com.google.gson
Class Gson

java.lang.Object
  com.google.gson.Gson

public final class Gson
extends Object

This is the main class for using Gson. Gson is typically used by first constructing a Gson instance and then invoking toJson(Object) or fromJson(String, Class) methods on it.

You can create a Gson instance by invoking new Gson() if the default configuration is all you need. You can also use GsonBuilder to build a Gson instance with various configuration options such as versioning support, pretty printing, custom JsonSerializers, JsonDeserializers, and InstanceCreators.

Here is an example of how Gson is used for a simple Class:

 Gson gson = new Gson(); // Or use new GsonBuilder().create();
 MyType target = new MyType();
 String json = gson.toJson(target); // serializes target to Json
 MyType target2 = gson.fromJson(json, MyType.class); // deserializes json into target2
 

If the object that your are serializing/deserializing is a ParameterizedType (i.e. contains at least one type parameter and may be an array) then you must use the toJson(Object, Type) or fromJson(String, Type) method. Here is an example for serializing and deserialing a ParameterizedType:

 Type listType = new TypeToken<List<String>>() {}.getType();
 List<String> target = new LinkedList<String>();
 target.add("blah");

 Gson gson = new Gson();
 String json = gson.toJson(target, listType);
 List<String> target2 = gson.fromJson(json, listType);
 

See the Gson User Guide for a more complete set of examples.

Author:

Inderjeet Singh, Joel Leitch, Jesse Wilson

See Also:

TypeToken

Constructor Summary

Gson()
Constructs a Gson object with default configuration.

Method Summary

<T> T	fromJson(JsonElement json, Class<T> classOfT) This method deserializes the Json read from the specified parse tree into an object of the specified type.
<T> T	fromJson(JsonElement json, Type typeOfT) This method deserializes the Json read from the specified parse tree into an object of the specified type.
<T> T	fromJson(JsonReader reader, Type typeOfT) Reads the next JSON value from reader and convert it to an object of type typeOfT.
<T> T	fromJson(Reader json, Class<T> classOfT) This method deserializes the Json read from the specified reader into an object of the specified class.
<T> T	fromJson(Reader json, Type typeOfT) This method deserializes the Json read from the specified reader into an object of the specified type.
<T> T	fromJson(String json, Class<T> classOfT) This method deserializes the specified Json into an object of the specified class.
<T> T	fromJson(String json, Type typeOfT) This method deserializes the specified Json into an object of the specified type.
<T> TypeAdapter<T>	getAdapter(Class<T> type) Returns the type adapter for type.
<T> TypeAdapter<T>	getAdapter(TypeToken<T> type) Returns the type adapter for type.
<T> TypeAdapter<T>	getDelegateAdapter(TypeAdapterFactory skipPast, TypeToken<T> type) This method is used to get an alternate type adapter for the specified type.
String	toJson(JsonElement jsonElement) Converts a tree of JsonElements into its equivalent JSON representation.
void	toJson(JsonElement jsonElement, Appendable writer) Writes out the equivalent JSON for a tree of JsonElements.
void	toJson(JsonElement jsonElement, JsonWriter writer) Writes the JSON for jsonElement to writer.
String	toJson(Object src) This method serializes the specified object into its equivalent Json representation.
void	toJson(Object src, Appendable writer) This method serializes the specified object into its equivalent Json representation.
String	toJson(Object src, Type typeOfSrc) This method serializes the specified object, including those of generic types, into its equivalent Json representation.
void	toJson(Object src, Type typeOfSrc, Appendable writer) This method serializes the specified object, including those of generic types, into its equivalent Json representation.
void	toJson(Object src, Type typeOfSrc, JsonWriter writer) Writes the JSON representation of src of type typeOfSrc to writer.
JsonElement	toJsonTree(Object src) This method serializes the specified object into its equivalent representation as a tree of JsonElements.
JsonElement	toJsonTree(Object src, Type typeOfSrc) This method serializes the specified object, including those of generic types, into its equivalent representation as a tree of JsonElements.
String	toString()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Gson

public Gson()

Constructs a Gson object with default configuration. The default configuration has the following settings:

• The JSON generated by toJson methods is in compact representation. This means that all the unneeded white-space is removed. You can change this behavior with GsonBuilder.setPrettyPrinting().
• The generated JSON omits all the fields that are null. Note that nulls in arrays are kept as is since an array is an ordered list. Moreover, if a field is not null, but its generated JSON is empty, the field is kept. You can configure Gson to serialize null values by setting GsonBuilder.serializeNulls().
• Gson provides default serialization and deserialization for Enums, Map, URL, URI, Locale, Date, BigDecimal, and BigInteger classes. If you would prefer to change the default representation, you can do so by registering a type adapter through GsonBuilder.registerTypeAdapter(Type, Object).
• The default Date format is same as DateFormat.DEFAULT. This format ignores the millisecond portion of the date during serialization. You can change this by invoking GsonBuilder.setDateFormat(int) or GsonBuilder.setDateFormat(String).
• By default, Gson ignores the Expose annotation. You can enable Gson to serialize/deserialize only those fields marked with this annotation through GsonBuilder.excludeFieldsWithoutExposeAnnotation().
• By default, Gson ignores the Since annotation. You can enable Gson to use this annotation through GsonBuilder.setVersion(double).
• The default field naming policy for the output Json is same as in Java. So, a Java class field versionNumber will be output as "versionNumber" in Json. The same rules are applied for mapping incoming Json to the Java classes. You can change this policy through GsonBuilder.setFieldNamingPolicy(FieldNamingPolicy).
• By default, Gson excludes transient or static fields from consideration for serialization and deserialization. You can change this behavior through GsonBuilder.excludeFieldsWithModifiers(int...).

Method Detail

getAdapter

public <T> TypeAdapter<T> getAdapter(TypeToken<T> type)

Returns the type adapter for type.

Throws:

IllegalArgumentException - if this GSON cannot serialize and deserialize type.

getDelegateAdapter

public <T> TypeAdapter<T> getDelegateAdapter(TypeAdapterFactory skipPast,
                                             TypeToken<T> type)

This method is used to get an alternate type adapter for the specified type. This is used to access a type adapter that is overridden by a TypeAdapterFactory that you may have registered. This features is typically used when you want to register a type adapter that does a little bit of work but then delegates further processing to the Gson default type adapter. Here is an example:

Let's say we want to write a type adapter that counts the number of objects being read from or written to JSON. We can achieve this by writing a type adapter factory that uses the getDelegateAdapter method:

 class StatsTypeAdapterFactory implements TypeAdapterFactory {
    public int numReads = 0;
    public int numWrites = 0;
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
      final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
      return new TypeAdapter<T>() {
        public void write(JsonWriter out, T value) throws IOException {
          ++numWrites;
          delegate.write(out, value);
        }
        public T read(JsonReader in) throws IOException {
          ++numReads;
          return delegate.read(in);
        }
      };
    }
  }
   

This factory can now be used like this:

 StatsTypeAdapterFactory stats = new StatsTypeAdapterFactory();
  Gson gson = new GsonBuilder().registerTypeAdapterFactory(stats).create();
  // Call gson.toJson() and fromJson methods on objects
  System.out.println("Num JSON reads" + stats.numReads);
  System.out.println("Num JSON writes" + stats.numWrites);
  

Note that since you can not override type adapter factories for String and Java primitive types, our stats factory will not count the number of String or primitives that will be read or written.

Parameters:

skipPast - The type adapter factory that needs to be skipped while searching for a matching type adapter. In most cases, you should just pass this (the type adapter factory from where getDelegateAdapter(com.google.gson.TypeAdapterFactory, com.google.gson.reflect.TypeToken) method is being invoked).

type - Type for which the delegate adapter is being searched for.

Since:

2.2

getAdapter

public <T> TypeAdapter<T> getAdapter(Class<T> type)

Returns the type adapter for type.

Throws:

IllegalArgumentException - if this GSON cannot serialize and deserialize type.

toJsonTree

public JsonElement toJsonTree(Object src)

This method serializes the specified object into its equivalent representation as a tree of JsonElements. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJsonTree(Object, Type) instead.

Parameters:

src - the object for which Json representation is to be created setting for Gson

Returns:

Json representation of src.

Since:

1.4

toJsonTree

public JsonElement toJsonTree(Object src,
                              Type typeOfSrc)

This method serializes the specified object, including those of generic types, into its equivalent representation as a tree of JsonElements. This method must be used if the specified object is a generic type. For non-generic objects, use toJsonTree(Object) instead.

Parameters:

src - the object for which JSON representation is to be created

typeOfSrc - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
 

Returns:

Json representation of src

Since:

1.4

toJson

public String toJson(Object src)

This method serializes the specified object into its equivalent Json representation. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJson(Object, Type) instead. If you want to write out the object to a Writer, use toJson(Object, Appendable) instead.

Parameters:

src - the object for which Json representation is to be created setting for Gson

Returns:

Json representation of src.

toJson

public String toJson(Object src,
                     Type typeOfSrc)

This method serializes the specified object, including those of generic types, into its equivalent Json representation. This method must be used if the specified object is a generic type. For non-generic objects, use toJson(Object) instead. If you want to write out the object to a Appendable, use toJson(Object, Type, Appendable) instead.

Parameters:

src - the object for which JSON representation is to be created

typeOfSrc - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
 

Returns:

Json representation of src

toJson

public void toJson(Object src,
                   Appendable writer)
            throws JsonIOException

This method serializes the specified object into its equivalent Json representation. This method should be used when the specified object is not a generic type. This method uses Object.getClass() to get the type for the specified object, but the getClass() loses the generic type information because of the Type Erasure feature of Java. Note that this method works fine if the any of the object fields are of generic type, just the object itself should not be of a generic type. If the object is of generic type, use toJson(Object, Type, Appendable) instead.

Parameters:

src - the object for which Json representation is to be created setting for Gson

writer - Writer to which the Json representation needs to be written

Throws:

JsonIOException - if there was a problem writing to the writer

Since:

1.2

toJson

public void toJson(Object src,
                   Type typeOfSrc,
                   Appendable writer)
            throws JsonIOException

This method serializes the specified object, including those of generic types, into its equivalent Json representation. This method must be used if the specified object is a generic type. For non-generic objects, use toJson(Object, Appendable) instead.

Parameters:

src - the object for which JSON representation is to be created

typeOfSrc - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfSrc = new TypeToken<Collection<Foo>>(){}.getType();
 

writer - Writer to which the Json representation of src needs to be written.

Throws:

JsonIOException - if there was a problem writing to the writer

Since:

1.2

toJson

public void toJson(Object src,
                   Type typeOfSrc,
                   JsonWriter writer)
            throws JsonIOException

Writes the JSON representation of src of type typeOfSrc to writer.

Throws:

JsonIOException - if there was a problem writing to the writer

toJson

public String toJson(JsonElement jsonElement)

Converts a tree of JsonElements into its equivalent JSON representation.

Parameters:

jsonElement - root of a tree of JsonElements

Returns:

JSON String representation of the tree

Since:

1.4

toJson

public void toJson(JsonElement jsonElement,
                   Appendable writer)
            throws JsonIOException

Writes out the equivalent JSON for a tree of JsonElements.

Parameters:

jsonElement - root of a tree of JsonElements

writer - Writer to which the Json representation needs to be written

Throws:

JsonIOException - if there was a problem writing to the writer

Since:

1.4

toJson

public void toJson(JsonElement jsonElement,
                   JsonWriter writer)
            throws JsonIOException

Writes the JSON for jsonElement to writer.

Throws:

JsonIOException - if there was a problem writing to the writer

fromJson

public <T> T fromJson(String json,
                      Class<T> classOfT)
           throws JsonSyntaxException

This method deserializes the specified Json into an object of the specified class. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(String, Type). If you have the Json in a Reader instead of a String, use fromJson(Reader, Class) instead.

Type Parameters:

T - the type of the desired object

Parameters:

json - the string from which the object is to be deserialized

classOfT - the class of T

Returns:

an object of type T from the string. Returns null if json is null.

Throws:

JsonSyntaxException - if json is not a valid representation for an object of type classOfT

fromJson

public <T> T fromJson(String json,
                      Type typeOfT)
           throws JsonSyntaxException

This method deserializes the specified Json into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(String, Class) instead. If you have the Json in a Reader instead of a String, use fromJson(Reader, Type) instead.

Type Parameters:

T - the type of the desired object

Parameters:

json - the string from which the object is to be deserialized

typeOfT - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
 

Returns:

an object of type T from the string. Returns null if json is null.

Throws:

JsonParseException - if json is not a valid representation for an object of type typeOfT

JsonSyntaxException - if json is not a valid representation for an object of type

fromJson

public <T> T fromJson(Reader json,
                      Class<T> classOfT)
           throws JsonSyntaxException,
                  JsonIOException

This method deserializes the Json read from the specified reader into an object of the specified class. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(Reader, Type). If you have the Json in a String form instead of a Reader, use fromJson(String, Class) instead.

Type Parameters:

T - the type of the desired object

Parameters:

json - the reader producing the Json from which the object is to be deserialized.

classOfT - the class of T

Returns:

an object of type T from the string. Returns null if json is at EOF.

Throws:

JsonIOException - if there was a problem reading from the Reader

JsonSyntaxException - if json is not a valid representation for an object of type

Since:

1.2

fromJson

public <T> T fromJson(Reader json,
                      Type typeOfT)
           throws JsonIOException,
                  JsonSyntaxException

This method deserializes the Json read from the specified reader into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(Reader, Class) instead. If you have the Json in a String form instead of a Reader, use fromJson(String, Type) instead.

Type Parameters:

T - the type of the desired object

Parameters:

json - the reader producing Json from which the object is to be deserialized

typeOfT - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
 

Returns:

an object of type T from the json. Returns null if json is at EOF.

Throws:

JsonIOException - if there was a problem reading from the Reader

JsonSyntaxException - if json is not a valid representation for an object of type

Since:

1.2

fromJson

public <T> T fromJson(JsonReader reader,
                      Type typeOfT)
           throws JsonIOException,
                  JsonSyntaxException

Reads the next JSON value from reader and convert it to an object of type typeOfT. Returns null, if the reader is at EOF. Since Type is not parameterized by T, this method is type unsafe and should be used carefully

Throws:

JsonIOException - if there was a problem writing to the Reader

JsonSyntaxException - if json is not a valid representation for an object of type

fromJson

public <T> T fromJson(JsonElement json,
                      Class<T> classOfT)
           throws JsonSyntaxException

This method deserializes the Json read from the specified parse tree into an object of the specified type. It is not suitable to use if the specified class is a generic type since it will not have the generic type information because of the Type Erasure feature of Java. Therefore, this method should not be used if the desired type is a generic type. Note that this method works fine if the any of the fields of the specified object are generics, just the object itself should not be a generic type. For the cases when the object is of generic type, invoke fromJson(JsonElement, Type).

Type Parameters:

T - the type of the desired object

Parameters:

json - the root of the parse tree of JsonElements from which the object is to be deserialized

classOfT - The class of T

Returns:

an object of type T from the json. Returns null if json is null.

Throws:

JsonSyntaxException - if json is not a valid representation for an object of type typeOfT

Since:

1.3

fromJson

public <T> T fromJson(JsonElement json,
                      Type typeOfT)
           throws JsonSyntaxException

This method deserializes the Json read from the specified parse tree into an object of the specified type. This method is useful if the specified object is a generic type. For non-generic objects, use fromJson(JsonElement, Class) instead.

Type Parameters:

T - the type of the desired object

Parameters:

json - the root of the parse tree of JsonElements from which the object is to be deserialized

typeOfT - The specific genericized type of src. You can obtain this type by using the TypeToken class. For example, to get the type for Collection<Foo>, you should use:

 Type typeOfT = new TypeToken<Collection<Foo>>(){}.getType();
 

Returns:

an object of type T from the json. Returns null if json is null.

Throws:

JsonSyntaxException - if json is not a valid representation for an object of type typeOfT

Since:

1.3

toString

public String toString()

Overrides:

toString in class Object