Java - 获取JSONObject时的NullPointerException

Question

我正在尝试使用Lukencode的RestClient类在我的Android应用程序中实现登录功能.我有一个名为UserFunctions通过RestClient检索JSONObject 的类:

public class UserFunctions {

    private RestClient restClient;
    private String json;
    private Context mContext;

    private static String login_tag = "login";
    private static String register_tag = "register";

    // constructor
    public UserFunctions(Context context){
        restClient = new RestClient("http://10.0.2.2:81/HGourmet/user");
        mContext = context;
    }

    /**
     * function make Login Request
     * @param email
     * @param password
     * */
   public JSONObject loginUser(String email, String password){
        // Building Parameters      
        restClient.AddParam("tag", login_tag);
        restClient.AddParam("email", email);
        restClient.AddParam("password", password);

        // getting JSON Object
        try{
    restClient.Execute(RequestMethod.POST);
    }catch(Exception e){
        e.printStackTrace();
    }               
    json = restClient.getResponse();

        JSONObject jObj = null;
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }
        return jObj;
    }

以下是我在logcat中得到的:

01-06 09:01:05.805: E/Trace(1819): error opening trace file: No such file or directory (2)
01-06 09:02:05.485: E/AndroidRuntime(1819): FATAL EXCEPTION: main
01-06 09:02:05.485: E/AndroidRuntime(1819): java.lang.NullPointerException
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONTokener.nextValue(JSONTokener.java:94)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONObject.<init>(JSONObject.java:154)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at org.json.JSONObject.<init>(JSONObject.java:171)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.hanu.hgourmet.library.UserFunctions.loginUser(UserFunctions.java:49)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.hanu.hgourmet.LoginActivity$1.onClick(LoginActivity.java:56)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.view.View.performClick(View.java:4202)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.view.View$PerformClick.run(View.java:17340)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Handler.handleCallback(Handler.java:725)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Handler.dispatchMessage(Handler.java:92)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.os.Looper.loop(Looper.java:137)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at android.app.ActivityThread.main(ActivityThread.java:5039)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at java.lang.reflect.Method.invokeNative(Native Method)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at java.lang.reflect.Method.invoke(Method.java:511)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-06 09:02:05.485: E/AndroidRuntime(1819):     at dalvik.system.NativeStart.main(Native Method)

我真的希望有人能帮我找出这个问题的原因.非常感谢你.

Answer 1

我不会扩展你应该如何做到这一点,而只是为什么它发生.根据定义,AsyncTask是异步的.这意味着当您执行它时,它会在不同的线程中并行执行.所以你不能指望它在execute()被调用后立即被终止(否则它将被称为SyncTask,并且没有任何理由存在).

见AsyncTask烤面包机.当你启动烤面包机(执行AsyncTask)时,它会烤面包,但不会立即返回烤好的面包.烤面包需要一些时间.虽然它是烘烤你的面包,你可以做其他事情(因此不会完全冻结UI线程).当它完成烘烤你的面包时,它会通知你!面包是烤的.这是同样的事情.json当AsyncTask通知您已完成执行时,您应该只开始使用(烤面包).