Toward Polyglot Programming on the JVM

In my previous post titled Treating Javascript as a first class language, I wrote about how the Java Virtual Machine (JVM) is evolving with new languages and frameworks like Groovy, Grails, Scala, Akka, and the Play Framework. In this post, I report on my experience in learning and evaluating these emerging technologies and their roles in the Java ecosystem.

A KangaRoo on the JVM

On a previous project, I used Spring Roo to jumpstart the software development process. Spring Roo was created by Ben Alex, an Australian engineer who is also the creator of Spring Security. Spring Roo was a big productivity boost and generated a significant amount of code and configuration based on the specification of the domain model. Spring Roo automatically generated the following:

• The domain entities with support for JPA annotations.
• Repository and service layers. In addition to JPA, Spring Roo also supports NoSQL persistence for MongoDB based on the Spring Data repository abstraction.
• A web layer with Spring MVC controllers and JSP views with support for Tiles-based layout, theming, and localization. The JSP views were subsequently replaced with a combination of Thymeleaf (a next generation server-side HTML5 template engine) and Twitter Boostrap to support a Responsive Web Design (RWD) approach. Roo also supports GWT and JSF.
• REST and JSON remoting for all domain types.
• Basic configuration for Spring Security, Spring Web Flow, Spring Integration, JMS, Email, and Apache Solr.
• Entity mocking, automatic generation of test data ("Data on Demand"),  in-container integration testing, and end-to-end Selenium integration tests.
• A Maven build file for the project and full integration with Spring STS.
• Deployment to Cloud Foundry.

Roo also supports other features such as database reverse engineering and Ajax . Another benefit of using Roo is that it helped enforce Spring best practices and other architectural concerns such as proper application layering.

For my future projects, I am looking forward to taking developer's productivity and innovation to the next level. There are several criteria in my mind:

• Being able to do more with less. This means being able to write code that is concise, expressive, requires less configuration and boilerplate coding, and is easier to understand and maintain (particularly for difficult concerns like concurrency which is a key factor in scalability).
• Interoperability with the Java language and being able to run on the JVM, so that I can take advantage of the larger and rich Java ecosystem of tools and frameworks.
• Lastly, my interest in responsive, massively scalable, and fault-tolerant systems has picked up recently.

Getting Groovy

Maven has been a very powerful build system for several projects that I have worked on. My goal now is to support continuous delivery pipelines as a pattern for achieving high quality software. Large open source projects like Hibernate, Spring, and Android have already moved to Gradle. Gradle builds are written in a Groovy DSL and are more concise than Maven POM files which are based on a more verbose XML syntax. Gradle supports Java, Groovy, and Scala out-of-the box. It also has other benefits like incremental builds, multi-project builds, and plugins for other essential development tools like Eclipse, Jenkins, SonarQube, Ivy, and Artifactory.

Grails is a full-stack framework based on Groovy, leveraging its concise syntax (which includes Closures), dynamic language programming, metaprogramming, and DSL support. The core principle of Grails is "convention over configuration". Grails also integrates well with existing and popular Java projects like Spring Security, Hibernate, and Sitemesh. Roo generates code at development time and makes use of AOP. Grails on the other hand generates code at run-time, allowing the developer to do more with less code. The scaffolding mechanism is very similar in Roo and Grails.

Grails has its own view technology called Groovy Server Pages (GSP) and its own ORM implementation called Grails Object Relational Mapping (GORM) which uses Hibernate under the hood. There is also decent support for REST/JSON and URL routing to controller actions. This makes it easy to use Grails together with Javascript MVC frameworks like AngularJS in creating more responsive user experiences based on the Single Page Application (SPA) architectural pattern.

There are many factors that can influence the decision to use Roo vs. Grails (e.g., the learning curve associated with Groovy and Grails for a traditional Java team). There is also a new high-productivity framework called Spring Boot that is emerging as part of the soon to be released Spring Framework 4.0.

Becoming Reactive

I am also interested in massively scalable and fault-tolerant systems. This is no longer a requirement solely for big internet players like Google, Twitter, Yahoo, and LinkedIn that need to scale to millions of users. These requirements (including response time and up time) are also essential in mission-critical applications such as healthcare.

The recently published "Reactive Manifesto" makes the case for a new breed of applications called "Reactive Applications". According to the manifesto, the Reactive Application architecture allows developers to build "systems that are event-driven, scalable, resilient, and responsive." That is the premise of the other two prominent languages on the JVM: Scala and Clojure. They are based on a different programming paradigm (than traditional OOP) called Functional Programming that is becoming very popular in the multi-core era.

Twitter uses Scala and has open-sourced some of their internal Scala resources like "Effective Scala" and "Scala School". One interesting framework based on Scala is Akka, a concurrency framework built on the Actor Model.

The Play Framework 2 is a full-stack web application framework based on Scala which is currently used by LinkedIn (which has over 225 millions registered users worldwide). In addition to its elegant design, Play's unique benefits include:

• An embedded Java NIO (New I/O) non-blocking server based on JBoss Netty, providing the ability to call collaborating services asynchronously without relying on thread pools to handle I/O. This new breed of servers is called "Evented Servers" (NodeJS is another implementation) as opposed to the old "Threaded Servers". Older frameworks like Spring MVC use a threaded and synchronous approach which is more difficult to scale.
• The ability to make changes to the source code and just refresh the browser page to see the changes (this is called hot reload).
• Type-safe Scala templates (errors are displayed in the browser during development).
• Integrated support for Akka which provides (among other benefits) fault-tolerance, the ability to quickly recover from failure.
• Asynchronous responses (based on the concepts of "Future" and "Promise" also found in AngularJS), caching, iteratees (for processing large streams of data), and support for real-time push-based technologies like WebSockets and Server-Sent Events.

The biggest challenge in moving to Scala is that the move to Functional Programming can be a significant learning curve for developers with a traditional OOP background in Java. Functional Programming is not new. Languages like Lisp and Haskell are functional programming languages. More recently, XML processing languages like XSLT and XQuery have adopted functional programming ideas.

Bringing Clojure to the JVM

Clojure is a dialect of LISP and a dynamically-type functional programming language which compiles to JVM bytecode. Clojure supports multithreaded programming and immutable data structures. One interesting application of Clojure is Incanter, a statistical computing and data visualization environment enabling big data analysis on the JVM.

Treating Javascript as a first-class language

With the emergence of the Single Page Application (SPA) architecture as an approach to creating more fluid and responsive user experiences in the browser, Javascript is gaining prominence as a platform for modern application development. Paypal, a large online payment service, announced recently that it has achieved significant performance and productivity gains by shifting its server-side development from Java to Javascript. From a software architecture and development perspective, what do expressions like "Javascript as a first-class language" or "Javascript as a platform" actually mean?

Let's consider a well-established first-class language and platform like Java. By the way, I still consider Java a strong and safe bet for developing applications. What makes Java strong is not just the language, but the rich ecosystem of free and open source tools and frameworks built around it (e.g., Eclipse, Tomcat, JBoss Application Server, Drools, Maven, Jenkins, Solr, Hibernate, Spring, Hadoop to name just a few). The JVM is evolving with new languages and frameworks like Groovy, Grails, Clojure, Scala, Akka, and the Play Framework which aim to enhance developer's productivity. It is also well-known that big internet companies like Twitter have achieved significant gains in performance, scalability, and other architectural concerns by shifting a lot of back-end code from Ruby on Rails to the JVM. There are a number of architectural patterns and software development practices that have been adopted over the years in successfully building quality Java applications. These include:

• Design patterns such as the Gang of Four (GoF), Dependency Injection, Model View Controller (MVC), Enterprise Integration Patterns (EIP), Domain Driven Design (DDD), and modularity patterns like those based on OSGi.
• Test-Driven Development (TDD) using tools like JUnit, TestNG, Mockito (mocking), Cucumber-JVM (for behavior-driven development or BDD), and Selenium (for automated end-to-end testing).
• Build tools like Maven and Gradle.
• Static analysis with tools like FindBugs, Checkstyle, PMD, and Sonar.
• Continuous integration and delivery with tools like Jenkins.
• Performance testing with JMeter.
• Web application vulnerability testing with Burp.

As we move to a rich client application paradigm based on Javascript and the Single Page Application (SPA) architecture, it is clear that Javascript can no longer be considered a toy language for front-end developers and so we need to bring the same engineering discipline to Javascript. As I said previously, the JVM remains my platform of choice for back-end development. For example, I find that AngularJS (a client-side Javascript MVC framework) works well with Spring back-end capabilities (like Spring Security and REST support in Spring MVC, HATEOAS, or Grail). However, I also keep an eye on server-side Javascript frameworks like Node.js.

The good news is that the community is coming up with patterns, tools, and practices that are helping elevate Javascript to the status of first-class language. The following is a list of patterns and tools that I find interesting and promising so far:

• Javascript design patterns including the application of the GoFs to Javascript. The MVC and Dependency Injection patterns are both implemented in AngularJS, my favorite Javascript MVC framework. There are also modularity patterns like Asyncronous Module Definition (AMD) supported by RequireJS.
• Functional programming support in Javascript (e.g., higher-order functions and closures) is emerging as a best practice in writing quality Javascript code. 
• Behavior-Driven Development (BDD) testing with Jasmine.
• Static analysis with Javascript code quality tools like JSLint and JSHint.
• Build with Grunt, a Javascript task runner.
• Karma, a test runner for Javascript.
• Protractor, an end-to-end test framework built on top of Selenium WedDriverJS.
• Single Page Applications are subject to common web application vulnerabilities like Cookie Snooping, Cross-Site Scripting (XSS), Cross-Site Request Forgery (CSRF), and JSON Injection. Security is mainly the responsibility of the server, although client-side frameworks like AngularJS also provide some features to enhance the security of Single Page Applications.