android - getJSONfromURL - 空指针异常

Question

我做了一个 Activity “消息”，它应该从给定的 URL 获取 JSON 数据。我尝试制作一个循环来打印 json 数据，但问题出在其他地方。我在 JSONArray，“json”上收到 NullPointerException。

消息类:

public class Messages extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    TextView tv = (TextView) findViewById(R.id.tv_messages);
    JSONArray json = JSONfunctions.getJSONfromURL("http://docs.blackberry.com/sampledata.json");

    tv.setText(json.length());
}

JSON 函数类:

public class JSONfunctions {

    public static JSONArray getJSONfromURL(String url) {
        InputStream is = null;
        String result = "";
        JSONArray jArray = null;

        // http post
        try {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(url);
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();

        } catch (Exception e) {
            Log.e("log_tag", "Error in http connection " + e.toString());
        }

        // convert response to string
        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();
            result = sb.toString();
        } catch (Exception e) {
            Log.e("log_tag get data string ",
                    "Error converting result " + e.toString());
        }

        try {

            jArray = new JSONArray(result);
        } catch (JSONException e) {
            Log.e("log_tag create object ",
                    "Error parsing data " + e.toString());
        }

        return jArray;
    }
}

错误:

08:11:43.683: Error in http connection android.os.NetworkOnMainThreadException 08:11:43.693: Error converting result java.lang.NullPointerException: lock == null 08:11:43.703: Error parsing data org.json.JSONException: End of input at character 0 of 08:11:43.723: Shutting down VM 08:11:43.733: threadid=1: thread exiting with uncaught exception (group=0x40a70930) 08:11:43.923: ATAL EXCEPTION: main 08:11:43.923: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sound/com.example.sound.Messages}: java.lang.NullPointerException 08:11:43.923: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 08:11:43.923: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 08:11:43.923: at android.app.ActivityThread.access$600(ActivityThread.java:141) 08:11:43.923: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 08:11:43.923: at android.os.Handler.dispatchMessage(Handler.java:99) 08:11:43.923: at android.os.Looper.loop(Looper.java:137) 08:11:43.923: at android.app.ActivityThread.main(ActivityThread.java:5039) 08:11:43.923: at java.lang.reflect.Method.invokeNative(Native Method) 08:11:43.923: at java.lang.reflect.Method.invoke(Method.java:511) 08:11:43.923: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 08:11:43.923: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 08:11:43.923: at dalvik.system.NativeStart.main(Native Method) 08:11:43.923: Caused by: java.lang.NullPointerException 08:11:43.923: at com.example.sound.Messages.onCreate(Messages.java:18) 08:11:43.923: at android.app.Activity.performCreate(Activity.java:5104) 08:11:43.923: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 08:11:43.923: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

第18行是:

tv.setText(json.length());

在出现上述错误之前，我还得到了很多:

Unexpected value from nativeGetEnabledTags: 0

我正在使用的 URL(用于测试目的，http://docs.blackberry.com/sampledata.json)已启动并正常工作。我是 Android 开发和 JSON 的新手。提前致谢。

Answer 1

这是因为要么你正在主线程上执行网络操作，而 Android 版本 >= 3.0 不允许。

解决这个问题要么使用

 StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
 StrictMode.setThreadPolicy(policy); 

或

Use AsyncTask 

阅读自http://developer.android.com/reference/android/os/AsyncTask.html