Why to use ByteArrayInputStream rather than byte[] in Java

As I understand ByteArrayInputStream is used to read byte[] data. Why should I use it rather than simple byte[] (for example reading it from DB).

What is the different between them?

If the input is always a byte[], then you're right, there's often no need for the stream. And if you don't need it, don't use it. One additional advantage of a ByteArrayInputStream is that it serves as a very strong indication that you intend the bytes to be read-only (since the stream doesn't provide an interface for changing them), though it's important to note that a programmer can often still access the bytes directly, so you shouldn't use that in a situation where security is a concern.

But if it's sometimes a byte[], sometimes a file, sometimes a network connection, etc, then you need some sort of abstraction for "a stream of bytes, and I don't care where they come from." That's what an InputStream is. When the source happens to be a byte array, ByteArrayInputStream is a good InputStream to use.

This is helpful in many situations, but to give two concrete examples:

• You're writing a library that takes bytes and processes them somehow (maybe it's an image processing library, for instance). Users of your library may supply bytes from a file, or from a byte[] in memory, or from some other source. So, you provide an interface that accepts an InputStream — which means that if what they have is a byte[], they need to wrap it in a ByteArrayInputStream.
• You're writing code that reads a network connection. But to unit test that code, you don't want to have to open up a connection; you want to just supply some bytes in the code. So the code takes an InputStream, and your test provides a ByteArrayInputStream.