Eclipse Vert.x异步编程指南：Java开发者实战

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《Eclipse Vert.x指南：Java开发者的手册》是一本由Julien Ponge、Thomas Segismont和Julien Viet编写的针对Java开发者的权威指南，着重于介绍如何利用Vert.x进行异步编程，尤其是在高并发场景下的微服务开发。Vert.x的核心理念是事件驱动和非阻塞I/O模型，这意味着通过较少的内核线程，应用程序能够处理大量的并发请求并实现平滑扩展。本书第1章首先概述了该指南的目的，解释了什么是Vert.x，它如何区别于传统的并发解决方案。核心概念包括线程模型和事件总线，后者是Vert.x架构中的关键组件，用于在各个组件间传递消息和协调工作。第二部分深入探讨了如何使用Vert.x构建一个最小可行的wiki应用。这部分内容包括了Maven项目的初始化，如何添加必要的依赖，以及如何设计和实现verticle（Vert.x中的基本执行单元）。读者会学习到关于Vert.x Future对象和回调函数的基础知识，这些是处理异步操作的重要工具。章节2详细介绍了wiki verticle的生命周期管理，如数据库初始化、日志处理、HTTP服务器设置等。HTTP路由处理器的实现也得到了详尽的阐述，包括索引页面、渲染页面、创建和保存页面以及删除页面的处理器。最后，如何运行整个应用被详细说明。第三部分讨论了将verticle模块化和重用的方法，旨在帮助开发者设计可维护且复用的架构。这涉及到了技术上的最佳实践，包括如何划分职责、设计良好的接口以及处理依赖关系。《Eclipse Vert.x for Java Developers》是一本实战性强的教程，适合希望学习和掌握Vert.x框架进行高效并发编程和微服务开发的Java开发者。通过阅读这本书，读者不仅能理解异步编程的核心原理，还能学会如何将其应用到实际项目中，提升应用的性能和扩展性。

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 is 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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