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，其中我添加的唯一代码是 onCreate() 方法中的这段代码（我还在清单中添加了 INTERNET 权限）：

@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，我看到try 块打印的最后一件事是executing，之后抛出了 NullPointerException。

我的代码松散地基于this example 的开头，但我看不出我在做什么会导致 NullPointerException。

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

打印出来的错误信息是

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 NullPointerException，但如果您调试应用程序，您会发现异常是：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，都应该放在一个线程中。理想情况下使用AsyncTask。查看此链接：http://www.techblogistech.com/2011/11/how-to-fix-the-android-networkonmainthreadexception/

希望这会有所帮助。