Jackson注解 @JsonInclude

落日映苍穹つ 2022-05-08 12:44 209阅读 0赞

@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.
 *
 * 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}.
 * 
 * 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).
 *
 * 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).
 *
 * 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.
 *
 * 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.
 *
 * 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 excluded
 * (that is, filtered out); if <code>false</code> value is included.
 *
 * @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。

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