open-nomad/website/source/docs/drivers/java.html.md

layout	page_title	sidebar_current	description
docs	Drivers: Java	docs-drivers-java	The Java task driver is used to run Jars using the JVM.

Java Driver

Name: java

The java driver is used to execute Java applications packaged into a Java Jar file. The driver requires the Jar file to be accessible from the Nomad client via the artifact downloader.

Task Configuration

task "webservice" {
  driver = "java"

  config {
    jar_path    = "local/example.jar"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }
}  

The java driver supports the following configuration in the job spec:

• class - (Optional) The name of the class to run. If jar_path is specified and the manifest specifies a main class, this is optional. If shipping classes rather than a Jar, please specify the class to run and the class_path.

• class_path - (Optional) The class_path specifies the class path used by Java to lookup classes and Jars.

• jar_path - (Optional) The path to the downloaded Jar. In most cases this will just be the name of the Jar. However, if the supplied artifact is an archive that contains the Jar in a subfolder, the path will need to be the relative path (subdir/from_archive/my.jar).

• args - (Optional) A list of arguments to the Jar's main method. References to environment variables or any interpretable Nomad variables will be interpreted before launching the task.

• jvm_options - (Optional) A list of JVM options to be passed while invoking java. These options are passed without being validated in any way by Nomad.

Examples

A simple config block to run a Java Jar:

task "web" {
  driver = "java"

  config {
    jar_path    = "local/hello.jar"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }

  # Specifying an artifact is required with the "java" driver. This is the
  # mechanism to ship the Jar to be run.
  artifact {
    source = "https://internal.file.server/hello.jar"

    options {
      checksum = "md5:123445555555555"
    }
  }
}

A simple config block to run a Java class:

task "web" {
  driver = "java"

  config {
    class       = "Hello"
    class_path  = "${NOMAD_TASK_DIR}"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }

  # Specifying an artifact is required with the "java" driver. This is the
  # mechanism to ship the Jar to be run.
  artifact {
    source = "https://internal.file.server/Hello.class"

    options {
      checksum = "md5:123445555555555"
    }
  }
}

Client Requirements

The java driver requires Java to be installed and in your system's $PATH. On Linux, Nomad must run as root since it will use chroot and cgroups which require root privileges. The task must also specify at least one artifact to download, as this is the only way to retrieve the Jar being run.

Client Attributes

The java driver will set the following client attributes:

• driver.java - Set to 1 if Java is found on the host node. Nomad determines this by executing java -version on the host and parsing the output
• driver.java.version - Version of Java, ex: 1.6.0_65
• driver.java.runtime - Runtime version, ex: Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
• driver.java.vm - Virtual Machine information, ex: Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)

Here is an example of using these properties in a job file:

job "docs" {
  # Only run this job where the JVM is higher than version 1.6.0.
  constraint {
    attribute = "${driver.java.version}"
    operator  = ">"
    value     = "1.6.0"
  }
}

Resource Isolation

The resource isolation provided varies by the operating system of the client and the configuration.

On Linux, Nomad will attempt to use cgroups, namespaces, and chroot to isolate the resources of a process. If the Nomad agent is not running as root, many of these mechanisms cannot be used.

As a baseline, the Java jars will be run inside a Java Virtual Machine, providing a minimum amount of isolation.