Eclipse Vert.x：Java开发者异步编程指南

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" Vert.x for Java 开发者指南是一本面向Java开发者的详细教程，它介绍了如何使用Vert.x这一高性能的异步开发框架。该框架基于事件驱动模型，旨在提供高效的并发处理能力，适合构建现代微服务和分布式系统。本文档涵盖了核心的Vert.x概念，如线程模型和事件总线，并通过一个最小化的维基百科应用实例，演示了如何使用Vert.x进行实际开发。此外，还讨论了如何将应用程序重构为独立且可重用的verticles，以提高模块化和可维护性。" 在深入探讨之前，我们先了解一些基本概念： 1. **关于Vert.x**：Vert.x是一个轻量级、高性能的运行时平台，允许开发者以反应式编程风格编写应用程序。它支持多种编程语言，但主要针对Java社区。Vert.x的核心特点是其非阻塞I/O模型和事件循环机制，这使得它可以高效地处理大量并发连接。 2. **异步编程与事件驱动**：在Vert.x中，开发者使用事件驱动模型来编写代码，这意味着程序不会等待某个操作完成，而是通过回调函数或Future对象来处理结果。这种方式可以避免线程阻塞，从而充分利用系统资源。 3. **事件总线(Eventbus)**：事件总线是Vert.x的一个关键特性，它允许不同的verticles（Vert.x中的轻量级进程）之间进行通信。这种解耦的通信方式使得组件之间的交互变得简单且可扩展。接下来，文档通过一个简单的维基百科应用示例，逐步展示了如何使用Vert.x： - **启动Maven项目**：设置一个新的Maven项目，引入Vert.x的依赖库，为项目构建基础结构。 - **verticle的架构**：了解verticle的基本结构，它是Vert.x中的基本执行单元，负责处理特定任务。 - **未来对象(Future)和回调(Callback)**：学习如何在Vert.x中处理异步操作，如数据库交互，通过Future和回调来处理结果。 - **初始化阶段**：包括数据库初始化、日志配置和HTTP服务器的启动，这些都是构建维基应用的关键步骤。 - **HTTP路由器处理器**：定义路由规则，如处理主页请求、渲染页面、创建新页面、保存页面和删除页面等操作。 - **运行应用**：最后，文档指导读者如何运行这个基于Vert.x的维基应用。 3. **重构为独立verticles**：为了提高代码的可复用性和模块化，文档探讨了如何将应用程序拆分为独立的verticles，每个verticle负责特定功能，这样可以更好地管理和扩展应用程序。通过这个指南，Java开发者能够掌握Vert.x的基本用法，理解其异步编程模型，并学会如何利用事件总线进行组件间的通信。这将帮助他们构建出更高效、可扩展的现代应用程序。

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.

Eventually steps completes: setHandler defines a handler to be called upon completion. In our case

we simply want to complete startFuture with steps and use the completer method to obtain a

handler. This is equivalent to:

Future<Void> steps = prepareDatabase().compose(v -> startHttpServer());

steps.setHandler(ar -> { ①

Ê if (ar.succeeded()) {

Ê startFuture.complete();

Ê } else {

Ê startFuture.fail(ar.cause());

Ê }

});

①

ar is of type AsyncResult<Void>. AsyncResult<T> is used to pass the result of an asynchronous

processing and may either yield a value of type T on success or a failure exception if the

processing failed.

2.5.1. Database initialization

The wiki database schema consists of a single table Pages with the following columns:

Column Type Description

Integer Primary key

Name

Characters Name of a wiki page, must be

unique

Content

Text Markdown text of a wiki page

The database operations will be typical create, read, update, delete operations. To get us started, we

simply store the corresponding SQL queries as static fields of the MainVerticle class. Note that they

are written in a SQL dialect that HSQLDB understands, but that other relational databases may not

necessarily support:

private static final String SQL_CREATE_PAGES_TABLE = "create table if not exists Pages (Id integer identity primary key,

Name varchar(255) unique, Content clob)";

private static final String SQL_GET_PAGE = "select Id, Content from Pages where Name = ?"; ①

private static final String SQL_CREATE_PAGE = "insert into Pages values (NULL, ?, ?)";

private static final String SQL_SAVE_PAGE = "update Pages set Content = ? where Id = ?";

private static final String SQL_ALL_PAGES = "select Name from Pages";

private static final String SQL_DELETE_PAGE = "delete from Pages where Id = ?";

①

The ? in the queries are placeholders to pass data when executing queries, and the Vert.x JDBC

client prevents from SQL injections.

The application verticle needs to keep a reference to a JDBCClient object (from the

io.vertx.ext.jdbc package) that serves as the connection to the database. We do so using a field in

MainVerticle, and we also create a general-purpose logger from the org.slf4j package:

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