我试图从数据库中获取一些数据，但是我得到了“致命错误”.AsyncTask＃2＆＃39;。我真的很新java和android工作室，因此我不知道我做了什么是正确的方法去做它。但简而言之，这就是我试图从数据库中读取JsonObject的内容。：
设置inputput流（con.getInputStream） 将其转换为字符串 并将其发送到JSONobject进行读取然后返回。
我正在使用API 23，而且我还没有真正发现很多/从网上可用的内容中了解多少。帮助将非常感谢。
public class fetchUserDataAsyncTask extends AsyncTask<Void, Void, User> {
User user;
GetUserCallback userCallback;
fetchUserDataAsyncTask(User user, GetUserCallback userCallback) {
this.user = user;
this.userCallback = userCallback;
}
@Override
protected User doInBackground(Void... params) {
//Use HashMap, it works similar to NameValuePair
Map<String, String> dataToSend = new HashMap<>();
dataToSend.put("username", user.username);
dataToSend.put("password", user.password);
//Encoded String - we will have to encode string by our custom method (Very easy)
String encodedStr = getEncodedData(dataToSend);
User returneduser = null; //for the JSONobject
try {
//Converting address String to URL
URL url = new URL(Server_Address + "FetchUserData.php");
//Opening the connection (Not setting or using CONNECTION_TIMEOUT)
HttpURLConnection con = (HttpURLConnection) url.openConnection();
//Post Method
con.setRequestMethod("POST");
//To enable inputting values using POST method
//(Basically, after this we can write the dataToSend to the body of POST method
con.setDoOutput(true);
OutputStreamWriter writer = new OutputStreamWriter(con.getOutputStream());
//Writing dataToSend to outputstreamwriter
writer.write(encodedStr);
//Sending the data to the server - This much is enough to send data to server
//But to read the response of the server, you will have to implement the procedure below
writer.flush();
//Attempt to read data and shit but doesnt work
StringBuilder sb = new StringBuilder();
BufferedReader reader = new BufferedReader(new InputStreamReader(con.getInputStream()));
String line = null;
while ((line = reader.readLine()) != null) { //Read till there is something available
sb.append(line + "\n"); //Reading and saving line by line - not all at once
}
line = sb.toString(); //Saving complete data received in string, you can do it differently
try {
JSONObject jObject = new JSONObject(line);//result in the ().
if (jObject.length() == 0) {
returneduser = null;
}
else {
try {
String name = jObject.getString("name");
String teamname = jObject.getString("teamname");
String username = jObject.getString("username");
String password = jObject.getString("password");
String emailad = jObject.getString("emailad");
returneduser = new User(name, teamname, username, emailad, password);
} catch (JSONException e) {
e.printStackTrace();
}
}
} catch (JSONException e){
e.printStackTrace();
}
}catch (IOException e){
e.printStackTrace();
}
return returneduser;
}
@Override
protected void onPostExecute(User returneduser) {
progressDialog.dismiss();
userCallback.done(returneduser);
super.onPostExecute(returneduser);
}
//get encoded data (edit)
(private String getEncodedData(Map<String, String> data) {
StringBuilder sb = new StringBuilder();
for (String key : data.keySet()) {
String value = null;
try {
value = URLEncoder.encode(data.get(key), "UTF-8");
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
if (sb.length() > 0)
sb.append("&");
sb.append(key + "=" + value);
}
return sb.toString();
})
}
错误堆栈：
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #2
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: Process: pk.com.gameon.myapplication, PID: 8414
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: java.lang.RuntimeException: An error occurred while executing doInBackground()
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at android.os.AsyncTask$3.done(AsyncTask.java:309)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.FutureTask.run(FutureTask.java:242)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.lang.Thread.run(Thread.java:818)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at libcore.net.UriCodec.encode(UriCodec.java:132)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.net.URLEncoder.encode(URLEncoder.java:57)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at pk.com.gameon.myapplication.ServerRequest.getEncodedData(ServerRequest.java:134)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at pk.com.gameon.myapplication.ServerRequest.access$000(ServerRequest.java:29)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at pk.com.gameon.myapplication.ServerRequest$fetchUserDataAsyncTask.doInBackground(ServerRequest.java:168)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at pk.com.gameon.myapplication.ServerRequest$fetchUserDataAsyncTask.doInBackground(ServerRequest.java:148)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at android.os.AsyncTask$2.call(AsyncTask.java:295)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.FutureTask.run(FutureTask.java:237)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
12-02 23:36:23.546 8414-8443/pk.com.gameon.myapplication E/AndroidRuntime: at java.lang.Thread.run(Thread.java:818)
12-02 23:36:24.617 8414-8441/pk.com.gameon.myapplication E/Surface: getSlotFromBufferLocked: unknown buffer: 0xab753b90
12-02 23:36:25.453 8414-8441/pk.com.gameon.myapplication E/Surface: getSlotFromBufferLocked: unknown buffer: 0xab755100
答案 0 :(得分：1)
这是你的罪魁祸首：
java.lang.NullPointerException: Attempt to invoke virtual method 'int
java.lang.String.length()' on a null object reference
你在AsyncTask中调用它：
if (jObject.length() == 0) {
returneduser = null;
}
相反，请将其更改为：
if(jObject == null || jObject.length() == 0) {
returneduser = null;
}
这样在检查时它是空的。此外，您可以通过在代码中设置断点并通过Android Studio或Intellij或Eclipse运行调试器来验证这一点，具体取决于您使用的是什么。这是您最好的选择，我建议您尝试使用这些无法轻易识别的错误。在大多数情况下，您将快速看到错误，并可以查看断点范围内所有变量的值，以验证它们是否符合预期。
另一种效率较低的方法是在任何地方删除Log.i（String，String）行以跟踪发生的情况以及应用程序的状态，然后将其转换为log cat输出。