我正在尝试将JSON文件(已经创建)解析为java对象。
我尝试使用JsonReader和bufferedReader,但是每次尝试都返回null
public class Sheep implements Serializable {
@SerializedName("localId")
@Expose
private int localId;
@SerializedName("globalId")
@Expose
private int globalId;
@SerializedName("birthDate")
@Expose
private Date birthDate = null;
@SerializedName("sellingDate")
@Expose
private Date sellingDate = null;
@SerializedName("deathDate")
@Expose
private Date deathDate = null;
@SerializedName("lastpregnancyDate")
@Expose
private Date lastpregnancyDate = null;
@SerializedName("lastDoctorCheck")
@Expose
private Date lastDoctorCheck = null;
@SerializedName("alive")
@Expose
private boolean alive;
@SerializedName("sold")
@Expose
private boolean sold;
@SerializedName("pregnant")
@Expose
private boolean pregnant;
@SerializedName("mother")
@Expose
private int mother;
@SerializedName("father")
@Expose
private int father;
@SerializedName("husband")
@Expose
private int husband;
@SerializedName("allDoctorChecks")
@Expose
private List<Date> allDoctorChecks = null;
@SerializedName("allpregnancies")
@Expose
private List<Date> allpregnancies = null;
@SerializedName("children")
@Expose
private List<Integer> children = null;
@SerializedName("allHusbands")
@Expose
private List<Integer> allHusbands = null;
}
public class Farm implements Serializable {
@SerializedName("allSheeps")
@Expose
private List<Sheep> allSheeps;
}
,在这里您可以看到我尝试解析JSON的两种方法。实际上,我不确切知道两者之间的区别,我只是在某人针对类似问题的答案中看到了第二个问题
1。
public class DataBase {
private static Farm farm;
public static void main(String[] args){
...
Gson gson = new Gson();
BufferedReader br = null;
try {
br = new BufferedReader(new FileReader("farm.json"));
Farm parsedFarm = gson.fromJson(br, Farm.class);
System.out.println(parsedFarm);
}catch(Exception e){
System.out.println(e.fillInStackTrace());
}
...
}
2。
public class DataBase {
private static Farm farm;
public static void main(String[] args){
...
Gson gson = new Gson();
JsonReader jsonReader = null;
Type farmType = new TypeToken<List<Sheep>>(){}.getType();
try {
jsonReader = new JsonReader(new FileReader("farm.json"));
List<Sheep> parsedFarm = gson.fromJson(jsonReader , farmType);
System.out.println(parsedFarm);
}catch(Exception e){
System.out.println(e.fillInStackTrace());
}
...
}
在我得到的两种方法的解析结束时:
java.lang.NullPointerException
{
"allSheeps": [
{
"localId": 0,
"globalId": 10,
"birthDate": "Jan 12, 3913, 12:00:00 AM",
"alive": true,
"sold": false,
"pregnant": false,
"mother": -1,
"father": -1,
"husband": -1
},
{
"localId": 1,
"globalId": 20,
"birthDate": "Jan 12, 3913, 12:00:00 AM",
"alive": true,
"sold": false,
"pregnant": false,
"mother": -1,
"father": -1,
"husband": -1
},
{
"localId": 2,
"globalId": 200,
"birthDate": "Jan 12, 3913, 12:00:00 AM",
"alive": true,
"sold": false,
"pregnant": false,
"mother": 0,
"father": 1,
"husband": -1
},
{
"localId": 3,
"globalId": 300,
"birthDate": "Jan 12, 3913, 12:00:00 AM",
"alive": true,
"sold": false,
"pregnant": false,
"mother": 1,
"father": 2,
"husband": -1
},
{
"localId": 4,
"globalId": 400,
"birthDate": "Jan 12, 3913, 12:00:00 AM",
"alive": true,
"sold": false,
"pregnant": false,
"mother": 3,
"father": 3,
"husband": -1
}
]
}
完整的跟踪记录:
"C:\Program Files\Java\jdk-12.0.1\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.1.2\lib\idea_rt.jar=49271:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.1.2\bin" -Dfile.encoding=UTF-8 -classpath C:\Users\saleh\Desktop\farm\out\production\farm;C:\Users\saleh\.m2\repository\com\google\code\gson\gson\2.8.5\gson-2.8.5.jar com.company.DB.DataBase
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.google.gson.internal.reflect.UnsafeReflectionAccessor (file:/C:/Users/saleh/.m2/repository/com/google/code/gson/gson/2.8.5/gson-2.8.5.jar) to field java.text.SimpleDateFormat.serialVersionOnStream
WARNING: Please consider reporting this to the maintainers of com.google.gson.internal.reflect.UnsafeReflectionAccessor
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Exception in thread "main" java.lang.NullPointerException
at com.company.Farm.Sheep.toString(Sheep.java:237)
at java.base/java.lang.String.valueOf(String.java:3042)
at java.base/java.lang.StringBuilder.append(StringBuilder.java:168)
at java.base/java.util.AbstractCollection.toString(AbstractCollection.java:473)
at java.base/java.lang.String.valueOf(String.java:3042)
at java.base/java.lang.StringBuilder.append(StringBuilder.java:168)
at com.company.Farm.Farm.toString(Farm.java:60)
at java.base/java.lang.String.valueOf(String.java:3042)
at java.base/java.io.PrintStream.println(PrintStream.java:897)
at com.company.DB.DataBase.main(DataBase.java:66)
Process finished with exit code 1
答案 0 :(得分:1)
在这两种方法中,您都只初始化了文件读取对象,但尚未开始从文件读取对象。
在方法1中,如果要读取文件,则应使用以下内容。
setTimeout(function() {
this.secondFunction();
}.bind(this), 2000);
然后,您必须解析字符串 line 。在您的情况下,实际上没有从文件读取任何内容以进行解析,这就是为什么它返回空异常的原因。无论如何,我不能保证是否可以使用 Gson parse ,因为它可以逐行读取而不是 Json节点。
因此,如果您再次重复上述错误,第二种方法将更方便使用。请参考附件中的链接(Gson - JsonReader Doc)以更好地理解方法2。
编辑:下面提到了与原始问题的第二种方法相关的示例代码片段,该代码片段是从上面的link中提取出来的,如果可能不再起作用,则无论如何。
String line = null; while((line = br.readLine()) != null) { System.out.println(line); }