GSON的流式反序列化
自动方式
Gson提供了fromJson()和toJson() 两个直接用于解析和生成的方法，前者实现反序列化，后者实现了序列化。同时每个方法都提供了重载方法，我常用的总共有5个。
Gson.toJson(Object);
Gson.fromJson(Reader,Class);
Gson.fromJson(String,Class);
Gson.fromJson(Reader,Type);
Gson.fromJson(String,Type);
自动方式就是使用：多个formJson的重载方法而已，就是这么简单，粗暴。
手动方式
手动的方式就是使用stream包下的JsonReader类来手动实现反序列化，和Android中使用pull解析XML是比较类似的。
private static void parseJsonStr() throws IOException {
String json = "{\"name\":\"gakki\",\"age\":18,\"address\":\"冲绳岛\"}";
JsonReader jsonReader = new JsonReader(new StringReader(json));
jsonReader.beginObject(); //throws IOException
User user = new User();
while (jsonReader.hasNext()) {
String name = jsonReader.nextName();
switch (name) {
case "name":
user.setName(jsonReader.nextString());
break;
case "age":
user.setAge(jsonReader.nextInt());
break;
case "address":
user.setAddress(jsonReader.nextString());
break;
default:
break;
}
}
jsonReader.endObject();
System.out.println(user);
}
结果：
User{name='gakki', age=18, address='冲绳岛'}
数组的例子：
private static void parseJsonStrArray() throws IOException {
String json = "[{\"name\":\"gakki\",\"age\":18,\"address\":\"冲绳岛\"}," +
"{\"name\":\"liuzhuo\",\"age\":20,\"address\":\"梦幻岛\"}," +
"{\"name\":\"gakkij\",\"age\":22,\"address\":\"东京\"}]";
JsonReader jsonReader = new JsonReader(new StringReader(json));
jsonReader.beginArray(); //throws IOException
List<User> userList = new ArrayList<>();
while (jsonReader.hasNext()) {
User user = new User();
jsonReader.beginObject();
while (jsonReader.hasNext()) {
String name = jsonReader.nextName();
switch (name) {
case "name":
user.setName(jsonReader.nextString());
break;
case "age":
user.setAge(jsonReader.nextInt());
break;
case "address":
user.setAddress(jsonReader.nextString());
break;
default:
break;
}
}
jsonReader.endObject();
userList.add(user);
}
jsonReader.endArray();
System.out.println(userList);
}
结果：
[User{name='gakki', age=18, address='冲绳岛'}, User{name='liuzhuo', age=20, address='梦幻岛'}, User{name='gakkij', age=22, address='东京'}]
注意：每个一个begin都必须在最后接一个end结束，beginObject 对应：endObject；beginArray 对应endArray。
通过hasNext来判断是否有下一个元素，nextName取出当前的key，nextXXX来取出对应类型的value值【必须先nextName，才能nextXXX】。
其实自动反序列化最终也是采用：JsonReader的方式：
GSON的流式序列化
自动方式
toJson()
提示：PrintStream(System.out) 、StringBuilder、StringBuffer和*Writer都实现了Appendable接口。
User user = new User("gakki",18,"冲绳岛");
Gson gson = new Gson();
gson.toJson(user,new PrintStream(System.out));
这样就是直接把序列化的结果打印到控制台上：
{"age":18,"userName":"gakki","address":"冲绳岛"}
手动方式
JsonWriter writer = new JsonWriter(new OutputStreamWriter(System.out));
writer.beginObject() // throws IOException
.name("name").value("怪盗kidou")
.name("age").value(24)
.name("email").nullValue() //演示null
.endObject(); // throws IOException
writer.flush(); // throws IOException
//{"name":"怪盗kidou","age":24,"email":null}
GsonBuilder的简单使用
序列化null值
serializeNulls()
public static void parsePojoForNull() {
Gson gson1 = new Gson();
Gson gson = new GsonBuilder().serializeNulls().create();
User user = new User();
String user1 = gson1.toJson(user);
String user2 = gson.toJson(user);
System.out.println(user1);
System.out.println(user2);
}
{"age":0}
{"age":0,"userName":null,"address":null}
age 是 int，默认值是0，不是 null。
序列化格式化
setPrettyPrinting()
public static void formatJsonStr() {
User user = new User("gakki", 18, "冲绳岛");
Gson gson = new Gson();
String user1 = gson.toJson(user);
Gson gson2 = new GsonBuilder().setPrettyPrinting().create();
String user2 = gson2.toJson(user);
System.out.println(user1);
System.out.println(user2);
}
{"age":18,"userName":"gakki","address":"冲绳岛"}
{
"age": 18,
"userName": "gakki",
"address": "冲绳岛"
}
时间格式化
setDateFormat()
public static void formatJsonStrDate() {
User user = new User("gakki", 18, "冲绳岛", new Date());
Gson gson = new Gson();
String user1 = gson.toJson(user);
Gson gson2 = new GsonBuilder().setDateFormat("yyyy-MM-dd HH:mm:ss").setPrettyPrinting().create();
String user2 = gson2.toJson(user);
System.out.println(user1);
System.out.println(user2);
}
{"age":18,"userName":"gakki","address":"冲绳岛","birthday":"Dec 15, 2019 8:47:03 PM"}
{
"age": 18,
"userName": "gakki",
"address": "冲绳岛",
"birthday": "2019-12-15 20:47:03"
}
时间的反序列化：
public static void parseJsonStrDate() {
String json = "{\"age\":18,\"userName\":\"gakki\",\"address\":\"冲绳岛\",\"birthday\":\"Dec 15, 2019 8:47:03 PM\"}";
String json2 = "{\"age\":18,\"userName\":\"gakki\",\"address\":\"冲绳岛\",\"birthday\":\"2019-06-11 11:11:11\"}";
Gson gson = new Gson();
User user = gson.fromJson(json, User.class);
System.out.println(user);
Gson gson2 = new GsonBuilder().setDateFormat("yyyy-MM-dd HH:mm:ss").create();
User user1 = gson2.fromJson(json2, User.class);
System.out.println(user1);
}
User{age=18, name='gakki', address='冲绳岛', birthday=Sun Dec 15 20:47:03 CST 2019}
User{age=18, name='gakki', address='冲绳岛', birthday=Tue Jun 11 11:11:11 CST 2019}
注意：json1 和 json2 的birthday的字符串格式的区别！！！
其他类似的，就不一一列举了：
Gson gson = new GsonBuilder()
//序列化null
.serializeNulls()
// 设置日期时间格式，另有2个重载方法
// 在序列化和反序化时均生效
.setDateFormat("yyyy-MM-dd")
// 禁此序列化内部类
.disableInnerClassSerialization()
//生成不可执行的Json（多了 )]}' 这4个字符）
.generateNonExecutableJson()
//禁止转义html标签
.disableHtmlEscaping()
//格式化输出
.setPrettyPrinting()
.create();
注意：内部类(Inner Class)和嵌套类(Nested Class)的区别
