Android应用程序在接收intent捆绑包时崩溃
Android app crashing on receiving intent bundle?
我会说我是一个完全的新手，这是我的第一个项目，我试着到处搜索来回答我的问题。
我在安卓系统上设置了登录屏幕。一旦用户点击登录按钮并且登录名/密码正确，（在PHP/SQL中验证）它就会向下一个活动发送意向/捆绑包。
以下是发送意向/捆绑包的代码：
Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
Intent intent2 = new Intent(this, Homepage.class);
intent2.putExtra("username2", username2);
startActivity(intent);
以下是接收下一个活动的意向捆绑包的代码：
Intent intent2 = getIntent();
String usernamefromlogin = intent2.getExtras().getString("username2");
String url5 = url.concat(usernamefromlogin);
TextView text = (TextView) findViewById(R.id.errorchecking);
text.setText(url5);
我使用concat（）将URL放在intent bundle之前，因为该URL将用作PHP文件的_GET命令（但我跑题了）。目前，我已经将其设置为将文本放入TextView中，这样我就可以看到最终结果，但在我到达这一点之前，应用程序就崩溃了。
当我点击登录按钮时，我的应用程序崩溃。这是我得到的Logcat：
11-30 20:06:39.449: E/AndroidRuntime(4578): FATAL EXCEPTION: main
11-30 20:06:39.449: E/AndroidRuntime(4578): Process: com.sencide, PID: 4578
11-30 20:06:39.449: E/AndroidRuntime(4578): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sencide/com.sencide.Homepage}: java.lang.NullPointerException
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread.access$700(ActivityThread.java:135)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.os.Handler.dispatchMessage(Handler.java:102)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.os.Looper.loop(Looper.java:137)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread.main(ActivityThread.java:4998)
11-30 20:06:39.449: E/AndroidRuntime(4578): at java.lang.reflect.Method.invokeNative(Native Method)
11-30 20:06:39.449: E/AndroidRuntime(4578): at java.lang.reflect.Method.invoke(Method.java:515)
11-30 20:06:39.449: E/AndroidRuntime(4578): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
11-30 20:06:39.449: E/AndroidRuntime(4578): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
11-30 20:06:39.449: E/AndroidRuntime(4578): at dalvik.system.NativeStart.main(Native Method)
11-30 20:06:39.449: E/AndroidRuntime(4578): Caused by: java.lang.NullPointerException
11-30 20:06:39.449: E/AndroidRuntime(4578): at com.sencide.Homepage.onCreate(Homepage.java:46)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.Activity.performCreate(Activity.java:5243)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-30 20:06:39.449: E/AndroidRuntime(4578): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
11-30 20:06:39.449: E/AndroidRuntime(4578): ... 11 more
11-30 20:06:42.569: I/Process(4578): Sending signal. PID: 4578 SIG: 9
这是我的全部代码：
AndroidLogin.java
public class AndroidLogin extends Activity implements OnClickListener {
Button ok,back,exit;
TextView result;
String thisisausername;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
// Login button clicked
ok = (Button)findViewById(R.id.btn_login);
ok.setOnClickListener(this);
result = (TextView)findViewById(R.id.tbl_result);
}
public void postLoginData() {
// Create a new HttpClient and Post Header
HttpClient httpclient = new DefaultHttpClient();
// login.php returns true if username and password match in db
HttpPost httppost = new HttpPost("http://10.0.2.2/login.php");
try {
// Add user name and password
EditText uname = (EditText)findViewById(R.id.txt_username);
String username = uname.getText().toString();
EditText pword = (EditText)findViewById(R.id.txt_password);
String password = pword.getText().toString();
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("username", username));
nameValuePairs.add(new BasicNameValuePair("password", password));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
// Execute HTTP Post Request
Log.w("SENCIDE", "Execute HTTP Post Request");
HttpResponse response = httpclient.execute(httppost);
String str = inputStreamToString(response.getEntity().getContent()).toString();
Log.w("SENCIDE", str);
if(str.toString().equalsIgnoreCase("true"))
{
Log.w("SENCIDE", "TRUE");
result.setText("Login Successful! Please Wait...");
}else
{
Log.w("SENCIDE", "FALSE");
result.setText(str);
}
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
private StringBuilder inputStreamToString(InputStream is) {
String line = "";
StringBuilder total = new StringBuilder();
// Wrap a BufferedReader around the InputStream
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
// Read response until the end
try {
while ((line = rd.readLine()) != null) {
total.append(line);
}
} catch (IOException e) {
e.printStackTrace();
}
// Return full string
return total;
}
public void RegisterButton(View view) {
String url = "http://10.0.2.2/register.php";
Intent i = new Intent(Intent.ACTION_VIEW);
i.setData(Uri.parse(url));
startActivity(i);
}
@Override
public void onClick(View view) {
postLoginData();
// turns the text in the textview "Tbl_result" into a text string called "tblresult"
TextView tblresult = (TextView) findViewById(R.id.tbl_result);
// If "tblresult" text string matches the string "Login Successful! Please Wait..." exactly, it will switch to next activity
if (tblresult.getText().toString().equals("Login Successful! Please Wait...")) {
Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);
}
}
}
主页.Java
public class Homepage extends Activity {
//URL to get JSON Arrays
public static String url = "http://x.x.x.x/SQL.php?username=123";
//JSON Node Names
private static final String TAG_USER = "users";
private static final String TAG_ID = "uid";
private static final String TAG_NAME = "fullname";
private static final String TAG_DISPLAY = "displayname";
private static final String TAG_EMAIL = "email";
private static final String TAG_PW = "password";
private static final String TAG_CREATED = "created_at";
private static final String TAG_UPDATED = "updated_at";
JSONArray user = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//Get login name from EditText in login screen and concatenate it to PHP user-name for _GET
Intent intent2 = getIntent();
String usernamefromlogin = intent2.getExtras().getString("username2");
String url5 = url.concat(usernamefromlogin);
TextView text = (TextView) findViewById(R.id.errorchecking);
text.setText(url5);
setContentView(R.layout.reshomepage);
// Creating new JSON Parser
JSONParser jParser = new JSONParser();
// Getting JSON from URL
JSONObject json = jParser.getJSONFromUrl(url);
try {
// Getting JSON Array
user = json.getJSONArray(TAG_USER);
JSONObject c = user.getJSONObject(0);
// Storing JSON item in a Variable
String uid = c.getString(TAG_ID);
String name = c.getString(TAG_NAME);
String display = c.getString(TAG_DISPLAY);
String email = c.getString(TAG_EMAIL);
String pw = c.getString(TAG_PW);
String created = c.getString(TAG_CREATED);
String updated = c.getString(TAG_UPDATED);
//Importing TextView
final TextView uid1 = (TextView)findViewById(R.id.tvuid);
final TextView name1 = (TextView)findViewById(R.id.tvfullname);
final TextView display1 = (TextView)findViewById(R.id.tvdisplayname);
final TextView email1 = (TextView)findViewById(R.id.tvemail);
final TextView pw1 = (TextView)findViewById(R.id.tvpassword);
final TextView created1 = (TextView)findViewById(R.id.tvcreated_at);
final TextView updated1 = (TextView)findViewById(R.id.tvupdated_at);
//Set JSON Data in TextView
uid1.setText(uid);
name1.setText(name);
display1.setText(display);
email1.setText(email);
pw1.setText(pw);
created1.setText(created);
updated1.setText(updated);
} catch (JSONException e) {
e.printStackTrace();
}
}
}
JSONParser.java
public class JSONParser {
static InputStream is = null;
static JSONObject jObj = null;
static String json = "";
// constructor
public JSONParser() {
}
public JSONObject getJSONFromUrl(String url) {
// Making HTTP request
try {
// defaultHttpClient
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpPost httpPost = new HttpPost(url);
HttpResponse httpResponse = httpClient.execute(httpPost);
HttpEntity httpEntity = httpResponse.getEntity();
is = httpEntity.getContent();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(
is, "iso-8859-1"), 8);
StringBuilder sb = new StringBuilder();
String line = null;
while ((line = reader.readLine()) != null) {
sb.append(line + "'n");
}
is.close();
json = sb.toString();
} catch (Exception e) {
Log.e("Buffer Error", "Error converting result " + e.toString());
}
// try parse the string to a JSON object
try {
jObj = new JSONObject(json);
} catch (JSONException e) {
Log.e("JSON Parser", "Error parsing data " + e.toString());
}
// return JSON String
return jObj;
}
}
Main.XML（第一个登录屏幕）
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<!-- Text at Top !-->
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/lbl_top"
android:textSize="16sp"
android:typeface="sans"
android:text="Please Login"
/>
<!-- Username Text Above Field !-->
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/lbl_username"
android:text="User Name (e-mail)"
/>
<!-- Username Field !-->
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textSize="18sp"
android:id="@+id/txt_username"
/>
<!-- Password Text Above Field !-->
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/lbl_password"
android:text="Password"
/>
<!-- Password Field !-->
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:password="true"
android:id="@+id/txt_password"
/>
<!-- Login Button !-->
<Button
android:layout_width="match_parent"
android:layout_height="35dp"
android:id="@+id/btn_login"
android:text="Login"
android:onClick="LoginScreenButton"
/>
<!-- Register Button !-->
<Button
android:layout_width="match_parent"
android:layout_height="35dp"
android:id="@+id/register"
android:text="New Registration"
android:onClick="RegisterButton"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/tbl_result"
android:textColor="#00FF00"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/tbl_result2"
android:textColor="#00FF00"
/>
</LinearLayout>
reshomepage.XML（我试图将意向发送到的第二个屏幕）
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
>
<TextView
android:id="@+id/tvuid"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvfullname"
android:layout_width="match_parent"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvdisplayname"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvemail"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvpassword"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvcreated_at"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/tvupdated_at"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/errorchecking"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
</LinearLayout>
这里的问题是创建了两个intent（intent和intent2），然后将用户名保存在intent2中，但正在调用startActivity（intent）。
因此，基本上，删除其中一个意图，您的代码应该看起来像这样：
Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);
让我知道的进展情况
这里的问题是您已经将用户名2额外附加到intent2，但您没有使用intent2。
您开始您的活动，然后在传入的意向中查找额外的"用户名2"，因为您发送的意向不包含额外的"用户名称2"，您从login获得的用户名被分配了一个null值，并导致您在堆栈跟踪中看到的null指针异常。
只需去掉第二个意图，并将字符串额外放在启动活动时使用的字符串中：
Intent intent = new Intent(this, Homepage.class);
EditText uname2 = (EditText)findViewById(R.id.txt_username);
String username2 = uname2.getText().toString();
intent.putExtra("username2", username2);
startActivity(intent);
为了安全起见，在你逃跑并使用它之前，也许要确保意图有额外的内容
Intent intent = getIntent();
if (intent.hasExtra("username2") {
String usernamefromlogin = intent.getExtras().getString("username2");
String url5 = url.concat(usernamefromlogin);
TextView text = (TextView) findViewById(R.id.errorchecking);
text.setText(url5);
} else {
// ...appropriate error handling goes here!
}
祝你好运！
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