Netty 服务器关闭期间的异常

Question

I have application running on Tomcat. I use Netty 4 for websocket handling. Netty server run in ServletContextListener in contextInitialized method and stop in contextDestroyed.

This my class for Netty server:

public class WebSocketServer {

    private final int port;
    private final EventLoopGroup bossGroup;
    private final EventLoopGroup workerGroup;
    private Channel serverChannel;

    public WebSocketServer(int port) {
        this.port = port;
        bossGroup = new NioEventLoopGroup(1);
        workerGroup = new NioEventLoopGroup();
    }

    public void run() throws Exception {
        final ServerBootstrap b = new ServerBootstrap();
        b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)
                .childHandler(new WebSocketServerInitializer());
        serverChannel = b.bind(port).sync().channel();
        System.out.println("Web socket server started at port " + port + '.');
        System.out
                .println("Open your browser and navigate to http://localhost:"
                        + port + '/');
    }

    public void stop() {
        if (serverChannel != null) {
            ChannelFuture chFuture = serverChannel.close();
            chFuture.addListener(new ChannelFutureListener() {

                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    shutdownWorkers();

                }
            });
        } else {
            shutdownWorkers();
        }
    }

    private void shutdownWorkers() {
        bossGroup.shutdownGracefully();
        workerGroup.shutdownGracefully();
    }
}

It's work fine after running, but when I try stop Tomcat I get exception:

INFO: Illegal access: this web application instance has been stopped already. Could not load io.netty.util.concurrent.DefaultPromise$3. The eventual following stack trace is caused by an error thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access, and has no functional impact. java.lang.IllegalStateException at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1610) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1569) at io.netty.util.concurrent.DefaultPromise.notifyListeners(DefaultPromise.java:592) at io.netty.util.concurrent.DefaultPromise.setSuccess(DefaultPromise.java:403) at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:139) at java.lang.Thread.run(Thread.java:662)

After Tomcat hangs up. What can be reason?

Answer 1

I assume you call shutdownWorkers() somewhere from Servlet.destroy() or use some other mechanism that ensures your Server goes down when servlet stops / unloads.

Then you need to do

void shutdownWorkers() {
    Future fb = trbossGroup.shutdownGracefully();
    Future fw = workerGroup.shutdownGracefully();
    try {
        fb.await();
        fw.await();
    } catch (InterruptedException ignore) {}
}

It is because shutdownGracefully() returns a Future, and, well, without waiting for it to come, you leave things that try to close the connections in very stressful environment. It also makes sense to first initiate all shutdown's and then wait till futures are awailable, this way it all runs in parallel and happens faster.

It fixed the issue for me. Obviously, you can make it nicer to your system without swallowing InterruptedException and wrapping each call in a nice method and putting reasonable timeout for each await() . Nice excercise in general, but in reality most probably you wouldn't care at this point in your code.

Side note: and yes, for WebSockets you will be better off with Tomcat's native, standards-compliant and robust implementation. Netty is awseome for many other things, but would be a wrong tool here.