Android应用无法启动活动componentinfo
我是一个新的Android程序员，最近，我的很多项目都得到这个错误：
07-31 23:45:19.592: ERROR/AndroidRuntime(716): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.Main}: java.lang.NullPointerException 任何人都可以帮我解决这个问题吗？
以下是整个logcat堆栈跟踪：
07-31 23:45:18.512: INFO/ActivityManager(63): Starting activity: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.amrit.musifind/.Main } 07-31 23:45:18.682: INFO/ActivityManager(63): Start proc com.amrit.musifind for activity com.amrit.musifind/.Main: pid=716 uid=10025 gids={3003} 07-31 23:45:18.962: INFO/jdwp(716): received file descriptor 20 from ADB 07-31 23:45:19.102: DEBUG/ddm-heap(716): Got feature list request 07-31 23:45:19.572: DEBUG/AndroidRuntime(716): Shutting down VM 07-31 23:45:19.572: WARN/dalvikvm(716): threadid=3: thread exiting with uncaught exception (group=0x4001aa28) 07-31 23:45:19.572: ERROR/AndroidRuntime(716): Uncaught handler: thread main exiting due to uncaught exception 07-31 23:45:19.592: ERROR/AndroidRuntime(716): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.Main}: java.lang.NullPointerException 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread.access$2100(ActivityThread.java:116) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.os.Handler.dispatchMessage(Handler.java:99) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.os.Looper.loop(Looper.java:123) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread.main(ActivityThread.java:4203) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at java.lang.reflect.Method.invokeNative(Native Method) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at java.lang.reflect.Method.invoke(Method.java:521) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at dalvik.system.NativeStart.main(Native Method) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): Caused by: java.lang.NullPointerException 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at com.amrit.musifind.Main.onCreate(Main.java:44) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364) 07-31 23:45:19.592: ERROR/AndroidRuntime(716): ... 11 more 07-31 23:45:19.612: INFO/Process(63): Sending signal. PID: 716 SIG: 3 07-31 23:45:19.612: INFO/dalvikvm(716): threadid=7: reacting to signal 3 07-31 23:45:19.642: INFO/dalvikvm(716): Wrote stack trace to '/data/anr/traces.txt' 07-31 23:45:21.422: INFO/Process(716): Sending signal. PID: 716 SIG: 9 07-31 23:45:21.442: INFO/ActivityManager(63): Process com.amrit.musifind (pid 716) has died. 07-31 23:45:21.583: WARN/UsageStats(63): Unexpected resume of com.android.launcher while already resumed in com.amrit.musifind 07-31 23:45:21.712: WARN/InputManagerService(63): Window already focused, ignoring focus gain of: com.android.internal.view.IInputMethodClient$Stub$Proxy@439559d0 07-31 23:45:23.482: DEBUG/AndroidRuntime(725): >>>>>>>>>>>>>> AndroidRuntime START <<<<<<<<<<<<<< 07-31 23:45:23.482: DEBUG/AndroidRuntime(725): CheckJNI is ON 07-31 23:45:23.812: DEBUG/AndroidRuntime(725): --- registering native functions --- 07-31 23:45:23.832: INFO/jdwp(725): received file descriptor 20 from ADB 07-31 23:45:24.193: DEBUG/ddm-heap(725): Got feature list request 07-31 23:45:25.052: DEBUG/AndroidRuntime(725): Shutting down VM 07-31 23:45:25.052: DEBUG/dalvikvm(725): DestroyJavaVM waiting for non-daemon threads to exit 07-31 23:45:25.062: DEBUG/dalvikvm(725): DestroyJavaVM shutting VM down 07-31 23:45:25.062: DEBUG/dalvikvm(725): HeapWorker thread shutting down 07-31 23:45:25.062: DEBUG/dalvikvm(725): HeapWorker thread has shut down 07-31 23:45:25.073: DEBUG/jdwp(725): JDWP shutting down net... 07-31 23:45:25.073: DEBUG/jdwp(725): +++ peer disconnected 07-31 23:45:25.073: INFO/dalvikvm(725): Debugger has detached; object registry had 1 entries 07-31 23:45:25.093: DEBUG/dalvikvm(725): VM cleaning up 07-31 23:45:25.113: DEBUG/dalvikvm(725): LinearAlloc 0x0 used 665652 of 4194304 (15%) 07-31 23:47:22.712: DEBUG/dalvikvm(104): GC freed 9173 objects / 524184 bytes in 149ms
下面是第一个活动：start.java
package com.amrit.musifind; import java.io.BufferedReader; import java.io.InputStreamReader; import org.apache.http.HttpResponse; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; import com.amrit.musifind.R; import android.app.ListActivity; import android.content.Intent; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.AdapterView; import android.widget.AdapterView.OnItemClickListener; import android.widget.ListAdapter; import android.widget.ListView; import android.widget.SimpleAdapter; import android.widget.Toast; import android.app.Activity; import android.os.Bundle; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.EditText; import android.widget.TextView; public abstract class start extends Activity implements OnClickListener { Button buttonGo, buttonReset; EditText EditTextinput; String input = ""; String ur = "http://www.tastekid.com/ask/ws?q="; String l = "&f=musifin2125&k=mjjlnzkyzwuz&format=JSON"; String url = "" ; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); buttonGo = (Button) findViewById(R.id.buttonGo); buttonReset = (Button) findViewById(R.id.buttonReset); EditTextinput = (EditText) findViewById(R.id.EditTextinput); //Button listener buttonReset.setOnClickListener(this); buttonGo.setOnClickListener(this); } public void onClick (View src){ switch(src.getId()){ case R.id.buttonGo: input = EditTextinput.getText().toString(); url = ur + input + l ; Intent Main = new Intent(this, ToClass.class); Main.putExtra("userurl", url); startActivity(Main); break; case R.id.buttonReset: EditTextinput.setText(""); break; } } }
第二个活动：Main.java
package com.amrit.musifind; import java.util.ArrayList; import java.util.HashMap; import org.json.JSONArray; import org.json.JSONException; import org.json.JSONObject; import org.w3c.dom.Document; import org.w3c.dom.Element; import org.w3c.dom.NodeList; import com.amrit.musifind.JSONfunctions; import com.amrit.musifind.R; import android.app.ListActivity; import android.content.Intent; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.AdapterView; import android.widget.AdapterView.OnItemClickListener; import android.widget.ListAdapter; import android.widget.ListView; import android.widget.SimpleAdapter; import android.widget.Toast; import android.app.Activity; import android.os.Bundle; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.EditText; import android.widget.TextView; public class Main extends ListActivity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.listplaceholder); Intent intent = getIntent(); String url = intent.getExtras().getString("userurl"); ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>> (); JSONObject json = JSONfunctions.getJSONfromURL(url); try{ JSONObject earthquakes = json.getJSONObject("Similar"); JSONArray info = earthquakes.getJSONArray("Results"); for (int i = 0; i < info.length(); i++) { HashMap<String, String> map = new HashMap<String, String>(); JSONObject e = info.getJSONObject(i); map.put("id", String.valueOf(i)); map.put("name", "Name:" + e.getString("Name")); map.put("type", "Type: " + e.getString("Type")); mylist.add(map); } JSONArray results = json.getJSONArray("Results"); for(int i=0;i<results.length();i++){ HashMap<String, String> map = new HashMap<String, String>(); JSONObject e = results.getJSONObject(i); map.put("id", String.valueOf(i)); map.put("name", "Name:" + e.getString("name")); map.put("type", "Type: " + e.getString("type")); mylist.add(map); } }catch(JSONException e) { Log.e("log_tag", "Error parsing data "+e.toString()); } ListAdapter adapter = new SimpleAdapter(this, mylist , R.layout.main, new String[] { "name", "type" }, new int[] { R.id.item_title, R.id.item_subtitle }); setListAdapter(adapter); final ListView lv = getListView(); lv.setTextFilterEnabled(true); lv.setOnItemClickListener(new OnItemClickListener() { public void onItemClick(AdapterView<?> parent, View view, int position, long id) { @SuppressWarnings("unchecked") HashMap<String, String> o = (HashMap<String, String>) lv.getItemAtPosition(position); Toast.makeText(Main.this, "ID '" + o.get("id") + "' was clicked.", Toast.LENGTH_SHORT).show(); } }); } }
最后，我的清单文件：AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.amrit.musifind" android:versionCode="1" android:versionName="1.0"> <uses-sdk android:minSdkVersion="4" /> <application android:icon="@drawable/icon" android:label="@string/app_name"> <activity android:name=".start" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.start" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <activity android:name= ".Main" > <intent-filter> <action android:name="android.intent.action.Main" /> </intent-filter> </activity> </application> <uses-permission android:name="android.permission.INTERNET" /> </manifest>
任何帮助非常感谢，没有解决这个问题，我不能testing我的应用程序了！
您的空指针exception似乎是在这一行：
String url = intent.getExtras().getString("userurl");
因为intent.getExtras()在intent没有任何额外的时候返回null。
你必须认识到这段代码：
Intent Main = new Intent(this, ToClass.class); Main.putExtra("userurl", url); startActivity(Main);
不会启动您在Main.java中编写的活动，它将尝试启动一个名为ToClass的活动，如果该活动不存在，您的应用程序将崩溃。
此外，没有"android.intent.action.start"这样的东西，所以清单应该看起来更像是：
<activity android:name=".start" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <activity android:name= ".Main"> </activity>
我希望这可以解决您遇到的一些问题，但我强烈build议您查看一些“开始”的Android开发教程，并从那里build立。
