shaz shaz - 2 months ago 12
Java Question

Need to wait for asynchronous api callback before I return from method in Java

import java.util.concurrent.CountDownLatch;

import quickfix.Initiator;


public class UserSession {
private final CountDownLatch latch = new CountDownLatch(1);

public String await() {
try {
System.out.println("waiting...");
if (latch.await(5, TimeUnit.SECONDS))
System.out.println("released!");
else
System.out.println("timed out");
return secret;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
System.out.println(e.getMessage());
e.printStackTrace();
}
return null;
}

public void countdown(String s) {
System.out.println("In countdown: "+s+ ". Latch count: "+latch.getCount());
secret = s;
latch.countDown();
System.out.println("Latch count: "+latch.getCount());
}
}


public class LogonHandler extends AbstractHandler {

public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
Map<String,String[]> query = request.getParameterMap();

if (query.containsKey("method")) {
if (query.get("method")[0].compareTo(method) == 0) {
baseRequest.setHandled(true);
response.getWriter().println(logon(query));
}
}
else
baseRequest.setHandled(false);
}

private String logon(Map<String,String[]> query) {
if (query.containsKey("username") && query.containsKey("password") && query.containsKey("sendercompid")) {

app.mapUser(query.get("sendercompid")[0], new UserSession(query.get("username")[0], query.get("password")[0]));

SessionID session = new SessionID(new BeginString("FIX.4.4"), new SenderCompID(query.get("sendercompid")[0]), new TargetCompID("PARFX"));

try {
ThreadedSocketInitiator tsi = new ThreadedSocketInitiator(app, app.getFileStoreFactory(), settings, app.getLogFactory(), app.getMessageFactory());
UserSession userSession = new UserSession(query.get("username")[0], query.get("password")[0]);
userSession.setInitiator(tsi);

tsi.start();
return userSession.await();
} catch (ConfigError e) {
// TODO Auto-generated catch block
e.printStackTrace();
return e.toString();
}
}
return "fail";
}
}


public class QuickfixjApplication implements Application {
private Map<String,UserSession> users = new HashMap<String,UserSession>();

public void mapUser(String s, UserSession u) {
users.put(s, u);
}

public void toAdmin(Message message, SessionID sessionId) {

try {
if (message.getHeader().getField(new StringField(MsgType.FIELD)).valueEquals(Logon.MSGTYPE)) {
UserSession user = users.get(sessionId.getSenderCompID());
message.setField(new Username(user.getUsername()));
message.setField(new Password(user.getPassword()));
}
} catch (FieldNotFound e) {
e.printStackTrace();
}
}

public void fromAdmin(Message message, SessionID sessionId)
throws FieldNotFound, IncorrectDataFormat, IncorrectTagValue, RejectLogon {

if (message.getHeader().getField(new StringField(MsgType.FIELD)).valueEquals(Logon.MSGTYPE)) {
System.out.println(message.toString());
UserSession user = users.get(sessionId.getSenderCompID());
user.countdown(message.toString());
}
}
}


Ok, I've tried to only include the minimum amount of code here. There are three interesting classes, UserSession is the internal glue between the Jetty handler and the QuickFix/j application.

The LogonHandler receives an HTTP logon request and tries to log a user onto a QuickFix/j application session.

QuickFix/j is sending a logon message to a FIX server, this logon request / response is asynchronous. The HTTP logon request is of course synchronous. So we have to wait for the reply from the FIX server before we return from the HTTP request. I do this using CountDownLatch and this UserSession object.

When I create the QuickFix/j session object I also create a UserSession object and add it to a map (that happens in the LogonHandler logon method).

There are two callbacks in the QuickFix/j application object, toAdmin() and fromAdmin(). In fromAdmin() I check if the message is a logon response and if it is I call a method of UserSession to countdown the latch. In debugging the code I see that the fromAdmin() method is hit, the UserSession object is found in the map and the countdown() method is called and the latch.getCount() goes from 1 to 0, but the latch.await() method in UserSession await() never returns. It always times out.

Answer

You could use CountDownLatch like this:

public class LogonHandler implements Handler {
    private final CountDownLatch loginLatch = new CountDownLatch (1);

    private boolean callbackResults;

    public void serverResponseCallback(boolean result) {
        callbackResults = result;
        loginLatch.countDown ();
    }

    public boolean tryLogon(Credentials creds) throws InterruptedException {
        SomeServer server = new SomeServer(address);
        server.tryLogon (creds.getName (), creds.getPass ());
        loginLatch.await ();
        return callbackResults;
    }
}

If you want to limit waiting time by, for example, 5 seconds, then instead of loginLatch.await () use the following:

if (loginLatch.await (5L, TimeUnit.SECONDS))
    return callbackResults;
else
    return false; // Timeout exceeded
Comments