Java - 如何从inputStream（socket / socketServer）读取未知数量的字节？

Question

Looking to read in some bytes over a socket using an inputStream. The bytes sent by the server may be of variable quantity, and the client doesn't know in advance the length of the byte array. How may this be accomplished?

byte b[]; 
sock.getInputStream().read(b);

This causes a 'might not be initialized error' from the Net BzEAnSZ. Help.

Answer 1

You need to expand the buffer as needed , by reading in chunks of bytes, 1024 at a time as in this example code I wrote some time ago

    byte[] resultBuff = new byte[0];
    byte[] buff = new byte[1024];
    int k = -1;
    while((k = sock.getInputStream().read(buff, 0, buff.length)) > -1) {
        byte[] tbuff = new byte[resultBuff.length + k]; // temp buffer size = bytes already read + bytes last read
        System.arraycopy(resultBuff, 0, tbuff, 0, resultBuff.length); // copy previous bytes
        System.arraycopy(buff, 0, tbuff, resultBuff.length, k);  // copy current lot
        resultBuff = tbuff; // call the temp buffer as your result buff
    }
    System.out.println(resultBuff.length + " bytes read.");
    return resultBuff;

Answer 2

Assuming the sender closes the stream at the end of the data:

ByteArrayOutputStream baos = new ByteArrayOutputStream();

byte[] buf = new byte[4096];
while(true) {
  int n = is.read(buf);
  if( n < 0 ) break;
  baos.write(buf,0,n);
}

byte data[] = baos.toByteArray();

Answer 3

Read an int, which is the size of the next segment of data being received. Create a buffer with that size, or use a roomy pre-existing buffer. Read into the buffer, making sure it is limited to the aforeread size. Rinse and repeat :)

If you really don't know the size in advance as you said, read into an expanding ByteArrayOutputStream as the other answers have mentioned. However, the size method really is the most reliable.

Answer 4

The simple answer is:

byte b[] = byte[BIG_ENOUGH];
int nosRead = sock.getInputStream().read(b);

where BIG_ENOUGH is big enough.

---

But in general there is a big problem with this. A single read call is not guaranteed to return all that the other end has written.

• If the nosRead value is BIG_ENOUGH , your application has no way of knowing for sure if there are more bytes to come; the other end may have sent exactly BIG_ENOUGH bytes ... or more than BIG_ENOUGH bytes. In the former case, you application will block (for ever) if you try to read. In the latter case, your application has to do (at least) another read to get the rest of the data.

• If the nosRead value is less than BIG_ENOUGH , your application still doesn't know. It might have received everything there is, part of the data may have been delayed (due to network packet fragmentation, network packet loss, network partition, etc), or the other end might have blocked or crashed part way through sending the data.

The best answer is that EITHER your application needs to know beforehand how many bytes to expect, OR the application protocol needs to somehow tell the application how many bytes to expect or when all bytes have been sent.

Possible approaches are:

• the application protocol uses fixed message sizes (not applicable to your example)
• the application protocol message sizes are specified in message headers
• the application protocol uses end-of-message markers
• the application protocol is not message based, and the other end closes the connection to say that is the end .

Without one of these strategies, your application is left to guess, and is liable to get it wrong occasionally.

Then you use multiple read calls and (maybe) multiple buffers.

Answer 5

Without re-inventing the wheel, using Apache Commons:

IOUtils.toByteArray(inputStream);

For example, complete code with error handling:

    public static byte[] readInputStreamToByteArray(InputStream inputStream) {
    if (inputStream == null) {
        // normally, the caller should check for null after getting the InputStream object from a resource
        throw new FileProcessingException("Cannot read from InputStream that is NULL. The resource requested by the caller may not exist or was not looked up correctly.");
    }
    try {
        return IOUtils.toByteArray(inputStream);
    } catch (IOException e) {
        throw new FileProcessingException("Error reading input stream.", e);
    } finally {
        closeStream(inputStream);
    }
}

private static void closeStream(Closeable closeable) {
    try {
        if (closeable != null) {
            closeable.close();
        }
    } catch (Exception e) {
        throw new FileProcessingException("IO Error closing a stream.", e);
    }
}

Where FileProcessingException is your app-specific meaningful RT exception that will travel uninterrupted to your proper handler w/o polluting the code in between.

Answer 6

Stream all Input data into Output stream. Here is working example:

    InputStream inputStream = null;
    byte[] tempStorage = new byte[1024];//try to read 1Kb at time
    int bLength;
    try{

        ByteArrayOutputStream outputByteArrayStream =  new ByteArrayOutputStream();     
        if (fileName.startsWith("http"))
            inputStream = new URL(fileName).openStream();
        else
            inputStream = new FileInputStream(fileName);            

        while ((bLength = inputStream.read(tempStorage)) != -1) {
                outputByteArrayStream.write(tempStorage, 0, bLength);
        }
        outputByteArrayStream.flush();
        //Here is the byte array at the end
        byte[] finalByteArray = outputByteArrayStream.toByteArray();
        outputByteArrayStream.close();
        inputStream.close();
    }catch(Exception e){
        e.printStackTrace();
        if (inputStream != null) inputStream.close();
    }

Answer 7

Here is a simpler example using ByteArrayOutputStream...

        socketInputStream = socket.getInputStream();
        int expectedDataLength = 128; //todo - set accordingly/experiment. Does not have to be precise value.
        ByteArrayOutputStream baos = new ByteArrayOutputStream(expectedDataLength);
        byte[] chunk = new byte[expectedDataLength];
        int numBytesJustRead;
        while((numBytesJustRead = socketInputStream.read(chunk)) != -1) {
            baos.write(chunk, 0, numBytesJustRead);
        }
        return baos.toString("UTF-8");

However, if the server does not return a -1, you will need to detect the end of the data some other way - eg, maybe the returned content always ends with a certain marker (eg, ""), or you could possibly solve using socket.setSoTimeout(). (Mentioning this as it is seems to be a common problem.)

Answer 8

这是一个迟到的答案和自我广告，但任何人都可以查看这个问题： https ： //github.com/GregoryConrad/SmartSocket

Answer 9

This question is 7 years old, but i had a similiar problem, while making a NIO and OIO compatible system (Client and Server might be whatever they want, OIO or NIO).

This was quit the challenge, because of the blocking InputStreams.

I found a way, which makes it possible and i want to post it, to help people with similiar problems.

Reading a byte array of dynamic sice is done here with the DataInputStream , which kann be simply wrapped around the socketInputStream. Also, i do not want to introduce a specific communication protocoll (like first sending the size of bytes, that will be send), because i want to make this as vanilla as possible. First of, i have a simple utility Buffer class, which looks like this:

import java.util.ArrayList;
import java.util.List;

public class Buffer {

    private byte[] core;
    private int capacity;

    public Buffer(int size){
        this.capacity = size;
        clear();
    }

    public List<Byte> list() {
        final List<Byte> result = new ArrayList<>();
        for(byte b : core) {
            result.add(b);
        }

        return result;
    }

    public void reallocate(int capacity) {
        this.capacity = capacity;
    }

    public void teardown() {
        this.core = null;
    }

    public void clear() {
        core = new byte[capacity];
    }

    public byte[] array() {
        return core;
    }
}

This class only exists, because of the dumb way, byte <=> Byte autoboxing in Java works with this List. This is not realy needed at all in this example, but i did not want to leave something out of this explanation.

Next up, the 2 simple, core methods. In those, a StringBuilder is used as a "callback". It will be filled with the result which has been read and the amount of bytes read will be returned. This might be done different of course.

private int readNext(StringBuilder stringBuilder, Buffer buffer) throws IOException {
    // Attempt to read up to the buffers size
    int read = in.read(buffer.array());
    // If EOF is reached (-1 read)
    // we disconnect, because the
    // other end disconnected.
    if(read == -1) {
        disconnect();
        return -1;
    }
    // Add the read byte[] as
    // a String to the stringBuilder.
    stringBuilder.append(new String(buffer.array()).trim());
    buffer.clear();

    return read;
}

private Optional<String> readBlocking() throws IOException {
    final Buffer buffer = new Buffer(256);
    final StringBuilder stringBuilder = new StringBuilder();
    // This call blocks. Therefor
    // if we continue past this point
    // we WILL have some sort of
    // result. This might be -1, which
    // means, EOF (disconnect.)
    if(readNext(stringBuilder, buffer) == -1) {
        return Optional.empty();
    }
    while(in.available() > 0) {
        buffer.reallocate(in.available());
        if(readNext(stringBuilder, buffer) == -1) {
            return Optional.empty();
        }
    }

    buffer.teardown();

    return Optional.of(stringBuilder.toString());
}

The first method readNext will fill the buffer, with byte[] from the DataInputStream and return the amount bytes read this way.

In the secon method, readBlocking , i utilized the blocking nature, not to worry about consumer-producer-problems . Simply readBlocking will block, untill a new byte-array is received. Before we call this blocking method, we allocate a Buffer-size. Note, i called reallocate after the first read (inside the while loop). This is not needed. You can safely delete this line and the code will still work. I did it, because of the uniqueness of my problem.

The 2 things, i did not explain in more detail are: 1. in (the DataInputStream and the only short varaible here, sorry for that) 2. disconnect (your disconnect routine)

All in all, you can now use it, this way:

// The in has to be an attribute, or an parameter to the readBlocking method
DataInputStream in = new DataInputStream(socket.getInputStream());
final Optional<String> rawDataOptional = readBlocking();
rawDataOptional.ifPresent(string -> threadPool.execute(() -> handle(string)));

This will provide you with a way of reading byte arrays of any shape or form over a socket (or any InputStream realy). Hope this helps!

Answer 10

Either:

1. Have the sender close the socket after transferring the bytes. Then at the receiver just keep reading until EOS.

2. Have the sender prefix a length word as per Chris's suggestion, then read that many bytes.

3. Use a self-describing protocol such as XML, Serialization, ...

Answer 11

Use BufferedInputStream , and use the available() method which returns the size of bytes available for reading, and then construct a byte[] with that size. Problem solved. :)

BufferedInputStream buf = new BufferedInputStream(is);  
int size = buf.available();