一、引入gson版本
com.google.code.gson gson 2.8.5
二、两种使用方式
2.1基本使用
Gson gson = new Gson();
2.2自定义
Gson gson = new GsonBuilder() .xxxx() .create();
常用使用如下替换xxxx()方法:
- setPrettyPrinting()，作用设置json的输出格式;
- setFieldNamingPolicy(FieldNamingPolicy.IDENTITY)，作用设置序列化字段策略,常用策略;
2.1.FieldNamingPolicy.IDENTITY 默认属性，不改变字段的属性 2.2.FieldNamingPolicy.LOWER_CASE_WITH_DASHES 是字段属性小写，并加上-为分割 2.3.FieldNamingPolicy.LOWER_CASE_WITH_DOTS 是字段属性小写，并加上.为分割 2.4. FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES 是字段属性小写，并 加上_为分割 2.5.FieldNamingPolicy.UPPER_CAMEL_CASE 每个单词首字母大写 2.6.FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES 每个首字母大写并以空格隔开
注：这些策略不会影响标记@SerializedName注解的字段
3. serializeNulls()，作用序列化空字段
4.setExclusionStrategies(new ExclusionStrategy())，作用设置字段是否需要序列化，如果shouldSkipClass(Class) 或shouldSkipField(attribute)方法返回true，将不会序列化和反序列化该类型（字段）
5.excludeFieldsWithoutExposeAnnotation()，需要配合@Expose注解来使用 ，忽略某个字段进行初始化
6.
excludeFieldsWithModifiers(Modifier.STATIC) Modifier.STATIC， 不序列化被 static修饰的字段Modifier.TRANSIENT,不序列化被transient修饰的字段，Modifier.VOLATILE 不序列化被volatile修饰的字段
7.registerTypeAdapter(Type, Object)自定义序列化和反序列化方法
三、常用注解
3.1 @since 指定该字段在序列化和反序列化时在哪个版本显示
public class Employee{
private Integer id;
@Since(1.0)
private String firstName;
@Since(1.1)
private String lastName;
@Since(1.2)
private String email;
}
Employee employeeObj = new Employee(1, "Lokesh", "Gupta", "[email protected]");
Gson gson = new GsonBuilder()
.setVersion(1.1)
.create();
System.out.println(gson.toJson(employeeObj));
只序列化1.1及之前的版本的字段
{
"id": 1,
"firstName": "Lokesh",
"lastName": "Gupta"
}
3.2.@SerializedName按照指定的字段名进行序列化和反序列化 序列化时email以emailId输出，反序列化的时候以emailAddress输入 代码如下
public class Employee
{
private Integer id;
private String firstName;
private String lastName;
@SerializedName(value = "emailId", alternate = "emailAddress")
private String email;
}
//序列化代码
Employee emp = new Employee(1001, "Lokesh", "Gupta", "[email protected]");
Gson gson = new GsonBuilder().setPrettyPrinting().create();
System.out.println(gson.toJson(emp));
//序列化输出
{
"id": 1001,
"firstName": "Lokesh",
"lastName": "Gupta",
"emailId": "[email protected]"
}
//反序列化
String json = "{'id': 1001,"
+ "'firstName': 'Lokesh',"
+ "'lastName': 'Gupta',"
+ "'email': '[email protected]',"
+ "'emailAddress': '[email protected]'}";
Gson gson = new GsonBuilder().setPrettyPrinting().create();
Employee emp = gson.fromJson(json, Employee.class);
System.out.println(emp);
//反序列化输出
Employee [id=1001, firstName=Lokesh, lastName=Gupta, [email protected]]
3.3.@Expose序列化和反序列化是否忽略该字段 @Expose(serialize = false)
private String lastName; @Expose (serialize = false, deserialize = false) private String emailAddress; Gson gson = new GsonBuilder() .excludeFieldsWithoutExposeAnnotation() .create(); 注使用new Gson()来执行 toJson() and fromJson() 方法时不生效的， transient 与 @Expose (serialize = false, deserialize = false)有相同的效果
四、自定义序列化和反序列化
4.1实现JsonSerializer接口
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
public class BooleanSerializer implements JsonSerializer {
public JsonElement serialize(Boolean aBoolean, Type type,
JsonSerializationContext jsonSerializationContext)
{
if(aBoolean){
return new JsonPrimitive(1);
}
return new JsonPrimitive(0);
}
}
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class Main
{
public static void main(String[] args) throws Exception
{
Employee emp = new Employee(1, "Lokesh", "Gupta", "[email protected]", true);
Gson gson = new GsonBuilder()
.registerTypeAdapter(Boolean.class, new BooleanSerializer())
.setPrettyPrinting()
.create();
String json = gson.toJson(emp);
System.out.println(json);
}
}
//输出
{
"id": 1,
"firstName": "Lokesh",
"lastName": "Gupta",
"email": "[email protected]",
"active": 1
}
4.2JsonDeserializer反序列化接口
//Employee.java
public class Employee
{
private Integer id;
private String firstName;
private String lastName;
private String email;
private LocalDate dob;
}
//EmployeeDeserializer.java
public class EmployeeDeserializer implements JsonDeserializer
{
@Override
public Employee deserialize(JsonElement json, Type typeOfT,
JsonDeserializationContext context) throws JsonParseException
{
JsonObject jsonObject = json.getAsJsonObject();
LocalDate localDate = LocalDate.of(
jsonObject.get("year").getAsInt(),
jsonObject.get("month").getAsInt(),
jsonObject.get("day").getAsInt()
);
return new Employee(
jsonObject.get("id").getAsInt(),
jsonObject.get("firstName").getAsString(),
jsonObject.get("lastName").getAsString(),
jsonObject.get("email").getAsString(),
localDate);
}
}
public class Main{
public static void main(String[] args) throws Exception
{
String json = "{'id': 1001,"
+ "'firstName': 'Lokesh',"
+ "'lastName': 'Gupta',"
+ "'email': '[email protected]', "
+ "'day': 11, "
+ "'month': 8, "
+ "'year': 2019}";
Gson gson = new GsonBuilder()
.registerTypeAdapter(Employee.class, new EmployeeDeserializer())
.create();
Employee employee = gson.fromJson(json, Employee.class);
System.out.println(employee);
}
}
五、gson反序列化数组，List，Set，Map，如果这些类型作为成员属性和序列化时无须特殊处理
import java.lang.reflect.Type;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
1.数组
String userJson = "[{'name': 'Alex','id': 1}, "
+ "{'name': 'Brian','id':2}, "
+ "{'name': 'Charles','id': 3}]";
Gson gson = new Gson();
User[] userArray = gson.fromJson(userJson, User[].class);
//2.List
String userJson = "[{'name': 'Alex','id': 1}, "
+ "{'name': 'Brian','id':2}, "
+ "{'name': 'Charles','id': 3}]";
Gson gson = new Gson();
Type userListType = new TypeToken>(){}.getType();
ArrayList userArray = gson.fromJson(userJson, userListType);
//3.Set
Gson gson = new Gson();
Type setType = new TypeToken>(){}.getType();
Set userSet = gson.fromJson(jsonString, setType);
//4.Map
HashMap employeeMap = new HashMap<>();
employeeMap.put(1, new Employee(1l, "Alex", LocalDate.of(1990, 01, 01)));
employeeMap.put(2, new Employee(2l, "Bob", LocalDate.of(1990, 02, 01)));
//Deep clone
Gson gson = new Gson();
String jsonString = gson.toJson(employeeMap);
Type type = new TypeToken>(){}.getType();
HashMap clonedMap = gson.fromJson(jsonString, type);
参考学习：
https://howtodoinjava.com/learningpaths/gson/
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