Android 尝试使用AsyncTask避免网络读取错误
我正在尝试实现一个异步任务来运行与网络相关的任务。但它似乎不起作用，我一直得到nullpointerexception我正在使用懒惰列表Android 尝试使用AsyncTask避免网络读取错误,android,android-asynctask,lazy-loading,Android,Android Asynctask,Lazy Loading,我正在尝试实现一个异步任务来运行与网络相关的任务。但它似乎不起作用，我一直得到nullpointerexception我正在使用懒惰列表 public class Homepage extends ActionBarActivity{ ListView list; LazyAdapter adapter; private static String url = "http://192.168.1.6/webservice/events.php"; static
public class Homepage extends ActionBarActivity{
ListView list;
LazyAdapter adapter;
private static String url = "http://192.168.1.6/webservice/events.php";
static final String TAG_IMG = "event_img";
static final String TAG_SPONSER= "sponser";
static final String TAG_TITLE= "title";
static final String TAG_LOCATION="event_location";
static final String TAG_TIME="event_time";
static final String TAG_ENDTIME="event_endtime";
static final String TAG_WHOINVITED="event_whoinvited";
static final String TAG_MESSAGE="message";
static final String TAG_DRESSCODE="event_dresscode";
//private List<String> _list = null;
//private String[] mStrings = null;
ViewFlipper flippy;
ArrayList<HashMap<String, String>> eventList = new ArrayList<HashMap<String, String>>();
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.homepage);
//StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
//StrictMode.setThreadPolicy(policy);
jsonThread jsonresult = new jsonThread();
jsonresult.execute();
adapter=new LazyAdapter(this, eventList);
Button b=(Button)findViewById(R.id.button1);
b.setOnClickListener(listener);
}//end of onCreate\\\
private class jsonThread extends AsyncTask<String, String, JSONArray>{
@Override
protected JSONArray doInBackground(String... params) {
// TODO Auto-generated method stub
ServiceHandler sh = new ServiceHandler();
String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);
JSONArray jArray = null;
JSONObject json;
//_list = new ArrayList<String>();
try {
json = new JSONObject(jsonStr);
jArray = json.getJSONArray("posts");
Log.d("jarray", "jarray post "+jArray);
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return jArray;
}
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
}
@Override
protected void onPostExecute(JSONArray result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
for(int u=0;u<result.length();u++){
HashMap<String, String> map = new HashMap<String, String>();
JSONObject json_data;
try {
json_data = result.getJSONObject(u);
String myevent_img = json_data.getString(TAG_IMG);
String sponser = json_data.getString(TAG_SPONSER);
String title = json_data.getString(TAG_TITLE);
String location = json_data.getString(TAG_LOCATION);
String time = json_data.getString(TAG_TIME);
String endtime = json_data.getString(TAG_ENDTIME);
String message = json_data.getString(TAG_MESSAGE);
String whoinvited = json_data.getString(TAG_WHOINVITED);
String dresscode = json_data.getString(TAG_DRESSCODE);
//_list.add(myevent_img);
map.put(TAG_SPONSER, sponser);
map.put(TAG_TITLE, title);
map.put(TAG_LOCATION, location);
map.put(TAG_TIME, time);
map.put(TAG_ENDTIME, endtime);
map.put(TAG_MESSAGE, message);
map.put(TAG_WHOINVITED, whoinvited);
map.put(TAG_DRESSCODE, dresscode);
map.put(TAG_IMG, myevent_img);
Log.d("jobj ", "event_img"+myevent_img);
Log.d("hashmap ", "sponser "+map.get("sponser").toString());
// need to put this to transfer the list to mstrings.
//mStrings=new String[_list.size()];
//mStrings = _list.toArray(mStrings);
eventList.add(map);
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
list=(ListView)findViewById(R.id.list);
list.setAdapter(adapter);
}
}
}
public class Homepage extends ActionBarActivity{
ListView list;
LazyAdapter adapter;
private static String url = "http://192.168.1.6/webservice/events.php";
static final String TAG_IMG = "event_img";
static final String TAG_SPONSER= "sponser";
static final String TAG_TITLE= "title";
static final String TAG_LOCATION="event_location";
static final String TAG_TIME="event_time";
static final String TAG_ENDTIME="event_endtime";
static final String TAG_WHOINVITED="event_whoinvited";
static final String TAG_MESSAGE="message";
static final String TAG_DRESSCODE="event_dresscode";
//private List<String> _list = null;
//private String[] mStrings = null;
ViewFlipper flippy;
ArrayList<HashMap<String, String>> eventList = new ArrayList<HashMap<String, String>>();
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.homepage);
jsonThread jsonresult = new jsonThread();
jsonresult.execute();
Button b=(Button)findViewById(R.id.button1);
b.setOnClickListener(listener);
}//end of onCreate\\\
private class jsonThread extends AsyncTask<String, String, ArrayList<HashMap<String, String>>>{
@Override
protected ArrayList<HashMap<String,String>> doInBackground(String... params) {
// TODO Auto-generated method stub
ServiceHandler sh = new ServiceHandler();
String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);
JSONArray jArray = null;
JSONObject json;
//_list = new ArrayList<String>();
try {
json = new JSONObject(jsonStr);
jArray = json.getJSONArray("posts");
Log.d("jarray", "jarray post "+jArray);
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
for(int u=0;u<jArray.length();u++){
HashMap<String, String> map = new HashMap<String, String>();
JSONObject json_data;
try {
json_data = jArray.getJSONObject(u);
String myevent_img = json_data.getString(TAG_IMG);
String sponser = json_data.getString(TAG_SPONSER);
String title = json_data.getString(TAG_TITLE);
String location = json_data.getString(TAG_LOCATION);
String time = json_data.getString(TAG_TIME);
String endtime = json_data.getString(TAG_ENDTIME);
String message = json_data.getString(TAG_MESSAGE);
String whoinvited = json_data.getString(TAG_WHOINVITED);
String dresscode = json_data.getString(TAG_DRESSCODE);
//_list.add(myevent_img);
map.put(TAG_SPONSER, sponser);
map.put(TAG_TITLE, title);
map.put(TAG_LOCATION, location);
map.put(TAG_TIME, time);
map.put(TAG_ENDTIME, endtime);
map.put(TAG_MESSAGE, message);
map.put(TAG_WHOINVITED, whoinvited);
map.put(TAG_DRESSCODE, dresscode);
map.put(TAG_IMG, myevent_img);
Log.d("jobj ", "event_img"+myevent_img);
Log.d("hashmap ", "sponser "+map.get("sponser").toString());
// need to put this to transfer the list to mstrings.
//mStrings=new String[_list.size()];
//mStrings = _list.toArray(mStrings);
eventList.add(map);
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return eventList;
}
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
}
@Override
protected void onPostExecute(ArrayList<HashMap<String,String>> result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
list=(ListView)findViewById(R.id.list);
adapter=new LazyAdapter(Homepage.this, eventList);
list.setAdapter(adapter);
}
}
}
当您尝试在
onPostExecute（）适配器设置为ListView
时，为null
移动
在执行之前执行onPostExecute（）
list.setAdapter(adapter);
另外，您的for循环
中可能应该有doInBackground（）
不要像你指出的那样使用严格模式。你可以在文档和大量关于它的帖子中看到原因
查看日志和评论后编辑
移动
在设置侦听器之前，从onPostExecute（）
到onCreate（）
。这导致您的NPE
发布stacktrace。您已经尝试过了。如果我把adapter=newlazyadapter（这个，eventList）
转换为onPostExecute，它编译的唯一方法是将其更改为Homepage.this。但这给了我一个空指针。我试着把我的for循环放到doInBackground中，但仍然不起作用。啊，是的，我错过了，但现在已经编辑了。但这就是它需要的方式，因为任务完成后才会填充eventList
。我会以这种方式运行它，并从中发布日志，这样我们就可以看到什么是null
。这可能是许多因素造成的，但如果没有stacktrace是不可能说的每个人似乎都在谈论的stacktrace是什么？这是logcat的另一个词吗？是的，logcat输出我已经发布了logcat，并且我在doInBackground
中加入了for循环。请看一看。
04-05 22:24:34.204: W/dalvikvm(5680): threadid=1: thread exiting with uncaught exception (group=0x41465700)
04-05 22:24:34.524: E/AndroidRuntime(5680): FATAL EXCEPTION: main
04-05 22:24:34.524: E/AndroidRuntime(5680): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.wordpress.yourhappening.happening/com.wordpress.yourhappening.happening.Homepage}: java.lang.NullPointerException
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.os.Handler.dispatchMessage(Handler.java:99)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.os.Looper.loop(Looper.java:137)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread.main(ActivityThread.java:5103)
04-05 22:24:34.524: E/AndroidRuntime(5680): at java.lang.reflect.Method.invokeNative(Native Method)
04-05 22:24:34.524: E/AndroidRuntime(5680): at java.lang.reflect.Method.invoke(Method.java:525)
04-05 22:24:34.524: E/AndroidRuntime(5680): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
04-05 22:24:34.524: E/AndroidRuntime(5680): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-05 22:24:34.524: E/AndroidRuntime(5680): at dalvik.system.NativeStart.main(Native Method)
04-05 22:24:34.524: E/AndroidRuntime(5680): Caused by: java.lang.NullPointerException
04-05 22:24:34.524: E/AndroidRuntime(5680): at com.wordpress.yourhappening.happening.Homepage.onCreate(Homepage.java:94)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.Activity.performCreate(Activity.java:5133)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-05 22:24:34.524: E/AndroidRuntime(5680): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
04-05 22:24:34.524: E/AndroidRuntime(5680): ... 11 more
adapter=new LazyAdapter(Homepage.this, eventList);
list.setAdapter(adapter);
list=(ListView)findViewById(R.id.list);