Module tools.jackson.databind
Package tools.jackson.databind

Class ValueDeserializer<T>

java.lang.Object
tools.jackson.databind.ValueDeserializer<T>
All Implemented Interfaces:
NullValueProvider
Direct Known Subclasses:
AbstractDeserializer, ErrorThrowingDeserializer, StdDeserializer, TypeWrappedDeserializer, ValueDeserializer.None
public abstract class ValueDeserializer<T> extends Object implements NullValueProvider
Abstract class that defines API used by ObjectMapper and ObjectReader to deserialize Objects of arbitrary types from JSON, using provided JsonParser (within current read context of DeserializationContext. Deserializers use delegation so that calls typically end up as a stack of calls through deserializer hierarchy based on POJO properties.

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 resolve(tools.jackson.databind.DeserializationContext) which can locate dependent deserializers. This is important to allow dynamic overrides of deserializers; separate call interface is needed to separate resolution of dependent 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 override createContextual(tools.jackson.databind.DeserializationContext, tools.jackson.databind.BeanProperty) which allows specialization of deserializers: it is passed information on property, and can create a newly configured deserializer for handling that particular property.
Resolution of deserializers occurs before contextualization.

NOTE: In Jackson 2.x was named JsonDeserializer

  • Constructor Details

    • ValueDeserializer

      public ValueDeserializer()

  • Method Details

    • resolve

      public void resolve(DeserializationContext ctxt)
      Method called after deserializer instance has been constructed (and registered as necessary by provider objects), but before it has returned it to the caller. Called object can then resolve its dependencies to other types, including self-references (direct or indirect).
      Parameters:
      ctxt - Context to use for accessing configuration, resolving secondary deserializers

    • createContextual

      public ValueDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property)
      Method called to see if a different (or differently configured) deserializer is needed to deserialize values of specified property. Note that instance that this method is called on is typically shared one and as a result method should NOT modify this instance but rather construct and return a new instance. This instance should only be returned as-is, in case it is already suitable for use.
      Parameters:
      ctxt - Deserialization context to access configuration, additional deserializers that may be needed by this deserializer
      property - Method, field or constructor parameter that represents the property (and is used to assign deserialized value). Should be available; but there may be cases where caller cannot provide it and null is passed instead (in which case impls usually pass 'this' deserializer as is)
      Returns:
      Deserializer to use for deserializing values of specified property; may be this instance or a new instance.

    • deserialize

      public abstract T deserialize(JsonParser p, DeserializationContext ctxt) throws JacksonException
      Method that can be called to ask implementation to deserialize JSON content into the value type this serializer handles. Returned instance is to be constructed by method itself.

      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 (p.currentToken() == JsonToken.START_OBJECT) {
         p.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).

      Handling null values (JsonToken.VALUE_NULL)
      : Note that this method is never called for the JSON null literal to avoid every deserializer from having to handle null values. Instead, the getNullValue(DeserializationContext) method is called to produce a null value. To influence null handling, custom deserializers should override getNullValue(DeserializationContext) and usually also getNullAccessPattern().

      Parameters:
      p - Parser used for reading JSON content
      ctxt - Context that can be used to access information about this deserialization activity.
      Returns:
      Deserialized value
      Throws:
      JacksonException

    • deserialize

      public T deserialize(JsonParser p, DeserializationContext ctxt, T intoValue) throws JacksonException
      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. 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)

      Throws:
      JacksonException

    • deserializeWithType

      public Object deserializeWithType(JsonParser p, DeserializationContext ctxt, TypeDeserializer typeDeserializer) throws JacksonException
      Deserialization called when type being deserialized is defined to contain additional type identifier, to allow for correctly instantiating correct subtype. This can be due to annotation on type (or its supertype), or due to global settings without annotations.

      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.

      Parameters:
      typeDeserializer - Deserializer to use for handling type information
      Throws:
      JacksonException

    • deserializeWithType

      public Object deserializeWithType(JsonParser p, DeserializationContext ctxt, TypeDeserializer typeDeserializer, T intoValue) throws JacksonException
      Method similar to deserializeWithType(JsonParser,DeserializationContext,TypeDeserializer) but called when merging value. Considered "bad merge" by default implementation, but if MapperFeature.IGNORE_MERGE_FOR_UNMERGEABLE is enabled will simple delegate to deserializeWithType(JsonParser, DeserializationContext, TypeDeserializer).
      Throws:
      JacksonException

    • unwrappingDeserializer

      public ValueDeserializer<T> unwrappingDeserializer(DeserializationContext ctxt, 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.

      Default implementation just returns 'this' indicating that no unwrapped variant exists

    • replaceDelegatee

      public ValueDeserializer<?> replaceDelegatee(ValueDeserializer<?> delegatee)
      Method that can be called to try to replace deserializer this deserializer delegates calls to. If not supported (either this deserializer does not delegate anything; or it does not want any changes), should either throw UnsupportedOperationException (if operation does not make sense or is not allowed); or return this deserializer as is.

    • handledType

      public Class<?> handledType()
      Method for accessing concrete physical type of values this deserializer produces. Note that this information is not guaranteed to be exact -- it may be a more generic (super-type) -- but it should not be incorrect (return a non-related type).

      Default implementation will return null, which means almost same same as returning Object.class would; that is, that nothing is known about handled type.

      Returns:
      Physical type of values this deserializer produces, if known; null if not

    • logicalType

      public LogicalType logicalType()
      Method for accessing logical type of values this deserializer produces. Typically used for further configuring handling of values, for example, to find which coercions are legal.
      Returns:
      Logical type of values this deserializer produces, if known; null if not

    • isCachable

      public 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).

      Note that cached instances are still contextualized on per-property basis (but note that resolve(DeserializationContext)d just once!) 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.

    • getDelegatee

      public ValueDeserializer<?> getDelegatee()
      Accessor that can be used to determine if this deserializer uses another deserializer for actual deserialization, by delegating calls. If so, will return immediate delegate (which itself may delegate to further deserializers); otherwise will return null.
      Returns:
      Deserializer this deserializer delegates calls to, if null; null otherwise.

    • getKnownPropertyNames

      public 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. This is only to be used for error reporting and diagnostics purposes (most commonly, to accompany "unknown property" exception).

    • getNullValue

      public Object getNullValue(DeserializationContext ctxt)
      Method that can be called to determine value to be used for representing null values (values deserialized when JSON token is JsonToken.VALUE_NULL). Usually this is simply Java null, but for some types (especially primitives) it may be necessary to use non-null values.

      This method may be called once, or multiple times, depending on what getNullAccessPattern() returns.

      Default implementation simply returns null.

      Specified by:
      getNullValue in interface NullValueProvider

    • getNullAccessPattern

      public AccessPattern getNullAccessPattern()
      This method may be called in conjunction with calls to getNullValue(DeserializationContext), to check whether it needs to be called just once (static values), or each time empty value is needed.

      Default implementation indicates that the "null value" to use for input null does not vary across uses so that getNullValue(DeserializationContext) need not be called more than once per deserializer instance. This information may be used as optimization.

      Specified by:
      getNullAccessPattern in interface NullValueProvider

    • getAbsentValue

      public Object getAbsentValue(DeserializationContext ctxt)
      Method called to determine placeholder value to be used for cases where no value was obtained from input but we must pass a value nonetheless: the common case is that of Creator methods requiring passing a value for every parameter. Usually this is same as getNullValue(tools.jackson.databind.DeserializationContext) (which in turn is usually simply Java null), but it can be overridden for specific types: most notable scalar types must use "default" values.

      This method needs to be called every time a determination is made.

      Default implementation simply calls getNullValue(tools.jackson.databind.DeserializationContext) and returns value.

      Specified by:
      getAbsentValue in interface NullValueProvider
      Since:
      2.13

    • getEmptyValue

      public Object getEmptyValue(DeserializationContext ctxt)
      Method called to determine value to be used for "empty" values (most commonly when deserializing from empty JSON Strings). Usually this is same as getNullValue(tools.jackson.databind.DeserializationContext) (which in turn is usually simply Java null), but it can be overridden for specific types. Or, if type should never be converted from empty String, method can also throw an exception.

      This method may be called once, or multiple times, depending on what getEmptyAccessPattern() returns.

      Default implementation simply calls getNullValue(tools.jackson.databind.DeserializationContext) and returns value.

    • getEmptyAccessPattern

      public AccessPattern getEmptyAccessPattern()
      This method may be called in conjunction with calls to getEmptyValue(DeserializationContext), to check whether it needs to be called just once (static values), or each time empty value is needed.

    • getObjectIdReader

      public ObjectIdReader getObjectIdReader(DeserializationContext ctxt)
      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.

      Default implementation returns null, as support cannot 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).

      Returns:
      ObjectIdReader used for resolving possible Object Identifier value, instead of full value serialization, if deserializer can do that; null if no Object Id is expected.

    • findBackReference

      public SettableBeanProperty findBackReference(String refName)
      Method needed by BeanDeserializerFactory to properly link managed- and back-reference pairs.

    • supportsUpdate

      public Boolean supportsUpdate(DeserializationConfig config)
      Introspection method that may be called to see whether deserializer supports update of an existing value (aka "merging") or not. Return value should either be Boolean.FALSE if update is not supported at all (immutable values); Boolean.TRUE if update should usually work (regular POJOs, for example), or null if this is either not known, or may sometimes work.

      Information gathered is typically used to either prevent merging update for property (either by skipping, if based on global defaults; or by exception during deserializer construction if explicit attempt made) if Boolean.FALSE returned, or inclusion if Boolean.TRUE is specified. If "unknown" case (null returned) behavior is to exclude property if global defaults used; or to allow if explicit per-type or property merging is defined.

      Default implementation returns null to allow explicit per-type or per-property attempts.