Gson 解析教程

Posted 2020-09-17

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Gson 是google解析Json的一个开源框架,同类的框架fastJson,JackJson等等

本人fastJson用了两年，也是从去年才开始接触Gson,希望下面的总结会对博友有用,至于Gson与FastJson的对比,其实半斤八两的问题,不再赘述

第一步:在AS下面创建Java工程并添加gson依赖

compile ‘com.google.code.gson:gson:2.6.2‘

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Gson的实例化方式:

1:Gson gson=newGson();

2:通过GsonBuilder 可以配置多种选项

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gson = new GsonBuilder()
.setLenient()// json宽松
.enableComplexMapKeySerialization()//支持Map的key为复杂对象的形式
.serializeNulls() //智能null
.setPrettyPrinting()// 调教格式
.disablehtmlEscaping() //默认是GSON把HTML 转义的
.create();

Gson的基本用法:

JavaBean转换Json字符串

gson提供 publicString toJson(Object src) 方法可以将对象转换成jsonStr

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package com.xuan.gson;
import com.google.gson.Gson;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 10:39
*/
public class GsonTest1 {
public static class Student {
private String name;
private int age;
//省略setter getter equals
}
private static void log(String msg) {
System.out.println(msg);
}
public static void main(String[] args) throws Exception {
Gson gson = new Gson();
Student student = new Student();
student.setName("xuanyouwu");
student.setAge(26);
String jsonStr = gson.toJson(student);
log("---->javabean convert jsonStr:" + jsonStr);
}
}

运行结果:

---->javabean convert jsonStr:{"name":"xuanyouwu","age":26}

List Map转Json字符串

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package com.xuan.gson;
import com.google.gson.Gson;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 10:39
*/
public class GsonTest1 {
private static void log(String msg) {
System.out.println(msg);
}
public static void main(String[] args) throws Exception {
Gson gson = new Gson();
List<String> list = Arrays.asList("1", "a", "3", "rt", "5");
log("---->list convert jsonStr:" + gson.toJson(list));
Map<String, Object> content = new HashMap<String, Object>();
content.put("name", "xuanyouwu");
content.put("age", "26");
log("---->map convert jsonStr:" + gson.toJson(content));
}
}

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运行结果:

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---->list convert jsonStr:["1","a","3","rt","5"]
---->map convert jsonStr:{"name":"xuanyouwu","age":"26"}

Json字符串转JavaBean

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String studentJsonStr="{\"name\":\"xuanyouwu\",\"age\":26}";
Student student1 = gson.fromJson(studentJsonStr, Student.class);
log("------->json convert JavaBean:"+student1);

运行结果:

------->json convert JavaBean:Student{name=‘xuanyouwu‘, age=26}

Json字符串转List

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String listJsonStr="[\"1\",\"a\",\"3\",\"rt\",\"5\"]";

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Type type = new TypeToken<ArrayList<String>>() {
}.getType();
ArrayList<String> sList=gson.fromJson(listJsonStr, type);
log("------->json convert List:"+sList);

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运行结果:

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------->json convert List:[1, a, 3, rt, 5]

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</pre><pre code_snippet_id="1687762" snippet_file_name="blog_20160518_11_939636" name="code" class="java"> Gson封装的类型体系

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Gson里面有一个非常有意思的抽象基类JsonElement,他的继承体系:

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<img src="http://img.blog.csdn.net/20160518135136864?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center" alt="" />

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JsonObject等同于org的JSONObject,JsonArray也类似,

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JsonNull 其实就是null 字段

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JsonNull jsonNull=new JsonNull();//构造方法过时，推荐INSTANCE

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JsonNull jsonNull=JsonNull.INSTANCE;
log("-----"+jsonNull);

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运行结果:

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-----null

JsonPrimitive非常有意思,我们知道如果json转换成字符串可能包含引号的转义，但是通过JsonPrimative我们可以获得为转义的字符串，看实例：

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package com.xuan.gson;
import com.google.gson.JsonNull;
import com.google.gson.JsonPrimitive;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 11:20
*/
public class GsonTest2 {
private static void log(String msg) {
System.out.println(msg);
}
public static void main(String[] args) throws Exception {
String studentJsonStr="{\"name\":\"xuanyouwu\",\"age\":26}";
log("------>studentJsonStr:"+studentJsonStr);
JsonPrimitive jsonPrimitive=new JsonPrimitive(studentJsonStr);
log("------>jsonPrimitive:"+jsonPrimitive);
log("------>jsonPrimitive:"+jsonPrimitive.toString());
log("------>jsonPrimitive:"+jsonPrimitive.getAsString());
JsonPrimitive jsonPrimitive2=new JsonPrimitive("this is String");
log("------>jsonPrimitive2:"+jsonPrimitive2);
log("------>jsonPrimitive2:"+jsonPrimitive2.toString());
log("------>jsonPrimitive2:"+jsonPrimitive2.getAsString());
}
}

运行结果:

------>studentJsonStr:{"name":"xuanyouwu","age":26}
------>jsonPrimitive:"{\"name\":\"xuanyouwu\",\"age\":26}"
------>jsonPrimitive:"{\"name\":\"xuanyouwu\",\"age\":26}"
------>jsonPrimitive:{"name":"xuanyouwu","age":26}
------>jsonPrimitive2:"this is String"
------>jsonPrimitive2:"this is String"
------>jsonPrimitive2:this is String

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创建JsonObject

通过addPropert(key,value)可以向jsonObject中添加字段跟hashMap类似

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JsonObject jsonObject=new JsonObject();
jsonObject.addProperty("name","xuanyouwu");
jsonObject.addProperty("age",26);
log("------>create jsonObject:"+jsonObject);

运行结果:

------>create jsonObject:{"name":"xuanyouwu","age":26}

创建JsonArray

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JsonArray jsonElements=new JsonArray();
jsonElements.add("a");
jsonElements.add("b");
jsonElements.add("c");
jsonElements.add("d");
log("------>create jsonArray:"+jsonElements);

运行结果:

------>create jsonArray:["a","b","c","d"]

JsonObject 嵌套数组或者说嵌套JsonArray

通过JsonObject的add(key,JsonElement)可以为jsonObject 添加一个数组的字段

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JsonObject jsonObject2=new JsonObject();
jsonObject2.addProperty("name","xuanyouwu");
jsonObject2.addProperty("age",26);
JsonArray jsonElements2=new JsonArray();
jsonElements2.add("骑车");
jsonElements2.add("打游戏");
jsonElements2.add("看电视");
jsonObject2.add("hobby",jsonElements2);
log("------>create jsonObject inner JsonArray:"+jsonObject2);

运行结果:

------>create jsonObject inner JsonArray:{"name":"xuanyouwu","age":26,"hobby":["骑车","打游戏","看电视"]}

Gson注解

在Gson中有五类注解

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重命名注解:SerializedName

作用:转换关键字key,json转换成JavaBean时,json字段的key 默认必须和我们声明类的字段名称一样,当服务器端返回了关键字怎么办，比如key 为new switch这样,我们在声明类的时候不能写这样的字段,可能你想服务器端改动，他可能要改数据库，但是我告诉你，做服务端的大部分不愿意改动他的json,是很自私的!这时候重命名注解都排上用场了第二种场景:服务器端返回的json 的key 简直太丑，或者太长,你想简化,my_parent_name,可以简化成mpn 比较优雅简介

实例:

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运行结果: ------>user:User{name=‘zhangsan‘, age=26, isnew=1}

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运行结果: ------>user2:User2{name=‘zhangsan‘, age=26, pn=‘zhangsanf‘}

作用2:结合alternate 提供多种备用字段key来解析,@SerializedName(value ="desc",alternate = {"other","note"}) 如果json中有other就会解析成desc 如果有note也会解析成desc,注意1:value中的值不能出现在alternate中,注意2:alternate的备选字段会后面的替换前面的

实例：

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package com.xuan.gson;
import com.google.gson.Gson;
import com.google.gson.annotations.SerializedName;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 11:20
*/
public class GsonTest4 {
private static void log(String msg) {
System.out.println(msg);
}
public static class User {
public String name;
public int age;
@SerializedName(value = "desc",alternate = {"other","note"})
public String desc;
@Override
public String toString() {
return "User{" +
"name=‘" + name + ‘\‘‘ +
", age=" + age +
", desc=‘" + desc + ‘\‘‘ +
‘}‘;
}
}
public static void main(String[] args) throws Exception {
String jsonFromServer = "{\n" +
" \"age\": 26,\n" +
" \"other\": \"成都人\",\n" +
" \"name\": \"zhangsan\"\n" +
"}";
Gson gson = new Gson();
User user = gson.fromJson(jsonFromServer, User.class);
log("------>user:" + user);
String jsonFromServer2 = "{\n" +
" \"age\": 26,\n" +
" \"note\": \"成都人\",\n" +
" \"name\": \"zhangsan\"\n" +
"}";
User user2 = gson.fromJson(jsonFromServer2, User.class);
log("------>user:" + user2);
//包括desc 与note note在desc之后
String jsonFromServer3="{\n" +
" \"age\": 26,\n" +
" \"desc\": \"desc成都人\",\n" +
" \"name\": \"zhangsan\",\n" +
" \"note\": \"note成都人\"\n" +
"}";
User user3 = gson.fromJson(jsonFromServer3, User.class);
log("------>user:" + user3);
//包括desc 与note note在desc之前
String jsonFromServer4="{\n" +
" \"age\": 26,\n" +
" \"note\": \"note成都人\",\n" +
" \"name\": \"zhangsan\",\n" +
" \"desc\": \"desc成都人\"\n" +
"}";
User user4 = gson.fromJson(jsonFromServer4, User.class);
log("------>user:" + user4);
}
}

运行结果:

------>user:User{name=‘zhangsan‘, age=26, desc=‘成都人‘}
------>user:User{name=‘zhangsan‘, age=26, desc=‘成都人‘}
------>user:User{name=‘zhangsan‘, age=26, desc=‘note成都人‘}
------>user:User{name=‘zhangsan‘, age=26, desc=‘desc成都人‘}

Gson @Expose过滤注解

源码:默认既可以序列化又可以反序列化

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@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Expose {
public boolean serialize() default true;
public boolean deserialize() default true;
}

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</pre><pre code_snippet_id="1687762" snippet_file_name="blog_20160518_25_4965554" name="code" class="java">可以排除不需要序列化的字段,需要配合GsonBuilder使用

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<pre style="font-family: 宋体; font-size: 9pt; background-color: rgb(255, 255, 255);"><pre name="code" class="java"> Gson gson = new GsonBuilder()
.excludeFieldsWithoutExposeAnnotation()
.create();

  不添加@Expose注解的字段将不会解析:

   分为以下几种情况:

    1:不添加@Expose注解等同于@Expose(deserialize = false,serialize = false) 不做任何解析

    2:@Expose(deserialize = true,serialize = false) 只解析用用，也就是反序列化可以，序列化不可以

    3:@Expose(deserialize = false,serialize = true) 序列化可以，反序列化不行

    4:@Expose(deserialize = true,serialize = true) 既可以序列化，也可以反序列化

    实例:将分别演示这四种情况

  不添加@Expose注解:等同于@Expose(deserialize = false,serialize = false)

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运行结果:

------>反序列化:User{name=‘null‘}
------>序列化:{}

  添加@Expose注解

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运行结果:

------>反序列化:User{name=‘zhangsan‘}
------>序列化:{"name":"zhangsan2"}

@Expose注解只序列化

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运行结果:

------>反序列化:User{name=‘null‘}------>序列化:{"name":"zhangsan2"}

@Expose 只反序列化

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运行结果:

------>反序列化:User{name=‘zhangsan‘}
------>序列化:{}

@Since(float v)注解版本控制

结合GsonBuilder.setVersion(n)使用当n>=v时才会序列化解析

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package com.xuan.gson;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.Since;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 11:20
*/
public class GsonTest6 {
private static void log(String msg) {
System.out.println(msg);
}
public static class User {
@Since(2)
public String name;
@Override
public String toString() {
return "User{" +
"name=‘" + name + ‘\‘‘ +
‘}‘;
}
}
public static void main(String[] args) throws Exception {
String jsonFromServer = "{\"name\": \"zhangsan\"}";
Gson gson = new GsonBuilder()
.setVersion(1)//版本为1
.create();
User user1 = gson.fromJson(jsonFromServer, User.class);
log("------>反序列化v=1:" + user1);
User user1_1 = new User();
user1_1.name = "zhangsan2";
String userStr = gson.toJson(user1_1);
log("------>序列化v=1:" + userStr);
Gson gson2 = new GsonBuilder()
.setVersion(2)//版本为2
.create();
User user2 = gson2.fromJson(jsonFromServer, User.class);
log("------>反序列化v=2:" + user2);
User user2_1 = new User();
user2_1.name = "zhangsan2";
String userStr2_1 = gson2.toJson(user2_1);
log("------>序列化v=2:" + userStr2_1);
Gson gson3 = new GsonBuilder()
.setVersion(3)//版本为3
.create();
User user3 = gson3.fromJson(jsonFromServer, User.class);
log("------>反序列化v=3:" + user3);
User user3_1 = new User();
user3_1.name = "zhangsan2";
String userStr3_1 = gson3.toJson(user3_1);
log("------>序列化v=3:" + userStr3_1);
}
}

运行结果:

------>反序列化v=1:User{name=‘null‘}
------>序列化v=1:{}
------>反序列化v=2:User{name=‘zhangsan‘}
------>序列化v=2:{"name":"zhangsan2"}
------>反序列化v=3:User{name=‘zhangsan‘}
------>序列化v=3:{"name":"zhangsan2"}

@Util(float v)注解版本控制

当gson的setVersion(n) n<v 才解析

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package com.xuan.gson;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.Until;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 11:20
*/
public class GsonTest6 {
private static void log(String msg) {
System.out.println(msg);
}
public static class User {
@Until(2)
public String name;
@Override
public String toString() {
return "User{" +
"name=‘" + name + ‘\‘‘ +
‘}‘;
}
}
public static void main(String[] args) throws Exception {
String jsonFromServer = "{\"name\": \"zhangsan\"}";
Gson gson = new GsonBuilder()
.setVersion(1)//版本为1
.create();
User user1 = gson.fromJson(jsonFromServer, User.class);
log("------>反序列化v=1:" + user1);
User user1_1 = new User();
user1_1.name = "zhangsan2";
String userStr = gson.toJson(user1_1);
log("------>序列化v=1:" + userStr);
Gson gson2 = new GsonBuilder()
.setVersion(2)//版本为2
.create();
User user2 = gson2.fromJson(jsonFromServer, User.class);
log("------>反序列化v=2:" + user2);
User user2_1 = new User();
user2_1.name = "zhangsan2";
String userStr2_1 = gson2.toJson(user2_1);
log("------>序列化v=2:" + userStr2_1);
Gson gson3 = new GsonBuilder()
.setVersion(3)//版本为3
.create();
User user3 = gson3.fromJson(jsonFromServer, User.class);
log("------>反序列化v=3:" + user3);
User user3_1 = new User();
user3_1.name = "zhangsan2";
String userStr3_1 = gson3.toJson(user3_1);
log("------>序列化v=3:" + userStr3_1);
}
}

运行结果:

------>反序列化v=1:User{name=‘zhangsan‘}
------>序列化v=1:{"name":"zhangsan2"}
------>反序列化v=2:User{name=‘null‘}
------>序列化v=2:{}
------>反序列化v=3:User{name=‘null‘}
------>序列化v=3:{}

Gson 高级用法

相信看过retrofit2.0 源码的同学都知道,其中有一个GsonConverterFactory 里面的用法十分精炼老成,这里来一段源码

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据说使用TypeAdapter 效率更高,本人还未对比测试,暂时放后吧,TypeAdapter是什么玩意呢？

在源码中备注了一句 Converts Java objects to and from JSON 就是对象json之间的互相转换接替了T 泛型类的序列化和反序列化的逻辑

从源码中我们看到区分了2.1版本之前后之后的用法，2.1版本之前可以自定义adapter:

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public class PointAdapter extends TypeAdapter<Point> {
* public Point read(JsonReader reader) throws IOException {
* if (reader.peek() == JsonToken.NULL) {
* reader.nextNull();
* return null;
* }
* String xy = reader.nextString();
* String[] parts = xy.split(",");
* int x = Integer.parseInt(parts[0]);
* int y = Integer.parseInt(parts[1]);
* return new Point(x, y);
* }
* public void write(JsonWriter writer, Point value) throws IOException {
* if (value == null) {
* writer.nullValue();
* return;
* }
* String xy = value.getX() + "," + value.getY();
* writer.value(xy);
* }
* }}

使用

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GsonBuilder builder = new GsonBuilder();
* builder.registerTypeAdapter(Point.class, new PointAdapter());
* // if PointAdapter didn‘t check for nulls in its read/write methods, you should instead use
* // builder.registerTypeAdapter(Point.class, new PointAdapter().nullSafe());
* ...
* Gson gson = builder.create();

在2.1版本之后更推荐直接插入泛型就使用

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// String json = "{‘origin‘:‘0,0‘,‘points‘:[‘1,2‘,‘3,4‘]}";
// TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
// Graph graph = graphAdapter.fromJson(json);
// }</pre>
// And an example for serialization: <pre> {@code
//
// Graph graph = new Graph(...);
// TypeAdapter<Graph> graphAdapter = gson.getAdapter(Graph.class);
// String json = graphAdapter.toJson(graph);
// }</pre>

   实例:使用TypeAdapter 来序列化和反序列化

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运行结果:

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------>序列化:{"name":"xuanyouwu","age":26}
------>反序列化:User{name=‘xuanyouwu‘, age=26}

Gson的容错机制:

   为什么要容错了,在javaBean中声明了int类型的age 如果服务端 返回的是"" 空字符串怎么办呢？崩溃？

   如果json格式不规范如 {name=zhangsan,age:26,hobby=null}  发现不是普通的key value

  容错实现方式1：

  1:创建Gson的方式

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gson = new GsonBuilder()
.setLenient()// json宽松
.create();

  2:使用JsonReader

   JsonReader jsonReader = gson.newJsonReader(value.charStream());

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jsonReader.<span style="font-family: 宋体; font-size: 9pt;">setLenient(true)</span>

3:自定义TypeAdapter

4:使用注解JsonAdapter,其实也是自定义Adapter

1.2归为一类由框架实现,基本json大格式规范,键值对不标准,多引号的问题等等,而不报错停止解析,但是功能相对较弱

能解决bug

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com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON<span style="font-family:宋体; font-size:9pt; background-color:rgb(228,228,255)"> </span>

3,4归为一类,都属于自定义adapter,但是3与gson绑定,4使用注解和字段绑定

实例：

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运行结果:

------->jsonFromServer:{
"age": "",
"name": "zhangsan"
}
------>默认Gson 解析异常:com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: empty String
------>自定义adapter 解析:User{name=‘zhangsan‘, age=0}
------>自定义adapter 解析2:User{name=‘zhangsan‘, age=0}

可以看到 age是空字符串但是不影响整体的解析流程,这对客户端是十分友好的

基于注解的方式,上面的方式倾向于整体,注解的方式倾向于字段

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运行结果:

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------->jsonFromServer:{
"age": "",
"name": "zhangsan"
}
------>gson 解析异常:com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: empty String
------>JsonAdapter 注解解析:User{name=‘zhangsan‘, age=0}

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可以看到我们成功地跳过服务端返回json不合理的坑了吧

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</pre><pre code_snippet_id="1687762" snippet_file_name="blog_20160519_47_1535429" name="code" class="java">不得不吐槽啊，像这种服务器端错误，应该完全归结服务端json不合理返回，

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往往老板都会找客户端原因，怎么又崩溃了？怎么又不显示了？老板不会关心是不是数据返回不合理的问题的！

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做客户端需要相当的承受能力哈,同意的点个赞哈

[java] view plain copy

</pre><p>其实上面的方式保险是很保险，但是需要维护的地方多，代码量大，我们介绍另外一种方式:JsonSerializer与JsonDeserializer 之关系单方面的处理，可以全局注册某个类型的处理：看实例：</p><p><pre name="code" class="java">package com.xuan.gson;
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.JsonParseException;
import java.lang.reflect.Type;
/**
* @author xuanyouwu
* @email [email protected]
* @time 2016-05-18 11:20
*/
public class GsonTest10 {
private static void log(String msg) {
System.out.println(msg);
}
public static class User {
public String name;
public int age;
@Override
public String toString() {
return "User{" +
"name=‘" + name + ‘\‘‘ +
", age=" + age +
‘}‘;
}
}
public static void main(String[] args) throws Exception {
JsonDeserializer<Integer> jsonDeserializer = new JsonDeserializer<Integer>() {
@Override
public Integer deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
try {
return json.getAsInt();
} catch (NumberFormatException e) {
return 0;
}
}
};
Gson gson = new GsonBuilder()
.registerTypeAdapter(int.class, jsonDeserializer)
.create();
String jsonStrFromServer = "{\n" +
" \"age\": \"\",\n" +
" \"name\": \"zhangsan\"\n" +
"}";
log("------->jsonFromServer:" + jsonStrFromServer);
try {
User user = gson.fromJson(jsonStrFromServer, User.class);
log("------> JsonDeserializer<Integer> 解析:" + user);
} catch (Exception e) {
log("------> JsonDeserializer<Integer> 解析异常:" + e);
}
Gson gson1=new Gson();
try {
User user1 = gson1.fromJson(jsonStrFromServer, User.class);
log("------> 默认gson 解析:" + user1);
} catch (Exception e) {
log("------> 默认gson 解析异常:" + e);
}
}
}

运行结果:

------->jsonFromServer:{
"age": "",
"name": "zhangsan"
}
------> JsonDeserializer<Integer> 解析:User{name=‘zhangsan‘, age=0}
------> 默认gson 解析异常:com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: empty String

这样定义全局的反序列化工具就能避免解析异常

from: http://blog.csdn.net/axuanqq/article/details/51441590

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