将一个文件的内容获取出来，为何不怕文件过长而溢出？

Question

明白了，弄个大文件，看到底跑到哪个环节去了。
public String getContentSD(Context context, String fileName) {
        boolean bPermission = isHavePermission(context, "android.permission.READ_EXTERNAL_STORAGE");
        Log.d(TAG, "getContentSD() bPermission = "+bPermission);
        if (!bPermission) {
            return null;
        }        String path = getMsgPathSD(context)
                + (fileName.endsWith(".txt") ? fileName : (fileName + ".txt"));
        File file = new File(path);
        if (!file.exists()) {
            return null;
        }        InputStream inputStream = null;
        InputStreamReader inputStreamReader = null;
        BufferedReader buffer = null;
        StringBuilder strApp = null;        try {
            inputStream = new FileInputStream(file);
            inputStreamReader = new InputStreamReader(inputStream);            buffer = new BufferedReader(inputStreamReader);
            strApp = new StringBuilder();            String line = null;
            while ((line = buffer.readLine()) != null) {
                strApp.append(line);
            }            buffer.close();
            buffer = null;
            inputStreamReader.close();
            inputStreamReader = null;
            inputStream.close();
            inputStream = null;            return strApp.toString();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (null != buffer) {
                    buffer.close();
                    buffer = null;
                }
                if (null != inputStreamReader) {
                    inputStreamReader.close();
                    inputStreamReader = null;
                }
                if (null != inputStream) {
                    inputStream.close();
                    inputStream = null;
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return null;
    }