NullPointerException：使用GSON在JAVA中解析JSON

Question

我希望通过java使用Api GSON解析一个JSON文件，以获取JSON文件的最后字段：

json：

{
    "Teleservice_1" : {
        "Record_1" : {
            "method_name" : "mehdi",
            "method_params": ["param1",2,"param3"]
        },
        "Record_2" : {
            "method_name" : "mkyong",
            "method_params": [3,"param2"]
        },
        "Record_3" : {
            "method_name" : "amine",
            "method_params": [3,"param1","param2"]
        }
    },
    "Teleservice_2" : {
        "Record_11" : {
            "method_name" : "mehdi1",
            "method_params": ["param11",22,"param33"]
        },
        "Record_22" : {
            "method_name" : "mkyong1",
            "method_params": [33,"param22"]
        },
        "Record_33" : {
            "method_name" : "amine1",
            "method_params": [33,"param11","param22"]
        }
    },
    "Teleservice_3" : {
        "Record_111" : {
            "method_name" : "mehdi2",
            "method_params": ["param111",222,"param333"]
        },
        "Record_222" : {
            "method_name" : "mkyong2",
            "method_params": [333,"param222"]
        },
        "Record_333" : {
            "method_name" : "amine2",
            "method_params": [333,"param111","param222"]
        }
    }
}

java：

import java.util.HashMap;

public class ListTeleServices {

    private HashMap<String, TeleService> listTeleServices;

    public ListTeleServices() {

    }

    public TeleService getTeleService(String teleserviceName) {
        if(this.listTeleServices.get(teleserviceName) != null) 
            return this.listTeleServices.get(teleserviceName);
        else
            return null;
    }
}

import java.util.HashMap;

public class TeleService {

    private HashMap<String, Record> listRecords;

    public TeleService() {

    }

    public Record getRecord(String recordName) {
        if(this.listRecords.get(recordName) != null) 
            return this.listRecords.get(recordName);
        else
            return null;
    }
}

public class Record {

    private String method_name;
    private Object[] method_parameters; 

    public Record(String methodName, Object[] methodParameters) {
        this.method_name = new String(methodName);
        this.method_parameters = methodParameters;
    }

    public String getMethodName() {
        return this.method_name;
    }

    public Object[] getMethodParameters() {
        return this.method_parameters;
    }

    public void setMethodName(String methodName) {
        this.method_name = methodName;
    }

    public void setMethodParameters(Object[] methodParameters) {
        this.method_parameters = methodParameters;
    }
}

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import com.google.gson.Gson;


public class JSONMainParse {

    public static void main(String[] args) throws FileNotFoundException {

        BufferedReader br = new BufferedReader(new FileReader("/Users/Mehdi/Desktop/descriptor.json"));
        Gson gson = new Gson();
        ListTeleServices teleservices = gson.fromJson(br, ListTeleServices.class);
        String methodName = teleservices.getTeleService("Teleservice_2").getRecord("Record_33").getMethodName();

        System.out.println(methodName);
    }
}

if(this.listTeleServices.get(teleserviceName) != null) 

String methodName = teleservices.getTeleService("Teleservice_2").getRecord("Record_33").getMethodName();

Answer 1

您使用的类比解析JSON响应所需的要多！您可以删除类ListTeleServices和TeleService，只保留Record类。

Gson gson = new Gson();
Type mapOfMapsType = new TypeToken<Map<String, Map<String, Record>>>() {}.getType();
Map<String, Map<String, Record>> map = gson.fromJson(br, mapOfMapsType);

最后，为了获得方法名，您必须使用：

String methodName = map.get("Teleservice_2").get("Record_33").getMethodName();

当您使用类ListTeleServices解析JSON时：

ListTeleServices teleservices = gson.fromJson(br, ListTeleServices.class);

null

另一方面，如果直接使用map > ，Gson请参见：

>

您传递了map > 的类型，JSON响应以对象{}...到目前为止一切都还好！（请记住映射只是一个对象）

然后继续解析JSON，并且：

null

private HashMap<String, Record> Teleservice_1;
private HashMap<String, Record> Teleservice_2;
private HashMap<String, Record> Teleservice_3;

@SerializedName("method_params")
private Object[] method_parameters;