我正在尝试连接到我的XAMPP服务器，并使用以下类与MySQL数据库进行交互。但是，错误指出我在以下行收到NullPointerException：
result = imService.createNewGroup(newGroupName);
在CreateGroup类中。应该注意的是，在用户将文本输入到对话框中并且从那里开始服务之后，也会立即调用CreateGroup类。我对服务和网络连接还比较陌生，但是我有什么遗漏应该允许在尝试发送.createGroup命令之前至少验证服务是否已连接？
CreateGroup Class：
public class CreateGroup extends Activity {
private static final String SERVER_RES_RES_SIGN_UP_SUCCESFULL = "1";
private static final String SERVER_RES_SIGN_UP_USERNAME_CRASHED = "2";
private Manager imService;
private Handler handler = new Handler();
String newGroupName;
public ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
imService = ((MessagingService.IMBinder) service).getService();
}
public void onServiceDisconnected(ComponentName className) {
imService = null;
Toast.makeText(CreateGroup.this, R.string.local_service_stopped,
Toast.LENGTH_SHORT).show();
}
};
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
bindService(new Intent(CreateGroup.this, MessagingService.class),
mConnection, Context.BIND_AUTO_CREATE);
// Getting intent and info from the dialog
Intent i = getIntent();
Bundle extras = i.getExtras();
newGroupName = extras.getString("groupName");
Thread thread = new Thread() {
String result = new String();
@Override
public void run() {
// Send group name to the messaging
// service
try {
result = imService.createNewGroup(newGroupName);
} catch (NullPointerException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Log.d("problem", "The value of result is " + result.toString());
handler.post(new Runnable() {
@Override
public void run() {
if (result == null) {
Toast.makeText(getApplicationContext(),
"It's null, not working", Toast.LENGTH_LONG)
.show();
}
if (result != null
&& result
.equals(SERVER_RES_RES_SIGN_UP_SUCCESFULL)) {
Toast.makeText(getApplicationContext(),
R.string.signup_successfull,
Toast.LENGTH_LONG).show();
// showDialog(SIGN_UP_SUCCESSFULL);
} else if (result != null
&& result
.equals(SERVER_RES_SIGN_UP_USERNAME_CRASHED)) {
Toast.makeText(getApplicationContext(),
R.string.signup_username_crashed,
Toast.LENGTH_LONG).show();
// showDialog(SIGN_UP_USERNAME_CRASHED);
} else // if
// (result.equals(SERVER_RES_SIGN_UP_FAILED))
{
Toast.makeText(getApplicationContext(),
R.string.signup_failed, Toast.LENGTH_LONG)
.show();
// showDialog(SIGN_UP_FAILED);
}
}
});
}
};
thread.start();
}
“createGroup”方法的服务器案例：
case "createGroup":
$SQLtest = "insert into groups(groupName, uniqueGroup, createTime)
VALUES('TestGroup', 1234567891, NOW())";
error_log("$SQLtest", 3 , "error_log");
if($result = $db -> query($SQLtest))
{
$out = SUCCESSFUL;
}
else
{
$out = FAILED;
}
break;
Messaging Service和createGroup方法：
public class MessagingService extends Service implements Manager, Updater {
// private NotificationManager mNM;
public static String USERNAME;
public static final String TAKE_MESSAGE = "Take_Message";
public static final String FRIEND_LIST_UPDATED = "Take Friend List";
public static final String MESSAGE_LIST_UPDATED = "Take Message List";
public ConnectivityManager conManager = null;
private final int UPDATE_TIME_PERIOD = 15000;
private String rawFriendList = new String();
private String rawMessageList = new String();
SocketerInterface socketOperator = new Socketer(this);
private final IBinder mBinder = new IMBinder();
private String username;
private String password;
private boolean authenticatedUser = false;
// timer to take the updated data from server
private Timer timer;
private StorageManipulater localstoragehandler;
private NotificationManager mNM;
public class IMBinder extends Binder {
public Manager getService() {
return MessagingService.this;
}
}
@Override
public void onCreate() {
mNM = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
localstoragehandler = new StorageManipulater(this);
// Display a notification about us starting. We put an icon in the
// status bar.
// showNotification();
conManager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
new StorageManipulater(this);
// Timer is used to take the friendList info every UPDATE_TIME_PERIOD;
timer = new Timer();
Thread thread = new Thread() {
@Override
public void run() {
Random random = new Random();
int tryCount = 0;
while (socketOperator.startListening(10000 + random
.nextInt(20000)) == 0) {
tryCount++;
if (tryCount > 10) {
// if it can't listen a port after trying 10 times, give
// up...
break;
}
}
}
};
thread.start();
}
@Override
public IBinder onBind(Intent intent) {
return mBinder;
}
@Override
public String createNewGroup(String groupName) throws NullPointerException, UnsupportedEncodingException {
String params = "action=createGroup";
String result = socketOperator.sendHttpRequest(params);
return result;
}
}
答案 0 :(得分：11)
因为您的代码具有固有的竞争条件。还是一个邪恶的人。
改为这样：
public void onCreate(Bundle savedInstanceState) {
bindService(new Intent(CreateGroup.this, MessagingService.class),
mConnection, Context.BIND_AUTO_CREATE);
// but do not start thread here!
}
public ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
imService = ((MessagingService.IMBinder) service).getService();
startCommunicationThread(); // <----------------------- only here can you start comm. thread
}
public void onServiceDisconnected(ComponentName className) {
imService = null;
Toast.makeText(CreateGroup.this, R.string.local_service_stopped,
Toast.LENGTH_SHORT).show();
}
};
void startCommunicationThread() {
Thread thread = new Thread() {
String result = new String();
@Override
public void run() {
try {
result = imService.createNewGroup(newGroupName);
..........
}
如果您希望代码更安全，请使用连接状态字段：
public ServiceConnection mConnection = new ServiceConnection() {
volatile boolean isConnected;
public void onServiceConnected(ComponentName className, IBinder service) {
isConnected = true; // <---------------------
imService = ((MessagingService.IMBinder) service).getService();
startCommunicationThread();
}
public void onServiceDisconnected(ComponentName className) {
isConnected = false; // <---------------
imService = null;
Toast.makeText(CreateGroup.this, R.string.local_service_stopped,
Toast.LENGTH_SHORT).show();
}
};
并且从startCommunicationThread中连接poll以确保没有突然断开连接。
答案 1 :(得分：0)
在我的项目中MessagingService.IMBinder MessagingService给出错误是否有我要导入的java类。