Gson?Soeasy.

1.概述

這篇文章主要講述了Gson的使用.包括從最基礎(chǔ)的基本類(lèi)型的序列化,到對(duì)象,數(shù)組,集合,再到Gson注解,Gson Builder,再到格式化,自定義序列化與反序列化等內(nèi)容.
另外文章篇幅較長(zhǎng),建議挑選所需部分查看.所有例子都提供了完整源碼,在文章的后面.

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2.Gson是什么?

(1)JSON

JSON全稱(chēng)為JavaScript Object Notation,一種輕量級(jí)的數(shù)據(jù)交換格式.

類(lèi)似于XML但比XML更小,更易解析.

(2)Gson

Gson是Google提供的可以使Java對(duì)象與JSON互轉(zhuǎn)的類(lèi)庫(kù),可將Java對(duì)象轉(zhuǎn)換為JSON,也可將JSON轉(zhuǎn)換成Java對(duì)象.

(3)Gson的好處

a.容易,高效,強(qiáng)大:Gson是Google管理的標(biāo)準(zhǔn)化庫(kù),經(jīng)過(guò)高度優(yōu)化,同時(shí)api簡(jiǎn)單,比如fromJSON(),toJSON().
b.無(wú)依賴(lài)性:不需要其他庫(kù),當(dāng)然jdk除外.
c.結(jié)果簡(jiǎn)單:轉(zhuǎn)換成的json易于閱讀.
d.支持泛型,支持內(nèi)部類(lèi).
e.開(kāi)源,免費(fèi)提供.

3.配置Gson環(huán)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


  com.google.code.gson
  gson
  2.8.6

(3)jar

點(diǎn)擊下載.

4.Gson基礎(chǔ)

(1)基礎(chǔ)類(lèi)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">
就是對(duì)應(yīng)輸出,沒(méi)啥好說(shuō)的.

(2)嵌套對(duì)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類(lèi):

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

Gson?So easy.
序列化出來(lái)的對(duì)象用{}表示.

反序列化時(shí),注意格式,注意名字對(duì)應(yīng),用單引號(hào)引起來(lái),還有char會(huì)自動(dòng)變?yōu)镾tring類(lèi)型,另外對(duì)于布爾類(lèi)型可以加單引號(hào)或不加單引號(hào),都可以正常反序列化.

(3)數(shù)組

a.普通數(shù)組

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.
對(duì)普通數(shù)組的話(huà),{}變成了[].
還有就是反序列化時(shí),默認(rèn)會(huì)在逗號(hào)后面添一個(gè)空格.

b.List

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

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

List 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>(){}.getType();

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

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

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

Gson?So easy.
序列化沒(méi)什么問(wèn)題,直接toJson即可,反序列化時(shí),需要配合java.lang.reflect.Type使用,明確指出所要轉(zhuǎn)換的類(lèi)型:

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

TypeToken中T為所需要的類(lèi)型,再把這個(gè)Type對(duì)象傳遞給fromJson即可完成轉(zhuǎn)換.
List序列化出來(lái)的用[]表示.

(4)Map

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

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

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

Gson?So easy.
Map同樣序列化時(shí)直接toJson,反序列化時(shí)使用java.lang.reflect.Type.Map序列化出來(lái)的用{}表示.

(5)Set

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

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

Gson?So easy.
set序列化出來(lái)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會(huì)忽略空值,在序列化時(shí)看不到null對(duì)應(yīng)的鍵值對(duì),反序列化時(shí),直接對(duì)應(yīng)為空.

(7)混合

一個(gè)對(duì)象里面包含了List,Map,Set,null.

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

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

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

    private Set nullSetField = null;
    private Map nullMapField = null;
    private List 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.
這個(gè)就不反序列化了,可以看到對(duì)象,Map用{},List,Set用[].與上面的一致.

5.Gson注解

(1)@SerializedName

SerializedName注解有兩個(gè)參數(shù),分別是value與alternate.SerializedName由注解名字可以知道與序列化成的Json名字有關(guān).

默認(rèn)情況下,json的鍵名與對(duì)象的字段名一致,@SerializedName可以解決序列化/反序列化時(shí)json鍵名與對(duì)象字段名不一致的問(wèn)題,使其將json可以正確映射到對(duì)應(yīng)的字段.

a.單獨(dú)使用value

單獨(dú)使用一個(gè)參數(shù)時(shí),即@SerializedName("xxx")或@SerializedName(value="xxx"),序列化時(shí),對(duì)象字段會(huì)變?yōu)锧SerializedName中的值,反序列化時(shí),若不是@SerializedName()中的值則不會(huì)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.
最后一個(gè)反序列化不成功,因?yàn)?name'不對(duì)應(yīng)@SerializedName()中的'this is a name'.

b.同時(shí)使用value與alternate

同時(shí)使用兩者可以解決上面的問(wèn)題,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就是為反序列化準(zhǔn)備的,若找不到value中對(duì)應(yīng)的值,則尋找alternate對(duì)應(yīng)的值,找不到再設(shè)為null.

Gson?So easy.

若同時(shí)存在value與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));

則以"最晚"出現(xiàn)的值為標(biāo)準(zhǔn).

Gson?So easy.

(2)@Expose

@Expose可以忽略某個(gè)字段,有兩個(gè)參數(shù):

serialize
deserialize

默認(rèn)情況下都是true,分別表示序列化與反序列化.

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不允許序列化,age不允許反序列化,email不允許序列化與反序列化,nums既允許也允許反序列化.(此時(shí)效果等于沒(méi)加@Expose)
輸出:
Gson?So easy.
咦?好像沒(méi)用的樣子?
因?yàn)镚son對(duì)象會(huì)默認(rèn)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與email,輸出的是age與email的默認(rèn)值.

6.Gson Builder

前面的例子基本上都是通過(guò)

Gson gson = new Gson();

來(lái)直接實(shí)例化一個(gè)Gson來(lái)使用Gson的,使用Gson Builder可以設(shè)置Gson的某些屬性,使用其中的create()返回一個(gè)Gson.

(1)基礎(chǔ)

創(chuàng)建一個(gè)簡(jiǎn)單的Gson,通過(guò)create()創(chuàng)建:

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

(2)命名規(guī)則

a.使用系統(tǒng)命名規(guī)則

使用

gsonBuilder.setFieldNamingPolicy();

設(shè)置json中鍵的命名規(guī)則.6個(gè)值可選:
Gson?So easy.

IDENTITY:相同,json中的鍵名與字段名相同.
LOWER_CASE_WITH_DASHES:在原來(lái)字段的大寫(xiě)字母前加-,并且把大寫(xiě)變成小寫(xiě)
LOWER_CASE_WITH_DOTS:在原來(lái)字段的大寫(xiě)字母前加.,并且把大寫(xiě)變成小寫(xiě)
LOWER_CASE_WITHUNDERSCORES:在原來(lái)字段的大寫(xiě)字母前加,并且把大寫(xiě)變成小寫(xiě)
UPPER_CAMEL_CASE:首字母大寫(xiě)
UPPER_CAMEL_CASE_WITH_SPACES:在原來(lái)字段的大寫(xiě)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,若某個(gè)字段有了@SerializedName,則這個(gè)字段遵循@SerializedName的策略.

b.自定義命名規(guī)則

重寫(xiě)FieldNamingStrategy中的translateName(Field field),把自定義的FieldNamingStrategy傳遞給GsonBuilder的setFieldNamingStrategy().
比如想要前綴加上某人的名字:

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

默認(rèn)情況下,Gson實(shí)例不允許序列化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

前面已經(jīng)接觸到了@Expose這樣的排除策略,下面看看更加強(qiáng)大的配合GsonBuilder使用的排除策略.主要有四種:屬性名排除,類(lèi)型排除,修飾符排除,@Expose排除.

a.屬性名排除

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結(jié)尾的字段.下面是重點(diǎn):

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

重寫(xiě)的shouldSkipField從名字可以看出跳過(guò)某些字段,返回true表示跳過(guò),即排除這個(gè)字段.上面的例子中若名字以e結(jié)尾則跳過(guò),因此輸出:
Gson?So easy.

b.類(lèi)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;
    }
}

重寫(xiě)的shouldSkipClass表示要跳過(guò)的類(lèi),這里跳過(guò)了String,只剩下num.

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

Gson?So easy.

c.修飾符排除

這個(gè)不用重寫(xiě)方法了,直接使用GsonBuilder的excludeFieldsWithModifiers(),參數(shù)是java.lang.reflect.Modifier:
Gson?So easy.
這個(gè)可選的比較多就不一一列舉了,只選了一個(gè)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

這個(gè)準(zhǔn)確來(lái)說(shuō)是排除沒(méi)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

這是有關(guān)于反序列化時(shí)JSON的容錯(cuò)機(jī)制的問(wèn)題,因?yàn)橥ǔ?lái)說(shuō)將一個(gè)對(duì)象通過(guò)Gson轉(zhuǎn)換成json沒(méi)什么大問(wèn)題,但是將json轉(zhuǎn)換為對(duì)象的時(shí)候就...就難說(shuō)了,因?yàn)椴恢朗欠穹蠘?biāo)準(zhǔn)的json格式,因此Gson提供了一定的容錯(cuò)機(jī)制,就是Lenient.

Lenient翻譯過(guò)來(lái)是"寬容的"的意思,可以通過(guò):

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

開(kāi)啟.
Gson內(nèi)部有一個(gè)JsonReader類(lèi),默認(rèn)情況下,JsonReader是嚴(yán)格的且僅接受符合RFC 4627標(biāo)準(zhǔn)的json(RFC 4627標(biāo)準(zhǔn)可以看看這里),設(shè)置為lenient后可以"容忍"以下幾種錯(cuò)誤:

a.以 ) ] } ' \n 開(kāi)頭
b.多個(gè)頂層值.
c.任何類(lèi)型的頂層值
d.數(shù)字可能是NaNs或infinites
e.以行注釋//或#結(jié)尾
f.以一個(gè)換行符結(jié)束
g.C風(fēng)格的注釋/**/,可能會(huì)嵌套
h.鍵/字符串沒(méi)有引號(hào)或者單引號(hào)
i.數(shù)組元素以;分隔
j.不必要的數(shù)組分隔符,"默認(rèn)"null為省略值,比如[1,,2],"默認(rèn)"第二個(gè)元素為null
k.鍵值以=或=>分隔而非使用:
l.鍵值對(duì)以;分隔而非使用,

設(shè)置setLenient()后,Gson會(huì)盡可能解析有錯(cuò)誤的json,若實(shí)在無(wú)能為力,會(huì)拋出MalformedJsonException異常.
下面是一個(gè)不嚴(yán)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.
下面是一個(gè)異常的例子:

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

Gson?So easy.

(6)Floats & Doubles

先看一個(gè)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;
    }
}

會(huì)報(bào)錯(cuò):
Gson?So easy.
它說(shuō)Infinity在JSON標(biāo)準(zhǔn)是一個(gè)非法值.想要重寫(xiě)這個(gè)行為需要使用GsonBuilder.serializeSpecialFloatingPointValues().
因?yàn)镴SON規(guī)范不允許NaN,-Infinity,Infinity,因此會(huì)報(bào)錯(cuò).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;
    }
}

直接輸出Infinity:
Gson?So easy.

double也類(lèi)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

通過(guò)@Since與@Until添加版本控制,控制某個(gè)版本在序列化與反序列化時(shí)忽略或忽略某個(gè)字段.
@Since表示從某個(gè)版本開(kāi)始這個(gè)字段不忽略,@Until表示這個(gè)版本后將忽略該字段.

需要配合GsonBuilder的setVersion使用,設(shè)定版本號(hào).

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的范圍包含了左區(qū)間端點(diǎn),@Until的范圍不包含右區(qū)間端點(diǎn).

(8)格式化日期

可以使用setDateForamt()來(lái)格式化日期輸出:

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

第一個(gè)函數(shù)的參數(shù)為DateFormat中的常量值,第二個(gè)函數(shù)的參數(shù)是類(lèi)似SimpleDateFormat中的String,第三個(gè)函數(shù)的參數(shù)與第一個(gè)類(lèi)似,分開(kāi)日期與時(shí)間設(shè)置.
再說(shuō)以下setPrettyPrinting()這個(gè)函數(shù),看名字就知道,美化打印的.會(huì)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進(jìn)階

(1)枚舉

枚舉其實(shí)也與普通的字段類(lèi)似,序列化與反序列化正常操作.

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,例如對(duì)于月份,可以使用1,2,3來(lái)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

這里主要說(shuō)一下反序列化泛型.因?yàn)樾蛄谢盒?..直接toJson()就好了.
想想這樣的情景:有一個(gè)待解析的Json String,類(lèi)型為L(zhǎng)ist,因此想有一個(gè)這樣的方法:

public  List fromJSON(String json,Class cls);

好了,怎么寫(xiě)呢,之前用的是TypeToken()實(shí)現(xiàn)反序列化:

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

答案就是使用ParameterizedType:重寫(xiě)里面的getActualTypeArguments(),getOwnerTpye()與getRawType():

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

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

        List 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  List fromJSON(String json,Class 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};
}

返回實(shí)際參數(shù)類(lèi)型數(shù)組,在這里是String.class與Integer.class.

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

返回聲明這個(gè)類(lèi)型的類(lèi)或接口,在這里是List.class.

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

返回其成員之一的類(lèi)型,就是說(shuō)如果完全限定類(lèi)名為A.B,則返回A,在這里沒(méi)有A,因此為null.
定義好實(shí)現(xiàn)ParameterizedTpye接口的類(lèi)后,把它傳遞給fromJson()作為第二參數(shù),構(gòu)造方法的參數(shù)為L(zhǎng)ist中的T.class.

(4)自定義序列化

自定義序列化一般用于自定義簡(jiǎn)化json.
比如有一個(gè)User類(lèi):

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,比如只是需要第一個(gè)nums[0],上面的@Expose,@SerializedName等注解都用不上,這時(shí)需要使用JsonSerializer自定義序列化,重寫(xiě)其中的:

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

可以返回一個(gè)JsonObject,這里的JsonObject可以自定義添加屬性(即鍵值對(duì)).
需要配合GsonBuilder使用,創(chuàng)建了自己的JsonSerializer后,把它傳遞給GsonBuilder的registerTypeAdapter():

public class SerializeCustomTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        JsonSerializer serializer = new JsonSerializer() {
            @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)自定義反序列化

自定義反序列化主要就是針對(duì)有多余鍵值對(duì)的json,比如User只需要name字段,但是傳來(lái)的json包含了諸如age這樣的鍵,因此只需挑選所需的進(jìn)行反序列化.與序列化類(lèi)似,首先定義自己的JsonDeserializer,重寫(xiě)其中的:

public T deserialize(JsonElement,Type,JsonDeserializationContext);

返回一個(gè)自定義的反序列化的對(duì)象.最后在GsonBuilder中registerTypeAdapter()即可.

public class DeserializedCustomTest
{
    public static void main(String[] args) {
        GsonBuilder gsonBuilder = new GsonBuilder();
        JsonDeserializer deserializer = new JsonDeserializer() {
            @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)默認(rèn)實(shí)例

可以設(shè)置反序列化時(shí)的默認(rèn)值,通過(guò)構(gòu)造方法實(shí)現(xiàn).比如json中沒(méi)有所需要的字段的值,默認(rèn)情況下為null,如果想要不為null,可以設(shè)定默認(rèn)值,對(duì)于對(duì)象可以設(shè)定構(gòu)造方法.

通過(guò)實(shí)現(xiàn)InstanceCreator來(lái)實(shí)現(xiàn),重寫(xiě)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
    {
        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沒(méi)有為BB類(lèi)設(shè)定值,采用默認(rèn)值.

(7)@JsonAdapter

這個(gè)是自定義序列化/反序列化的注解,可以簡(jiǎn)化JsonSerializer與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與JsonDeserializer要簡(jiǎn)單.
首先使一個(gè)類(lèi)實(shí)現(xiàn)JsonSerializer接口,接著把這個(gè)類(lèi)作為要注解的字段的@JsonAdapter的參數(shù):

class Serializer implements JsonSerializer
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
    {
        @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.
反序列化也類(lèi)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
    {
        @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.
實(shí)現(xiàn)JsonDeserializer,重寫(xiě)deserialize(),在所需的類(lèi)上添加注解.

注意@JsonAdapter注解只能添加在類(lèi)上.

8.源碼

所有例子的完整源碼.
github.
碼云.
Gson官方github.

9.參考鏈接

1.Gson Tutorial Series by Future Studio

2.gson完全教程

3.gson教程

4.gson用戶(hù)指南

5.Gson User guide

6.gson

網(wǎng)站題目：Gson?Soeasy.
文章出自：http://m.jiaotiyi.com/article/jioepd.html

網(wǎng)站建設(shè)知識(shí)

Gson?Soeasy.

1.概述

2.Gson是什么?

(1)JSON

(2)Gson

(3)Gson的好處

3.配置Gson環(huán)境

(1)Gradle

(2)Maven

(3)jar

4.Gson基礎(chǔ)

(1)基礎(chǔ)類(lèi)型

(2)嵌套對(duì)象

(3)數(shù)組

a.普通數(shù)組

b.List

(4)Map

(5)Set

(6)null

(7)混合

5.Gson注解

(1)@SerializedName

a.單獨(dú)使用value

b.同時(shí)使用value與alternate

(2)@Expose

6.Gson Builder

(1)基礎(chǔ)

(2)命名規(guī)則

a.使用系統(tǒng)命名規(guī)則

b.自定義命名規(guī)則

(3)null

(4)排除

a.屬性名排除

b.類(lèi)型排除

c.修飾符排除

d.@Expose排除

(5)Lenient

(6)Floats & Doubles

(7)模型版本化

(8)格式化日期

7.Gson進(jìn)階

(1)枚舉

(2)泛型

(4)自定義序列化

(5)自定義反序列化

(6)默認(rèn)實(shí)例

(7)@JsonAdapter

8.源碼

9.參考鏈接

其他資訊