Gson?So easy.

发布时间：2020-08-03 03:17:46 来源：网络阅读：233 作者：2578612215 栏目：编程语言

1.概述

这篇文章主要讲述了Gson的使用.包括从最基础的基本类型的序列化,到对象,数组,集合,再到Gson注解,Gson Builder,再到格式化,自定义序列化与反序列化等内容.
另外文章篇幅较长,建议挑选所需部分查看.所有例子都提供了完整源码,在文章的后面.

2.Gson是什么?

(1)JSON

JSON全称为JavaScript Object Notation,一种轻量级的数据交换格式.

类似于XML但比XML更小,更易解析.

(2)Gson

Gson是Google提供的可以使Java对象与JSON互转的类库,可将Java对象转换为JSON,也可将JSON转换成Java对象.

(3)Gson的好处

a.容易,高效,强大:Gson是Google管理的标准化库,经过高度优化,同时api简单,比如fromJSON(),toJSON().
b.无依赖性:不需要其他库,当然jdk除外.
c.结果简单:转换成的json易于阅读.
d.支持泛型,支持内部类.
e.开源,免费提供.

3.配置Gson环境

目前最新的是2.8.6版本.

(1)Gradle

dependencies {
  implementation 'com.google.code.gson:gson:2.8.6'
}

(2)Maven

<dependency>
  <groupId>com.google.code.gson</groupId>
  <artifactId>gson</artifactId>
  <version>2.8.6</version>
</dependency>

(3)jar

点击下载.

4.Gson基础

(1)基础类型

int intJSON = 1;
long longJSON = 1L;
double doubleJSON = 1.0;
float floatJSON = 1.0f;
byte byteJSON = 1;
char charJSON = 'G';
short shortJSON = 1;
boolean boooleanJSON = true;
System.out.println(gson.toJson(intJSON));
System.out.println(gson.toJson(longJSON));
System.out.println(gson.toJson(doubleJSON));
System.out.println(gson.toJson(floatJSON));
System.out.println(gson.toJson(byteJSON));
System.out.println(gson.toJson(charJSON));
System.out.println(gson.toJson(shortJSON));
System.out.println(gson.toJson(boooleanJSON));

System.out.println("----------------------------------------");

System.out.println(gson.fromJson("1",Integer.class));
System.out.println(gson.fromJson("1.0",Double.class));
System.out.println(gson.fromJson("1",Long.class));
System.out.println(gson.fromJson("true",Boolean.class));
System.out.println(gson.fromJson("B",Character.class));

Gson?So easy. cdn.xitu.io/2019/12/9/16ee6c9f1e5fa1c5?w=455&h=304&f=png&s=5737">
就是对应输出,没啥好说的.

(2)嵌套对象

Gson gson = new Gson();
System.out.println(gson.toJson(new A()));

String str = "{'field':'gggg','field2':33,'field3':'G','field4':'true'}";
A a = gson.fromJson(str,A.class);

System.out.println(a.getField1());
System.out.println(a.getField2());
System.out.println(a.getField3());
System.out.println(a.getField4());

A类:

class A
{
    private String field1 = "123";
    private int field2 = 1;
    private char field3 = 'X';
    private Boolean field4 = true;
}

Gson?So easy.
序列化出来的对象用{}表示.

反序列化时,注意格式,注意名字对应,用单引号引起来,还有char会自动变为String类型,另外对于布尔类型可以加单引号或不加单引号,都可以正常反序列化.

(3)数组

a.普通数组

int [] a = new int []{1,2,3};
double [] b = new double []{1.0,2.0,3.0};
String [] c = new String []{"123","456"};

System.out.println(gson.toJson(a));
System.out.println(gson.toJson(b));
System.out.println(gson.toJson(c));

int [] aa = gson.fromJson("[6,7,8]",int [].class);
double [] bb = gson.fromJson("[6.0,8.0,9.0]",double [].class);
String [] cc = gson.fromJson("['123123','5464']",String [].class);

System.out.println(Arrays.toString(aa));
System.out.println(Arrays.toString(bb));
System.out.println(Arrays.toString(cc));

Gson?So easy.
对普通数组的话,{}变成了[].
还有就是反序列化时,默认会在逗号后面添一个空格.

b.List

List<String> list = new ArrayList<>();
list.add("123");
list.add("456");

System.out.println(gson.toJson(list));

List<AAAA> list2 = new ArrayList<>();
list2.add(new AAAA());
list2.add(new AAAA("45345",8888,false));

System.out.println(gson.toJson(list2));
System.out.println("---------------------------------");

Type type = new TypeToken<List<String>>(){}.getType();

String strList = "['234234','45457']";
List<String> list3 = gson.fromJson(strList, type);
System.out.println(list3);

String strList2 = "[" +
    "{'field1':'9999','field2':666,'field3':'true'}" +
    "," +
    "{'field1':'sdlkfkl','field2':-234234,'field3':'false'}" +
    "]";
List<AAAA> list4 = gson.fromJson(strList2, new TypeToken<List<AAAA>>(){}.getType());
System.out.println(list4);

class AAAA 
{
    private String field1 = "123";
    private int field2 = 5;
    private boolean field3 = true;
}

Gson?So easy.
序列化没什么问题,直接toJson即可,反序列化时,需要配合java.lang.reflect.Type使用,明确指出所要转换的类型:

Type type = new TypeToken<List<String>>(){}.getType();

TypeToken<T>中T为所需要的类型,再把这个Type对象传递给fromJson即可完成转换.
List序列化出来的用[]表示.

(4)Map

Map<String,Integer> map = new HashMap<>();
map.put("123",88);
map.put("2349",999);

System.out.println(gson.toJson(map));

String str = "{'123':23423423,'9999':-234234}";
Map<String,Integer> map2 = gson.fromJson(str, new TypeToken<Map<String,Integer>>(){}.getType());
System.out.println(map2.get("123").toString());
System.out.println(map2.get("9999").toString());

Gson?So easy.
Map同样序列化时直接toJson,反序列化时使用java.lang.reflect.Type.Map序列化出来的用{}表示.

(5)Set

Set<String> set = new HashSet<>();
set.add("123123");
set.add("2349594");
set.add("-234()@#$@#");
System.out.println(gson.toJson(set));

String str = "['38483','@*#$(@#$)','SD<FGDF>G']";
Set<String> set2 = gson.fromJson(str, new TypeToken<Set<String>>(){}.getType());
set2.stream().forEach(System.out::println);

Gson?So easy.
set序列化出来的用[]表示.

(6)null

System.out.println(gson.toJson(new AAA()));
String str = "{'field2':333,'field3':null}";
System.out.println(gson.fromJson(str, AAA.class));

class AAA
{
    private String field1 = null;
    private Integer field2 = null;
    private Double field3 = 3.0;

    @Override
    public String toString()
    {
        return "field1:"+field1+",field2:"+field2+",field3:"+field3;
    }
}

Gson?So easy.
Gson会忽略空值,在序列化时看不到null对应的键值对,反序列化时,直接对应为空.

(7)混合

一个对象里面包含了List,Map,Set,null.

System.out.println(gson.toJson(new Test()));

class Test
{
    private List<String> listField = new ArrayList<>();
    private Map<String,Double> mapField = new HashMap<>();
    private Set<Integer> setField = new HashSet<>();

    private List<User> userListFiled = new ArrayList<>();
    private Map<String,User> userMapField = new HashMap<>();
    private Set<User> userSetField = new HashSet<>();

    private Set<String> nullSetField = null;
    private Map<String,User> nullMapField = null;
    private List<User> nullListField = null;

    private Long longField = 23423423423L;
    private int intField = 234234;
    private Double doubleField = 234234.23423;
    private User userField = new User();

    {
        listField.add("234234");
        mapField.put("23432",-234.0);
        setField.add(-23423);

        userListFiled.add(new User("3459",-23423,new int []{4,5,5}));
        userListFiled.add(new User());
        userMapField.put("()",new User());
        userMapField.put("------", new User("345345",3434,new int []{4,44}));
        userSetField.add(new User());
        userSetField.add(new User());
    }
}

class User
{
    private String name = "noName";
    private int age = 0;
    private int [] nums = new int[]{2,3,4};

    public User()
    {

    }

    public User(String name,int age,int [] nums)
    {
        this.name = name;
        this.age = age;
        this.nums = nums;
    }
}

Gson?So easy.
这个就不反序列化了,可以看到对象,Map用{},List,Set用[].与上面的一致.

5.Gson注解

(1)@SerializedName

SerializedName注解有两个参数,分别是value与alternate.SerializedName由注解名字可以知道与序列化成的Json名字有关.

默认情况下,json的键名与对象的字段名一致,@SerializedName可以解决序列化/反序列化时json键名与对象字段名不一致的问题,使其将json可以正确映射到对应的字段.

a.单独使用value

单独使用一个参数时,即@SerializedName("xxx")或@SerializedName(value="xxx"),序列化时,对象字段会变为@SerializedName中的值,反序列化时,若不是@SerializedName()中的值则不会反序列化:

public class SerializedNameValueTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new SerializedNameValueTest().new User()));

        String str = "{'this is a name':'what????','this is an age':13,'email':'33333@222.com'}";
        System.out.println(gson.fromJson(str,User.class));

        String str2 = "{'name':'askldfklaslk','age':-222,'email':'234234@23423.com'}";
        System.out.println(gson.fromJson(str2, User.class));
    }

    class User
    {
        @SerializedName("this is a name")
        private String name = "123";

        @SerializedName("this is an age")
        private int age = 0;

        private String email = "xxx@xxx.com";

        private double[] nums = new double[] { 1.0, 2.0 };

        @Override
        public String toString() {
            return "name:" + name + ",age:" + age + ",eamil:" + email + ",nums:" + Arrays.toString(nums);
        }
    }
}

Gson?So easy.
最后一个反序列化不成功,因为'name'不对应@SerializedName()中的'this is a name'.

b.同时使用value与alternate

同时使用两者可以解决上面的问题,即name字段--->json中的'this is a name'--->name字段:

public class SerializedNameValueAndAlternateTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new SerializedNameValueAndAlternateTest().new User()));

        String str = "{'this is a name':'what????','this is an age':13,'email':'33333@222.com'}";
        System.out.println(gson.fromJson(str, User.class));

        String str2 = "{'name':'askldfklaslk','age':-222,'email':'234234@23423.com'}";
        System.out.println(gson.fromJson(str2, User.class));
    }

    class User {
        @SerializedName(value = "this is a name",alternate = "name")
        private String name = "123";

        @SerializedName(value = "this is an age",alternate = "age")
        private int age = 0;

        private String email = "xxx@xxx.com";

        private double[] nums = new double[] { 1.0, 2.0 };

        @Override
        public String toString() {
            return "name:" + name + ",age:" + age + ",eamil:" + email + ",nums:" + Arrays.toString(nums);
        }
    }
}

alternate就是为反序列化准备的,若找不到value中对应的值,则寻找alternate对应的值,找不到再设为null.

Gson?So easy.

若同时存在value与alternate:

String str3 = "{'name':'altername','this is a name':'value'}";
System.out.println(gson.fromJson(str3, User.class));
String str4 = "{'this is a name':'value','name':'altername'}";
System.out.println(gson.fromJson(str4, User.class));

则以"最晚"出现的值为标准.

Gson?So easy.

(2)@Expose

@Expose可以忽略某个字段,有两个参数:

serialize
deserialize

默认情况下都是true,分别表示序列化与反序列化.

System.out.println(gson.toJson(new User()));

class User
{
    @Expose(serialize = false)
    private String name = "123";

    @Expose(deserialize = false)
    private int age = 0;

    @Expose(serialize = false,deserialize = false)
    private String email = "xxx@xxx.com";

    @Expose(serialize = true,deserialize = true)
    private double [] nums = new double [] {1.0,2.0};
}

name不允许序列化,age不允许反序列化,email不允许序列化与反序列化,nums既允许也允许反序列化.(此时效果等于没加@Expose)
输出:
Gson?So easy.
咦?好像没用的样子?
因为Gson对象会默认忽略@Expose,想要@Expose生效需要使用Gson Builder:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.excludeFieldsWithoutExposeAnnotation();
Gson gson = gsonBuilder.create();
System.out.println(gson.toJson(new User()));

String str = "{'name':'234','age':-3,'email':'23423','nums':[3,4,3]}";
System.out.println(gson.fromJson(str, User.class));

class User
{
    @Expose(serialize = false)
    private String name = "123";

    @Expose(deserialize = false)
    private int age = 0;

    @Expose(serialize = false,deserialize = false)
    private String email = "xxx@xxx.com";

    @Expose(serialize = true,deserialize = true)
    private double [] nums = new double [] {1.0,2.0};

    @Override
    public String toString()
    {
        return "name:"+name+",age:"+age+",eamil:"+email+",nums:"+Arrays.toString(nums);
    }
}

Gson?So easy.
不能反序列化age与email,输出的是age与email的默认值.

6.Gson Builder

前面的例子基本上都是通过

Gson gson = new Gson();

来直接实例化一个Gson来使用Gson的,使用Gson Builder可以设置Gson的某些属性,使用其中的create()返回一个Gson.

(1)基础

创建一个简单的Gson,通过create()创建:

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

(2)命名规则

a.使用系统命名规则

使用

gsonBuilder.setFieldNamingPolicy();

设置json中键的命名规则.6个值可选:
Gson?So easy.

IDENTITY:相同,json中的键名与字段名相同.
LOWER_CASE_WITH_DASHES:在原来字段的大写字母前加-,并且把大写变成小写
LOWER_CASE_WITH_DOTS:在原来字段的大写字母前加.,并且把大写变成小写
LOWER_CASE_WITHUNDERSCORES:在原来字段的大写字母前加,并且把大写变成小写
UPPER_CAMEL_CASE:首字母大写
UPPER_CAMEL_CASE_WITH_SPACES:在原来字段的大写字母前加空格

public class NamingRulesTest
{
    public static void main(String[] args) {
        NamingRulesTest n = new NamingRulesTest();
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.IDENTITY);
        System.out.println(gsonBuilder.create().toJson(n.new User()));

        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_DASHES);
        System.out.println(gsonBuilder.create().toJson(n.new User()));

        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_DOTS);
        System.out.println(gsonBuilder.create().toJson(n.new User()));

        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES);
        System.out.println(gsonBuilder.create().toJson(n.new User()));

        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE);
        System.out.println(gsonBuilder.create().toJson(n.new User()));

        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES);
        System.out.println(gsonBuilder.create().toJson(n.new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private String Name_Name_name = "123";
        @SuppressWarnings("unused")
        private int _age_age_Age_age = 645;
        @SuppressWarnings("unused")
        private double numsNumsNums = 34.45;
    }
}

Gson?So easy.
注意,若某个字段有了@SerializedName,则这个字段遵循@SerializedName的策略.

b.自定义命名规则

重写FieldNamingStrategy中的translateName(Field field),把自定义的FieldNamingStrategy传递给GsonBuilder的setFieldNamingStrategy().
比如想要前缀加上某人的名字:

public class CustomNamingRulesTest
{
    public static void main(String[] args) {
        FieldNamingStrategy myNamingStrategy = new FieldNamingStrategy(){
            @Override
            public String translateName(Field field) {
                return "kr"+field.getName();
            }
        };

        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setFieldNamingStrategy(myNamingStrategy);
        Gson gson = gsonBuilder.create();
        System.out.println(gson.toJson(new CustomNamingRulesTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private String name = "123";
        @SuppressWarnings("unused")
        private int num = 5;
    }
}

Gson?So easy.

(3)null

默认情况下,Gson实例不允许序列化null,如果想要序列化null,借助GsonBuilder的serializeNulls()方法:

public class NullTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeNulls();
        gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES);
        System.out.println(gsonBuilder.create().toJson(new NullTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private String thisIsANullField;
        @SuppressWarnings("unused")
        private Integer andThisIsANullFieldToo;
    }
}

Gson?So easy.

(4)排除

前面已经接触到了@Expose这样的排除策略,下面看看更加强大的配合GsonBuilder使用的排除策略.主要有四种:属性名排除,类型排除,修饰符排除,@Expose排除.

a.属性名排除

public class ExclusionNameTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeNulls();
        ExclusionStrategy exclusionName = new ExclusionStrategy(){
            @Override
            public boolean shouldSkipField(FieldAttributes f)
            {
                return f.getName().endsWith("e");
            }

            @Override
            public boolean shouldSkipClass(Class<?> arg0) {
                return false;
            }
        };
        gsonBuilder.setExclusionStrategies(exclusionName);

        System.out.println(gsonBuilder.create().toJson(new ExclusionNameTest().new User()));
    }

    class User
    {
        private String name;
        private int num;
    }
}

排除字段名以e结尾的字段.下面是重点:

public boolean shouldSkipField(FieldAttributes f)
{
    return f.getName().endsWith("e");
}

重写的shouldSkipField从名字可以看出跳过某些字段,返回true表示跳过,即排除这个字段.上面的例子中若名字以e结尾则跳过,因此输出:
Gson?So easy.

b.类型排除

public class ExclusionTypeTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeNulls();
        ExclusionStrategy exclusionType = new ExclusionStrategy(){

            @Override
            public boolean shouldSkipField(FieldAttributes arg0) {
                return false;
            }

            @Override
            public boolean shouldSkipClass(Class<?> cls) {
                return cls == String.class;
            }
        };
        gsonBuilder.setExclusionStrategies(exclusionType);

        System.out.println(gsonBuilder.create().toJson(new ExclusionNameTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private String name;
        @SuppressWarnings("unused")
        private int num;
    }
}

重写的shouldSkipClass表示要跳过的类,这里跳过了String,只剩下num.

@Override
public boolean shouldSkipClass(Class<?> cls) {
    return cls == String.class;
}

Gson?So easy.

c.修饰符排除

这个不用重写方法了,直接使用GsonBuilder的excludeFieldsWithModifiers(),参数是java.lang.reflect.Modifier:
Gson?So easy.
这个可选的比较多就不一一列举了,只选了一个final的例子:

public class ExclusionModifierTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.excludeFieldsWithModifiers(Modifier.FINAL);
        System.out.println(gsonBuilder.create().toJson(new User()));
    }
}

class User
{
    @SuppressWarnings("unused")
    private final String name = "123";
    @SuppressWarnings("unused")
    private static int num;
}

Gson?So easy.

d.@Expose排除

这个准确来说是排除没有被@Expose注解的字段:

public class ExclusionExposeTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.excludeFieldsWithoutExposeAnnotation();
        System.out.println(gsonBuilder.create().toJson(new ExclusionExposeTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private String name;
        @SuppressWarnings("unused")
        private int num;
        @Expose(serialize = true,deserialize = true)
        private int age;
    }
}

Gson?So easy.

(5)Lenient

这是有关于反序列化时JSON的容错机制的问题,因为通常来说将一个对象通过Gson转换成json没什么大问题,但是将json转换为对象的时候就...就难说了,因为不知道是否符合标准的json格式,因此Gson提供了一定的容错机制,就是Lenient.

Lenient翻译过来是"宽容的"的意思,可以通过:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.setLenient();

开启.
Gson内部有一个JsonReader类,默认情况下,JsonReader是严格的且仅接受符合RFC 4627标准的json(RFC 4627标准可以看看这里),设置为lenient后可以"容忍"以下几种错误:

a.以 ) ] } ' \n 开头
b.多个顶层值.
c.任何类型的顶层值
d.数字可能是NaNs或infinites
e.以行注释//或#结尾
f.以一个换行符结束
g.C风格的注释/**/,可能会嵌套
h.键/字符串没有引号或者单引号
i.数组元素以;分隔
j.不必要的数组分隔符,"默认"null为省略值,比如[1,,2],"默认"第二个元素为null
k.键值以=或=>分隔而非使用:
l.键值对以;分隔而非使用,

设置setLenient()后,Gson会尽可能解析有错误的json,若实在无能为力,会抛出MalformedJsonException异常.
下面是一个不严格的json的例子:

public class LenientTest
{
    public static void main(String[] args)
    {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setLenient();

        String str = "{'name'='234';'num'=6}";
        System.out.println(gsonBuilder.create().fromJson(str,User.class));
    }

    class User
    {
        private String name;
        private int num;

        @Override
        public String toString()
        {
            return "name:"+name+",num:"+num;
        }
    }
}

Gson?So easy.
下面是一个异常的例子:

String errorStr = "{'name'=????,,,,,,}";
System.out.println(gsonBuilder.create().fromJson(errorStr,User.class));

Gson?So easy.

(6)Floats & Doubles

先看一个例子:

public class FloatTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new FloatTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private float f = Float.POSITIVE_INFINITY;
    }
}

会报错:
Gson?So easy.
它说Infinity在JSON标准是一个非法值.想要重写这个行为需要使用GsonBuilder.serializeSpecialFloatingPointValues().
因为JSON规范不允许NaN,-Infinity,Infinity,因此会报错.下面使用GsonBuilder:

public class FloatTest
{
    public static void main(String[] args) {
        // Gson gson = new Gson();
        // System.out.println(gson.toJson(new FloatTest().new User()));

        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeSpecialFloatingPointValues();
        System.out.println(gsonBuilder.create().toJson(new FloatTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private float f = Float.POSITIVE_INFINITY;
    }
}

直接输出Infinity:
Gson?So easy.

double也类似:

public class DoubleTest
{
    public static void main(String[] args)
    {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeSpecialFloatingPointValues();
        System.out.println(gsonBuilder.create().toJson(new DoubleTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private double ni = Double.NEGATIVE_INFINITY;
        @SuppressWarnings("unused")
        private double pi = Double.POSITIVE_INFINITY;
        @SuppressWarnings("unused")
        private double nan = Double.NaN;
    }
}

Gson?So easy.

(7)模型版本化

通过@Since与@Until添加版本控制,控制某个版本在序列化与反序列化时忽略或忽略某个字段.
@Since表示从某个版本开始这个字段不忽略,@Until表示这个版本后将忽略该字段.

需要配合GsonBuilder的setVersion使用,设定版本号.

public class VersionTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.serializeNulls();
        System.out.println(gsonBuilder.setVersion(0.9).create().toJson(new VersionTest().new User()));
        System.out.println(gsonBuilder.setVersion(1.0).create().toJson(new VersionTest().new User()));
        System.out.println(gsonBuilder.setVersion(1.4).create().toJson(new VersionTest().new User()));
        System.out.println(gsonBuilder.setVersion(1.5).create().toJson(new VersionTest().new User()));
    }

    class User
    {
        @Since(1.0)
        private String name;
        @Until(1.4)
        private int num;
    }
}

Gson?So easy.
@Since的范围包含了左区间端点,@Until的范围不包含右区间端点.

(8)格式化日期

可以使用setDateForamt()来格式化日期输出:

gsonBuilder.setDateFormat(int style);
gsonBuilder.setDateFormat(String pattern);
gsonBuilder.setDateFormat(int dateStyle,int timeStyle);

第一个函数的参数为DateFormat中的常量值,第二个函数的参数是类似SimpleDateFormat中的String,第三个函数的参数与第一个类似,分开日期与时间设置.
再说以下setPrettyPrinting()这个函数,看名字就知道,美化打印的.会加上空格.

public class FormatTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();

        System.out.println(gsonBuilder.setDateFormat(DateFormat.SHORT, DateFormat.SHORT).create().toJson(new FormatTest().new User()));
        System.out.println(gsonBuilder.setDateFormat(DateFormat.SHORT,DateFormat.LONG).create().toJson(new FormatTest().new User()));
        System.out.println(gsonBuilder.setDateFormat(DateFormat.MEDIUM, DateFormat.MEDIUM).create().toJson(new FormatTest().new User()));
        System.out.println(gsonBuilder.setDateFormat(DateFormat.LONG, DateFormat.MEDIUM).create().toJson(new FormatTest().new User()));
        System.out.println(gsonBuilder.setDateFormat(DateFormat.FULL, DateFormat.FULL).create().toJson(new FormatTest().new User()));
        System.out.println(gsonBuilder.setDateFormat("yyyy-MM-dd HH:mm:ss").create().toJson(new FormatTest().new User()));
    }

    class User
    {
        @SuppressWarnings("unused")
        private Date date = new Date();
    }
}

Gson?So easy.

7.Gson进阶

(1)枚举

枚举其实也与普通的字段类似,序列化与反序列化正常操作.

public class EnumTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new EnumTest().new User()));

        String str = "{\"season\":\"SUMMER\",\"name\":\"8888\"}";
        System.out.println(gson.fromJson(str,User.class));
    }

    enum Season
    {
        SPRING,
        SUMMER,
        AUTUMN,
        WINTER
    }

    class User
    {
        private Season season = Season.SPRING;
        private String name = "123";

        @Override
        public String toString()
        {
            return "season:"+season.toString()+",name:"+name;
        }
    }
}

Gson?So easy.
主要配合@SerializedName()使用,例如对于月份,可以使用1,2,3来表示:

public class EnumTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new EnumTest().new User()));

        String str = "{\"season\":\"SUMMER\",\"name\":\"8888\",\"month\":\"2\"}";
        System.out.println(gson.fromJson(str,User.class));
    }

    enum Season
    {
        SPRING,
        SUMMER,
        AUTUMN,
        WINTER
    }

    class User
    {
        private Season season = Season.SPRING;
        private String name = "123";
        private Month month = Month.January;

        @Override
        public String toString()
        {
            return "season:"+season.toString()+",name:"+name+",month:"+month.toString();
        }
    }

    enum Month
    {
        @SerializedName("1")
        January,
        @SerializedName("2")
        February,
        @SerializedName("3")
        March,
        @SerializedName("4")
        April,
        @SerializedName("5")
        May,
        @SerializedName("6")
        June,
        @SerializedName("7")
        July,
        @SerializedName("8")
        August,
        @SerializedName("9")
        September,
        @SerializedName("10")
        October,
        @SerializedName("11")
        November,
        @SerializedName("12")
        December
    }
}

Gson?So easy.

(2)泛型

这里主要说一下反序列化泛型.因为序列化泛型...直接toJson()就好了.
想想这样的情景:有一个待解析的Json String,类型为List<T>,因此想有一个这样的方法:

public <T> List<T> fromJSON(String json,Class<T> cls);

好了,怎么写呢,之前用的是TypeToken()实现反序列化:

List<Integer> integerList = new ArrayList<>();
integerList.add(3);
integerList.add(-999);
gson.toJson(integerList, new TypeToken<List<Integer>>() {}.getType());

答案就是使用ParameterizedType:重写里面的getActualTypeArguments(),getOwnerTpye()与getRawType():

public class GenericsTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();
        Gson gson = gsonBuilder.create();

        List<Integer> integerList = new ArrayList<>();
        integerList.add(3);
        integerList.add(-999);
        gson.toJson(integerList, new TypeToken<List<Integer>>() {}.getType());

        List<String> stringList = new ArrayList<>();
        stringList.add("12321");
        stringList.add("(@)#$@#");
        String t1 = gson.toJson(integerList);
        String t2 = gson.toJson(stringList);

        new GenericsTest().fromJSON(t1, Integer.class).stream().forEach(System.out::println);
        new GenericsTest().fromJSON(t2, String.class).stream().forEach(System.out::println);
    }

    public <T> List<T> fromJSON(String json,Class<T> cls)
    {
        return new Gson().fromJson(json,new ParameterizedTypeImpl(cls));
    }

    class ParameterizedTypeImpl implements ParameterizedType
    {
        private Class cls;
        public ParameterizedTypeImpl(Class cls)
        {
            this.cls = cls;
        }

        @Override
        public Type[] getActualTypeArguments()
        {
            return new Type[]{cls};
        }

        @Override
        public Type getRawType()
        {
            return List.class;
        }

        @Override
        public Type getOwnerType()
        {
            return null;
        }
    }
}

Gson?So easy.

@Override
public Type[] getActualTypeArguments()
{
    return new Type[]{cls};
}

返回实际参数类型数组,在这里是String.class与Integer.class.

@Override
public Type getRawType()
{
    return List.class;
}

返回声明这个类型的类或接口,在这里是List.class.

@Override
public Type getOwnerType()
{
    return null;
}

返回其成员之一的类型,就是说如果完全限定类名为A.B,则返回A,在这里没有A,因此为null.
定义好实现ParameterizedTpye接口的类后,把它传递给fromJson()作为第二参数,构造方法的参数为List<T>中的T.class.

(4)自定义序列化

自定义序列化一般用于自定义简化json.
比如有一个User类:

class User
{
    private String name = "123";
    private String email = "xxx@xxx.com";
    private int [] nums = new int [] {1,2,3};
}

想要不序列化name,可以使用@Expose(serialize = false),但是想要部分序列化nums,比如只是需要第一个nums[0],上面的@Expose,@SerializedName等注解都用不上,这时需要使用JsonSerializer自定义序列化,重写其中的:

public JsonElement serialize(T t,Type type,JsonSerializationContext context)

可以返回一个JsonObject,这里的JsonObject可以自定义添加属性(即键值对).
需要配合GsonBuilder使用,创建了自己的JsonSerializer<T>后,把它传递给GsonBuilder的registerTypeAdapter():

public class SerializeCustomTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        JsonSerializer<User> serializer = new JsonSerializer<SerializeCustomTest.User>() {
            @Override
            public JsonElement serialize(User u,Type type,JsonSerializationContext context)
            {
                JsonObject object = new JsonObject();
                object.addProperty("name",u.getName());
                object.addProperty("email",u.getEmail());
                object.addProperty("nums",u.getNums()[0]);
                return object;
            }
        };
        gsonBuilder.registerTypeAdapter(User.class,serializer);
        System.out.println(gsonBuilder.create().toJson(new SerializeCustomTest().new User()));
    }

    class User
    {
        private String name = "123";
        private String email = "xxx@xxx.com";
        private int [] nums = new int [] {1,2,3};

        public int [] getNums()
        {
            return nums;
        }

        public String getName()
        {
            return name;
        }

        public String getEmail()
        {
            return email;
        }
    }
}

Gson?So easy.

(5)自定义反序列化

自定义反序列化主要就是针对有多余键值对的json,比如User只需要name字段,但是传来的json包含了诸如age这样的键,因此只需挑选所需的进行反序列化.与序列化类似,首先定义自己的JsonDeserializer<T>,重写其中的:

public T deserialize(JsonElement,Type,JsonDeserializationContext);

返回一个自定义的反序列化的对象.最后在GsonBuilder中registerTypeAdapter()即可.

public class DeserializedCustomTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        JsonDeserializer<User> deserializer = new JsonDeserializer<DeserializedCustomTest.User>() {
            @Override
            public User deserialize(JsonElement json,Type type,JsonDeserializationContext context)
            {
                JsonObject object = json.getAsJsonObject();
                User user = new DeserializedCustomTest().new User(object.get("name").getAsString());
                return user;
            }
        };
        gsonBuilder.registerTypeAdapter(User.class, deserializer);

        String str = "{\"aaa\":\"bbbb\",\"name\":\"this is a name\",\"age\":\"444\"}";
        System.out.println(gsonBuilder.create().fromJson(str,User.class));
    }

    class User
    {
        private String name;
        private int age;

        public User(String name)
        {
            this.name = name;
        }

        @Override
        public String toString()
        {
            return "name:"+name+",age:"+age;
        }
    }
}

Gson?So easy.

(6)默认实例

可以设置反序列化时的默认值,通过构造方法实现.比如json中没有所需要的字段的值,默认情况下为null,如果想要不为null,可以设定默认值,对于对象可以设定构造方法.

通过实现InstanceCreator<T>来实现,重写其中的createInstance方法,再配合GsonBuilder的registerTypeAdapter().

public class InstanceCustomTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.registerTypeAdapter(
            AA.class,
            new InstanceCustomTest().new AAInstance(
                new InstanceCustomTest().new BB("123","456")
            )
        );

        String str = "{\"name\":\"123123\"}";
        System.out.println(gsonBuilder.create().fromJson(str, AA.class));
    }

    class AAInstance implements InstanceCreator<AA>
    {
        private BB bb;
        public AAInstance(BB bb)
        {
            this.bb = bb;
        }

        @Override
        public AA createInstance(Type type)
        {
            return new AA(bb);
        }
    }

    class AA
    {
        private String name = "AA";
        private BB bb;

        public AA(BB bb)
        {
            this.bb = bb;
        }

        @Override
        public String toString()
        {
            return "name:"+name+",bb:"+bb.toString();
        }
    }

    class BB
    {
        private String field1;
        private String field2;

        public BB(String field1,String field2)
        {
            this.field1 = field1;
            this.field2 = field2;
        }

        @Override
        public String toString()
        {
            return "field1:"+field1+",field2:"+field2;
        }
    }
}

json没有为BB类设定值,采用默认值.

(7)@JsonAdapter

这个是自定义序列化/反序列化的注解,可以简化JsonSerializer与JsonDeserializer.另外不用配合GsonBuilder使用,直接使用new Gson().toJson()/fromJson()即可,比直接使用JsonSerializer与JsonDeserializer要简单.
首先使一个类实现JsonSerializer<T>接口,接着把这个类作为要注解的字段的@JsonAdapter的参数:

class Serializer implements JsonSerializer<User>
class ContainUser
{
    private String userId = "x";
    @JsonAdapter(Serializer.class)
    private User user = new User();
}

public class JsonAdapterSerializeTest
{
    public static void main(String[] args)
    {
        Gson gson = new Gson();
        System.out.println(gson.toJson(new JsonAdapterSerializeTest().new ContainUser()));
    }

    class Serializer implements JsonSerializer<User>
    {
        @Override
        public JsonElement serialize(User u,Type type,JsonSerializationContext context)
        {
            JsonObject object = new JsonObject();
            object.addProperty("name",u.getName());
            object.addProperty("email",u.getEmail());
            object.addProperty("nums",u.getNums()[0]);
            return object;
        }
    }

    class ContainUser
    {
        private String userId = "x";
        @JsonAdapter(Serializer.class)
        private User user = new User();
    }

    class User
    {
        private String name = "123";
        private String email = "xxx@xxx.com";
        private int [] nums = new int [] {1,2,3};

        public int [] getNums()
        {
            return nums;
        }

        public String getName()
        {
            return name;
        }

        public String getEmail()
        {
            return email;
        }
    }
}

Gson?So easy.
反序列化也类似.

public class JsonAdapterDeserializeTest
{
    public static void main(String[] args) {
        Gson gson = new Gson();
        String str = "{\"aaaa\":\"bbbb\",\"name\":\"this is a name\",\"age\":\"444\"}";
        System.out.println(gson.fromJson(str,User.class));
    }

    class Deserialize implements JsonDeserializer<User>
    {
        @Override
        public User deserialize(JsonElement json,Type type,JsonDeserializationContext context)
        {
            JsonObject object = json.getAsJsonObject();
            User user = new JsonAdapterDeserializeTest().new User(object.get("name").getAsString());
            return user;
        }
    }

    @JsonAdapter(Deserialize.class)
    class User
    {
        private String name = "123";
        public User()
        {

        }
        public User(String name)
        {
            this.name = name;
        }

        @Override
        public String toString()
        {
            return "name:"+name;
        }
    }
}

Gson?So easy.
实现JsonDeserializer<T>,重写deserialize(),在所需的类上添加注解.

注意@JsonAdapter注解只能添加在类上.

8.源码

所有例子的完整源码.
github.
码云.
Gson官方github.

9.参考链接

1.Gson Tutorial Series by Future Studio

2.gson完全教程

3.gson教程

4.gson用户指南

5.Gson User guide

6.gson

向AI问一下细节

Gson?So easy.

1.概述

2.Gson是什么?

(1)JSON

(2)Gson

(3)Gson的好处

3.配置Gson环境

(1)Gradle

(2)Maven

(3)jar

4.Gson基础

(1)基础类型

(2)嵌套对象

(3)数组

a.普通数组

b.List

(4)Map

(5)Set

(6)null

(7)混合

5.Gson注解

(1)@SerializedName

a.单独使用value

b.同时使用value与alternate

(2)@Expose

6.Gson Builder

(1)基础

(2)命名规则

a.使用系统命名规则

b.自定义命名规则

(3)null

(4)排除

a.属性名排除

b.类型排除

c.修饰符排除

d.@Expose排除

(5)Lenient

(6)Floats & Doubles

(7)模型版本化

(8)格式化日期

7.Gson进阶

(1)枚举

(2)泛型

(4)自定义序列化

(5)自定义反序列化

(6)默认实例

(7)@JsonAdapter

8.源码

9.参考链接

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