Question

I am writing a content distribution network in java. I have a Link class to manage sockets between two nodes in the system. There are two programs, RouterNode and DiscoveryNode.

When a router node starts up, the first thing it does is try to initialize a connection to the discovery node:

public RouterNode(int num)
{
    myNumber = num;
    input = new Scanner(System.in);

    try {
        discoveryServer = new Socket("MONDAY-PC", 60111);
        myServerLink = new Link(this, discoveryServer);
    } catch (IOException e) {
        System.out.println("Socket could not be opened. Node terminating.");
        System.exit(-1);
    }

There is more to the constructor, but my problem keeps my program from getting past this try block.

The constructor of the link class (called on the line ‘myServerLink = new Link(this, discoveryServer); ) looks as such:

public Link(Node n, Socket s)
{
    parentNode = n;
    regSocket = s;
    try {
        out =  new DataOutputStream(regSocket.getOutputStream());
        in = new DataInputStream(regSocket.getInputStream());
    } catch (IOException e) {
        System.out.println("Data Streams could not be created on the link.");
        e.printStackTrace();
    }


    new Thread(new LinkListenerThread(this, in)).run();
}

where the last line of this constructor starts a new thread that is designed to listen on the socket for incoming messages that are being passed.

The run() method in LinkListenerThread stars as follows:

@Override
public void run()
{
    byte[] message;     

    System.out.println("Link now active and running.");
    while(!done)
    {
        System.out.println("attempting to read from socket...");

        try {

            // read now many bytes the following message will be
            byte[] messageLengthBytes = new byte[4];
            in.read(messageLengthBytes, 0, 4);

My problem is that once I instantiate the link from the router node, it’s execution stops from what seems to be the LinkListenerThread blocking it when it calls in.read(). This listener is running on a separate thread so I am not sure if this is actually something strange with threads, or it is just an example of my lack of experience with them.

I have another instance in my program where I am reading on a separate thread

Could this be caused because the node classes aren’t explicitly implementing runnable and therefore are not on their own threads?

Any help is greatly appreciated.

Edit 1: I have made the Node classes implement Runnable and starting them on their own threads, but it still locks up when the in.read() is called;

Answer 1

You intend to start a thread by calling Thread.run() method instead of Thread.start(). And this makes your program single-threaded, then the I/O ops block the only thread(the main thread) in the try-catch clause if there is no data coming in or the while loop never ends.

Just use Thread.start() to start a thread can fix your problem.