Gson - 学习
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Google 的 Gson 库,Gson 是一个非常强大的库,可以将 JSON 格式的数据转化成 Java 对象,也支持将 Java 对象转成 JSON 数据格式。
Gson 依赖
本文将会快速开始使用 Gson 进行序列化操作。由于很多读者都是 android 开发人员,我们会提供 Java 环境中的 Gson。在开始之前,我们需要引入 Gson 库到自己的项目中,最新的版本是 2.8.0.
Gradle 项目添加
compile ‘com.google.code.gson:gson:2.8.0‘
Maven 项目添加
<dependencies>
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.8.0</version>
<scope>compile</scope>
</dependency>
</dependencies>
基础
UserSimple userObject = new UserSimple( "Norman", "[email protected]", 26, true ); Gson gson = new Gson(); String userJson = gson.toJson(userObject);
反序列化:
String userJson = "{‘age‘:26,‘email‘:‘[email protected]‘,‘isDeveloper‘:true,‘name‘:‘Norman‘}"; Gson gson = new Gson(); UserSimple userObject = gson.fromJson(userJson, UserSimple.class);
数组反序列化:
public class Founder {
String name;
int flowerCount;
}
String founderJson = "[{‘name‘: ‘Christian‘,‘flowerCount‘: 1}, {‘name‘: ‘Marcus‘, ‘flowerCount‘: 3}, {‘name‘: ‘Norman‘, ‘flowerCount‘: 2}]"; Gson gson = new Gson(); Founder[] founderArray = gson.fromJson(founderJson, Founder[].class);
列表List反序列化:
String founderJson = "[{‘name‘: ‘Christian‘,‘flowerCount‘: 1}, {‘name‘: ‘Marcus‘, ‘flowerCount‘: 3}, {‘name‘: ‘Norman‘, ‘flowerCount‘: 2}]"; Gson gson = new Gson(); Type founderListType = new TypeToken<ArrayList<Founder>>(){}.getType(); List<Founder> founderList = gson.fromJson(founderJson, founderListType);
列表作为对象的一部分:反序列化时不需要传递 TypeToken
public class GeneralInfo { String name; String website; List<Founder> founders; } String generalInfoJson = "{‘name‘: ‘Future Studio Dev Team‘, ‘website‘: ‘https://futurestud.io‘, ‘founders‘: [{‘name‘: ‘Christian‘, ‘flowerCount‘: 1 }, {‘name‘: ‘Marcus‘,‘flowerCount‘: 3 }, {‘name‘: ‘Norman‘,‘flowerCount‘: 2 }]}"; Gson gson = new Gson(); GeneralInfo generalInfoObject = gson.fromJson(generalInfoJson, GeneralInfo.class);
Map反序列化:
public class AmountWithCurrency { String currency; int amount; } String dollarJson = "{ ‘1$‘: { ‘amount‘: 1, ‘currency‘: ‘Dollar‘}, ‘2$‘: { ‘amount‘: 2, ‘currency‘: ‘Dollar‘}, ‘3€‘: { ‘amount‘: 3, ‘currency‘: ‘Euro‘} }"; Gson gson = new Gson(); Type amountCurrencyType = new TypeToken<HashMap<String, AmountWithCurrency>>(){}.getType(); HashMap<String, AmountWithCurrency> amountCurrency = gson.fromJson(dollarJson, amountCurrencyType);
Map 数据结构没有根元素,如 "[",我们就可以像解析 List 那用解析嵌套的 Map。
Set反序列化
String founderJson = "[{‘name‘: ‘Christian‘,‘flowerCount‘: 1}, {‘name‘: ‘Marcus‘, ‘flowerCount‘: 3}, {‘name‘: ‘Norman‘, ‘flowerCount‘: 2}]"; Gson gson = new Gson(); Type founderSetType = new TypeToken<HashSet<Founder>>(){}.getType(); HashSet<Founder> founderSet = gson.fromJson(founderJson, founderSetType);
泛型序列化:每种数据类型需要 new TypeToken 才能解析成功
Gson gson = new Gson(); List<Integer> integerList = new ArrayList<>(); integerList.add(1); integerList.add(2); integerList.add(3); List<String> stringList = new ArrayList<>(); stringList.add("1"); stringList.add("2"); stringList.add("3"); Type integerType = new TypeToken<List<Integer>>() {}.getType(); Type stringType = new TypeToken<List<String>>() {}.getType(); String integerJson = gson.toJson(integerList, integerType); String stringJson = gson.toJson(stringList, stringType);
@SerializedName
public class UserSimple { @SerializedName("fullName") String name; String email; boolean isDeveloper; int age; }
@SerializedName(value = "fullName", alternate = "username") private String name;
SerializedName 接受两个参数,value、alternate。
SerializedName 改变了默认序列化和默认反序列化的字段取值,序列化时就是 value 的名称就是字段的名称,alternate 属性是用在反序列化上。
GsonBuider
public class UserNaming { String Name; String email_of_developer; boolean isDeveloper; int _ageOfDeveloper; }
命名规则:
FieldNamingPolicy.IDENTITY:完全匹配我们 Java model 中的字段名,不管你有没有设置其他注解属性
GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.IDENTITY); Gson gson = gsonBuilder.create(); UserNaming user = new UserNaming("Norman", "[email protected]", true, 26); String usersJson = gson.toJson(user);
输出:
{ "Name": "Norman", "_ageOfDeveloper": 26, "email_of_developer": "[email protected]", "isDeveloper": true }
FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES:将修改生成的 JSON 中的字段名,格式将全部变成小写,并且每个单词用“_” 分割
{ "name": "Norman", "_age_of_developer": 26, "email_of_developer": "[email protected]", "is_developer": true }
FieldNamingPolicy.LOWER_CASE_WITH_DASHES:每个单词用“-” 分隔
FieldNamingPolicy.UPPER_CAMEL_CASE:规则是每个单词的第一个字母都要大写,其他不变
{ "Name": "Norman", "_AgeOfDeveloper": 26, "Email_of_developer": "[email protected]", "IsDeveloper": true }
FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES:每个单词的第一个字母会大写、每个单词使用空格分隔、含有 “_” 的链接的不会在使用空格
{ "Name": "Norman", "_Age Of Developer": 26, "Email_of_developer": "[email protected]", "Is Developer": true }
FieldNamingStrategy 自定义规则
FieldNamingStrategy customPolicy = new FieldNamingStrategy() { @Override public String translateName(Field f) { return f.getName().replace("_", ""); } }; GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setFieldNamingStrategy(customPolicy); Gson gson = gsonBuilder.create(); UserNaming user = new UserNaming("Norman", "[email protected]", true, 26); String usersJson = gson.toJson(user);
序列化null
GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.serializeNulls(); //重点 Gson gson = gsonBuilder.create(); UserSimple user = new UserSimple("Norman", null, 26, true); String usersJson = gson.toJson(user);
输出:
{ "age": 26, "email": null, "isDeveloper": true, "name": "Norman" }
忽略策略
ExclusionStrategies 将 Date 和 boolean 类型的字段忽略、shouldSkipField 是用来忽略单个字段的,如果你想要忽略带有 “_” 的字段
GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setExclusionStrategies(new ExclusionStrategy() { @Override public boolean shouldSkipField(FieldAttributes f) { return f.getName().contains("_"); } @Override public boolean shouldSkipClass(Class<?> incomingClass) { return incomingClass == Date.class || incomingClass == boolean.class; } }); Gson gson = gsonBuilder.create(); UserDate user = new UserDate("Norman", "[email protected]", 26, true); String usersJson = gson.toJson(user);
上面的例子使用的是 setExclusionStrategies 方法,不管是序列化还是反序列化都会起作用,如果我们只想其中一个起作用,选择调下面的方法就行了:
- addSerializationExclusionStrategy()
- addDeserializationExclusionStrategy()
用法和 ExclusionStrategy 的实现一样,可重写两个方法实现。
基于 Modifiers 的忽略规则:
public class UserModifier { private String name; private transient String email; private static boolean isDeveloper; private final int age; }
如果你想忽略 final 和 static 类型的字段, 保留 transient 类型的字段
GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.excludeFieldsWithModifiers(Modifier.STATIC, Modifier.FINAL); Gson gson = gsonBuilder.create(); UserModifier user = new UserModifier("Norman", "[email protected]", 26, true); String usersJson = gson.toJson(user);
注解 @Expose
public class UserSimple { @Expose() String name; // equals serialize & deserialize @Expose(serialize = false, deserialize = false) String email; // equals neither serialize nor deserialize @Expose(serialize = false) int age; // equals only deserialize @Expose(deserialize = false) boolean isDeveloper; // equals only serialize }
根据 @Expose 的用法,UserSimple 序列化 JSON 输出只有 name 和 isDeveloper,其他连个字段就不会被输出,因为 serialize 都是 false;
反序列化的话,只有 email 和 isDeveloper 被忽略,因为 deserialize = false
transient :使用 transient 来描述字段,将不能被序列化和反序列化
Gson 内部使用的是 JsonReader 类,看源码能发现里面有一个 lenient 的属性,默认是 false,也就是说默认值接受标准的 JSON 格式数据,如果数据有问题,将抛出异常解析失败。
JsonReader 也提供了设置 lenient 属性的方法,来忽略一些不标准的 JSON 数据格式。
特殊类型 Floats & Doubles
如果你 Java 对象中包含一个正常的 Floats 或者 Doubles 类型的数据,是可以正常序列化得到 JSON的,如果你传入 Float.POSITIVE_INFINITY 值,Gson 将会抛出异常,因为这个值是不能符合 JSON 标准的GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.serializeSpecialFloatingPointValues(); Gson gson = gsonBuilder.create(); UserFloat userFloat = new UserFloat("Norman", Float.POSITIVE_INFINITY); String usersJson = gson.toJson(userFloat); System.out.println("userJson:" + usersJson);
自定义序列化:
public class UserSubscription { String name; String email; int age; boolean isDeveloper; // new! List<Merchant> merchantList; } public class Merchant { private int Id; private String name; // possibly more properties }
JsonSerializer<Merchant> serializer = new JsonSerializer<Merchant>() { @Override public JsonElement serialize(Merchant src, Type typeOfSrc, JsonSerializationContext context) { JsonObject jsonMerchant = new JsonObject(); jsonMerchant.addProperty("Id", src.getId()); return jsonMerchant; } }; GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.registerTypeAdapter(Merchant.class, serializer); Gson customGson = gsonBuilder.create(); String customJSON = customGson.toJson(subscription);
自定义序列化List:
JsonSerializer<List<Merchant>> serializer = new JsonSerializer<List<Merchant>>() { @Override public JsonElement serialize(List<Merchant> src, Type typeOfSrc, JsonSerializationContext context) { JsonObject jsonMerchant = new JsonObject(); List<String> merchantIds = new ArrayList<>(src.size()); for (Merchant merchant : src) { merchantIds.add("" + merchant.getId()); } String merchantIdsAsString = TextUtils.join(",", merchantIds); jsonMerchant.addProperty("Ids", merchantIdsAsString); return jsonMerchant; } } GsonBuilder gsonBuilder = new GsonBuilder(); Type merchantListType = new TypeToken<List<Merchant>>() {}.getType(); gsonBuilder.registerTypeAdapter(merchantListType, serializer); Gson customGson = gsonBuilder.create(); String customJSON = customGson.toJson(subscription); //结果 { "age": 26, "email": "[email protected]", "isDeveloper": true, "merchantList": { "Ids": "23,42" }, "name": "Norman" }
简化:
JsonSerializer<List<Merchant>> serializer = new JsonSerializer<List<Merchant>>() { @Override public JsonElement serialize(List<Merchant> src, Type typeOfSrc, JsonSerializationContext context) { JsonArray jsonMerchant = new JsonArray(); for (Merchant merchant : src) { jsonMerchant.add("" + merchant.getId()); } return jsonMerchant; } }
//结果
{
"age": 26,
"email": "[email protected]",
"isDeveloper": true,
"merchantList": [
"23",
"42"
],
"name": "Norman"
}
自定义反序列化:
前三个表示年月日,是一个日期结构,后面四个字段表示一个 model 信息
//模型 public class UserDate { private String name; private String email; private boolean isDeveloper; private int age; private Date registerDate; } //Json { "year": 116, "month": 5, "day": 21, "age": 26, "email": "[email protected]", "isDeveloper": true, "name": "Norman" }
JsonDeserializer<UserDate> deserializer = new JsonDeserializer<UserDate>() { @Override public UserDate deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException { JsonObject jsonObject = json.getAsJsonObject(); Date date = new Date( jsonObject.get("year").getAsInt(), jsonObject.get("month").getAsInt(), jsonObject.get("day").getAsInt() ); return new UserDate( jsonObject.get("name").getAsString(), jsonObject.get("email").getAsString(), jsonObject.get("isDeveloper").getAsBoolean(), jsonObject.get("age").getAsInt(), date ); } }; GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.registerTypeAdapter(UserDate.class, deserializer); Gson customGson = gsonBuilder.create(); UserDate customObject = customGson.fromJson(userJson, UserDate.class);
自定义对象示例创建:
//Json { "age": 26, "email": "[email protected]", "isDeveloper": true, "name": "Norman" } //Model public class UserContext { private String name; private String email; private boolean isDeveloper; private int age; // additional attribute, which is not part of the data model private Context context; public UserContext(Context context) { this.context = context; } }
如果你还是用原来的方式来反序列化,那么得到的 Context 肯定是null。你需要在映射之前的构造函数中来创建 Context,这是就需要用到 InstanceCreators。
String userSimpleJson = ...; // the JSON from the server, see above GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.registerTypeAdapter( UserContext.class, new UserContextInstanceCreator(getApplicationContext()) ); Gson customGson = gsonBuilder.create(); UserContext customObject = customGson.fromJson(userSimpleJson, UserContext.class); private class UserContextInstanceCreator implements InstanceCreator<UserContext> { private Context context; public UserContextInstanceCreator(Context context) { this.context = context; } @Override public UserContext createInstance(Type type) { // create new object with our additional property UserContext userContext = new UserContext(context); // return it to gson for further usage return userContext; } }
@JsonAdapter 注解:
public class MerchantListSerializer implements JsonSerializer<List<Merchant>> { @Override public JsonElement serialize(List<Merchant> src, Type typeOfSrc, JsonSerializationContext context) { JsonArray jsonMerchant = new JsonArray(); for (Merchant merchant : src) { jsonMerchant.add("" + merchant.getId()); } return jsonMerchant; } } public class UserSubscriptionAnnotation { String name; String email; int age; boolean isDeveloper; // new! @JsonAdapter(MerchantListSerializer.class) List<Merchant> merchantList; }
只有是类的形式,我们才能使用注解 @JsonAdapter 来添加 MerchantListSerializer。就像之前的一些注解用法一样,并添加到你需要序列化的 Java model 中。
转载自:
作者:無名小子的杂货铺
链接:https://www.jianshu.com/p/215708d00015
來源:简书
简书著作权归作者所有,任何形式的转载都请联系作者获得授权并注明出处。
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