public abstract class JsonDeserializer<T> extends Object
ObjectMapper
(and
other chained JsonDeserializer
s too) to deserialize Objects of
arbitrary types from JSON, using provided JsonParser
.
Custom deserializers should usually not directly extend this class,
but instead extend StdDeserializer
(or its subtypes like StdScalarDeserializer
).
If deserializer is an aggregate one -- meaning it delegates handling of some
of its contents by using other deserializer(s) -- it typically also needs
to implement ResolvableDeserializer
,
which can locate dependant deserializers. This is important to allow dynamic
overrides of deserializers; separate call interface is needed to separate
resolution of dependant deserializers (which may have cyclic link back
to deserializer itself, directly or indirectly).
In addition, to support per-property annotations (to configure aspects
of deserialization on per-property basis), deserializers may want
to implement
ContextualDeserializer
,
which allows specialization of deserializers: call to
ContextualDeserializer.createContextual(com.fasterxml.jackson.databind.DeserializationContext, com.fasterxml.jackson.databind.BeanProperty)
is passed information on property, and can create a newly configured
deserializer for handling that particular property.
If both
ResolvableDeserializer
and
ContextualDeserializer
are implemented, resolution of deserializers occurs before
contextualization.
Modifier and Type | Class and Description |
---|---|
static class |
JsonDeserializer.None
This marker class is only to be used with annotations, to
indicate that no deserializer is configured.
|
Constructor and Description |
---|
JsonDeserializer() |
Modifier and Type | Method and Description |
---|---|
abstract T |
deserialize(com.fasterxml.jackson.core.JsonParser p,
DeserializationContext ctxt)
Method that can be called to ask implementation to deserialize
JSON content into the value type this serializer handles.
|
T |
deserialize(com.fasterxml.jackson.core.JsonParser p,
DeserializationContext ctxt,
T intoValue)
Alternate deserialization method (compared to the most commonly
used,
deserialize(JsonParser, DeserializationContext) ),
which takes in initialized value instance, to be
configured and/or populated by deserializer. |
Object |
deserializeWithType(com.fasterxml.jackson.core.JsonParser p,
DeserializationContext ctxt,
TypeDeserializer typeDeserializer)
Deserialization called when type being deserialized is defined to
contain additional type identifier, to allow for correctly
instantiating correct subtype.
|
SettableBeanProperty |
findBackReference(String refName)
Method needed by
BeanDeserializerFactory to properly link
managed- and back-reference pairs. |
JsonDeserializer<?> |
getDelegatee()
Accessor that can be used to determine if this deserializer uses
another deserializer for actual deserialization, by delegating
calls.
|
T |
getEmptyValue()
Method called to determine value to be used for "empty" values
(most commonly when deserializing from empty JSON Strings).
|
Collection<Object> |
getKnownPropertyNames()
Method that will
either return null to indicate that type being deserializers
has no concept of properties; or a collection of identifiers
for which
toString will give external property
name. |
T |
getNullValue()
Method that can be called to determine value to be used for
representing null values (values deserialized when JSON token
is
JsonToken.VALUE_NULL ). |
ObjectIdReader |
getObjectIdReader()
Accessor that can be used to check whether this deserializer
is expecting to possibly get an Object Identifier value instead of full value
serialization, and if so, should be able to resolve it to actual
Object instance to return as deserialized value.
|
Class<?> |
handledType()
Method for accessing type of values this deserializer produces.
|
boolean |
isCachable()
Method called to see if deserializer instance is cachable and
usable for other properties of same type (type for which instance
was created).
|
JsonDeserializer<?> |
replaceDelegatee(JsonDeserializer<?> delegatee)
Method that can be called to try to replace deserializer this deserializer
delegates calls to.
|
JsonDeserializer<T> |
unwrappingDeserializer(NameTransformer unwrapper)
Method that will return deserializer instance that is able
to handle "unwrapped" value instances
If no unwrapped instance can be constructed, will simply
return this object as-is.
|
public abstract T deserialize(com.fasterxml.jackson.core.JsonParser p, DeserializationContext ctxt) throws IOException, com.fasterxml.jackson.core.JsonProcessingException
Pre-condition for this method is that the parser points to the first event that is part of value to deserializer (and which is never JSON 'null' literal, more on this below): for simple types it may be the only value; and for structured types the Object start marker or a FIELD_NAME.
The two possible input conditions for structured types result from polymorphism via fields. In the ordinary case, Jackson calls this method when it has encountered an OBJECT_START, and the method implementation must advance to the next token to see the first field name. If the application configures polymorphism via a field, then the object looks like the following.
{ "@class": "class name", ... }Jackson consumes the two tokens (the @class field name and its value) in order to learn the class and select the deserializer. Thus, the stream is pointing to the FIELD_NAME for the first field after the @class. Thus, if you want your method to work correctly both with and without polymorphism, you must begin your method with:
if (jp.getCurrentToken() == JsonToken.START_OBJECT) { jp.nextToken(); }This results in the stream pointing to the field name, so that the two conditions align.
Post-condition is that the parser will point to the last event that is part of deserialized value (or in case deserialization fails, event that was not recognized or usable, which may be the same event as the one it pointed to upon call).
Note that this method is never called for JSON null literal, and thus deserializers need (and should) not check for it.
p
- Parsed used for reading JSON contentctxt
- Context that can be used to access information about
this deserialization activity.IOException
com.fasterxml.jackson.core.JsonProcessingException
public T deserialize(com.fasterxml.jackson.core.JsonParser p, DeserializationContext ctxt, T intoValue) throws IOException, com.fasterxml.jackson.core.JsonProcessingException
deserialize(JsonParser, DeserializationContext)
),
which takes in initialized value instance, to be
configured and/or populated by deserializer.
Method is not necessarily used (or supported) by all types
(it will not work for immutable types, for obvious reasons):
most commonly it is used for Collections and Maps.
It may be used both with "updating readers" (for POJOs) and
when Collections and Maps use "getter as setter".
Default implementation just throws
UnsupportedOperationException
, to indicate that types
that do not explicitly add support do not necessarily support
update-existing-value operation (esp. immutable types)
IOException
com.fasterxml.jackson.core.JsonProcessingException
public Object deserializeWithType(com.fasterxml.jackson.core.JsonParser p, DeserializationContext ctxt, TypeDeserializer typeDeserializer) throws IOException
Default implementation may work for some types, but ideally subclasses should not rely on current default implementation. Implementation is mostly provided to avoid compilation errors with older code.
typeDeserializer
- Deserializer to use for handling type informationIOException
public JsonDeserializer<T> unwrappingDeserializer(NameTransformer unwrapper)
Default implementation just returns 'this' indicating that no unwrapped variant exists
public JsonDeserializer<?> replaceDelegatee(JsonDeserializer<?> delegatee)
UnsupportedOperationException
(if operation does not
make sense or is not allowed); or return this deserializer as is.public Class<?> handledType()
Default implementation will return null, which means almost same
same as returning Object.class
would; that is, that
nothing is known about handled type.
public T getNullValue()
JsonToken.VALUE_NULL
). Usually this is simply
Java null, but for some types (especially primitives) it may be
necessary to use non-null values.
Note that deserializers are allowed to call this just once and then reuse returned value; that is, method is not guaranteed to be called once for each conversion.
Default implementation simply returns null.
public T getEmptyValue()
getNullValue()
(which in turn
is usually simply Java null), but it can be overridden
for types. Or, if type should never be converted from empty
String, method can also throw an exception.
Default implementation simple calls getNullValue()
and
returns value.
public Collection<Object> getKnownPropertyNames()
toString
will give external property
name.
This is only to be used for error reporting and diagnostics
purposes (most commonly, to accompany "unknown property"
exception).public boolean isCachable()
Note that cached instances are still resolved on per-property basis,
if instance implements ResolvableDeserializer
:
cached instance is just as the base. This means that in most cases it is safe to
cache instances; however, it only makes sense to cache instances
if instantiation is expensive, or if instances are heavy-weight.
Default implementation returns false, to indicate that no caching is done.
public ObjectIdReader getObjectIdReader()
Default implementation returns null, as support can not be implemented
generically. Some standard deserializers (most notably
BeanDeserializer
)
do implement this feature, and may return reader instance, depending on exact
configuration of instance (which is based on type, and referring property).
public JsonDeserializer<?> getDelegatee()
public SettableBeanProperty findBackReference(String refName)
BeanDeserializerFactory
to properly link
managed- and back-reference pairs.BeanDeserializerBase
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