java - Gson.json() 解析json文件时返回null

Question

我正在尝试将 JSON 文件(我已经创建的)解析为 java 对象。

我尝试过使用 JsonReader 和 bufferedReader 但每次尝试都返回 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。其实我也不知道这两者到底有什么区别，我只是在类似问题的某人的回答中看到了第二个

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());
    }
        ...

}

在对这两种方法的解析结束时，我得到:

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 stacetrace:

"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

Answer 1

好吧，在这两种方法中，您都只初始化了文件读取对象，但还没有开始读取它。

在方法 1 中，如果你想读取文件，你应该使用类似下面的内容。

String line = null;
while((line = br.readLine()) != null) {
   System.out.println(line);
}

然后你必须解析字符串line。在您的情况下，实际上没有从文件中读取任何内容来解析，这就是它返回空异常的原因。无论如何，我不能保证这种方法是否可以使用 Gson 解析，因为它是逐行读取而不是一次读取 Json 节点。

因此，如果您再次犯了与上述相同的错误，则第二种方法会更方便。请参阅随附的链接 ( Gson - JsonReader Doc ) 以更好地理解方法 2。

编辑:下面提到了与从 link 中提取的原始问题的第二种方法相关的示例代码片段。无论如何，如果它可能不再起作用。

String json = "{\"brand\" : \"Toyota\", \"doors\" : 5}";

JsonReader jsonReader = new JsonReader(new StringReader(json));

try {
    while(jsonReader.hasNext()){
        JsonToken nextToken = jsonReader.peek();
        System.out.println(nextToken);

        if(JsonToken.BEGIN_OBJECT.equals(nextToken)){

            jsonReader.beginObject();

        } else if(JsonToken.NAME.equals(nextToken)){

            String name  =  jsonReader.nextName();
            System.out.println(name);

        } else if(JsonToken.STRING.equals(nextToken)){

            String value =  jsonReader.nextString();
            System.out.println(value);

        } else if(JsonToken.NUMBER.equals(nextToken)){

            long value =  jsonReader.nextLong();
            System.out.println(value);

        }
    }
} catch (IOException e) {
    e.printStackTrace();
}