This section highlights the realtime resources available for Java developers.

Realtime Java Libraries

Realtime Cloud Messaging Java SDK – Part of the The Realtime® Framework, Realtime Cloud Messaging (aka ORTC) is a secure, fast and highly scalable cloud-hosted Pub/Sub real-time message broker for web and mobile apps. If your application has data that needs to be updated in the user’s interface as it changes (e.g. real-time stock quotes or ever changing social news feed) Realtime Cloud Messaging is the reliable, easy, unbelievably fast, “works everywhere” solution.

Atmosphere: The Asynchronous WebSocket/Comet Framework – The Atmosphere Framework contains client and server side components for building Asynchronous Web Applications. The majority of popular frameworks are either supporting Atmosphere or supported natively by the framework. The Atmosphere Framework supports all major Browsers and Servers.

OneBusAway: Java classes generated for GTFS-realtime extensions – This project contains Java libraries generated from the OneBusAway-specific and NYCT-specific GTFS-realtime protocol buffer extensions.

Intrinio: Java SDK for Real-Time Stock Prices – Intrinio provides real-time stock prices via a two-way WebSocket connection.

Firebase: Java Realtime Database Quickstart – The Java Firebase Database quickstart demonstrates how to connect to and use the Firebase Realtime Database using Java through a simple social blogging app. It will interoperate with the Web, iOS and Android database quickstarts.

jInstagram: A Java library for the Instagram API – An unofficial Java library for the Instagram API. Note : jInstagram uses code from the scribe-java library developed by Pablo Fernandez.

Symphony Java Client – The Symphony java client provides a real-time wrapper around the Symphony REST API’s to simplify the creation of chat sessions, room access, presence, messaging and more… The client provides a set of logical services representing supported features of the Symphony platform. Services support real-time events through feature based listeners and communication objects. Access is not limited to the services as all underlying Symphony client implementations are exposed for advanced use or creation of your own service.

Java SE APIs


  • Developing Realtime Software with Java SE APIs.
  • Author: Oracle, Kelvin Nilsen
  • August 2014



The use of Java SE APIs in the implementation of real-time systems is most appropriate for soft real-time development. Using Java SE for hard real-time development is also possible, but generally requires the use of more specialized techniques such as the use of NoHeapRealtimeThread abstractions, as described in the Real-Time Specification for Java (JSR 1), or the use of the somewhat simpler ManagedSchedulable abstractions of the Safety Critical Java Technology specification (JSR 302).

It is also important to distinguish real-time engineering, as it is described in this series, from performance engineering. An e-commerce web server, for example, might have been carefully engineered to support an average of 1,000 transactions per second. That is different from saying that every transaction must be completed in 1 ms. It could be that some transactions require hundreds of ms and others are completed in less than 1 ms, as long as the average of all transactions is 1 ms. It could also mean that each transaction requires an average of 4 ms from start to end, but the system has the ability to concurrently execute four transactions at a time.

The benefits of the Java language are especially valuable in real-time applications that are large, complex, and dynamic. Software engineers are motivated to select Java SE when their projects require dynamic code updates, coordination between multiple teams of developers, integration of independently developed software components, support for multiple hardware or operating system platforms, or support for multiple software configurations as product requirements evolve over multiple years or even decades.

Intro to Realtime Java


  • Real-Time Java: An Introduction
  • Author: O’Reilly, Peter Mikhalenko
  • May 2006



Real-time application development requires an API set and semantics that allow developers to correctly control the temporal behavior of application, i.e., how it will behave in real-world time. A real-time edition of Java must therefore provide some semantic JVM enhancements and a new API set appropriate for real-time applications. It is not surprising that the main obstacle in achieving real-time characteristics for Java is its garbage collector. A real-time garbage collector became a revolutionary and central component of Sun’s recently-released Java real-time edition RTS 1.0, although its first implementation does not include one (it is expected in the next release). Java RTS addresses other issues, making strong deterministic guarantees for thread scheduling, synchronization overhead, lock queuing order, class initialization, and maximum interrupt response latency. Java RTS is intended only for suitable underlying operating systems, which means that only a real-time operating system, such as QNX, is appropriate for implementing the JVM.

Realtime Specification for Java 2.0


  • Realtime Specification for Java 2.0 (RTSJ)
  • Author: Multiple, Aicas
  • May 2006 – June 2017



The goal of the Real-Time Specification for Java (RTSJ) is to support the use of Java technology in embedded and realtime systems. It provides a specification for refining the Java Language Specification and the Java Virtual Machine Specification and for providing an extended Application Programming Interface that facilitates the creation, verification, analysis, execution, and management of realtime Java programs such as control and sensor applications. The Java Virtual Machine and the Java Language were conceived as a portable environment for desktop and server applications. The emphasis has been on throughput and responsiveness. These are characteristics obtainable with time-sharing systems. For this conventional Java environment, it is more important that each task makes progress, than that a particular task completes within a predefined time slot. In a realtime system, the system tries to schedule the most critical task that is ready to run first. This task runs either until it is finished, or it needs to wait for some event or data, or a more critical task is released or a more critical task becomes schedulable after waiting for its event or data.

RTSJ Compatible VMs for Java


  • Realtime Specification for Java 2.0 (RTSJ)
  • Author: Ales Plsek
  • June 2009



Although real-time Java is becoming well known, the specific Virtual Machines implementing its functionalities are still known only to the community. There is no list of VMs supporting real-time features, furthermore, every VM is implementing a different subset of RTSJ. Therefore, I am posting a list of RTSJ-compatible VMs that are available up to this date (sorted by first release date).

Furthermore, other real-time, Java-like platforms have been developed, either commercial or academic projects. Usually, these VMs are not fully compliant with RTSJ.

Other Resources

  • Real-Time Java for the Enterprise
    • “Java, the language, and the Java Enterprise Edition platform have proved to be very popular for the development of enterprise applications. The ease-of-development, performance and reliability all make Java extremely attractive to developers. However, the Java platform does not support real-time applications, and even running a Java application on a real-time operating system will not make the application deterministic.”
  • Real-Time Java for Latency Critical Banking Applications
  • Exploit real-time Java’s unique features
    • “Real-time Java is a set of enhancements to the Java language that provide applications with a degree of real-time performance that exceeds that of standard Java technology. Real-time performance differs from traditional throughput performance, which is typically a measurement of the total number of instructions, tasks, or work that can be done over a fixed amount of time. Real-time performance focuses on the time an application requires to respond to external stimuli without exceeding given time constraints. In the case of hard real-time systems, such constraints must never be exceeded; softreal-time systems have a higher tolerance for violations. Real-time performance requires that the application itself gain control of the processor so that it can respond to stimuli, and that while responding to the stimuli the application’s code is not blocked from execution by competing processes within the virtual machine. Real-time Java delivers responsiveness previously unmet in Java applications.”

Realtime Infrastructure Services

  • Realtime API Infrastructure – Realtime API infrastructure specifically allows developers to build realtime data push into their existing APIs.  Typically, you would not need to modify your existing API contracts, as the streaming server would serve as a proxy. The proxy design allows these services to fit nicely within an API stack. This means it can inherit other facilities from your REST API, such as authentication, logging, throttling, etc. It can be combined with an API management system.  In the case of WebSocket messages being proxied out as HTTP requests, the messages may be handled statelessly by the backend. Messages from a single connection can even be load balanced across a set of backend instances.
    • Fanout/Pushpin – Fanout is a real-time API development kit that helps you push data to connected devices easily. Fanout is a cross between a reverse proxy and a message broker. Pushpin is the open source version.
    • – a SaaS API proxy tool that converts standard API requests into a streaming API. In other words, it provides a proxy as a service for any HTTP API by polling and acting as a streaming API.
    • LiveResource – LiveResource is a protocol specification and JavaScript reference library for receiving live updates of web resources. It has the following principles:
  • Realtime Application Infrastructure – Realtime app infrastructure sends data to browsers and clients. It typically uses pub/sub messaging, webhooks, and/or websockets — and is separate from an application or service’s main API.
    • Firebase – Firebase is a BaaS (Backend-as-a-Service) that allows developers to create web applications with no server-side programming required.
    • Pubnub – PubNub is a programmable Data Stream Network (DSN) and realtime infrastructure-as-a-service (IaaS) company. Primarily, they are a messaging solution hosted on a cloud service that allows developers to publish data instantly to one or multiple devices.
    • Pusher – Pusher is a hosted service that allows developers to add realtime bi-directional functionality via WebSockets (with HTTP-based fallbacks) to the web and mobile apps.
    • Ably – Ably is a realtime data delivery platform that provides creators the tools to create, deliver, and manage projects. Their main realtime functionality consists of pub/sub, presence, authentication, encryption, and connection state recovery.