getJSONfromURL - 空指针异常 [英] getJSONfromURL - Null Pointer Exception

问题描述

我做了一个活动消息"，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"上得到 NullPointerException.

I made an activity "Messages" which is supposed to get JSON data from a given URL. I tried making a loop to print the json data, but the problem was somewhere else. I am getting NullPointerException on the JSONArray, "json".

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());
}

JSONfunctions 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:http 连接错误android.os.NetworkOnMainThreadException 08:11:43.693: 转换错误结果 java.lang.NullPointerException: lock == null 08:11:43.703:解析数据时出错 org.json.JSONException: End of input at character 008:11:43.723: 关闭虚拟机 08:11:43.733: threadid=1: thread退出时出现未捕获的异常 (group=0x40a70930) 08:11:43.923: ATAL例外:主要 08:11:43.923:java.lang.RuntimeException:无法开始活动组件信息{com.example.sound/com.example.sound.Messages}:java.lang.NullPointerException 08:11:43.923: 在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)08:11:43.923:在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)08:11:43.923:在android.app.ActivityThread.access$600(ActivityThread.java:141)08:11:43.923:在android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)08:11:43.923: 在 android.os.Handler.dispatchMessage(Handler.java:99)08:11:43.923:在 android.os.Looper.loop(Looper.java:137) 08:11:43.923:在 android.app.ActivityThread.main(ActivityThread.java:5039)08:11:43.923: 在 java.lang.reflect.Method.invokeNative(Native Method)08:11:43.923: 在 java.lang.reflect.Method.invoke(Method.java:511)08:11:43.923:在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)08:11:43.923:在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)08:11:43.923: 在 dalvik.system.NativeStart.main(Native Method)08:11:43.923:引起:java.lang.NullPointerException 08:11:43.923:在 com.example.sound.Messages.onCreate(Messages.java:18) 08:11:43.923:在 android.app.Activity.performCreate(Activity.java:5104)08:11:43.923:在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)08:11:43.923:在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

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，我也得到了很多:

Prior to the above errors, I also get many:

Unexpected value from nativeGetEnabledTags: 0

我使用的 URL(出于测试目的，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.

The URL I'm using (for testing purposes, http://docs.blackberry.com/sampledata.json) is up and working. I'm new to Android Developing and JSON. Thanks in advance.

推荐答案

这是因为您正在主线程上执行网络操作，而 android 版本 >= 3.0 不允许.

This is because either you are performing network operation on main thread which in not allowed android version >= 3.0.

要解决这个问题，请使用

To solve this either use

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

或

Use AsyncTask 

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

这篇关于getJSONfromURL - 空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！