Google Gson简明教程

简介

Gson是Google出品的一个JSON解析library，它可以将任意的Java对象转换为JSON string，也可以将JSON string转换为对应的Java对象。
官方介绍如下：

Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object.

Gson can work with arbitrary Java objects including pre-existing objects that you do not have source code of.

maven依赖

<dependencies>    <!--  Gson: Java to Json conversion -->    <dependency>      <groupId>com.google.code.gson</groupId>      <artifactId>gson</artifactId>      <version>2.7</version>    </dependency></dependencies>

Gson

序列化&反序列化

通过Gson的toJson和fromJson方法来完成Serialization&Deserialization。

友情提示

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. Gson instances are Thread-safe so you can reuse them freely across multiple threads.

Gson是线程安全的（Thread-Safe），可以在多线程间共享使用。

基本类型

// SerializationGson gson = new Gson();gson.toJson(1);            // ==> 1gson.toJson("abcd");       // ==> "abcd"gson.toJson(new Long(10)); // ==> 10int[] values = { 1 };gson.toJson(values);       // ==> [1]// Deserializationint one = gson.fromJson("1", int.class);Integer one = gson.fromJson("1", Integer.class);Long one = gson.fromJson("1", Long.class);Boolean false = gson.fromJson("false", Boolean.class);String str = gson.fromJson("\"abc\"", String.class);String[] anotherStr = gson.fromJson("[\"abc\"]", String[].class);

简单对象

1、序列化

class User {  private long id;  private String name;  private int age;  private transient int gender;  User() {    // no-args constructor  }  //省略setter/getter方法}// SerializationUser user = new User();user.setId(3);user.setName("ricky");user.setAge(27);Gson gson = new Gson();String json = gson.toJson(user);  System.out.println(json);

输出结果：

{"id":3,"name":"ricky","age":27}

需要注意几点：

• 序列化对象推荐使用基本类型属性
• 默认情况下，属性值为null在序列化时会被忽略
• 默认情况下，transient修饰的属性在序列化和反序列化时会被忽略不输出。

2、反序列化

// DeserializationGson gson = new Gson();User obj2 = gson.fromJson(json, User.class);

复杂对象

class User {    private long id;    private String name;    private int age;    private transient int gender;    private Address address;    //省略getter/setter方法}// SerializationUser user = new User();user.setId(3);user.setName("ricky");user.setAge(27);Address address = new Address();address.setProvince("湖北省");address.setCity("武汉市");address.setDistrict("武昌区");user.setAddress(address);Gson gson = new Gson();String json = gson.toJson(user);  System.out.println(json);

输出结果：
{“id”:3,”name”:”ricky”,”age”:27,”address”:{“province”:”湖北省”,”city”:”武汉市”,”district”:”武昌区”}}

数组

Gson gson = new Gson();int[] ints = {1, 2, 3, 4, 5};String[] strings = {"abc", "def", "ghi"};// Serializationgson.toJson(ints);     // ==> [1,2,3,4,5]gson.toJson(strings);  // ==> ["abc", "def", "ghi"]// Deserializationint[] ints2 = gson.fromJson("[1,2,3,4,5]", int[].class); // ==> ints2 will be same as ints

Gson支持多维数组Serialization/Deserialization。

集合

基本类型

Gson gson = new Gson();Collection<Integer> ints = Lists.immutableList(1,2,3,4,5);// SerializationString json = gson.toJson(ints);  // ==> json is [1,2,3,4,5]// DeserializationType collectionType = new TypeToken<Collection<Integer>>(){}.getType();Collection<Integer> ints2 = gson.fromJson(json, collectionType);// ==> ints2 is same as ints

对象集合

List<User> list = new ArrayList<User>();        User user = new User();        user.setId(1);        user.setName("ricky");        user.setAge(27);        list.add(user);        Gson gson = new Gson();        // Serialization        String json = gson.toJson(list);          System.out.println(json);        // Deserialization        Type listType = new TypeToken<List<User>>(){}.getType();        List<User> userList = gson.fromJson(json, listType);        System.out.println(userList);

泛型类

class Foo<T> {  T value;}Gson gson = new Gson();Foo<Bar> foo = new Foo<Bar>();Type fooType = new TypeToken<Foo<Bar>>() {}.getType();gson.toJson(foo, fooType);gson.fromJson(json, fooType);

指定属性序列化名称

默认情况下，Gson使用属性名作为序列化名称，我们也可以使用SerializedName注解来为属性指定一个不同于属性名的名称。

class Person {    @SerializedName("first_name")    private String firstName;    @SerializedName("middle_name")    private String middleName;    @SerializedName("last_name")    private String lastName;    private int age;    //省略getter/setter方法}Person p = new Person();p.setFirstName("Shark");p.setMiddleName("Q");p.setLastName("ONeal");p.setAge(27);Gson gson = new GsonBuilder().serializeNulls().create();String json = gson.toJson(p);   //{"first_name":"Shark","middle_name":"Q","last_name":"ONeal","age":27}

控制Gson Serialization and Deserialization行为

JSON 输出格式

Gson gson = new GsonBuilder().setPrettyPrinting().create();String jsonOutput = gson.toJson(someObject);

序列化null属性

Gson gson = new GsonBuilder().serializeNulls().create();

自定义Serialization and Deserialization

有些时候Gson默认的Serialization and Deserialization满足不了我们的需求，例如DateTime，此时我们就需要自定义了。
Gson 允许我们注册自定义的serializers and deserializers. 这部分定义如下：

• Json Serializers: Need to define custom serialization for an object
• Json Deserializers: Needed to define custom deserialization for a
  type
• Instance Creators: Not needed if no-args constructor is available or
  a deserializer is registered

Excluding Fields From Serialization and Deserialization

1、修饰符

import java.lang.reflect.Modifier;Gson gson = new GsonBuilder()    .excludeFieldsWithModifiers(Modifier.STATIC)    .create();

2、@Expose注解

Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()

JsonParser

有些情况下，如果需要手工解析JSON string，可以使用com.google.gson.JsonParser类，如下：

String json = "{\"first_name\":\"Shark\",\"middle_name\":\"Q\",\"last_name\":\"ONeal\",\"age\":27}";        JsonParser parser = new JsonParser();        JsonObject jObj = parser.parse(json).getAsJsonObject();        String first_name = jObj.get("first_name").getAsString();   //Shark        System.out.println(first_name);

参考资料

https://github.com/google/gson/blob/master/UserGuide.md