特点:

1. 读写速度快

2. 解析简单

3. 轻量级

4. 独立于语言，平台

5. 具有自我描叙性

使用 Gson需要导包：

implementation 'com.google.code.gson:gson:2.8.4'

Gson基础使用案例一

 Gson gson = new Gson();
        //输出"1",单个字符也可以序列化
        System.out.println(gson.toJson("1"));
        //输出 "fw123"
        System.out.println(gson.toJson("fw123"));

        //打印结果：[1,2,3] ---这种形式是数组吗？
        int[] value = {1,2,3};
        System.out.println(gson.toJson(value));
        //上面的序列化结果都是转化成字符串类型的

        // 下面是反序列化的过程
        //这是一个泛型入参的函数，打印的结果: i 的值为: 1
        int i = gson.fromJson("1",int.class);
        System.out.println("i 的值为: " + i);


        //打印出来的值为： gsonBean的值为: {"mInt":1,"mStr":"22"}--- 这个是序列化格式
        String gsonBean = gson.toJson(new GsonBean(1,"22"));
        System.out.println("gsonBean的值为: " + gsonBean);

SerializedName注解的使用：

public class JsonAdepterTest {
    static class SerializedNameTestClass {
        @SerializedName("name") //给a设置Key值为：name
                String a;
        @SerializedName(value = "name1", alternate = {"name2", "name3"})
        String b;

        String c;

        public SerializedNameTestClass(String a, String b, String c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }

        public static void test() {
            SerializedNameTestClass target =
                     new SerializedNameTestClass("v1", "v2", "v3");
            Gson gson = new Gson();
            String json = gson.toJson(target);
            System.out.println(json);
        }
    }

    public static void main(String[] args) {
        SerializedNameTestClass.test();
    }
}
执行结果为：{"name":"v1","name1":"v2","c":"v3"}

JsonAdapter注解的使用：

 @JsonAdapter(UserJsonAdapter.class)
    static
    class User {
        public final String firstName, lastName;
        public User(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

        @Override
        public String toString() {
            return "User{" +
                    "firstName='" + firstName + '\'' +
                    ", lastName='" + lastName + '\'' +
                    '}';
        }
    }

    class UserJsonAdapter extends TypeAdapter<User> {
        @Override
        public void write(JsonWriter out, User user) throws IOException {
            out.beginObject();
            out.name("firstKey:");
            out.value(user.firstName);
            out.endObject();
            out.beginObject();
            out.name("secondKey");
            out.value(user.lastName);
            out.endObject();
        }
        @Override
        public User read(JsonReader in) throws IOException {
            in.beginObject();
            in.nextName();
            String nameParts1 = in.nextString();
            in.endObject();
            
            in.beginObject();
            in.nextName();
            String nameParts2 = in.nextString();
            in.endObject();
            return new User(nameParts1, nameParts2);
        }
    }

    public static void main(String[] args) {
        User user = new User("gg","Zero");
        Gson gson = new GsonBuilder()
                .setVersion(1.2).create();
        String gsonStr = gson.toJson(user);
        System.out.println("gsonStr=" + gsonStr);

        user = gson.fromJson(gsonStr,User.class);
        System.out.println("user: " + user);
    }

输出结果：
gsonStr={"firstKey:":"gg"}{"secondKey":"Zero"}
user: User{firstName='gg', lastName='Zero'}

public class GsonError1 {
        private String name;
        private List<AuthorsBean> authors;
        @Override
        public String toString() {
            return "GsonError1{" +
                    "name='" + name + '\'' +
                    ", authors=" + authors +
                    '}';
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public List<AuthorsBean> getAuthors() {
            return authors;
        }

        public void setAuthors(List<AuthorsBean> authors) {
            this.authors = authors;
        }

        public static class AuthorsBean {
            private String id;
            private String name;
            public String getId() {
                return id;
            }
            public void setId(String id) {
                this.id = id;
            }
            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            @Override
            public String toString() {
                return "AuthorsBean{" +
                        "id='" + id + '\'' +
                        ", name='" + name + '\'' +
                        '}';
            }
        }

    public static void test1() {
        //TODO:
        String json = "{\n" +
                "    \"name\": \"java\",\n" +
                "    \"authors\": [\n" +
                "        {\n" +
                "            \"id\": \"1'\",\n" +
                "            \"name\": \"Joshua Bloch'\"\n" +
                "        },\n" +
                "        {\n" +
                "            \"id\": \"2'\",\n" +
                "            \"name\": \"Tom\"\n" +
                "        }\n" +
                "    ]\n" +
                "}";
        Gson gson = new Gson();
        GsonError1 gsonError1 = gson.fromJson(json, GsonError1.class);
        System.out.println(gsonError1);
    }

    public static void main(String[] args) {
        test1();
    }

输出结果：
GsonError1{name='java', authors=[AuthorsBean{id='1'', name='Joshua Bloch''}, AuthorsBean{id='2'', name='Tom'}]}

String json = "{\n" +
                "    \"name\": \"java\",\n" +
                "    \"authors\": \"\"\n" +
                "}";
Gson gson = new Gson();
GsonError1 gsonError1 = gson.fromJson(json, GsonError1.class);
System.out.println(gsonError1);

如果上面的Json序列化形式不对，
就会上报错误：
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 3 column 17 path $.authors

为了解决由于Json中某些字段缺省的情况，方案一：

package com.android.serializable;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;

import java.lang.reflect.Type;
import java.util.Arrays;
import java.util.List;

public class GsonError1 {
        private String name;
        private List<AuthorsBean> authors;
        @Override
        public String toString() {
            return "GsonError1{" +
                    "name='" + name + '\'' +
                    ", authors=" + authors +
                    '}';
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public List<AuthorsBean> getAuthors() {
            return authors;
        }

        public void setAuthors(List<AuthorsBean> authors) {
            this.authors = authors;
        }

        public static class AuthorsBean {
            private String id;
            private String name;
            public String getId() {
                return id;
            }
            public void setId(String id) {
                this.id = id;
            }
            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            @Override
            public String toString() {
                return "AuthorsBean{" +
                        "id='" + id + '\'' +
                        ", name='" + name + '\'' +
                        '}';
            }
        }

    static class GsonError1Deserializer implements JsonDeserializer<GsonError1> {

        @Override
        public GsonError1 deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            final JsonObject jsonObject = json.getAsJsonObject();

            final JsonElement jsonTitle = jsonObject.get("name");
            final String name = jsonTitle.getAsString();

            JsonElement jsonAuthors = jsonObject.get("authors");

            GsonError1 gsonError1 = new GsonError1();

            if (jsonAuthors.isJsonArray()) {//如果数组类型，此种情况是我们需要的
                //关于context在文章最后有简单说明
                GsonError1.AuthorsBean[] authors = context.deserialize(jsonAuthors, GsonError1.AuthorsBean[].class);
                gsonError1.setAuthors(Arrays.asList(authors));
            } else {//此种情况为无效情况
                gsonError1.setAuthors(null);
            }
            gsonError1.setName(name);
            return gsonError1;
        }
    }

    static class AuthorDeserializer implements JsonDeserializer {

        @Override
        public Object deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            final JsonObject jsonObject = json.getAsJsonObject();

            final GsonError1.AuthorsBean author = new GsonError1.AuthorsBean();
            author.setId(jsonObject.get("id").getAsString());
            author.setName(jsonObject.get("name").getAsString());
            return author;
        }
    }

    public static void test3() {
        //TODO:
        String json = "{\n" +
                "    \"name\": \"java\",\n" +
                "    \"authors\": \"\"\n" +
                "}";

        GsonBuilder gsonBuilder = new GsonBuilder();

        //注册TypeAdapter
        gsonBuilder.registerTypeAdapter(GsonError1.class, new GsonError1Deserializer());
        gsonBuilder.registerTypeAdapter(GsonError1.AuthorsBean.class, new AuthorDeserializer());

        Gson gson = gsonBuilder.create();
        GsonError1 gsonError1 = gson.fromJson(json, GsonError1.class);

        System.out.println(gsonError1);
    }

    public static void main(String[] args) {
        test3();
    }


}

解决方法二： 参照上面自定义TypeAdapter

Gson的源码解析

JsonToken是枚举类型.---定义Json数据的类型

/**
 * A structure, name or value type in a JSON-encoded string.
 */
public enum JsonToken {

    /**
     * The opening of a JSON array. Written using {@link JsonWriter#beginObject}
     * and read using {@link JsonReader#beginObject}.
     */
    BEGIN_ARRAY,

    /**
     * The closing of a JSON array. Written using {@link JsonWriter#endArray}
     * and read using {@link JsonReader#endArray}.
     */
    END_ARRAY,

    /**
     * The opening of a JSON object. Written using {@link JsonWriter#beginObject}
     * and read using {@link JsonReader#beginObject}.
     */
    BEGIN_OBJECT,

    /**
     * The closing of a JSON object. Written using {@link JsonWriter#endObject}
     * and read using {@link JsonReader#endObject}.
     */
    END_OBJECT,

    /**
     * A JSON property name. Within objects, tokens alternate between names and
     * their values. Written using {@link JsonWriter#name} and read using {@link
     * JsonReader#nextName}
     */
    NAME,

    /**
     * A JSON string.
     */
    STRING,

    /**
     * A JSON number represented in this API by a Java {@code double}, {@code
     * long}, or {@code int}.
     */
    NUMBER,

    /**
     * A JSON {@code true} or {@code false}.
     */
    BOOLEAN,

    /**
     * A JSON {@code null}.
     */
    NULL,

    /**
     * The end of the JSON stream. This sentinel value is returned by {@link
     * JsonReader#peek()} to signal that the JSON-encoded value has no more
     * tokens.
     */
    END_DOCUMENT
}

?JsonElement

?