为什么在Android中执行此HttpGet会引发NullPointerException？

Question

我创建了一个新的Android项目中，我已经添加了唯一的代码是这样的代码在onCreate()方法（我还添加了INTERNET许可清单）：

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_http_get_test); 
    HttpClient client = new DefaultHttpClient(); 
    HttpGet request = new HttpGet(); 
    try 
    { 
     System.out.println("making uri"); 
     URI uri = new URI("http://w3mentor.com/"); 
     request.setURI(uri); 
     System.out.println("executing"); 
     client.execute(request); // This line throws the exception 
     System.out.println("Done!"); 
    } 
    catch(Exception e) 
    { 
     System.out.println(e.getMessage()); 
    } 
} 

在日志中，我请注意0​​块打印的最后一件事是executing，之后抛出了NullPointerException。

我松散地基于我的代码在this example的开头，但我不明白我在做什么不同，会导致NullPointerException。

当在调试器中运行时，我看到异常实际上是一个NetworkOnMainThreadException，当我尝试e.getMessage()时，显然会导致NullPointerException异常。

的错误信息被打印出来是

07-12 02:58:33.997: E/AndroidRuntime(983): FATAL EXCEPTION: main 
07-12 02:58:33.997: E/AndroidRuntime(983): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.whatever.http.test/com.whatever.http.test.HttpGetTest}: java.lang.NullPointerException 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread.access$600(ActivityThread.java:123) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.os.Looper.loop(Looper.java:137) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread.main(ActivityThread.java:4424) 
07-12 02:58:33.997: E/AndroidRuntime(983): at java.lang.reflect.Method.invokeNative(Native Method) 
07-12 02:58:33.997: E/AndroidRuntime(983): at java.lang.reflect.Method.invoke(Method.java:511) 
07-12 02:58:33.997: E/AndroidRuntime(983): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
07-12 02:58:33.997: E/AndroidRuntime(983): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
07-12 02:58:33.997: E/AndroidRuntime(983): at dalvik.system.NativeStart.main(Native Method) 
07-12 02:58:33.997: E/AndroidRuntime(983): Caused by: java.lang.NullPointerException 
07-12 02:58:33.997: E/AndroidRuntime(983): at com.android.internal.os.LoggingPrintStream.println(LoggingPrintStream.java:298) 
07-12 02:58:33.997: E/AndroidRuntime(983): at com.whatever.http.test.HttpGetTest.onCreate(HttpGetTest.java:33) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.Activity.performCreate(Activity.java:4465) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 
07-12 02:58:33.997: E/AndroidRuntime(983): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920) 
07-12 02:58:33.997: E/AndroidRuntime(983): ... 11 more 

Answer 1

问题不是一个NullPointerException异常，但如果您调试应用程序，你会发现，例外情况是：NetworkOnMainThreadException。

您很可能在较新的SDK（Honeycomb或更高版本）上运行您的应用程序，因此不鼓励这样做。

任何网络代码，你应该把一个线程。理想情况下，使用AsyncTask即可。检查此链接：http://www.techblogistech.com/2011/11/how-to-fix-the-android-networkonmainthreadexception/

希望这可以帮助。