Gson?So easy.
1.概述
這篇文章主要講述了Gson的使用.包括從最基礎的基本類型的序列化,到對象,數組,集合,再到Gson註解,Gson Builder,再到格式化,自定義序列化與反序列化等內容.html
另外文章篇幅較長,建議挑選所需部分查看.全部例子都提供了完整源碼,在文章的後面.java
2.Gson是什麼?
(1)JSON
JSON全稱爲JavaScript Object Notation,一種輕量級的數據交換格式.git
相似於XML但比XML更小,更易解析.github
(2)Gson
Gson是Google提供的可使Java對象與JSON互轉的類庫,可將Java對象轉換爲JSON,也可將JSON轉換成Java對象.json
(3)Gson的好處
- a.容易,高效,強大:Gson是Google管理的標準化庫,通過高度優化,同時api簡單,好比fromJSON(),toJSON().
- b.無依賴性:不須要其餘庫,固然jdk除外.
- c.結果簡單:轉換成的json易於閱讀.
- d.支持泛型,支持內部類.
- e.開源,免費提供.
3.配置Gson環境
目前最新的是2.8.6版本.api
(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
點擊下載.數組
4.Gson基礎
(1)基礎類型
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));
就是對應輸出,沒啥好說的.yii
(2)嵌套對象
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:maven
class A
{
private String field1 = "123";
private int field2 = 1;
private char field3 = 'X';
private Boolean field4 = true;
}
序列化出來的對象用{}表示.ide
反序列化時,注意格式,注意名字對應,用單引號引發來,還有char會自動變爲String類型,另外對於布爾類型能夠加單引號或不加單引號,均可以正常反序列化.
(3)數組
a.普通數組
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));
對普通數組的話,{}變成了[]. 還有就是反序列化時,默認會在逗號後面添一個空格.
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;
}
序列化沒什麼問題,直接toJson便可,反序列化時,須要配合java.lang.reflect.Type使用,明確指出所要轉換的類型:
Type type = new TypeToken<List<String>>(){}.getType();
TypeToken<T>中T爲所須要的類型,再把這個Type對象傳遞給fromJson便可完成轉換.
List序列化出來的用[]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());
Map一樣序列化時直接toJson,反序列化時使用java.lang.reflect.Type.Map序列化出來的用{}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);
set序列化出來的用[]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會忽略空值,在序列化時看不到null對應的鍵值對,反序列化時,直接對應爲空.
(7)混合
一個對象裏面包含了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;
}
}
這個就不反序列化了,能夠看到對象,Map用{},List,Set用[].與上面的一致.
5.Gson註解
(1)@SerializedName
SerializedName註解有兩個參數,分別是value與alternate.SerializedName由註解名字能夠知道與序列化成的Json名字有關.
默認狀況下,json的鍵名與對象的字段名一致,@SerializedName能夠解決序列化/反序列化時json鍵名與對象字段名不一致的問題,使其將json能夠正確映射到對應的字段.
a.單獨使用value
單獨使用一個參數時,即@SerializedName("xxx")或@SerializedName(value="xxx"),序列化時,對象字段會變爲@SerializedName中的值,反序列化時,若不是@SerializedName()中的值則不會反序列化:
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);
}
}
}
最後一個反序列化不成功,由於'name'不對應@SerializedName()中的'this is a name'.
b.同時使用value與alternate
同時使用二者能夠解決上面的問題,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就是爲反序列化準備的,若找不到value中對應的值,則尋找alternate對應的值,找不到再設爲null.
若同時存在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"出現的值爲標準.
(2)@Expose
@Expose能夠忽略某個字段,有兩個參數:
- serialize
- deserialize
默認狀況下都是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不容許序列化,age不容許反序列化,email不容許序列化與反序列化,nums既容許也容許反序列化.(此時效果等於沒加@Expose) 輸出:
咦?好像沒用的樣子?
由於Gson對象會默認忽略@Expose,想要@Expose生效須要使用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);
}
}
不能反序列化age與email,輸出的是age與email的默認值.
6.Gson Builder
前面的例子基本上都是經過
Gson gson = new Gson();
來直接實例化一個Gson來使用Gson的,使用Gson Builder能夠設置Gson的某些屬性,使用其中的create()返回一個Gson.
(1)基礎
建立一個簡單的Gson,經過create()建立:
GsonBuilder gsonBuilder = new GsonBuilder(); Gson gson = gsonBuilder.create();
(2)命名規則
a.使用系統命名規則
使用
gsonBuilder.setFieldNamingPolicy();
設置json中鍵的命名規則.6個值可選:
- IDENTITY:相同,json中的鍵名與字段名相同.
- LOWER_CASE_WITH_DASHES:在原來字段的大寫字母前加-,而且把大寫變成小寫
- LOWER_CASE_WITH_DOTS:在原來字段的大寫字母前加.,而且把大寫變成小寫
- LOWER_CASE_WITH_UNDERSCORES:在原來字段的大寫字母前加_,而且把大寫變成小寫
- UPPER_CAMEL_CASE:首字母大寫
- UPPER_CAMEL_CASE_WITH_SPACES:在原來字段的大寫字母前加空格
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;
}
}
注意,若某個字段有了@SerializedName,則這個字段遵循@SerializedName的策略.
b.自定義命名規則
重寫FieldNamingStrategy中的translateName(Field field),把自定義的FieldNamingStrategy傳遞給GsonBuilder的setFieldNamingStrategy().
好比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;
}
}
(3)null
默認狀況下,Gson實例不容許序列化null,若是想要序列化null,藉助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;
}
}
(4)排除
前面已經接觸到了@Expose這樣的排除策略,下面看看更增強大的配合GsonBuilder使用的排除策略.主要有四種:屬性名排除,類型排除,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結尾的字段.下面是重點:
public boolean shouldSkipField(FieldAttributes f)
{
return f.getName().endsWith("e");
}
重寫的shouldSkipField從名字能夠看出跳過某些字段,返回true表示跳過,即排除這個字段.上面的例子中若名字以e結尾則跳過,所以輸出:
b.類型排除
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;
}
}
重寫的shouldSkipClass表示要跳過的類,這裏跳過了String,只剩下num.
@Override
public boolean shouldSkipClass(Class<?> cls) {
return cls == String.class;
}
c.修飾符排除
這個不用重寫方法了,直接使用GsonBuilder的excludeFieldsWithModifiers(),參數是java.lang.reflect.Modifier:
這個可選的比較多就不一一列舉了,只選了一個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;
}
d.@Expose排除
這個準確來講是排除沒有被@Expose註解的字段:
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;
}
}
(5)Lenient
這是有關於反序列化時JSON的容錯機制的問題,由於一般來講將一個對象經過Gson轉換成json沒什麼大問題,可是將json轉換爲對象的時候就...就難說了,由於不知道是否符合標準的json格式,所以Gson提供了必定的容錯機制,就是Lenient.
Lenient翻譯過來是"寬容的"的意思,能夠經過:
GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.setLenient();
開啓.
Gson內部有一個JsonReader類,默認狀況下,JsonReader是嚴格的且僅接受符合RFC 4627標準的json(RFC 4627標準能夠看看這裏),設置爲lenient後能夠"容忍"如下幾種錯誤:
- a.以 ) ] } ' \n 開頭
- b.多個頂層值.
- c.任何類型的頂層值
- d.數字多是NaNs或infinites
- e.以行註釋//或#結尾
- f.以一個換行符結束
- g.C風格的註釋/**/,可能會嵌套
- h.鍵/字符串沒有引號或者單引號
- i.數組元素以;分隔
- j.沒必要要的數組分隔符,"默認"null爲省略值,好比[1,,2],"默認"第二個元素爲null
- k.鍵值以=或=>分隔而非使用:
- l.鍵值對以;分隔而非使用,
設置setLenient()後,Gson會盡量解析有錯誤的json,若實在無能爲力,會拋出MalformedJsonException異常.
下面是一個不嚴格的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;
}
}
}
下面是一個異常的例子:
String errorStr = "{'name'=????,,,,,,}";
System.out.println(gsonBuilder.create().fromJson(errorStr,User.class));
(6)Floats & Doubles
先看一個例子:
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;
}
}
會報錯:
它說Infinity在JSON標準是一個非法值.想要重寫這個行爲須要使用GsonBuilder.serializeSpecialFloatingPointValues().
由於JSON規範不容許NaN,-Infinity,Infinity,所以會報錯.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:
double也相似:
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;
}
}
(7)模型版本化
經過@Since與@Until添加版本控制,控制某個版本在序列化與反序列化時忽略或忽略某個字段.
@Since表示從某個版本開始這個字段不忽略,@Until表示這個版本後將忽略該字段.
須要配合GsonBuilder的setVersion使用,設定版本號.
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;
}
}
@Since的範圍包含了左區間端點,@Until的範圍不包含右區間端點.
(8)格式化日期
可使用setDateForamt()來格式化日期輸出:
gsonBuilder.setDateFormat(int style); gsonBuilder.setDateFormat(String pattern); gsonBuilder.setDateFormat(int dateStyle,int timeStyle);
第一個函數的參數爲DateFormat中的常量值,第二個函數的參數是相似SimpleDateFormat中的String,第三個函數的參數與第一個相似,分開日期與時間設置.
再說如下setPrettyPrinting()這個函數,看名字就知道,美化打印的.會加上空格.
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();
}
}
7.Gson進階
(1)枚舉
枚舉其實也與普通的字段相似,序列化與反序列化正常操做.
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;
}
}
}
主要配合@SerializedName()使用,例如對於月份,可使用1,2,3來表示:
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
}
}
(2)泛型
這裏主要說一下反序列化泛型.由於序列化泛型...直接toJson()就行了. 想一想這樣的情景:有一個待解析的Json String,類型爲List<T>,所以想有一個這樣的方法:
public <T> List<T> fromJSON(String json,Class<T> cls);
好了,怎麼寫呢,以前用的是TypeToken()實現反序列化:
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:重寫裏面的getActualTypeArguments(),getOwnerTpye()與getRawType():
public class GenericsTest
{
public static void main(String[] args) {
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.setPrettyPrinting();
Gson gson = gsonBuilder.create();
List<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;
}
}
}
@Override
public Type[] getActualTypeArguments()
{
return new Type[]{cls};
}
返回實際參數類型數組,在這裏是String.class與Integer.class.
@Override
public Type getRawType()
{
return List.class;
}
返回聲明這個類型的類或接口,在這裏是List.class.
@Override
public Type getOwnerType()
{
return null;
}
返回其成員之一的類型,就是說若是徹底限定類名爲A.B,則返回A,在這裏沒有A,所以爲null.
定義好實現ParameterizedTpye接口的類後,把它傳遞給fromJson()做爲第二參數,構造方法的參數爲List<T>中的T.class.
(4)自定義序列化
自定義序列化通常用於自定義簡化json. 好比有一個User類:
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,可使用@Expose(serialize = false),可是想要部分序列化nums,好比只是須要第一個nums[0],上面的@Expose,@SerializedName等註解都用不上,這時須要使用JsonSerializer自定義序列化,重寫其中的:
public JsonElement serialize(T t,Type type,JsonSerializationContext context)
能夠返回一個JsonObject,這裏的JsonObject能夠自定義添加屬性(即鍵值對).
須要配合GsonBuilder使用,建立了本身的JsonSerializer<T>後,把它傳遞給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;
}
}
}
(5)自定義反序列化
自定義反序列化主要就是針對有多餘鍵值對的json,好比User只須要name字段,可是傳來的json包含了諸如age這樣的鍵,所以只需挑選所需的進行反序列化.與序列化相似,首先定義本身的JsonDeserializer<T>,重寫其中的:
public T deserialize(JsonElement,Type,JsonDeserializationContext);
返回一個自定義的反序列化的對象.最後在GsonBuilder中registerTypeAdapter()便可.
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;
}
}
}
(6)默認實例
能夠設置反序列化時的默認值,經過構造方法實現.好比json中沒有所須要的字段的值,默認狀況下爲null,若是想要不爲null,能夠設定默認值,對於對象能夠設定構造方法.
經過實現InstanceCreator<T>來實現,重寫其中的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沒有爲BB類設定值,採用默認值.
(7)@JsonAdapter
這個是自定義序列化/反序列化的註解,能夠簡化JsonSerializer與JsonDeserializer.另外不用配合GsonBuilder使用,直接使用new Gson().toJson()/fromJson()便可,比直接使用JsonSerializer與JsonDeserializer要簡單.
首先使一個類實現JsonSerializer<T>接口,接着把這個類做爲要註解的字段的@JsonAdapter的參數:
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;
}
}
}
反序列化也相似.
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;
}
}
}
實現JsonDeserializer<T>,重寫deserialize(),在所需的類上添加註解.
注意@JsonAdapter註解只能添加在類上.
8.源碼
全部例子的完整源碼.
9.參考連接
1.Gson Tutorial Series by Future Studio
2.gson徹底教程
3.gson教程
4.gson用戶指南
6.gson
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