我在使用 JSON 对象从 Google Books API 检索诸如书名之类的信息时遇到了一些问题。下面是相关代码：
//Retrieves scanned data from barcode reader
public void onActivityResult(int requestCode, int resultCode, Intent intent) {
IntentResult scanningResult = IntentIntegrator.parseActivityResult(requestCode, resultCode, intent);
if (scanningResult != null) {
String scanContent = scanningResult.getContents();
String scanFormat = scanningResult.getFormatName();
if(scanContent!=null && scanFormat!=null && scanFormat.equalsIgnoreCase("EAN_13")){
String bookSearchString = "https://www.googleapis.com/books/v1/volumes?" +
"q=isbn:"+scanContent+"&key=564967988072"; //my key
new GetBookInfo().execute(bookSearchString);
String booktitle = booktitleText.getText().toString();
BookGenActivity.setBookTitle(booktitle);
}
else{
Toast toast = Toast.makeText(getApplicationContext(),
"Not a valid scan!", Toast.LENGTH_SHORT);
toast.show();
}
}
else{
Toast noData = Toast.makeText(getApplicationContext(),
"No scan data received!", Toast.LENGTH_SHORT);
noData.show();
}
}
这是获取书籍信息的类：
private class GetBookInfo extends AsyncTask<String, Void, String> {
//fetch book info
@Override
protected String doInBackground(String... bookURLs) {
//request book info
StringBuilder bookBuilder = new StringBuilder();
for (String bookSearchURL : bookURLs) {
//search urls
HttpClient bookClient = new DefaultHttpClient();
try {
//get the data
HttpGet bookGet = new HttpGet(bookSearchURL);
HttpResponse bookResponse = bookClient.execute(bookGet);
StatusLine bookSearchStatus = bookResponse.getStatusLine();
if (bookSearchStatus.getStatusCode()==200) {
//we have a result
HttpEntity bookEntity = bookResponse.getEntity();
InputStream bookContent = bookEntity.getContent();
InputStreamReader bookInput = new InputStreamReader(bookContent);
BufferedReader bookReader = new BufferedReader(bookInput);
String lineIn;
while ((lineIn=bookReader.readLine())!=null) {
bookBuilder.append(lineIn);
}
}
}
catch(Exception e){
e.printStackTrace();
}
}
return bookBuilder.toString();
}
protected void onPostExecute(String result) {
//parse search results
try{
//parse results
JSONObject resultObject = new JSONObject(result);
JSONArray bookArray = resultObject.getJSONArray("items");
JSONObject bookObject = bookArray.getJSONObject(0);
JSONObject volumeObject = bookObject.getJSONObject("volumeInfo");
try{
bookTextView.setText("TITLE: "+volumeObject.getString("title"));
}
catch(JSONException jse){
booktitleText.setText("Not Found");
jse.printStackTrace();
}
}
//If no result is found
catch (Exception e) {
//no result
e.printStackTrace();
bookTextView.setText("NOT FOUND");
}
}
}
在我扫描条形码并且应用程序崩溃后似乎出现 NullPointerException，但我无法找出原因。下面是堆栈跟踪：
08-02 11:34:56.171: W/System.err(6994): org.json.JSONException: End of input at character 0 of
08-02 11:34:56.171: W/System.err(6994): at org.json.JSONTokener.syntaxError(JSONTokener.java:450)
08-02 11:34:56.171: W/System.err(6994): at org.json.JSONTokener.nextValue(JSONTokener.java:97)
08-02 11:34:56.171: W/System.err(6994): at org.json.JSONObject.<init>(JSONObject.java:155)
08-02 11:34:56.171: W/System.err(6994): at org.json.JSONObject.<init>(JSONObject.java:172)
08-02 11:34:56.171: W/System.err(6994): at com.myapps.bscanr.BookActivity$GetBookInfo.onPostExecute(BookActivity.java:128)
08-02 11:34:56.171: W/System.err(6994): at com.myapps.bscanr.BookActivity$GetBookInfo.onPostExecute(BookActivity.java:1)
08-02 11:34:56.171: W/System.err(6994): at android.os.AsyncTask.finish(AsyncTask.java:632)
08-02 11:34:56.171: W/System.err(6994): at android.os.AsyncTask.access$600(AsyncTask.java:177)
08-02 11:34:56.171: W/System.err(6994): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
08-02 11:34:56.171: W/System.err(6994): at android.os.Handler.dispatchMessage(Handler.java:102)
08-02 11:34:56.171: W/System.err(6994): at android.os.Looper.loop(Looper.java:136)
08-02 11:34:56.171: W/System.err(6994): at android.app.ActivityThread.main(ActivityThread.java:5001)
08-02 11:34:56.171: W/System.err(6994): at java.lang.reflect.Method.invokeNative(Native Method)
08-02 11:34:56.171: W/System.err(6994): at java.lang.reflect.Method.invoke(Method.java:515)
08-02 11:34:56.171: W/System.err(6994): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
08-02 11:34:56.171: W/System.err(6994): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
08-02 11:34:56.171: W/System.err(6994): at dalvik.system.NativeStart.main(Native Method)
08-02 11:34:56.171: D/AndroidRuntime(6994): Shutting down VM
08-02 11:34:56.171: W/dalvikvm(6994): threadid=1: thread exiting with uncaught exception (group=0x4159dba8)
08-02 11:34:56.171: E/AndroidRuntime(6994): FATAL EXCEPTION: main
08-02 11:34:56.171: E/AndroidRuntime(6994): Process: com.myapps.bscanr, PID: 6994
08-02 11:34:56.171: E/AndroidRuntime(6994): java.lang.NullPointerException
08-02 11:34:56.171: E/AndroidRuntime(6994): at com.myapps.bscanr.BookActivity$GetBookInfo.onPostExecute(BookActivity.java:168)
08-02 11:34:56.171: E/AndroidRuntime(6994): at com.myapps.bscanr.BookActivity$GetBookInfo.onPostExecute(BookActivity.java:1)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.os.AsyncTask.finish(AsyncTask.java:632)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.os.AsyncTask.access$600(AsyncTask.java:177)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.os.Handler.dispatchMessage(Handler.java:102)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.os.Looper.loop(Looper.java:136)
08-02 11:34:56.171: E/AndroidRuntime(6994): at android.app.ActivityThread.main(ActivityThread.java:5001)
08-02 11:34:56.171: E/AndroidRuntime(6994): at java.lang.reflect.Method.invokeNative(Native Method)
08-02 11:34:56.171: E/AndroidRuntime(6994): at java.lang.reflect.Method.invoke(Method.java:515)
08-02 11:34:56.171: E/AndroidRuntime(6994): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
08-02 11:34:56.171: E/AndroidRuntime(6994): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
08-02 11:34:56.171: E/AndroidRuntime(6994): at dalvik.system.NativeStart.main(Native Method)
对此的任何指导将不胜感激！