How do I use H2O with Java?

There are two ways to use H2O with Java: using the REST API or embedding H2O within your Java application.

  • Using the REST API:

The simplest way is to call the REST API from your Java program to a remote cluster and should meet the needs of most users.

You can access the REST API documentation within Flow, or on our documentation site.

Flow, Python, and R all rely on the REST API to run H2O. For example, each action in Flow translates into one or more REST API calls. The script fragments in the cells in Flow are essentially the payloads for the REST API calls. Most R and Python API calls translate into a single REST API call.

To see how the REST API is used with H2O:

  • Using Chrome as your internet browser, open the developer tab while viewing the web UI. As you perform tasks, review the network calls made by Flow.

  • Write an R program for H2O using the H2O R package that uses h2o.startLogging() at the beginning. All REST API calls used are logged.

  • Embedding H2O within Your Java Application:

The second way to use H2O with Java is to embed H2O within your Java application. An example describing how to do this is available in the h2o-droplets repository.

How do I communicate with a remote cluster using the REST API?

Download the H2O distribution zip file from The distribution zip includes a file named This file contains all of the Java bindings necessary for communicating with a remote cluster via the REST API. The REST API documentation is available here.

I keep getting a message that I need to install Java. I have a supported version of Java installed, but I am still getting this message. What should I do?

This error message displays if the JAVA_HOME environment variable is not set correctly. The JAVA_HOME variable is likely points to Apple Java version 6 instead of Oracle Java version 8.

If you are running OS X 10.7 or earlier, enter the following in Terminal:

export JAVA_HOME=/Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin/Contents/Home``

If you are running OS X 10.8 or later, modify the launchd.plist by entering the following in Terminal:

cat << EOF | sudo tee /Library/LaunchDaemons/setenv.JAVA_HOME.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "">
  <plist version="1.0">
    <string>/Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home</string>