Package com.google.gson.internal.bind
Class MapTypeAdapterFactory
java.lang.Object
com.google.gson.internal.bind.MapTypeAdapterFactory
- All Implemented Interfaces:
TypeAdapterFactory
Adapts maps to either JSON objects or JSON arrays.
Maps as JSON objects
For primitive keys or when complex map key serialization is not enabled, this converts JavaMaps
to JSON Objects. This requires that map keys
can be serialized as strings; this is insufficient for some key types. For
example, consider a map whose keys are points on a grid. The default JSON
form encodes reasonably:
Map<Point, String> original = new LinkedHashMap<Point, String>();
original.put(new Point(5, 6), "a");
original.put(new Point(8, 8), "b");
System.out.println(gson.toJson(original, type));
The above code prints this JSON object:
{
"(5,6)": "a",
"(8,8)": "b"
}
But GSON is unable to deserialize this value because the JSON string name is
just the toString()
of the map key. Attempting to
convert the above JSON to an object fails with a parse exception:
com.google.gson.JsonParseException: Expecting object found: "(5,6)" at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler at com.google.gson.ObjectNavigator.navigateClassFields ...
Maps as JSON arrays
An alternative approach taken by this type adapter when it is required and complex map key serialization is enabled is to encode maps as arrays of map entries. Each map entry is a two element array containing a key and a value. This approach is more flexible because any type can be used as the map's key; not just strings. But it's also less portable because the receiver of such JSON must be aware of the map entry convention.Register this adapter when you are creating your GSON instance.
Gson gson = new GsonBuilder()
.registerTypeAdapter(Map.class, new MapAsArrayTypeAdapter())
.create();
This will change the structure of the JSON emitted by the code above. Now we
get an array. In this case the arrays elements are map entries:
[
[
{
"x": 5,
"y": 6
},
"a",
],
[
{
"x": 8,
"y": 8
},
"b"
]
]
This format will serialize and deserialize just fine as long as this adapter
is registered.-
Nested Class Summary
Nested Classes -
Field Summary
FieldsModifier and TypeFieldDescription(package private) final boolean
private final ConstructorConstructor
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Constructor Summary
ConstructorsConstructorDescriptionMapTypeAdapterFactory
(ConstructorConstructor constructorConstructor, boolean complexMapKeySerialization) -
Method Summary
Modifier and TypeMethodDescription<T> TypeAdapter<T>
Returns a type adapter fortype
, or null if this factory doesn't supporttype
.private TypeAdapter<?>
getKeyAdapter
(Gson context, Type keyType) Returns a type adapter that writes the value as a string.
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Field Details
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constructorConstructor
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complexMapKeySerialization
final boolean complexMapKeySerialization
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Constructor Details
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MapTypeAdapterFactory
public MapTypeAdapterFactory(ConstructorConstructor constructorConstructor, boolean complexMapKeySerialization)
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Method Details
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create
Description copied from interface:TypeAdapterFactory
Returns a type adapter fortype
, or null if this factory doesn't supporttype
.- Specified by:
create
in interfaceTypeAdapterFactory
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getKeyAdapter
Returns a type adapter that writes the value as a string.
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