这是我的代码它从url.i获取json时抛出java null指针异常已经在android清单中给出了Internet权限，现在在新线程中获取url，因为它不允许在主要线程中进行网络活动
package com.example.usa;
import java.util.ArrayList;
import java.util.HashMap;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
public class Home extends Activity {
// url to make request
private static String url = "http://api.androidhive.info/contacts/";
// JSON Node names
private static final String TAG_CONTACTS = "contacts";
private static final String TAG_ID = "id";
private static final String TAG_NAME = "name";
private static final String TAG_EMAIL = "email";
private static final String TAG_ADDRESS = "address";
private static final String TAG_GENDER = "gender";
private static final String TAG_PHONE = "phone";
private static final String TAG_PHONE_MOBILE = "mobile";
private static final String TAG_PHONE_HOME = "home";
private static final String TAG_PHONE_OFFICE = "office";
JSONArray contacts = null;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.home);
final ArrayList<HashMap<String, String>> contactList = new ArrayList<HashMap<String, String>>();
// Creating JSON Parser instance
final JSONParser jParser = new JSONParser();
final JSONObject json = null ;
// getting JSON string from URL
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Thread thread = new Thread()
{
@Override
public void run() {
try {
while(true) {
JSONObject json = jParser.getJSONFromUrl(url);
sleep(1000);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
thread.start();
//
finish();
}
}, 5000);
// JSONObject json = jParser.getJSONFromUrl(url);
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
try {
// Getting Array of Contacts
contacts = json.getJSONArray(TAG_CONTACTS);
// looping through All Contacts
for(int i = 0; i < contacts.length(); i++){
JSONObject c = contacts.getJSONObject(i);
// Storing each json item in variable
String id = c.getString(TAG_ID);
String name = c.getString(TAG_NAME);
String email = c.getString(TAG_EMAIL);
String address = c.getString(TAG_ADDRESS);
String gender = c.getString(TAG_GENDER);
// Phone number is agin JSON Object
JSONObject phone = c.getJSONObject(TAG_PHONE);
String mobile = phone.getString(TAG_PHONE_MOBILE);
String home = phone.getString(TAG_PHONE_HOME);
String office = phone.getString(TAG_PHONE_OFFICE);
///////////////////////////
Log.w("ID",id);
Log.w("Name",name);
Log.w("Email",email);
Log.w("Gender",gender);
Log.w("mobile",mobile);
Log.w("home",home);
Log.w("office",office);
Log.w("address",address);
///////////////////
// creating new HashMap
HashMap<String, String> map = new HashMap<String, String>();
// adding each child node to HashMap key => value
map.put(TAG_ID, id);
map.put(TAG_NAME, name);
map.put(TAG_EMAIL, email);
map.put(TAG_PHONE_MOBILE, mobile);
// adding HashList to ArrayList
contactList.add(map);
}
} catch (JSONException e) {
e.printStackTrace();
}
//
finish();
}
}, 5000);
// TODO Auto-generated method stub
}
}
这是堆栈跟踪
10-04 06:39:24.830: D/dalvikvm(777): GC_FOR_ALLOC freed 15K, 4% free 4156K/4288K, paused 42ms, total 45ms
10-04 06:39:24.850: I/dalvikvm-heap(777): Grow heap (frag case) to 5.635MB for 1536016-byte allocation
10-04 06:39:25.040: D/dalvikvm(777): GC_FOR_ALLOC freed <1K, 3% free 5655K/5792K, paused 180ms, total 180ms
10-04 06:39:30.280: D/AndroidRuntime(777): Shutting down VM
10-04 06:39:30.280: W/dalvikvm(777): threadid=1: thread exiting with uncaught exception (group=0x414c4700)
10-04 06:39:30.290: E/AndroidRuntime(777): FATAL EXCEPTION: main
10-04 06:39:30.290: E/AndroidRuntime(777): java.lang.NullPointerException
10-04 06:39:30.290: E/AndroidRuntime(777): at com.example.usa.Home$2.run(Home.java:105)
10-04 06:39:30.290: E/AndroidRuntime(777): at android.os.Handler.handleCallback(Handler.java:730)
10-04 06:39:30.290: E/AndroidRuntime(777): at android.os.Handler.dispatchMessage(Handler.java:92)
10-04 06:39:30.290: E/AndroidRuntime(777): at android.os.Looper.loop(Looper.java:137)
10-04 06:39:30.290: E/AndroidRuntime(777): at android.app.ActivityThread.main(ActivityThread.java:5103)
10-04 06:39:30.290: E/AndroidRuntime(777): at java.lang.reflect.Method.invokeNative(Native Method)
10-04 06:39:30.290: E/AndroidRuntime(777): at java.lang.reflect.Method.invoke(Method.java:525)
10-04 06:39:30.290: E/AndroidRuntime(777): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-04 06:39:30.290: E/AndroidRuntime(777): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-04 06:39:30.290: E/AndroidRuntime(777): at dalvik.system.NativeStart.main(Native Method)
10-04 06:39:34.957: D/dalvikvm(777): GC_FOR_ALLOC freed 261K, 6% free 6445K/6824K, paused 81ms, total 98ms
10-04 06:40:00.868: D/dalvikvm(777): GC_FOR_ALLOC freed 2688K, 36% free 5075K/7880K, paused 39ms, total 43ms
10-04 06:44:25.888: I/Process(777): Sending signal. PID: 777 SIG: 9
答案 0 :(得分：0)
你有这一行
JSONObject json = jParser.getJSONFromUrl(url);
但你也有一个方法变量json，我认为你打算稍后使用它。
答案 1 :(得分：0)
问题在于：
JSONObject json = jParser.getJSONFromUrl(url);
在第一个线程中，您正在从URL中检索数据并将其解析为json，在第一个线程之后不久，您尝试从JSON对象 检索数据，其中很可能没有任何数据，因为它仍然忙于检索数据。
所以这不是一个好主意。 Retrieve and get data from JSON array in the same thread. This will save you alot of trouble.
其次，您将json解析的数据存储在第一个线程中的 本地json对象中，而不是全局的 ，并尝试使用null json对象来获取TAG_CONTACT数据。 这会导致NULL指针异常