---
layout: docs
page_title: Configure the Java task driver
description: Nomad's Java task driver lets you run JAR files in your workloads. Review the Java task driver capabilities, plugin options, client requirements, and client attributes such as Java version and virtual machine. Learn how the Java task driver affects resource isolation and chroot.
---

# Configure the Java task 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](/nomad/docs/job-specification/artifact).

## Capabilities

The `java` driver implements the following [capabilities](/nomad/plugins/author/task-driver#capabilities-capabilities-error).

| Feature              | Implementation                |
| -------------------- | ----------------------------- |
| `nomad alloc signal` | false                         |
| `nomad alloc exec`   | false                         |
| filesystem isolation | none, chroot (only for linux) |
| network isolation    | host, group                   |
| volume mounting      | none, all (only for linux)    |

## Plugin Options

- `default_pid_mode` `(string: optional)` - Defaults to `"private"`. Set to
  `"private"` to enable PID namespace isolation for tasks by default, or `"host"` to
  disable isolation.

!> **Warning:** If set to `"host"`, other processes running as the same user will
be able to access sensitive process information like environment variables.

- `default_ipc_mode` `(string: optional)` - Defaults to `"private"`. Set to
  `"private"` to enable IPC namespace isolation for tasks by default,
  or `"host"` to disable isolation.

!> **Warning:** If set to `"host"`, other processes running as the same user will be
able to make use of IPC features, like sending unexpected POSIX signals.

- `allow_caps` - A list of allowed Linux capabilities. Defaults to

```hcl
["audit_write", "chown", "dac_override", "fowner", "fsetid", "kill", "mknod",
 "net_bind_service", "setfcap", "setgid", "setpcap", "setuid", "sys_chroot"]
```

  which is modeled after the capabilities allowed by [docker by default][docker_caps]
  (without [`NET_RAW`][no_net_raw]). Allows the operator to control which capabilities
  can be obtained by tasks using [`cap_add`][cap_add] and [`cap_drop`][cap_drop] options.
  Supports the value `"all"` as a shortcut for allow-listing all capabilities supported
  by the operating system.

!> **Warning:** Allowing more capabilities beyond the default may lead to
undesirable consequences, including untrusted tasks being able to compromise the
host system.

## 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:

```hcl
job "docs" {
  # Only run this job where the JVM is higher than version 1.6.0.
  constraint {
    attribute = "${attr.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.

Nomad can only use cgroups to control resources if all the required controllers
are available. If one or more required cgroups are unavailable, Nomad will
disable resource controls that require cgroups entirely. See the documentation
on [cgroup controller requirements][] for more details.

### Chroot

The chroot created on Linux is populated with data in the following
directories from the host machine:

```
[
  "/bin",
  "/etc",
  "/lib",
  "/lib32",
  "/lib64",
  "/run/resolvconf",
  "/sbin",
  "/usr",
]
```

The task's chroot is populated by linking or copying the data from the host into
the chroot. Note that this can take considerable disk space. Since Nomad v0.5.3,
the client manages garbage collection locally which mitigates any issue this may
create.

This list is configurable through the agent client
[configuration file](/nomad/docs/configuration/client#chroot_env).

## Next steps

[Use the Java task driver in a job](/nomad/docs/job-declare/task-driver/java).

[default_pid_mode]: /nomad/docs/job-declare/task-driver/java#default_pid_mode
[default_ipc_mode]: /nomad/docs/job-declare/task-driver/java#default_ipc_mode
[cap_add]: /nomad/docs/job-declare/task-driver/java#cap_add
[cap_drop]: /nomad/docs/job-declare/task-driver/java#cap_drop
[no_net_raw]: /nomad/docs/upgrade/upgrade-specific#nomad-1-1-0-rc1-1-0-5-0-12-12
[allow_caps]: /nomad/docs/job-declare/task-driver/java#allow_caps
[docker_caps]: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities
[cgroup controller requirements]: /nomad/docs/deploy/production/requirements#hardening-nomad
[volume_mount]: /nomad/docs/job-specification/volume_mount
[host volume]: /nomad/docs/configuration/client#host_volume-block
