NullPointerException:使用GSON在JAVA中解析JSON
我想使用Api GSON通过java解析JSON文件,以获取JSON文件的最后字段: descriptor.json:NullPointerException:使用GSON在JAVA中解析JSON,java,json,nullpointerexception,gson,Java,Json,Nullpointerexception,Gson,我想使用Api GSON通过java解析JSON文件,以获取JSON文件的最后字段: descriptor.json: { "Teleservice_1" : { "Record_1" : { "method_name" : "mehdi", "method_params": ["param1",2,"param3"] }, "Record_2" : { "method
{
"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"]
}
}
}
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;
}
}
ListTeleServices.java:
{
"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"]
}
}
}
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;
}
}
最后是我的解析器类,JSONMainParse.java:
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);
}
}
在我看来,它似乎是正确的,应该显示:“amine1”,但它在以下位置给我一个空点异常:
ListTeleServices.getTeleService(ListTeleServices.java:12),它是:
if(this.listTeleServices.get(teleserviceName) != null)
String methodName = teleservices.getTeleService("Teleservice_2").getRecord("Record_33").getMethodName();
在JSONMainParse.main(JSONMainParse.java:15)上,它是:
if(this.listTeleServices.get(teleserviceName) != null)
String methodName = teleservices.getTeleService("Teleservice_2").getRecord("Record_33").getMethodName();
你知道这件事吗?谢谢:)初始化
private HashMap<String, TeleService> listTeleServices;
私有HashMap listTeleServices;
作为
private HashMap listTeleServices=new HashMap();
在当前代码中,您正试图调用null
对象引用上的get()
,当您试图调用null对象上的实例方法时,该对象引用会引发错误。更改此设置:
private HashMap<String, TeleService> listTeleServices;
私有HashMap listTeleServices;
对此
private HashMap<String, TeleService> listTeleServices = new HashMap<String,TeleService>();
private HashMap listTeleServices=new HashMap();
解决方案:
您正在使用比解析JSON响应所需的更多的类!您可以删除您的类ListTeleServices
和TeleService
,只保留您的记录
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);
说明: 当您在此处使用类
ListTeleServices
解析JSON时:
ListTeleServices teleservices = gson.fromJson(br, ListTeleServices.class);
Gson所做的是分析类ListTeleServices
,并将其与JSON响应进行比较,因此它说:
ListTeleServices.class
,JSON响应以对象{}
开始。。。到目前为止一切都很好- 在类
中,它找到一个属性ListTeleServices
,它是某个对象(暂时不介意类型)李>ListTeleServices
- 然而,在JSON响应中,它找到了三个元素
、“Teleservice_1”
和“Teleservice_2”
,但它们都没有相同的名称“Teleservice_3”
,因此Gson跳过所有这些值,并将listTeleServices
赋值给属性null
李>listTeleServices
地图
,请参见:
Map
的类型,JSON响应以对象{}
开始。。。到目前为止一切都很好!(请记住,Map
只是一个对象)- 在
中,可以看到一定有一些键(字符串)和值(一些对象)对李>Map
- 在JSON响应中,它发现一些字符串对
,“Teleservice_1”
和“Teleservice_2”
,以及一些对象“Teleservice_3”
,因此它可以愉快地继续解析{}
private HashMap<String, Record> Teleservice_1;
private HashMap<String, Record> Teleservice_2;
private HashMap<String, Record> Teleservice_3;
为什么要用原始类型初始化它?谢谢,但它仍然在以下位置给我一个NullPointerException:
String methodName=teleservices.getTeleService(“Teleservice_2”).getRecord(“Record_33”).getMethodName()代码>现在编辑!!!我不在IDE前面,所以这样写是为了节省时间。只需检查teleservices
,teleservices.getTeleService(“Teleservice_2”)
,teleservices.getTeleService(“Teleservice_2”).getRecord(“Record_33”)
是否为null
!!!问题不在于地图的日化,Gson会为你做这件事!问题是JSON没有被正确解析…谢谢,是的teleservices
不是空的,但是teleservices.getTeleService(“Teleservice_2”)
返回空的…谢谢!它起作用了!但是我真的不明白为什么我们必须使用Map
而不是创建大量的类并使用它们的getter,我现在明白了!多谢各位!