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TypeAdapterFactory (Gson 2.3.1 API)
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Interface TypeAdapterFactory</H2>
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<DL>
<DT><PRE>public interface <B>TypeAdapterFactory</B></DL>
</PRE>
<P>
Creates type adapters for set of related types. Type adapter factories are
most useful when several types share similar structure in their JSON form.
<h3>Example: Converting enums to lowercase</h3>
In this example, we implement a factory that creates type adapters for all
enums. The type adapters will write enums in lowercase, despite the fact
that they're defined in <code>CONSTANT_CASE</code> in the corresponding Java
model: <pre> <code>public class LowercaseEnumTypeAdapterFactory implements TypeAdapterFactory {
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
Class<T> rawType = (Class<T>) type.getRawType();
if (!rawType.isEnum()) {
return null;
}
final Map<String, T> lowercaseToConstant = new HashMap<String, T>();
for (T constant : rawType.getEnumConstants()) {
lowercaseToConstant.put(toLowercase(constant), constant);
}
return new TypeAdapter<T>() {
public void write(JsonWriter out, T value) throws IOException {
if (value == null) {
out.nullValue();
} else {
out.value(toLowercase(value));
}
}
public T read(JsonReader reader) throws IOException {
if (reader.peek() == JsonToken.NULL) {
reader.nextNull();
return null;
} else {
return lowercaseToConstant.get(reader.nextString());
}
}
};
}
private String toLowercase(Object o) {
return o.toString().toLowerCase(Locale.US);
}
}
</code></pre>
<p>Type adapter factories select which types they provide type adapters
for. If a factory cannot support a given type, it must return null when
that type is passed to <A HREF="../../../com/google/gson/TypeAdapterFactory.html#create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)"><CODE>create(com.google.gson.Gson, com.google.gson.reflect.TypeToken<T>)</CODE></A>. Factories should expect <code>create()</code> to be called on them for many types and should return null for
most of those types. In the above example the factory returns null for
calls to <code>create()</code> where <code>type</code> is not an enum.
<p>A factory is typically called once per type, but the returned type
adapter may be used many times. It is most efficient to do expensive work
like reflection in <code>create()</code> so that the type adapter's <code>read()</code> and <code>write()</code> methods can be very fast. In this example the
mapping from lowercase name to enum value is computed eagerly.
<p>As with type adapters, factories must be <i>registered</i> with a <A HREF="../../../com/google/gson/GsonBuilder.html" title="class in com.google.gson"><CODE>GsonBuilder</CODE></A> for them to take effect: <pre> <code>GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapterFactory(new LowercaseEnumTypeAdapterFactory());
...
Gson gson = builder.create();
</code></pre>
If multiple factories support the same type, the factory registered earlier
takes precedence.
<h3>Example: composing other type adapters</h3>
In this example we implement a factory for Guava's <code>Multiset</code>
collection type. The factory can be used to create type adapters for
multisets of any element type: the type adapter for <code>Multiset<String></code> is different from the type adapter for <code>Multiset<URL></code>.
<p>The type adapter <i>delegates</i> to another type adapter for the
multiset elements. It figures out the element type by reflecting on the
multiset's type token. A <code>Gson</code> is passed in to <code>create</code> for
just this purpose: <pre> <code>public class MultisetTypeAdapterFactory implements TypeAdapterFactory {
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
Type type = typeToken.getType();
if (typeToken.getRawType() != Multiset.class
|| !(type instanceof ParameterizedType)) {
return null;
}
Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
return (TypeAdapter<T>) newMultisetAdapter(elementAdapter);
}
private <E> TypeAdapter<Multiset<E>> newMultisetAdapter(
final TypeAdapter<E> elementAdapter) {
return new TypeAdapter<Multiset<E>>() {
public void write(JsonWriter out, Multiset<E> value) throws IOException {
if (value == null) {
out.nullValue();
return;
}
out.beginArray();
for (Multiset.Entry<E> entry : value.entrySet()) {
out.value(entry.getCount());
elementAdapter.write(out, entry.getElement());
}
out.endArray();
}
public Multiset<E> read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
Multiset<E> result = LinkedHashMultiset.create();
in.beginArray();
while (in.hasNext()) {
int count = in.nextInt();
E element = elementAdapter.read(in);
result.add(element, count);
}
in.endArray();
return result;
}
};
}
}
</code></pre>
Delegating from one type adapter to another is extremely powerful; it's
the foundation of how Gson converts Java objects and collections. Whenever
possible your factory should retrieve its delegate type adapter in the
<code>create()</code> method; this ensures potentially-expensive type adapter
creation happens only once.
<P>
<P>
<DL>
<DT><B>Since:</B></DT>
<DD>2.1</DD>
</DL>
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<CODE><T> <A HREF="../../../com/google/gson/TypeAdapter.html" title="class in com.google.gson">TypeAdapter</A><T></CODE></FONT></TD>
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<TD><CODE><B><A HREF="../../../com/google/gson/TypeAdapterFactory.html#create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)">create</A></B>(<A HREF="../../../com/google/gson/Gson.html" title="class in com.google.gson">Gson</A> gson,
<A HREF="../../../com/google/gson/reflect/TypeToken.html" title="class in com.google.gson.reflect">TypeToken</A><T> type)</CODE>
<BR>
Returns a type adapter for <code>type</code>, or null if this factory doesn't
support <code>type</code>.</TD>
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<A NAME="create(com.google.gson.Gson, com.google.gson.reflect.TypeToken)"><!-- --></A><H3>
create</H3>
<PRE>
<T> <A HREF="../../../com/google/gson/TypeAdapter.html" title="class in com.google.gson">TypeAdapter</A><T> <B>create</B>(<A HREF="../../../com/google/gson/Gson.html" title="class in com.google.gson">Gson</A> gson,
<A HREF="../../../com/google/gson/reflect/TypeToken.html" title="class in com.google.gson.reflect">TypeToken</A><T> type)</PRE>
<DL>
<DD>Returns a type adapter for <code>type</code>, or null if this factory doesn't
support <code>type</code>.
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