this guide only uses callbacks in the first sections to ensure that the reader gets familiar with the
core concepts in Vert.x. It is also arguably easier to start with callbacks to draw a line between the
many sections of asynchronous code. Once it becomes evident in the sample code that callbacks do
not always lead to easily readable code, we will introduce the RxJava support to show how the
same asynchronous code can be better expressed by thinking in streams of processed events.
2.5. Wiki verticle initialization phases
To get our wiki running, we need to perform a 2-phases initialization:
1. we need to establish a JDBC database connection, and also make sure that the database schema
is in place, and
2. we need to start a HTTP server for the web application.
Each phase can fail (e.g., the HTTP server TCP port is already being used), and they should not run
in parallel as the web application code first needs the database access to work.
To make our code cleaner we will define 1 method per phase, and adopt a pattern of returning a
future / promise object to notify when each of the phases completes, and whether it did so
successfully or not:
private Future<Void> prepareDatabase() {
Ê Future<Void> future = Future.future();
Ê // (...)
Ê return future;
}
private Future<Void> startHttpServer() {
Ê Future<Void> future = Future.future();
Ê // (...)
Ê return future;
}
By having each method returning a future object, the implementation of the start method becomes
a composition:
@Override
public void start(Future<Void> startFuture) throws Exception {
Ê Future<Void> steps = prepareDatabase().compose(v -> startHttpServer());
Ê steps.setHandler(startFuture.completer());
}
When the future of prepareDatabase completes successfully, then startHttpServer is called and the
steps future completes depending of the outcome of the future returned by startHttpServer.
startHttpServer is never called if prepareDatabase encounters an error, in which case the steps
future is in a failed state and becomes completed with the exception describing the error.
11
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