所以我有一个问题。我一直试图让http请求工作很长一段时间，我已经创建了一个新线程，以便请求永远不会在主线程上运行。我已经成功地使它发生了HTTP请求，并且在请求线程中我做了它，因此它记录了它从http请求中获取的数据。现在，如果我想在方法之外使用这个jsonObject，它会给我带来错误。帮助
logcat的：
02-23 22:48:08.612 4239-4239/com.tamrefrank.www.listviewandroidstudio E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.tamrefrank.www.listviewandroidstudio, PID: 4239
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tamrefrank.www.listviewandroidstudio/com.tamrefrank.www.listviewandroidstudio.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONArray.toString()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONArray.toString()' on a null object reference
at com.tamrefrank.www.listviewandroidstudio.MainActivity.onCreate(MainActivity.java:38)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
02-23 22:48:09.332 4239-4254/com.tamrefrank.www.listviewandroidstudio I/log_tag﹕ [{"Name":"Cabinet","Location":"In Storage","Latitude":"40.83990100","Longitude":"14.25185000","pk":19587,"image_id":"2006AJ6728"},{"Name":"Panel","Location":"Medieval and Renaissance, room 8, case WS EXP","Latitude":"48.87287900","Longitude":"4.63710000","pk":11371,"image_id":"2006AP3163"},{"Name":"Wallpaper","Location":"British Galleries, room 122f, case 1","Latitude":"53.47962200","Longitude":"-2.24881000","pk":19492,"image_id":"2006AU6265"},{"Name":"Portrait miniature","Location":"British Galleries, room 58c, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":9491,"image_id":"2006AH0271"},{"Name":"Caricature","Location":"In Storage","Latitude":"53.02513900","Longitude":"-2.18676000","pk":29979,"image_id":"2006AE9142"},{"Name":"Card game","Location":"In Storage","Latitude":"52.88328900","Longitude":"-1.97685000","pk":26483,"image_id":"2006AU5916"},{"Name":"Tile","Location":"Islamic Middle East, room 42, case WW9","Latitude":"39.76586200","Longitude":"64.42234000","pk":30220,"image_id":"2006AY5617"},{"Name":"Sculpture","Location":"Sculpture, room 22, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":36026,"image_id":"2006AM7780"},{"Name":"Oil painting","Location":"Prints & Drawings Study Room, room 315, case 24, shelf R","Latitude":"52.88328900","Longitude":"-1.97685000","pk":6905,"image_id":"2006AU6480"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10895,"image_id":"2006AU8783"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10902,"image_id":"2006AU9234"}]
02-23 22:48:09.332 4239-4254/com.tamrefrank.www.listviewandroidstudio I/log_tag﹕ [{"Name":"Cabinet","Location":"In Storage","Latitude":"40.83990100","Longitude":"14.25185000","pk":19587,"image_id":"2006AJ6728"},{"Name":"Panel","Location":"Medieval and Renaissance, room 8, case WS EXP","Latitude":"48.87287900","Longitude":"4.63710000","pk":11371,"image_id":"2006AP3163"},{"Name":"Wallpaper","Location":"British Galleries, room 122f, case 1","Latitude":"53.47962200","Longitude":"-2.24881000","pk":19492,"image_id":"2006AU6265"},{"Name":"Portrait miniature","Location":"British Galleries, room 58c, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":9491,"image_id":"2006AH0271"},{"Name":"Caricature","Location":"In Storage","Latitude":"53.02513900","Longitude":"-2.18676000","pk":29979,"image_id":"2006AE9142"},{"Name":"Card game","Location":"In Storage","Latitude":"52.88328900","Longitude":"-1.97685000","pk":26483,"image_id":"2006AU5916"},{"Name":"Tile","Location":"Islamic Middle East, room 42, case WW9","Latitude":"39.76586200","Longitude":"64.42234000","pk":30220,"image_id":"2006AY5617"},{"Name":"Sculpture","Location":"Sculpture, room 22, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":36026,"image_id":"2006AM7780"},{"Name":"Oil painting","Location":"Prints & Drawings Study Room, room 315, case 24, shelf R","Latitude":"52.88328900","Longitude":"-1.97685000","pk":6905,"image_id":"2006AU6480"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10895,"image_id":"2006AU8783"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10902,"image_id":"2006AU9234"}]
02-23 22:48:32.712 4239-4239/com.tamrefrank.www.listviewandroidstudio I/Process﹕ Sending signal. PID: 4239 SIG: 9
代码：
package com.tamrefrank.www.listviewandroidstudio;
//All the imports happen when you auto complete. Often you may have erros when you copy paste things directly because libraries may not get imported
import android.os.AsyncTask;
import android.os.StrictMode;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.IOException;
import java.io.InputStream;
public class MainActivity extends ActionBarActivity {
private String [] drinks;
JSONArray jsonObject;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//set the content view to activity_main.xml
setContentView(R.layout.activity_main);
JSONArray object = new JSONArray();
object = getData();
Log.i("log_tag", jsonObject.toString());
// System.out.print("success: " + object.toString());
drinks = new String[3];
drinks[0] = "hi";
drinks[1] = "5";
drinks[2] = "3";
/**
* Lists require adapters-Adapters are the place that we store the various data items we want
to appear in the Lists. This will be handled in an adapter.
*/
ArrayAdapter<String> adapter;
//We will create the array adapter and reference it to our individual List item from the ListView and then to the array we just created- Drinks.
adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,drinks);
//We finally will connect the adapter to the List View
ListView drinkLists = (ListView) findViewById(R.id.listView);
//Set the drinks array to the adapter below
drinkLists.setAdapter(adapter);
}
public JSONArray getData(){
new Thread(new Runnable() {
public void run() {
HttpGet httpGet = new HttpGet("https://stampwatch.herokuapp.com/");
HttpClient client = new DefaultHttpClient();
HttpResponse response;
StringBuilder stringBuilder = new StringBuilder();
try {
response = client.execute(httpGet);
HttpEntity entity = response.getEntity();
InputStream stream = entity.getContent();
int b;
while ((b = stream.read()) != -1) {
stringBuilder.append((char) b);
}
} catch (ClientProtocolException e) {
} catch (IOException e) {
System.out.println(e);
}
try {
jsonObject = new JSONArray(stringBuilder.toString());
Log.i("log_tag", jsonObject.toString());
;
} catch (JSONException e) {
e.printStackTrace();
}
Log.i("log_tag", jsonObject.toString());
}
}).start();
return jsonObject;
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
答案 0 :(得分：2)
下面：
Log.i("log_tag", jsonObject.toString());
NPE因为jsonObject是null并尝试使用null对象为JSONObject调用toString()方法。
但是如果我用该对象发出请求，为什么对象为null？
因为jsonObject对象是在getData();方法中初始化的，其中一个Thread用于从api获取数据。意思是：
Log.i("log_tag", jsonObject.toString());
在启动Thread后执行，而不是等待，直到线程完成。