我创建了一个类，我希望通过该类从数据库中获取列表URL。但是我得到了这个错误：
03-30 15:05:35.599 3591-3591/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.imran.myapp, PID: 3591
java.lang.NullPointerException: Attempt to read from null array
at com.example.imran.myapp.gridgallery.onCreateView(gridgallery.java:55)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1026)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1207)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1572)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:493)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6145)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
03-30 15:05:35.599 1032-1045/? V/ApplicationPolicy: isApplicationStateBlocked userId 0 pkgname com.example.imran.myapp
03-30 15:05:35.599 1032-1045/? W/ActivityManager: Force finishing activity com.example.imran.myapp/.Home
03-30 15:05:35.609 1032-1045/? V/ApplicationPolicy: isApplicationStateBlocked userId 0 pkgname com.example.imran.myapp
03-30 15:05:35.909 1032-1032/? D/CrashAnrDetector: processName: com.example.imran.myapp
03-30 15:05:35.919 1032-1032/? D/CrashAnrDetector: broadcastEvent : com.example.imran.myapp data_app_crash
03-30 15:05:36.159 1032-1059/? W/ActivityManager: Activity pause timeout for ActivityRecord{3f2af340 u0 com.example.imran.myapp/.Home t3125 f}
03-30 15:05:37.019 1032-1521/? I/WindowState: WIN DEATH: Window{224e112b u0 com.example.imran.myapp/com.example.imran.myapp.Home}
03-30 15:05:37.079 1032-1528/? I/ActivityManager: Process com.example.imran.myapp (pid 3591)(adj 9) has died(67,606)
03-30 15:05:37.359 10199-10199/? D/AASAservice-UpdateReceiver: AASAUpdateReceiver: android.intent.action.PACKAGE_REPLACED, package = com.example.imran.myapp, uid = -1
03-30 15:05:37.379 2433-3619/? D/PkgBroadcastIntentOp: Received broadcast action=android.intent.action.PACKAGE_REPLACED and uri=com.example.imran.myapp
03-30 15:05:37.559 2433-3974/? D/k: Processing package: com.example.imran.myapp
03-30 15:05:37.559 2227-3976/? D/Compatibility: intentservice saw: Intent { act=android.intent.action.PACKAGE_REPLACED dat=package:com.example.imran.myapp flg=0x4000010 cmp=com.sec.android.app.soundalive/.compatibility.Compatibility$Receiver (has extras) } Bundle[{android.intent.extra.UID=10252, android.intent.extra.REPLACING=true, android.intent.extra.user_handle=0}]
03-30 15:05:37.589 2433-3974/? D/GassUtils: Found app info for package com.example.imran.myapp:1. Hash: 6b9333e031907d7a6a6c12cd9fdfa0d23bd13ee0f40c9617ddd005dc358321b0
03-30 15:05:37.589 2433-3974/? D/k: Found info for package com.example.imran.myapp in db.
这是我使用该课程的活动：
gridgallery.java
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_gridgallery, null);
Myserver myserver = new Myserver();
//postStringRequest(myserver.url + "/api/albums/getalbums.php", view);
AlbumsList albumsList = new AlbumsList(myserver.url+"/api/albums/getalbums.php",getContext(),view);
albumsList.connection(); // Do connection
Toast.makeText(getContext(),albumsList.imgsurls[1],Toast.LENGTH_LONG).show();
/*
GridView gv = (GridView)view.findViewById(R.id.fragment_gridgallery);
ImageAdapter myadapter = new ImageAdapter(getContext(),albumsList.imgurls());
gv.setAdapter(myadapter);
*/
return view;
}
AlbumsList.java
public class AlbumsList {
private String server;
Context mycontext;
public String[] imgsurls;
private View view;
AlbumsList(String serverurl, Context c,View uview){
mycontext = c;
server = serverurl;
view = uview;
}
public View connection(){
// Instantiate the RequestQueue.
RequestQueue queue = Volley.newRequestQueue(mycontext);
// Request a string response from the provided URL.
StringRequest stringRequest = new StringRequest(Request.Method.POST, server,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
//GridView gv = (GridView)view.findViewById(R.id.fragment_gridgallery);
final ArrayList<String> urls2 = new ArrayList<String>();
try {
JSONArray jsonObj = new JSONArray(response);
Myserver myserver = new Myserver();
for (int i=0;i<jsonObj.length();i++){
JSONObject c = jsonObj.getJSONObject(i);
String imgurl = myserver.url+"/images/thumbs/tn_"+c.getString("album_thumbnail");
urls2.add(imgurl);
}
final String myabc[] = urls2.toArray(new String[urls2.size()]); //Do not remove this
imgsurls = myabc;
Toast.makeText(mycontext, imgsurls[0], Toast.LENGTH_LONG).show(); //Interestingly this toast works and imgsurls[] is ok
} catch (JSONException e) {
e.printStackTrace();
Toast.makeText(mycontext,e.getLocalizedMessage(),Toast.LENGTH_LONG).show();
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(mycontext, "Unable to reach server", Toast.LENGTH_LONG).show();
}
}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
params.put("getalbum", "getalbum");
return params;
}
};
queue.add(stringRequest);
return view;
}
public String[] imgurls(){
return imgsurls;
}
}
在AlbumsList.java中，我想通过函数或变量返回URLS列表，但这两种方法都不起作用。有趣的是，url是从数据库中获取的。而AlbumsList.java Toast.makeText(mycontext, imgsurls[0], Toast.LENGTH_LONG).show();内的Toast也有效。然后我不知道为什么imgsurls没有返回到main函数。提前致谢。
答案 0 :(得分：1)
因为Volley请求异步。
因此，您何时调用albumsList.connection();，请求在后台运行，在完成之前，控制权将转到以下行：
Toast.makeText(getContext(),albumsList.imgsurls[1],Toast.LENGTH_LONG).show();
到那时，您的请求尚未完成。
答案 1 :(得分：0)
Volley以异步方式工作。这意味着它不会立即返回操作的结果。您试图在从服务器获取结果之前访问结果。
当实际传递结果时，您应该将视图初始化逻辑移动到onResponse()。