com.fasterxml.jackson.core/jackson-databind/2.11.0 : com/fasterxml/jackson/databind/JsonDeserializer.java

JsonDeserializer
http://github.com/FasterXML/jackson: General data-binding functionality for Jackson: works on core streaming API (FasterXML)
The Apache Software License, Version 2.0
Tatu Saloranta
Christopher Currie
Paul Brown
package com.fasterxml.jackson.databind;

import java.io.IOException;
import java.util.Collection;

import com.fasterxml.jackson.core.*;

import com.fasterxml.jackson.databind.deser.*;
import com.fasterxml.jackson.databind.deser.impl.ObjectIdReader;
import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
import com.fasterxml.jackson.databind.util.AccessPattern;
import com.fasterxml.jackson.databind.util.NameTransformer;

Abstract class that defines API used by ObjectMapper (and other chained JsonDeserializers 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 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. /**
 * Abstract class that defines API used by {@link ObjectMapper} (and
 * other chained {@link JsonDeserializer}s too) to deserialize Objects of
 * arbitrary types from JSON, using provided {@link JsonParser}.
 *<p>
 * Custom deserializers should usually not directly extend this class,
 * but instead extend {@link com.fasterxml.jackson.databind.deser.std.StdDeserializer}
 * (or its subtypes like {@link com.fasterxml.jackson.databind.deser.std.StdScalarDeserializer}).
 *<p>
 * 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 {@link com.fasterxml.jackson.databind.deser.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).
 *<p>
 * In addition, to support per-property annotations (to configure aspects
 * of deserialization on per-property basis), deserializers may want
 * to implement
 * {@link com.fasterxml.jackson.databind.deser.ContextualDeserializer},
 * which allows specialization of deserializers: call to
 * {@link com.fasterxml.jackson.databind.deser.ContextualDeserializer#createContextual}
 * is passed information on property, and can create a newly configured
 * deserializer for handling that particular property.
 *<p>
 * If both
 * {@link com.fasterxml.jackson.databind.deser.ResolvableDeserializer} and
 * {@link com.fasterxml.jackson.databind.deser.ContextualDeserializer}
 * are implemented, resolution of deserializers occurs before
 * contextualization.
 */
public abstract class JsonDeserializer<T>
    implements NullValueProvider // since 2.9
{
    /*
    /**********************************************************
    /* Main deserialization methods
    /**********************************************************
     */
    
    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).

Note that this method is never called for JSON null literal,
and thus deserializers need (and should) not check for it.
Params: p – Parsed used for reading JSON content
ctxt – Context that can be used to access information about
  this deserialization activity.
Returns: Deserialized value/**
     * 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.
     *<p>
     * 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.
     * </p>
     * <p>
     * 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.
     *  <pre>
     *      {
     *          "@class": "class name",
     *          ...
     *      }
     *  </pre>
     *  Jackson consumes the two tokens (the <tt>@class</tt> 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:
     *  <pre>
     *       if (p.currentToken() == JsonToken.START_OBJECT) {
     *         p.nextToken();
     *       }
     *  </pre>
     * This results in the stream pointing to the field name, so that
     * the two conditions align.
     * <p>
     * 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).
     *<p>
     * Note that this method is never called for JSON null literal,
     * and thus deserializers need (and should) not check for it.
     *
     * @param p Parsed used for reading JSON content
     * @param ctxt Context that can be used to access information about
     *   this deserialization activity.
     *
     * @return Deserialized value
     */
    public abstract T deserialize(JsonParser p, DeserializationContext ctxt)
        throws IOException, JsonProcessingException;

    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) /**
     * Alternate deserialization method (compared to the most commonly
     * used, {@link #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".
     *<p>
     * Default implementation just throws
     * {@link UnsupportedOperationException}, to indicate that types
     * that do not explicitly add support do not necessarily support
     * update-existing-value operation (esp. immutable types)
     */
    public T deserialize(JsonParser p, DeserializationContext ctxt, T intoValue)
        throws IOException
    {
        ctxt.handleBadMerge(this);
        return deserialize(p, ctxt);
    }

    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.
Params: typeDeserializer – Deserializer to use for handling type information/**
     * 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.
     *<p>
     * 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.
     * 
     * @param typeDeserializer Deserializer to use for handling type information
     */
    public Object deserializeWithType(JsonParser p, DeserializationContext ctxt,
            TypeDeserializer typeDeserializer)
        throws IOException
    {
        // We could try calling 
        return typeDeserializer.deserializeTypedFromAny(p, ctxt);
    }

    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). 
Since: 2.10/**
     * Method similar to {@link #deserializeWithType(JsonParser,DeserializationContext,TypeDeserializer)}
     * but called when merging value. Considered "bad merge" by default implementation,
     * but if {@link MapperFeature#IGNORE_MERGE_FOR_UNMERGEABLE} is enabled will simple delegate to
     * {@link #deserializeWithType(JsonParser, DeserializationContext, TypeDeserializer)}.
     *
     * @since 2.10
     */
    public Object deserializeWithType(JsonParser p, DeserializationContext ctxt,
            TypeDeserializer typeDeserializer, T intoValue)
        throws IOException
    {
        ctxt.handleBadMerge(this);
        return deserializeWithType(p, ctxt, typeDeserializer);
    }

    /*
    /**********************************************************
    /* Fluent factory methods for constructing decorated versions
    /**********************************************************
     */

    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
/**
     * 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.
     *<p>
     * Default implementation just returns 'this'
     * indicating that no unwrapped variant exists
     */
    public JsonDeserializer<T> unwrappingDeserializer(NameTransformer unwrapper) {
        return this;
    }

    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. 
Since: 2.1/**
     * 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 {@link UnsupportedOperationException} (if operation does not
     * make sense or is not allowed); or return this deserializer as is.
     * 
     * @since 2.1
     */
    public JsonDeserializer<?> replaceDelegatee(JsonDeserializer<?> delegatee) {
        throw new UnsupportedOperationException();
    }

    /*
    /**********************************************************
    /* Introspection methods for figuring out configuration/setup
    /* of this deserializer instance and/or type it handles
    /**********************************************************
     */

    Method for accessing 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.

Since: 2.3/**
     * Method for accessing 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).
     *<p>
     * Default implementation will return null, which means almost same
     * same as returning <code>Object.class</code> would; that is, that
     * nothing is known about handled type.
     *<p>
     * @since 2.3
     */
    public Class<?> handledType() { return null; }

    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 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.
/**
     * Method called to see if deserializer instance is cachable and
     * usable for other properties of same type (type for which instance
     * was created).
     *<p>
     * Note that cached instances are still resolved on per-property basis,
     * if instance implements {@link com.fasterxml.jackson.databind.deser.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.
     *<p>
     * Default implementation returns false, to indicate that no caching
     * is done.
     */
    public boolean isCachable() { return false; }

    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.
Since: 2.1/**
     * 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.
     * 
     * @return Deserializer this deserializer delegates calls to, if null;
     *   null otherwise.
     * 
     * @since 2.1
     */
    public JsonDeserializer<?> getDelegatee() {
        return null;
    }

    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).
Since: 2.0/**
     * Method that will
     * either return null to indicate that type being deserializers
     * has no concept of properties; or a collection of identifiers
     * for which <code>toString</code> will give external property
     * name.
     * This is only to be used for error reporting and diagnostics
     * purposes (most commonly, to accompany "unknown property"
     * exception).
     * 
     * @since 2.0
     */
    public Collection<Object> getKnownPropertyNames() {
        return null;
    }

    /*
    /**********************************************************
    /* Default NullValueProvider implementation
    /**********************************************************
     */

    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.
Since: 2.6 Added to replace earlier no-arguments variant/**
     * Method that can be called to determine value to be used for
     * representing null values (values deserialized when JSON token
     * is {@link JsonToken#VALUE_NULL}). Usually this is simply
     * Java null, but for some types (especially primitives) it may be
     * necessary to use non-null values.
     *<p>
     * This method may be called once, or multiple times, depending on what
     * {@link #getNullAccessPattern()} returns.
     *<p>
     * Default implementation simply returns null.
     * 
     * @since 2.6 Added to replace earlier no-arguments variant
     */
    @Override
    public T getNullValue(DeserializationContext ctxt) throws JsonMappingException {
        // Change the direction in 2.7
        return getNullValue();
    }

     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 "null value" to use for input null
is simply Java `null` for all deserializers, unless overridden by sub-classes.
This information may be used as optimization.
/**
     *  This method may be called in conjunction with calls to
     * {@link #getNullValue(DeserializationContext)}, to check whether it needs
     * to be called just once (static values), or each time empty value is
     * needed.
     *<p>
     * Default implementation indicates that "null value" to use for input null
     * is simply Java `null` for all deserializers, unless overridden by sub-classes.
     * This information may be used as optimization.
     */
    @Override
    public AccessPattern getNullAccessPattern() {
        // Default implementation assumes that the null value does not vary, which
        // is usually the case for most implementations. But it is not necessarily
        // `null`; so sub-classes may want to refine further.
        return AccessPattern.CONSTANT;
    }

    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. 
Since: 2.9/**
     * This method may be called in conjunction with calls to
     * {@link #getEmptyValue(DeserializationContext)}, to check whether it needs
     * to be called just once (static values), or each time empty value is
     * needed.
     *
     * @since 2.9
     */
    public AccessPattern getEmptyAccessPattern() {
        return AccessPattern.DYNAMIC;
    }

    /*
    /**********************************************************
    /* Other accessors
    /**********************************************************
     */
    
    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 (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 and returns value. 
Since: 2.6 Added to replace earlier no-arguments variant/**
     * Method called to determine value to be used for "empty" values
     * (most commonly when deserializing from empty JSON Strings).
     * Usually this is same as {@link #getNullValue} (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.
     *<p>
     * This method may be called once, or multiple times, depending on what
     * {@link #getEmptyAccessPattern()} returns.
     *<p>
     * Default implementation simply calls {@link #getNullValue} and
     * returns value.
     *
     * @since 2.6 Added to replace earlier no-arguments variant
     */
    public Object getEmptyValue(DeserializationContext ctxt) throws JsonMappingException {
        return getNullValue(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.
Since: 2.0/**
     * 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.
     *<p>
     * Default implementation returns null, as support cannot be implemented
     * generically. Some standard deserializers (most notably
     * {@link com.fasterxml.jackson.databind.deser.BeanDeserializer})
     * do implement this feature, and may return reader instance, depending on exact
     * configuration of instance (which is based on type, and referring property).
     * 
     * @return 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.
     * 
     * @since 2.0
     */
    public ObjectIdReader getObjectIdReader() { return null; }

    Method needed by BeanDeserializerFactory to properly link managed- and back-reference pairs. 
Since: 2.2 (was moved out of BeanDeserializerBase)/**
     * Method needed by {@link BeanDeserializerFactory} to properly link
     * managed- and back-reference pairs.
     * 
     * @since 2.2 (was moved out of <code>BeanDeserializerBase</code>)
     */
    public SettableBeanProperty findBackReference(String refName)
    {
        throw new IllegalArgumentException("Cannot handle managed/back reference '"+refName
                +"': type: value deserializer of type "+getClass().getName()+" does not support them");
    }

    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 deserialization 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.
Since: 2.9/**
     * 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 {@link Boolean#FALSE} if update is not supported at all (immutable values);
     * {@link Boolean#TRUE} if update should usually work (regular POJOs, for example),
     * or <code>null</code> if this is either not known, or may sometimes work.
     *<p>
     * Information gathered is typically used to either prevent merging update for
     * property (either by skipping, if based on global defaults; or by exception during
     * deserialization construction if explicit attempt made) if {@link Boolean#FALSE}
     * returned, or inclusion if {@link Boolean#TRUE} is specified. If "unknown" case
     * (<code>null</code> returned) behavior is to exclude property if global defaults
     * used; or to allow if explicit per-type or property merging is defined.
     *<p>
     * Default implementation returns <code>null</code> to allow explicit per-type
     * or per-property attempts.
     *
     * @since 2.9
     */
    public Boolean supportsUpdate(DeserializationConfig config) {
        return null;
    }

    /*
    /**********************************************************
    /* Deprecated methods
    /**********************************************************
     */

    Deprecated: Since 2.6 Use overloaded variant that takes context argument/**
     * @deprecated Since 2.6 Use overloaded variant that takes context argument
     */
    @Deprecated
    public T getNullValue() { return null; }

    Deprecated: Since 2.6 Use overloaded variant that takes context argument/**
     * @deprecated Since 2.6 Use overloaded variant that takes context argument
     */
    @Deprecated
    public Object getEmptyValue() { return getNullValue(); }

    /*
    /**********************************************************
    /* Helper classes
    /**********************************************************
     */

    This marker class is only to be used with annotations, to
indicate that no deserializer is configured.
 Specifically, this class is to be used as the marker for annotation JsonDeserialize /**
     * This marker class is only to be used with annotations, to
     * indicate that <b>no deserializer is configured</b>.
     *<p>
     * Specifically, this class is to be used as the marker for
     * annotation {@link com.fasterxml.jackson.databind.annotation.JsonDeserialize}
     */
    public abstract static class None extends JsonDeserializer<Object> {
        private None() { } // not to be instantiated
    }
}
Params:	p – Parsed used for reading JSON content ctxt – Context that can be used to access information about this deserialization activity.
Returns:	Deserialized value
Returns:	Deserializer this deserializer delegates calls to, if null; null otherwise.
Since:	2.1
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.
Since:	2.0
/

com.fasterxml.jackson.core/ jackson-databind/ 2.11.0/ com/fasterxml/jackson/databind/JsonDeserializer.java
