问题如下。我有一个登录活动（在 Android Studio 中），几天前运行良好。我不记得更改过任何内容，但是当我上次运行此应用程序时，应用程序在我单击登录按钮后立即关闭。最后一件事是关于 AsyncTask 预执行的祝酒。
我不明白为什么会出现 NullPointerException。
我的注册活动的代码几乎相同，并且工作正常。
这是日志：
05-28 16:04:52.395 1218-1232/system_process V/WindowManager﹕ addAppToken: AppWindowToken{5d89eb token=Token{23ccc93a ActivityRecord{2fe54865 u0 utanashati.reminder/.HomepageActivity t17}}} to stack=1 task=17 at 1
05-28 16:04:52.407 19927-19927/utanashati.reminder D/AndroidRuntime﹕ Shutting down VM
05-28 16:04:52.408 19927-19927/utanashati.reminder E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: utanashati.reminder, PID: 19927
java.lang.RuntimeException: Unable to start activity
ComponentInfo{utanashati.reminder/utanashati.reminder.HomepageActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at utanashati.reminder.HomepageActivity.onCreate(HomepageActivity.java:55)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5257)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
05-28 16:04:52.410 1218-1232/system_process W/ActivityManager﹕ Force finishing activity 1 utanashati.reminder/.HomepageActivity
05-28 16:04:52.411 1218-1232/system_process W/ActivityManager﹕ Force finishing activity 2 utanashati.reminder/.LoginActivity
EDIT 1
我睁大了眼睛，问题不在于LoginActivity，而在于HomepageActivity。这是代码：
import ...
public class HomepageActivity extends Activity implements AdapterView.OnItemSelectedListener {
protected EditText mAddTaskText;
protected Spinner mPrioritySpinner;
protected Button mAddTaskButton;
protected int intPriority = 0;
protected String taskText;
protected Timestamp taskTimestamp;
protected Task userTask;
protected JsonGenerator taskJSON;
@Override
protected void onCreate(Bundle savedInstanceState) { // Starts activity. The state can be restored from savedInstanceState
super.onCreate(savedInstanceState); // Calls the superclass method (IMPORTANT)
setContentView(R.layout.activity_homepage); // Sets layout from activity_homepage.xml
mPrioritySpinner = (Spinner) findViewById(R.id.prioritySpinner); // Creates an ArrayAdapter using the string array and a default spinner layout
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this,
R.array.priorityList, android.R.layout.simple_spinner_item); // Specifies the layout to use when the list of choices appears
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); // Applies the adapter to the spinner
mPrioritySpinner.setAdapter(adapter);
mPrioritySpinner.setOnItemSelectedListener(this);
mAddTaskText = (EditText) findViewById(R.id.addTaskEditText); // Finds View by its id in .xml file
mAddTaskButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(HomepageActivity.this, "Done!", Toast.LENGTH_LONG).show();
Calendar taskCalendar = Calendar.getInstance(); // Creates new calendar
long taskTime = taskCalendar.getTimeInMillis(); // Gets time in milliseconds
taskTimestamp = new Timestamp(taskTime); // Creates new Timestamp
taskText = mAddTaskText.getText().toString(); // Gets description of the task
userTask.setDate(taskTimestamp); // Sets date
userTask.setText(taskText); // Sets text
/* Creating JsonGenerator */
ObjectMapper mapper = new ObjectMapper();
try {
mapper.writeValue(taskJSON, userTask);
}
catch (IOException e) {
Toast.makeText(HomepageActivity.this, "Could not create JSON", Toast.LENGTH_LONG).show();
}
/* Getting out email and password */
String userPassword = ((EmailPassword) HomepageActivity.this.getApplication()).getPassword();
String userEmail = ((EmailPassword) HomepageActivity.this.getApplication()).getUserEmail();
Toast.makeText(HomepageActivity.this, userEmail + " " + userPassword, Toast.LENGTH_LONG).show();
/* HTTP stuff */
HttpPoster get = new HttpPoster();
get.execute(userEmail, userPassword, taskJSON.toString());
}
});
}
public int getData (String username, String password, String taskJSON) {
try {
HttpPost httpPost = new HttpPost("http://something.com/" + username + "/tasks");
String dataToEncode = username + ":" + password;
String encodedData = Base64.encodeToString(dataToEncode.getBytes(), Base64.NO_WRAP);
httpPost.setHeader("Authorization", encodedData);
try {
StringEntity taskEntity = new StringEntity(taskJSON, "UTF-8");
httpPost.setEntity(taskEntity);
}
catch (UnsupportedEncodingException e) {
Toast.makeText(HomepageActivity.this, "Unsupported encoding", Toast.LENGTH_LONG).show();
}
HttpClient client = new DefaultHttpClient();
HttpResponse response = client.execute(httpPost);
StatusLine statusLine = response.getStatusLine();
int statusCode = statusLine.getStatusCode();
if (statusCode == 200) {
return 1;
}
else if (statusCode == 404) { return 2; }
else if (statusCode == 500) { return 3; }
else if (statusCode == 409) { return 4; }
else { return statusCode; }
}
catch (IOException e) {
e.printStackTrace();
}
return 0;
}
public void onItemSelected(AdapterView<?> parent, View view,
int pos, long id) {
String priority = parent.getItemAtPosition(pos).toString(); // Gets chosen priority
Toast.makeText(HomepageActivity.this, priority, Toast.LENGTH_LONG).show();
while (!((priority.equals("Low")) || (priority.equals("Medium")) || (priority.equals("High")))) {
Toast.makeText(HomepageActivity.this, "Something bad happened. Try to choose again", Toast.LENGTH_LONG).show();
}
if (priority.equals("Low")) {
intPriority = 0;
}
else if (priority.equals("Medium")) {
intPriority = 1;
}
else if (priority.equals("High")) {
intPriority = 2;
}
userTask.setPriority(intPriority); // Sets chosen priority
}
public void onNothingSelected(AdapterView<?> parent) {
userTask.setPriority(intPriority); // Sets default priority ("0")
}
public class HttpPoster extends AsyncTask<String, Void, Integer> {
@Override
protected void onPreExecute() {
super.onPreExecute();
}
@Override
protected Integer doInBackground(String... params) {
return getData(params[0], params[1], params[3]);
}
@Override
protected void onPostExecute(Integer result) {
super.onPostExecute(result);
if (result == 1) {
Toast.makeText(HomepageActivity.this, "Login successful", Toast.LENGTH_LONG).show();
Intent takeUserHome = new Intent(HomepageActivity.this, HomepageActivity.class);
startActivity(takeUserHome);
}
else if (result == 2) {
Toast.makeText(HomepageActivity.this, "No such user", Toast.LENGTH_LONG).show();
}
else if (result == 3) {
Toast.makeText(HomepageActivity.this, "Internal server error: unable to send email", Toast.LENGTH_LONG).show();
}
else if (result == 4) {
Toast.makeText(HomepageActivity.this, "Task already exists", Toast.LENGTH_LONG).show();
}
else {
Toast.makeText(HomepageActivity.this, result.toString(), Toast.LENGTH_LONG).show();
}
}
}
}
和 XML 文件：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="utanashati.testapp.HomepageActivity">
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Add a new task..."
android:id="@+id/addTaskEditText"
android:nestedScrollingEnabled="false"
android:minLines="1"
android:maxLines="1" />
<Spinner
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/prioritySpinner"
android:layout_alignRight="@+id/addTaskButton"
android:layout_alignEnd="@+id/addTaskButton"
android:layout_below="@+id/addTaskEditText" />
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Add task"
android:id="@+id/addTaskButton"
android:layout_below="@+id/prioritySpinner"
android:layout_centerHorizontal="true" />
</RelativeLayout>
