Writing unsigned int of 4 bytes over network

问题

I have problem writing an unsigned 4 bytes int in java.

Either writing a long value in java has different result on 64 bit MacOS and 32 bit Linux (Ubuntu) OR Writing to network a 4 byte unsigned int has a problem.

The following call works perfectly on my local OSX

writeUInt32(999999,outputstream)

Reading it back gives me 999999

However when the application is deployed to a network writing a long value results in some other random number (I assume the endian has been switched?) and reading it gives me some other large number.

---------- The complete method stack is as below---------------

public void writeUInt32(long uint32,DataOutputStream stream) throws IOException {
        writeUInt16((int) (uint32 & 0xffff0000) >> 16,stream);
        writeUInt16((int) uint32 & 0x0000ffff,stream);
    }

public void writeUInt16(int uint16,DataOutputStream stream) throws IOException {
        writeUInt8(uint16 >> 8, stream);
        writeUInt8(uint16, stream);
    }

public void writeUInt8(int uint8,DataOutputStream stream) throws IOException {
        stream.write(uint8 & 0xFF);
    }

Edit: To add to the confusion writing to a file and then transporting it over the network sends me the correct value! So when outputstream points to a local file then it writes the correct values but when outputstream points to a ByteArrayOutputStream then the long value written is wrong.

回答1:

Just use DataOutput/InputStream.

To write, cast your long to int

public void writeUInt32(
      long uint32,
      DataOutputStream stream
    ) throws IOException
{
    stream.writeInt( (int) uint32 );
}

On read, use readInt, assign to long and mask top 32 bits to get unsigned value.

public long readUInt32(
      DataInputStream stream
    ) throws IOException
{
    long retVal = stream.readInt( );

    return retVal & 0x00000000FFFFFFFFL;
}

EDIT

From your questions, looks like you are confused about Java cast conversions and promotions for primitive types.

Read this section of Java Spec on Conversions and Promotions: http://java.sun.com/docs/books/jls/third_edition/html/conversions.html

来源：https://stackoverflow.com/questions/7830175/writing-unsigned-int-of-4-bytes-over-network

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java