How to save file in memory and read file output stream?

问题

When I save file I use:

File file = new File(filename);

But, since I no longer have privileges to write to folders, I would like rather to save it to memory and then read file to FileOutputStream.

I have read I can save file to memory with this approach:

new ByteArrayOutputStream();

How would whole code look like? I can't figure it out how to write it properly after upload is done.

Edit: I'm using Vaadins upload plugin:

public File file;

    public OutputStream receiveUpload(String filename,
                                      String mimeType) {

        // Create upload stream
        FileOutputStream fos = null; // Stream to write to
        file = null;

        if(StringUtils.containsIgnoreCase(filename, ".csv")){
            try {

                file = new File(filename);
                fos = new FileOutputStream(file);
            } catch (final java.io.FileNotFoundException e) {

                new Notification("Error", e.getMessage(), Notification.Type.WARNING_MESSAGE)
                    .show(Page.getCurrent());
                return null;
            }
        } else {

            new Notification("Document is not .csv file", Notification.Type.WARNING_MESSAGE)
                .show(Page.getCurrent());
            return null;
        }
        return fos; // Return the output stream to write to
    }

回答1:

public OutputStream receiveUpload(String filename,
                                  String mimeType) {

    ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

    return byteArrayOutputStream;
}

You can get the content written in the stream using:

byte[] dataWrittenInTheOutputStream = byteArrayOutputStream.toByteArray();

Or you can write the contents to another OutputStream:

byteArrayOutputStream.writeTo(System.out);

Or:

file = new File(filename);    
fos = new FileOutputStream(file);
byteArrayOutputStream.writeTo(fos);

回答2:

Please avoid using ByteArrayOutputStream/ByteArrayInputStream, because they are not chunked and creates huge arrays internally
Please avoid using methods .toByteArray(), because they will require additional array allocation
Large allocation is not so optimal than several smaller objects allocations, because the oldest generation will be used. See details in this answer.

What is my recommendation:

Use Buffer class from okio library. It is chunked in memory and support InputStream/OutputStream bindings
Use async way of data read to increase performance. However, I will show synchronous way.

So, short code will be:

Buffer buffer = new Buffer();
buffer.readFrom(previouslyCreatedFileStream); // of course, it is better to use    
buffer.writeTo(someOutputStream);

来源：https://stackoverflow.com/questions/37385908/how-to-save-file-in-memory-and-read-file-output-stream

标签

java

file

vaadin

java-io