Develop Reference

Reading n bytes atomically without blocking

I just asked a question about why my thread shut down wasn't working. It ended up being due to readLine() blocking my thread before the shutdown flag could be recognised. This was easy to fix by checking ready() before calling readLine().
However, I'm now using a DataInputStream to do the following in series:
int x = reader.readInt();
int y = reader.readInt();
byte[] z = new byte[y]
reader.readFully(z);
I know I could implement my own buffering which would check the running file flag while loading up the buffer. But I know this would be tedious. Instead, I could let the data be buffered within the InputStream class, and wait until I have my n bytes read, before executing a non-blocking read - as I know how much I need to read.
4 bytes for the first integer
4 bytes for the second integer y
and y bytes for the z byte array.
Instead of using ready() to check if there is a line in the buffer, is there some equivalent ready(int bytesNeeded)?

The available() method returns the amount of bytes in the InputStreams internal buffer.
So, one can do something like:
while (reader.available() < 4) checkIfShutdown();
reader.readInt();

You can use InputStream.available() to get an estimate of the amount of bytes that can be read. Quoting the Javadoc:
Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking, which may be 0, or 0 when end of stream is detected. The read might be on the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes.
In other words, if available() returns n, you know you can safely call read(n) without blocking. Note that, as the Javadoc states, the value returned is an estimate. For example, InflaterInputStream.available() will always return 1 if EOF isn't reached. Check the documentation of the InputStream subclass you will be using to ensure it meets your needs.

You are going to need to implement your own equivalent of BufferedInputStream. Either as a sole owner of an InputStream and a thread (possibly borrowed from a pool) to block in. Alternatively, implement with NIO.

available and read: can we actually be nonblocking while using blocking Java I/O?

Based on this question I would like to ask the following.
Assuming blocking I/O and I have a piece of code like the following:
byte[] data = new byte[10];
someInputStream.read(data)
This code snippet is going to block in the read call until it has some bytes to read. I'm familiar that read might actually read less bytes and the number of bytes read is going to be returned by the read method.
My question is this. Assume I have:
byte[] data = new byte[10];
if (someInputeStream.available() >= 10) {
someInputStream.read(data); // ***
}
Is *** line guaranteed to not block? Again, I'm aware that this read might still read less than 10 bytes.

It is guaranteed not to block.
From the Javadoc for InputStream, looking at the available() method:
Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream. The next invocation might be the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes.
(Emphasis mine.)
So it won't block, but (as you say) you might not get the full 10 bytes.
Note that this is assuming a single thread. If there are multiple threads, then of course another thread might have read from the stream between the available() and the read().

After calling read() my InputStream beacomes empty

im making a form to upload several images to a blob field in a mysql databases.
In a servlet i get all the images uploaded by users using a type="file".
Before inserting the images into the database, i have to check if the type="file" is empyt or not.
For doing this, according to the corrent structure of my code i do this:
if(allegatoInputStream.read()>-1){....
//allegatoInputStream is my InputStream
But, after i call the read() method, my InputStream becomes empty, so i have nothing to insert in my blob field.
i did something like this
InputStream InputStream_FOR_THE_CHECK = blablabla.getInputStream();
if(InputStream_FOR_THE_CHECK.read()>-1){....
But i don't think this is the right way to do what i want to do

You could wrap the stream in a PushbackInputStream, read one byte to see if there is content, then push back the byte if there is. As long as you then use the same instance of PushbackInputStream when you process the file content, that byte will then be part of the stream.

Your read() call reads a byte from the stream and throws it away. You need to store the result in a variable, test it for -1, and if it isn't, use it. It's the value of the next byte in the stream.

You can use inputStream.available(). From the JavaDoc:
Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream. The next invocation might be the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes.
Note that while some implementations of InputStream will return the total number of bytes in the stream, many will not. It is never correct to use the return value of this method to allocate a buffer intended to hold all data in this stream.
A subclass' implementation of this method may choose to throw an IOException if this input stream has been closed by invoking the close() method.
In your scenario:
if(allegatoInputStream.available()>0){....
//allegatoInputStream is my InputStream

InputDataStream.available() is always 0 Java Socket Client

Why does this part of my client code is always zero ?
InputStream inputStream = clientSocket.getInputStream();
int readCount = inputStream.available(); // >> IS ALWAYS ZERO
byte[] recvBytes = new byte[readCount];
ByteArrayOutputStream baos = new ByteArrayOutputStream();
int n = inputStream.read(recvBytes);
...

Presumably it's because no data has been received yet. available() tries to return the amount of data available right now without blocking, so if you call available() straight after making the connection, I'd expect to receive 0 most of the time. If you wait a while, you may well find available() returns a different value.
However, personally I don't typically use available() anyway. I create a buffer of some appropriate size for the situation, and just read into that:
byte[] data = new byte[16 * 1024];
int bytesRead = stream.read(data);
That will block until some data is available, but it may well return read than 16K of data. If you want to keep reading until you reach the end of the stream, you need to loop round.
Basically it depends on what you're trying to do, but available() is rarely useful in my experience.

From the java docs
Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream. The next invocation might be the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes.
Note that while some implementations of InputStream will return the total number of bytes in the stream, many will not. It is never correct to use the return value of this method to allocate a buffer intended to hold all data in this stream.
A subclass' implementation of this method may choose to throw an IOException if this input stream has been closed by invoking the close() method.
The available method for class InputStream always returns 0.
This method should be overridden by subclasses.
Here is a note to understand why it returns 0
In InputStreams, read() calls are said to be "blocking" method calls. That means that if no data is available at the time of the method call, the method will wait for data to be made available.
The available() method tells you how many bytes can be read until the read() call will block the execution flow of your program. On most of the input streams, all call to read() are blocking, that's why available returns 0 by default.
However, on some streams (such as BufferedInputStream, that have an internal buffer), some bytes are read and kept in memory, so you can read them without blocking the program flow. In this case, the available() method tells you how many bytes are kept in the buffer.

According to the documentation, available() only returns the number of bytes that can be read from the stream without blocking. It doesn't mean that a read operation won't return anything.
You should check this value after a delay, to see it increasing.

There are very few correct uses of available(), and this isn't one of them. In this case no data had arrived so it returned zero, which is what it's supposed to do.
Just read until you have what you need. It will block until data is available.

Channel.read returns -2

I'm using Java NIO to do socket operations. In the past when working with streams, the read call (where you are reading into a byte array, or in this case a ByteBuffer) returns the number of bytes read from the stream, or -1 if the stream was closed. So you basically can do
while(channel.read(buffer) != -1){
//do stuff
}
However, I noticed that I was killing my servers. When I added some logging statements, I noticed that the read() call was returning -2 at the end of the stream. According to the documentation:
Returns: The number of bytes read, possibly zero, or -1 if the channel
has reached end-of-stream
Has anybody experienced this before? I changed my code to loop on a value >0, but I wanted to make sure I understand what's going on.