java.lang.NullPointerException 问题
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【中文标题】java.lang.NullPointerException 问题【英文标题】：java.lang.NullPointerException Issue 【发布时间】：2013-01-10 01:34:16 【问题描述】：我意识到这通常是一个简单的问题，但我一直在查看本网站和其他网站上的其他类似问题，但无法修复代码，也无法准确查看错误来自何处。我的问题似乎是调用分配给按钮的 OnClickListener，但我又可能错了。我基本上是从另一个有效的应用程序中直接复制了这段代码（只做了很少的改动）。这个事实让我更加困惑。提前感谢您对这个相对简单的问题的帮助。
代码：
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.update_activity);
final Handler handler;
handler = new Handler();
final EditText getCustID = (EditText) findViewById(R.id.customer);
final EditText custvar1 = (EditText) findViewById(R.id.var1);
final EditText custvar2 = (EditText) findViewById(R.id.var2);
final EditText custvar3 = (EditText) findViewById(R.id.var3);
class sendGET implements Runnable
private String url;
public sendGET(String mUrl)
url = mUrl;
public void run()
try
//Declare HttpClient and HttpGet
HttpClient client = new DefaultHttpClient();
HttpGet httpget = new HttpGet(url);
//Run HttpGet
client.execute(httpget);
catch (ClientProtocolException e)
// TODO Auto-generated catch block
catch (IOException e)
// TODO Auto-generated catch block
e.printStackTrace();
class getCustomerData implements Runnable
public void run()
try
InputStream stream = null;
XMLparser xmlParser = new XMLparser();
List<Customer> customers = null;
String parseUrl = URL + getCustID.getText().toString();
try
stream = downloadUrl(parseUrl);
customers = xmlParser.parse(stream);
finally
stream.close();
final Customer data = customers.get(0);
handler.post(new Runnable()
public void run()
custvar1.setText(String.valueOf(data.var1));
custvar2.setText(String.valueOf(data.var2));
custvar3.setText(String.valueOf(data.var3));
);
catch (IOException e)
//return getResources().getString(R.string.connection_error);
catch (XmlPullParserException e)
//return getResources().getString(R.string.xml_error);
private Customer readCustomer(XmlPullParser parser) throws XmlPullParserException, IOException
parser.require(XmlPullParser.START_TAG, null, "customer");
String custID = null;
String var1 = null;
String var2 = null;
String var3 = null;
while(parser.next() != XmlPullParser.END_TAG)
if (parser.getEventType() != XmlPullParser.START_TAG)
continue;
String name = parser.getName();
if (name.equals("CustID"))
custID = readCustID(parser);
else if (name.equals("custvar1"))
var1 = readVar1(parser);
else if (name.equals("custvar2"))
var2 = readVar2(parser);
else if (name.equals("custvar3"))
var3 = readVar3(parser);
else
skip(parser);
return new Customer(custID, var1, var2, var3);
private String readCustID(XmlPullParser parser) throws IOException, XmlPullParserException
parser.require(XmlPullParser.START_TAG, null, "CustID");
final String custID = readText(parser);
parser.require(XmlPullParser.END_TAG, null, "CustID");
return custID;
private String readVar1(XmlPullParser parser) throws IOException, XmlPullParserException
parser.require(XmlPullParser.START_TAG, null, "custvar1");
String var1 = readText(parser);
parser.require(XmlPullParser.END_TAG, null, "custvar1");
return var1;
private String readVar2(XmlPullParser parser) throws IOException, XmlPullParserException
parser.require(XmlPullParser.START_TAG, null, "custvar2");
String var2 = readText(parser);
parser.require(XmlPullParser.END_TAG, null, "custvar2");
return var2;
private String readVar3(XmlPullParser parser) throws IOException, XmlPullParserException
parser.require(XmlPullParser.START_TAG, null, "custvar3");
String var3 = readText(parser);
parser.require(XmlPullParser.END_TAG, null, "custvar3");
return var3;
private String readText(XmlPullParser parser) throws IOException, XmlPullParserException
String result = "";
if (parser.next() == XmlPullParser.TEXT)
result = parser.getText();
parser.nextTag();
return result;
private void skip(XmlPullParser parser) throws IOException, XmlPullParserException
if (parser.getEventType() != XmlPullParser.START_TAG)
throw new IllegalStateException();
int depth = 1;
while (depth != 0)
switch (parser.next())
case XmlPullParser.END_TAG:
depth --;
case XmlPullParser.START_TAG:
depth++;
break;
class XMLparser
public List<Customer> parse(InputStream in) throws XmlPullParserException, IOException
try
XmlPullParser parser = Xml.newPullParser();
parser.setInput(in, null);
parser.nextTag();
return readXML(parser);
finally
in.close();
private List<Customer> readXML(XmlPullParser parser) throws XmlPullParserException, IOException
List<Customer> custData = new ArrayList<Customer>();
parser.require(XmlPullParser.START_TAG, null, "xml");
while (parser.next() != XmlPullParser.END_TAG)
if (parser.getEventType() != XmlPullParser.START_TAG)
continue;
String name = parser.getName();
//Look for customer tag
if (name.equals("customer"))
custData.add(readCustomer(parser));
else
skip(parser);
return custData;
private InputStream downloadUrl(String urlString) throws IOException
URL url = new URL(urlString);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(10000);
conn.setConnectTimeout(15000);
conn.setRequestMethod("GET");
conn.setDoInput(true);
//start the query
conn.connect();
InputStream stream = conn.getInputStream();
return stream;
final Button getData = (Button) findViewById(R.id.getData);
getData.setOnClickListener(new View.OnClickListener()
public void onClick(View v)
//Declare and start thread
final Runnable mGetData = new getCustomerData();
final Thread getData = new Thread(mGetData);
getData.start();
);
final Button submit = (Button) findViewById(R.id.Submit);
submit.setOnClickListener(new View.OnClickListener()
public void onClick(View v)
//set url
String url = "**removed**";
url += "var1=" + custvar1.getText().toString() + "&var2=" + custvar2.getText().toString() + "&var3=" + custvar3.getText().toString();
//Declare and start thread
final Runnable mConnect = new sendGET(url);
final Thread Connect = new Thread(mConnect);
Connect.start();
);
public class Customer
public final String custID;
public final String var1;
public final String var2;
public final String var3;
private Customer()
custID = null;
var1 = null;
var2 = null;
var3 = null;
private Customer(String custID, String var1, String var2, String var3)
this.custID = custID;
this.var1 = var1;
this.var2 = var2;
this.var3 = var3;
Logcat 错误：
01-09 19:20:53.478: W/dalvikvm(911): threadid=11: thread exiting with uncaught exception (group=0x40a13300)
01-09 19:20:53.478: E/androidRuntime(911): FATAL EXCEPTION: Thread-99
01-09 19:20:53.478: E/AndroidRuntime(911): java.lang.NullPointerException
01-09 19:20:53.478: E/AndroidRuntime(911): at us.rns.editdata.UpdateActivity$1getCustomerData.run(UpdateActivity.java:90)
01-09 19:20:53.478: E/AndroidRuntime(911): at java.lang.Thread.run(Thread.java:856)
01-09 19:20:54.408: W/IInputConnectionWrapper(911): showStatusIcon on inactive InputConnection
【问题讨论】：
这是在哪一行抛出异常？ 停止使用“logcat 调试”。附加调试器。停止异常（当它被抛出时）。检查相关行上的表达式以找出哪个为空。 好像是抛出异常就行了："stream.close();"如果我正确阅读调试器，但这对我来说更没有意义。 @Jim 这很有意义。您的 downloadUrl 方法失败，异常被捕获，此时stream 仍然为空。
那么我猜这意味着尝试进行字符串连接会导致 URL 出现错误。听起来对吗？
【参考方案1】：
您的 downloadUrl 方法失败。
InputStream stream = null;
XMLparser xmlParser = new XMLparser();
List<Customer> customers = null;
String parseUrl = URL + getCustID.getText().toString();
try
stream = downloadUrl(parseUrl);
customers = xmlParser.parse(stream);
finally
stream.close();
流对象设置为null，然后你分配stream = downloadUrl(parseUrl)。
downloadUrl 方法失败，没有为stream 分配任何内容。
然后您尝试在finally 子句中关闭流。
private InputStream downloadUrl(String urlString) throws IOException
URL url = new URL(urlString);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(10000);
conn.setConnectTimeout(15000);
conn.setRequestMethod("GET");
conn.setDoInput(true);
//start the query
conn.connect();
InputStream stream = conn.getInputStream();
return stream;
在这个方法上设置一个断点，并通过它进行调试。请返回它正在中断的确切行。我怀疑是conn.connect(); 或InputStream stream = conn.getInputStream();
我也会在 finally 子句中这样做：
finally
if(stream != null)
stream.close();
也可以是：
您需要移动所有这些：
final EditText getCustID = (EditText) findViewById(R.id.customer);
final EditText custvar1 = (EditText) findViewById(R.id.var1);
final EditText custvar2 = (EditText) findViewById(R.id.var2);
final EditText custvar3 = (EditText) findViewById(R.id.var3);
final Button getData = (Button) findViewById(R.id.getData);
final Button submit = (Button) findViewById(R.id.Submit);
进入 onResume，而不是 onCreate()
【讨论】：
@A--C 最后两个是类中的静态字段，这会导致 NullRefException 因为它们没有被实例化（因为View 还没有被绘制）
天哪，到处都是私人课程。我现在要逃跑了。
我目前没有 onResume 方法。那我应该创建一个吗？为什么这会对正在运行的代码产生影响？
@Jim A--C 可能是对的，但我总是将我所有的 UI/可运行的东西放在 onResume 中，因为这是视图显示给用户之前的最后一步。
onResume 应该是什么样子的？以上是关于java.lang.NullPointerException 问题的主要内容，如果未能解决你的问题，请参考以下文章
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