in what way is java.net.Socket threadsafe?

Question

I have a Socket that I am both reading and writing to, via BufferedReaders and BufferedWriters. I'm not sure which operations are okay to do from separate threads. I would guess that writing to the socket from two different threads at the same time is a bad idea. Same with reading off the socket from two different threads at the same time. What about reading on one thread while writing on another?

I ask because I want to have one thread blocked for a long time on a read as it waits for more data, but during this wait I also have occasional data to send on the socket. I'm not clear if this is threadsafe, or if I should cancel the read before I write (which would be annoying).

Answer 1

Java java.net.Socket is not actually thread safe: Open the Socket source, and look at the (let say) connected member field and how it is used. You will see that is not volatile, read and updated without synchrinization. This indicates that Socket class is not designed to be used by multiple threads. Though, there is some locks and synchronization there, it is not consistent.`

I recommend not to do it. Eventually, use buffers(nio), and do socket reads/writes in one thread

For details go the the discussionv

Answer 2

Very interesting, the nio SocketChannel writes are synchronized

http://www.docjar.com/html/api/sun/nio/ch/SocketChannelImpl.java.html

The old io Socket stuff depends on the OS so you would have to look at the OS native code to know for sure(and that may vary from OS to OS)...

Just look at java.net.SocketOutputStream.java which is what Socket.getOutputStream returns.

(unless of course I missed something).

oh, one more thing, they could have put synchronization in the native code in every JVM on each OS but who knows for sure. Only the nio is obvious that synchronization exists.

Answer 3

You actually read from InputStream and write to OutputStream. They are fairly independent and for as long as you serialize access to each of them you are ok.

You have to correlate, however, the data that you send with data that you receive. That's different from thread safety.

Answer 4

This is how socketWrite in native code, so it's not thread safe from the code

JNIEXPORT void JNICALL
Java_java_net_SocketOutputStream_socketWrite0(JNIEnv *env, jobject this,
                                              jobject fdObj,
                                              jbyteArray data,
                                              jint off, jint len) {
    char *bufP;
    char BUF[MAX_BUFFER_LEN];
    int buflen;
    int fd;

    if (IS_NULL(fdObj)) {
        JNU_ThrowByName(env, "java/net/SocketException", "Socket closed");
        return;
    } else {
        fd = (*env)->GetIntField(env, fdObj, IO_fd_fdID);
        /* Bug 4086704 - If the Socket associated with this file descriptor
         * was closed (sysCloseFD), the the file descriptor is set to -1.
         */
        if (fd == -1) {
            JNU_ThrowByName(env, "java/net/SocketException", "Socket closed");
            return;
        }

    }

    if (len <= MAX_BUFFER_LEN) {
        bufP = BUF;
        buflen = MAX_BUFFER_LEN;
    } else {
        buflen = min(MAX_HEAP_BUFFER_LEN, len);
        bufP = (char *)malloc((size_t)buflen);

        /* if heap exhausted resort to stack buffer */
        if (bufP == NULL) {
            bufP = BUF;
            buflen = MAX_BUFFER_LEN;
        }
    }

    while(len > 0) {
        int loff = 0;
        int chunkLen = min(buflen, len);
        int llen = chunkLen;
        (*env)->GetByteArrayRegion(env, data, off, chunkLen, (jbyte *)bufP);

        while(llen > 0) {
            int n = NET_Send(fd, bufP + loff, llen, 0);
            if (n > 0) {
                llen -= n;
                loff += n;
                continue;
            }
            if (n == JVM_IO_INTR) {
                JNU_ThrowByName(env, "java/io/InterruptedIOException", 0);
            } else {
                if (errno == ECONNRESET) {
                    JNU_ThrowByName(env, "sun/net/ConnectionResetException",
                                    "Connection reset");
                } else {
                    NET_ThrowByNameWithLastError(env, "java/net/SocketException",
                                                 "Write failed");
                }
            }
            if (bufP != BUF) {
                free(bufP);
            }
            return;
        }
        len -= chunkLen;
        off += chunkLen;
    }

    if (bufP != BUF) {
        free(bufP);
    }
}

Answer 5

You can have one thread reading the socket and another thread writing to it. You may want to have a number of threads write to the socket, in which case you have to serialize your access with synchronization or you could have a single writing thread which gets the data to write from a queue. (I prefer the former)

You can use non-blocking IO and share the reading and writing work in a single thread. However this is actually more complex and tricky to get right. If you want to do this I suggest you use a library to help you such as Netty or Mina.

Answer 6

Sockets are thread unsafe at the stream level. You have to provide synchronization. The only warranty is that you won't get copies of the exact same bytes in different read invocations no matter concurrency.

But at a Reader and, specially, Writer level, you might have some locking problems.

Anyway, you can handle read and write operations with the Socket's streams as if they were completely independent objects (they are, the only thing they share is their lifecyle).

Once you have provided correct synchronization among reader threads on one hand, and writer threads on the other hand, any number of readers and writers will be okay. This means that, yes, you can read on one thread and write on another (in fact that's very frequent), and you don't have to stop reading while writing.

One last advice: all of the operations involving threads have associated timeout, make sure that you handle the timeouts correctly.