Android getAssets（）NullPointerException從資產中加載Json文件

Question

我試圖從json文件中獲取數據，以便稍後可以使用它與aChartEngine。這是我使用的得到這個數據的方法：

public String loadJSONFromAsset() { 
     String json = null; 


     try { 



      InputStream is = this.getAssets().open(ficheiro); 

      int size = is.available(); 

      byte[] buffer = new byte[size]; 

      is.read(buffer); 

      is.close(); 

      json = new String(buffer, "UTF-8"); 


     } catch (IOException ex) { 
      ex.printStackTrace(); 
      return null; 
     } 
     return json; 

    } 

我使用的是其他類，其中我只是在ListView顯示JSON數據這種方法（即使沒有getAssets之前提供上下文（ ）），但是這次我不能得到這個工作。

的logcat：

05-20 10:26:14.108: E/AndroidRuntime(6388): FATAL EXCEPTION: main 
05-20 10:26:14.108: E/AndroidRuntime(6388): Process: com.example.euroxxi_testeficheiro, PID: 6388 
05-20 10:26:14.108: E/AndroidRuntime(6388): java.lang.IllegalStateException: Could not execute method of the activity 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.view.View$1.onClick(View.java:3969) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.view.View.performClick(View.java:4633) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.view.View$PerformClick.run(View.java:19330) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.os.Handler.handleCallback(Handler.java:733) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.os.Handler.dispatchMessage(Handler.java:95) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.os.Looper.loop(Looper.java:157) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.app.ActivityThread.main(ActivityThread.java:5356) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at java.lang.reflect.Method.invokeNative(Native Method) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at java.lang.reflect.Method.invoke(Method.java:515) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at dalvik.system.NativeStart.main(Native Method) 
05-20 10:26:14.108: E/AndroidRuntime(6388): Caused by: java.lang.reflect.InvocationTargetException 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at java.lang.reflect.Method.invokeNative(Native Method) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at java.lang.reflect.Method.invoke(Method.java:515) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.view.View$1.onClick(View.java:3964) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  ... 11 more 
05-20 10:26:14.108: E/AndroidRuntime(6388): Caused by: java.lang.NullPointerException 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at android.content.ContextWrapper.getAssets(ContextWrapper.java:88) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at com.example.euroxxi_testeficheiro.ChartTeste.loadJSONFromAsset(ChartTeste.java:53) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at com.example.euroxxi_testeficheiro.ChartTeste.getIntent(ChartTeste.java:91) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  at com.example.euroxxi_testeficheiro.MainActivity.ChartPlot(MainActivity.java:77) 
05-20 10:26:14.108: E/AndroidRuntime(6388):  ... 14 more 

這是完整的活動

package com.example.euroxxi_testeficheiro; 


import java.io.IOException; 
import java.io.InputStream; 
import java.text.SimpleDateFormat; 
import java.util.ArrayList; 
import java.util.Calendar; 
import java.util.Date; 
import java.util.HashMap; 

import org.achartengine.ChartFactory; 
import org.achartengine.chart.BarChart.Type; 
import org.achartengine.model.CategorySeries; 
import org.achartengine.model.XYMultipleSeriesDataset; 
import org.achartengine.renderer.XYMultipleSeriesRenderer; 
import org.achartengine.renderer.XYSeriesRenderer; 
import org.json.JSONArray; 
import org.json.JSONException; 
import org.json.JSONObject; 

import com.jjoe64.graphview.BarGraphView; 
import com.jjoe64.graphview.GraphViewSeries; 
import com.jjoe64.graphview.GraphView.GraphViewData; 

import android.app.Activity; 
import android.content.Context; 
import android.content.Intent; 
import android.graphics.Color; 
import android.os.Bundle; 
import android.util.DisplayMetrics; 
import android.util.TypedValue; 
import android.widget.EditText; 
import android.widget.LinearLayout; 
import android.widget.ListView; 

public class ChartTeste extends Activity{ 

    private static final String TAG_CENTRAL = "getCentrais_JSONResult"; 
    private static final String TAG_NOME = "Central"; 
    private static final String TAG_DATA = "Data"; 
    private static final String TAG_PRODUCAO = "Producao"; 
    String ficheiro = "getCentrais_JSON.json"; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_chart_teste); 

     getIntent(); 

    } 

    public String loadJSONFromAsset() { 
     String json = null; 


     try { 



      InputStream is = getApplicationContext().getAssets().open("getCentrais_JSON.json"); 

      int size = is.available(); 

      byte[] buffer = new byte[size]; 

      is.read(buffer); 

      is.close(); 

      json = new String(buffer, "UTF-8"); 


     } catch (IOException ex) { 
      ex.printStackTrace(); 
      return null; 
     } 
     return json; 

    } 



    public Intent getIntent(Context context) 
    { 

     XYMultipleSeriesDataset dataset = new XYMultipleSeriesDataset(); 
     XYMultipleSeriesRenderer mRenderer = new XYMultipleSeriesRenderer(); 

     try { 

      String nome; 
      int producao; 


      // contacts JSONArray 
      JSONArray centrais = null; 

      JSONObject jsonObj = new JSONObject(loadJSONFromAsset()); 


      // Getting JSON Array node 
      centrais = jsonObj.getJSONArray(TAG_CENTRAL); 

      int[] y={0}; 
      String[] xLabel = {""}; 

      // looping through All Contacts 
      for (int i = 0; i < centrais.length(); i++) { 
       JSONObject c = centrais.getJSONObject(i); 

       //String id = c.getString(TAG_ID); 
       nome = c.getString(TAG_NOME); 
       producao = c.getInt(TAG_PRODUCAO); 


       xLabel[i]=nome; 
       y[i]=producao; 

       mRenderer.addXTextLabel(y[i], xLabel[i]); 
      } 

      // Bar 1 

       CategorySeries series = new CategorySeries("Grafico de producao"); 
       for (int j = 0; j < y.length; j++) { 
        series.add("Bar " + (j+1), y[j]); 


       } 


       dataset.addSeries(series.toXYSeries()); 

       // This is how the "Graph" itself will look like 
       mRenderer.setChartTitle("Gráfico de produção"); 
       mRenderer.setXTitle("X VALUES"); 
       mRenderer.setYTitle("Y VALUES"); 


       mRenderer.setXLabels(0); 


       mRenderer.setAxesColor(Color.GREEN); 
       mRenderer.setLabelsColor(Color.RED); 

       // Customize bar 1 
       XYSeriesRenderer renderer = new XYSeriesRenderer(); 
       renderer.setDisplayChartValues(true); 
       renderer.setChartValuesSpacing((float) 0.5); 
       mRenderer.addSeriesRenderer(renderer); 
       //renderer.setChartTitleTextSize(titleSize); // if we change title text size, it will render off screen 
       mRenderer.setLabelsTextSize(25); 
       mRenderer.setLegendTextSize(25); 



     } catch (JSONException e) { 
      e.printStackTrace(); 
     } 

     Intent intent = ChartFactory.getBarChartIntent(context, dataset,mRenderer, Type.DEFAULT); 

     return intent; 
    } 

} 

Answer 1

堆棧跟蹤表明，ChartTeste進行實例化new。千萬不要用new實例化活動。

要麼考慮您是否需要其他活動，要麼使用Intent來實例化一個活動，例如，

Intent i = new Intent(MainActivity.this, ChartTeste.class); 
startActivity(i); 

Answer 2

試試這個，

public String loadJSONFromAsset() { 
     String json = null; 
     try { 

      InputStream is = getAssets().open("file_name.json"); 

      int size = is.available(); 

      byte[] buffer = new byte[size]; 

      is.read(buffer); 

      is.close(); 

      json = new String(buffer, "UTF-8"); 


     } catch (IOException ex) { 
      ex.printStackTrace(); 
      return null; 
     } 
     return json; 

    } 

這可能會幫助你

Answer 3

this 

指當前對象，在我看來，你應該使用

getApplicationContext()