How to create ByteArrayInputStream from a file in Java?

A ByteArrayInputStream is an InputStream wrapper around a byte array. This means you'll have to fully read the file into a byte[], and then use one of the ByteArrayInputStream constructors.

Can you give any more details of what you are doing with the ByteArrayInputStream? Its likely there are better ways around what you are trying to achieve.

Edit:
If you are using Apache FTPClient to upload, you just need an InputStream. You can do this;

String remote = "whatever";
InputStream is = new FileInputStream(new File("your file"));
ftpClient.storeFile(remote, is);

You should of course remember to close the input stream once you have finished with it.

This isn't exactly what you are asking, but is a fast way of reading files in bytes.

File file = new File(yourFileName);
RandomAccessFile ra = new RandomAccessFile(yourFileName, "rw"):
byte[] b = new byte[(int)file.length()];
try {
    ra.read(b);
} catch(Exception e) {
    e.printStackTrace();
}

//Then iterate through b

Use the FileUtils#readFileToByteArray(File) from Apache Commons IO, and then create the ByteArrayInputStream using the ByteArrayInputStream(byte[]) constructor.

public static ByteArrayInputStream retrieveByteArrayInputStream(File file) {
    return new ByteArrayInputStream(FileUtils.readFileToByteArray(file));
}

The general idea is that a File would yield a FileInputStream and a byte[] a ByteArrayInputStream. Both implement InputStream so they should be compatible with any method that uses InputStream as a parameter.

Putting all of the file contents in a ByteArrayInputStream can be done of course:

1. read in the full file into a byte[]; Java version >= 7 contains a convenience method called readAllBytes to read all data from a file;
2. create a ByteArrayInputStream around the file content, which is now in memory.

Note that this may not be optimal solution for very large files - all the file will stored in memory at the same point in time. Using the right stream for the job is important.