java - Android 写入文件时出现 NullPointerException

Question

当我尝试写入内部存储中的文件时，出现了 NullPointerException，但我不明白为什么。我阅读了所有文档并使用该代码进行测试，但没有进行。

我正在使用 HTTP GET 请求 拉取一个字符串(它是一行，但采用 JSON 格式)，并尝试将该字符串写入 JSON 文件在内部存储器中。

我知道 HTTP GET 请求工作正常，因为我能够将其记录到 LogCat 中，但我陷入了要将其写入 JSON 文件的部分。

主要 Activity

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_new_checklist);
    mGestureDetector = createGestureDetector(this);

//      JSONReader jsonReader = new JSONReader(this.getApplicationContext());
//      jsonReader.readJson();
//      jsonReader.getData();

    new BackgroundTask().execute("");

    try {
        writeToJSON();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

public void writeToJSON() throws IOException {
    String filename = "test.json";
    FileOutputStream fos = openFileOutput(filename, Context.MODE_PRIVATE);
    fos.write(data.getBytes());
    fos.close();
}

public class BackgroundTask extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... params) {
        HTTPRequest request = new HTTPRequest(1, 2);

        try {
            data = request.GetRequest();
            Log.v("testing", data);
//              request.writeToJSON();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

}

HTTP请求

    public String GetRequest() throws MalformedURLException, IOException {
    String charset = "UTF-8";
    URLConnection connection = new URL(listOfChecklistsURL).openConnection();
    InputStream response = connection.getInputStream();
    String contentType = connection.getHeaderField("Content-Type");
    for (String param : contentType.replace(" ", "").split(";")) {
        if (param.startsWith("charset=")) {
            charset = param.split("=", 2)[1];
            break;
        }
    }

    if (charset != null) {
        BufferedReader reader = new BufferedReader(new InputStreamReader(response, charset));
        try {
            for (String line; (line = reader.readLine()) != null;) {
//                  System.out.println(line);
                data = line;
            }
        }
        finally {
            try { reader.close(); } catch (IOException logOrIgnore) {}
        }
    }

    return data;
}

堆栈跟踪

12-24 16:32:12.159: E/AndroidRuntime(4301): FATAL EXCEPTION: main
12-24 16:32:12.159: E/AndroidRuntime(4301): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.medusa.checkit/com.medusa.checkit.NewChecklistActivity}: java.lang.NullPointerException
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.os.Looper.loop(Looper.java:137)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread.main(ActivityThread.java:4424)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at java.lang.reflect.Method.invokeNative(Native Method)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at java.lang.reflect.Method.invoke(Method.java:511)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at dalvik.system.NativeStart.main(Native Method)
12-24 16:32:12.159: E/AndroidRuntime(4301): Caused by: java.lang.NullPointerException
12-24 16:32:12.159: E/AndroidRuntime(4301):     at com.medusa.checkit.NewChecklistActivity.writeToJSON(NewChecklistActivity.java:52)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at com.medusa.checkit.NewChecklistActivity.onCreate(NewChecklistActivity.java:42)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.Activity.performCreate(Activity.java:4470)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
12-24 16:32:12.159: E/AndroidRuntime(4301):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
12-24 16:32:12.159: E/AndroidRuntime(4301):     ... 11 more

新编辑的代码 - 主要 Activity

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_new_checklist);
    mGestureDetector = createGestureDetector(this);


    new BackgroundTask();

}

public void writeToJSON() throws IOException {
    String filename = "test.json";
    FileOutputStream fos = openFileOutput(filename, Context.MODE_PRIVATE);
    fos.write(data.getBytes());
    fos.close();
}

public class BackgroundTask extends AsyncTask<Void, Void, Void> {

    protected Void doInBackground(Void... params) {
        HTTPRequest request = new HTTPRequest(1, 2);

        try {
            data = request.GetRequest();
            Log.v("testing", data);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    protected void onPostExecute() {
        try {
            writeToJSON();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

编辑:新的 onPostExecute 有效!忘记返回

protected void onPostExecute(Void result) {
        try {
            Log.v("onPostExecute", "writing to JSON");
            JSONWriter writer = new JSONWriter(context, data);
            writer.writeToJSON();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return;
    }

Answer 1

由于 AsyncTask 是异步的，因此 data 为 null，因为 AsyncTask 在该方法之前尚未完成叫做。从 AsyncTask 的 onPostExecute() 调用 writeToJSON()，以便在 data 具有值。

此外，doInBackground() 返回 null 但根据您的任务声明，

public class BackgroundTask extends AsyncTask<String, Void, String> {

onPostExecute() 应该是一个 String。最后一个 param 应该是 Void，或者您应该从 doInBackground() 返回一个 String。