读取内部存储文件在Android中花费太多时间

Question

I have a large JSON data around 20 MB (I create this data using JSON.stringify from JavaScript code). I 'm writing this JSON data to an internal storage file on Android Device and reading it later. So When I read the file it's taking too much time, I don't know whether its reading or not. One more thing I need to Read in the Main thread only.

The below code works fine if I pass data value "Hello World" in WriteFile method, But it fails with the large JSON

public String ReadFile()
{
    StringBuffer text = new StringBuffer();
    String FILE_NAME = "file.txt";
    try {
        BufferedReader bReader = new BufferedReader(new InputStreamReader(openFileInput(FILE_NAME)));
        String line;
        int count = 0;
        while ((line = bReader.readLine()) != null) {
            text.append(line + "\n");
            alert("Reading File: " + ++count);
        }

    } 
    catch (Exception e) {
        alert(e.toString());
    }

    return text.toString();
}

public String WriteFile(String data)
{
    String FILE_NAME = "file.txt";
    String result = "";
  try {
      FileOutputStream fos = openFileOutput(FILE_NAME, Context.MODE_PRIVATE);
      fos.write(data.toString().getBytes());
      result = "Success";
      fos.close();
    }
    catch (Exception e) {
        e.printStackTrace();
        result="Error";
   }

    return result;
}

I have added one alert in while loop also, but I cannot see any alert message. I have not seen even the Exception message also.

so there can be two problems.

1. There is something wrong in writing to file (But I don't know how to verify this? because I don't think there is any way to view internal storage file).

2. Something wrong in my reading code.

Update1:

If let's say I cannot read so large file in Java native code, then Is there any way to read an internal storage Android file from WebView JavaScript code?

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Application Requirement

I have an Android application, In which I have a WebView. I have copied the full javascript code (js and HTML files) to assets folder of the app. I'm writing to file from java native code and reading from java native code. I am getting all data from the server on app launch. My client has a very slow internet connection and its disconnected many times. So they want this app to be run in offline mode. Means app will get all the data at launch and We will store it somewhere and then read it throughout the app. If a user launches the app again it will get the old existing data. Actually, this data is very big so I'm storing it to the internal storage file.

Answer 1

First of all, the only way to be really sure why your code is taking a long time is to profile it. We can't do that for you.

But here are some performance tips relevant to your code:

1. Don't read the entire 20MB JSON file into the Java heap / RAM memory unless you really need to do it. (I am finding it difficult to understand why you are doing this. For example, a typical JSON parser will happily ¹ read input directly from a file. Or if you are reading this so that you can send this to a client on the other end of an HTTP connection, you should be able to stream the data.)

2. Reading a file a line at a time and then stitching the lines back together is unnecessary. It generates unnecessary garbage. Extra garbage means more work for the GC, which slows you down. If the lines are long, you have the added performance "hit" of using a internal StringBuilder to build each line.

   Reading to a recycled char[] , then appending the char[] content to the StringBuilder will be faster than appending lines.

3. Your StringBuilder will repeatedly "grow" its backing character array to accommodate the characters as you append them. This generates garbage and leads to unnecessary copying. (Implementations typically "grow" the array exponentially to avoid O(N^2) behavior. However the expansions still affect performance, and can result in up to 3 times the peak memory usage than is actually required.)

   One way to avoid this is to get an initial estimate of the number of characters you are going to add and set the StringBuilder "capacity" accordingly. You may be able to estimate the number of characters from the file size. (It depends on the encoding.)

4. Look for a way to do it using existing standard Java libraries; eg Files.copy and ByteArrayOutputStream , or Files.readAllBytes

5. Look for an existing 3rd-party library method; eg Apache Commons IO has an IOUtils.toString(Reader) method. The chances are that they will have spent a lot of time figuring out how to do this efficiently . Reusing a well engineered, well maintained library is likely to saves you time.

6. Don't put a trace print (I assume that is what alert is ...) in the middle of a loop that could be called millions of times. (Duh!)

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^{1 - Parser are cheerful once you get to know them :-)}