Undertow

This is a very fast Java web server. When using this web server, I met some problems but I could not find enough answers even on the stackOverflow. Therefore, I decided to write my experience down.

How to set parameters to each Servlet?

It is necessary for us to pass some parameters to Serlet functions. One way to do this is to use addInitParam when adding Servlet. Like the following code.

DeploymentInfo servletBuilder = deployment()
                    .setClassLoader(ServletServer.class.getClassLoader())
                    .setContextPath(MYAPP)
                    .setDeploymentName("test.war")
                    .addServlets(
                            servlet("MessageServlet", MessageServlet.class)
                                    .addInitParam("message", "Hello World")
                                    .addMapping("/*"),
                    )

In the example, we set parameter message to “Hello Word”. Then how to we read that in the Servlet funtion? We could set in the init.

//MessageServlet.java
    public static final String MESSAGE = "message";

    private String message;

    @Override
    public void init(final ServletConfig config) throws ServletException {
        super.init(config);
        message = config.getInitParameter(MESSAGE);
    }

Why we could not get any new request from doGet

There is a very strange question happened when we used Undertow. Although we got requests from load generator continuously, we could not see any from the doGet function.

It is because all listeners are pending at this time and we could not get any requests now. You could learn more from here. There is a limited threads opened for Undertow, if all WORKER_IO_THREADS threads were used, we will lose all incoming requests. We could set parameters for Undertow using the following codes.

XnioWorker worker = xnio.createWorker(OptionMap.builder()
    .set(Options.WORKER_IO_THREADS, ioThreads)
    .set(Options.WORKER_TASK_CORE_THREADS, workerThreads)
    .set(Options.WORKER_TASK_MAX_THREADS, workerThreads)
    .set(Options.TCP_NODELAY, true)
    .getMap());

Connecting to MySQL problems

We could use PreparedStatement to perform SQL query.

PreparedStatement statement = connection.prepareStatement(
             "SELECT * FROM World");
ResultSet resultSet = statement.executeQuery()

We also need to close them after using them. If you did not close them, there would occur out of memory problem. reference.