.6 included a wider range of "empty" values than
* either earlier (up to 2.5) or later (2.7 and beyond) types; specifically:
*<ul>
* <li>Default values of primitive types (like <code>0</code> for `int`/`java.lang.Integer`
* and `false` for `bool`/`Boolean`)
* </li>
* <li>Timestamp 0 for date/time types
* </li>
*</ul>
* With 2.7, definition has been tightened back to only containing types explained
* above (null, absent, empty String, empty containers), and now
* extended definition may be specified using {@link #NON_DEFAULT}.
*/
NON_EMPTY,
/**
* Meaning of this setting depends on context: whether annotation is
* specified for POJO type (class), or not. In latter case annotation
* is either used as the global default, or as property override.
*<p>
* When used for a POJO, definition is that only values that differ from
* the default values of POJO properties are included. This is done
* by creating an instance of POJO using zero-argument constructor,
* and accessing property values: value is used as the default value
* by using <code>equals()</code> method, except for the case where property
* has `null` value in which case straight null check is used.
*<p>
* When NOT used for a POJO (that is, as a global default, or as property
* override), definition is such that:
*<ul>
* <li>All values considered "empty" (as per {@link #NON_EMPTY}) are excluded</li>
* <li>Primitive/wrapper default values are excluded</li>
* <li>Date/time values that have timestamp (`long` value of milliseconds since
* epoch, see {@link java.util.Date}) of `0L` are excluded</li>
* </ul>
*/
NON_DEFAULT,
/**
* Value that indicates that separate `filter` Object (specified by
* {@link JsonInclude#valueFilter} for value itself, and/or
* {@link JsonInclude#contentFilter} for contents of structured types)
* is to be used for determining inclusion criteria.
* Filter object's <code>equals()</code> method is called with value
* to serialize; if it returns <code>true</code> value is <b>excluded</b>
* (that is, filtered out); if <code>false</code> value is <b>included</b>.
*
* @since 2.9
*/
CUSTOM,
/**
* Pseudo-value used to indicate that the higher-level defaults make
* sense, to avoid overriding inclusion value. For example, if returned
* for a property this would use defaults for the class that contains
* property, if any defined; and if none defined for that, then
* global serialization inclusion details.
*
* @since 2.6
*/
USE_DEFAULTS
;
}我在类上加了这个注解：
@JsonInclude(value = JsonInclude.Include.NON_EMPTY)
public class User {
private String id;
....如果User对象某些属性为空，则序列化时不会有这个字段。