Typesafe Config Introduction

Typesafe Config library: its basic structure, loading mechanism, parsing, substitutions, merging, inclusion, unit format support, IDE plugin, and more.


Recently I discovered an awesome Java library about configuration — Typesafe Config (GitHub). It supports Java properties, JSON and a human-friendly JSON superset. It can load resources from different places: files, URLs, classpath. It supports types, loading convention, substitutions, properties merging, etc. It’s why I’m very excited to show what I learned to you. After reading this article, you will understand:

  • The basic structure of the configuration
  • Its loading mechanism and different ways of parsing
  • Substitution
  • Include
  • Unit format support
  • IDE support

Now, let’s get started!


If you want to use Typesafe Config in Java project via Maven, you can do:


or via sbt:

libraryDependencies += "com.typesafe" % "config" % "1.4.1"

Basic Structure

Here are some examples of the basic usage of Typesafe Config. It supports types such as integer, long, boolean, double, string, list and object. There are two ways to write comments: using either // or #. Comments can be written inline at the tail of the line or they can be written in separated lines. More advanced features are presented in the following sections.

// This is a comment
# This is a comment, too

myInteger: 1234
myBoolean: true
myDouble: 1.000
myString: Hello

myList: [1, 2, 3]

userA {
  firstName: Mincong
  lastName: Huang
userB { firstName: Foo, lastName: Bar }
userC { firstName = Foo, lastName = Bar }

Type support. Configuration instance can retrieve the field value using syntax getXxx("key"), where Xxx is the type of the field value, such as getBoolean(), getInt(), getLong(), getDouble, getString(). If the target is a list, you can do getXxxList("key"), such as getBooleanList(), getIntList(), getLongList(), getDoubleList(), getStringList().

Flexibility. Typesafe Config uses interface com.typesafe.config.Config to represent configuration. Retrieving a sub-tree of the given Config returns another Config. This is very flexible. It allows you to define configuration anywhere in your code with minimal knowledge of the whole configuration structure. Therefore, the source code is well decoupled from the configuration. It has many benefits, such as having different configuration structures based on different implementations or making tests easy to write.

Immutable. com.typesafe.config.Config is immutable, whenever you want to change something, it creates a new instance for you. It means it is safe to use even in a multi-threaded situation.

Loading Mechanism

You can use ConfigFactory#load() to loads the available configuration. According to the official documentation, the standard behavior loads the following (first-listed are higher priority):

  • system properties
  • application.conf (all resources on classpath with this name)
  • application.json (all resources on classpath with this name)
  • application.properties (all resources on classpath with this name)
  • reference.conf (all resources on classpath with this name)

The idea is that libraries and frameworks should ship with a reference.conf in their JAR. Applications should provide an application.conf, or if they want to create multiple configurations in a single JVM, they could use ConfigFactory.load("myapp") to load their own myapp.conf.

Beside method ConfigFeature#load(), you can also use the following methods to create a config object. They are convenient for tesing:

// Create Config from String
Config c = ConfigFactory.parseString("foo: bar");
// Create Config from Map<String, ?>
Config c = ConfigFactory.parseMap(myMap);
// Create Config from File
Config c = ConfigFactory.parseFile(myFile);
// Get an empty Config
Config c = ConfigFactory.empty();


Another powerful feature of Typesafe config is its substitution. It helps you to avoid duplicating your logic. For example:

user {
  firstName: Mincong
  lastName: Huang
  fullName: ${user.firstName} ${user.lastName} // Mincong Huang

You can also take advantage of this for inheritance, optional system, or environment variable overrides. Checkout “Use of substitutions” of the official guide for more details.


Typesafe Config allows you to store configuration into separated files and use include keyword to include configuration of those files. Here is a concrete example for myApp which relies on the configuration of moduleA and moduleB.

$ tree .
├── moduleA.conf
├── moduleB.conf
└── myApp.conf

0 directories, 3 files
# file: moduleA.conf
moduleA {
  msg: "Hello from Module A"
# file: moduleB.conf
moduleB {
  msg: "Hello from Module B"
# file: myApp.conf
include "moduleA"
include "moduleB"

app {
  msg: "Hello from App"

// Results:
//   app.msg     => "Hello from App"
//   moduleA.msg => "Hello from Module A"
//   moduleB.msg => "Hello from Module B"

include feature merges the root object of the referenced object into the current one. If a key in the included object occurred before the include statement in the including object, the included key’s value overrides or merges with the earlier value, exactly as with duplicate keys found in a single file. When including file, extensions (.conf, .json, .properties) are not needed. You can also include configuration from URL or classpath. See the HOCON spec for more detail.

Unit Format

Typesafe Config supports unit form: Duration, Memory Size, and Period.

delay = 10 minutes

Duration. Gets a value as a duration in a specified TimeUnit. If the value is already a number, then it’s taken as milliseconds and then converted to the requested TimeUnit; if it’s a string, it’s parsed understanding units suffixes like “10m” or “5ns” as documented in the HOCON spec.

Java Unit
TimeUnit.MILLISECONDS ms, millis, milliseconds
TimeUnit.MICROSECONDS us, micros, microseconds
TimeUnit.NANOSECONDS ns, nanos, nanoseconds
TimeUnit.DAYS d, days
TimeUnit.HOURS h, hours
TimeUnit.SECONDS s, seconds
TimeUnit.MINUTES m, minutes

There are two ways to retrieve a duration in Java: either use Config#getDuration(String) or Config#getDuration(String, TimeUnit). The first one returns a java.time.Duration and the second one returns a long value corresponding to the requested time unit.

config.getDuration("delay"); // Duration.ofMinutes(10)
config.getDuration("delay", TimeUnit.MINUTE); // 10

Now, let’s take a look on memory size.

minSize = 128K

Memory Size. Gets a value as an amount of memory (parses special strings like “128M”). If the value is already a number, then it’s left alone; if it’s a string, it’s parsed understanding unit suffixes such as “128K”.

For powers of ten, exactly these strings are supported:

Bytes Unit
10 ^ 3 kB, kilobyte, kilobytes
10 ^ 6 MB, megabyte, megabytes
10 ^ 9 GB, gigabyte, gigabytes
10 ^ 12 TB, terabyte, terabytes
10 ^ 15 PB, petabyte, petabytes
10 ^ 18 EB, exabyte, exabytes
10 ^ 21 ZB, zettabyte, zettabytes
10 ^ 24 YB, yottabyte, yottabytes

For powers of two, exactly these strings are supported:

Bytes Unit
2 ^ 10 K, k, Ki, KiB, kibibyte, kibibytes
2 ^ 20 M, m, Mi, MiB, mebibyte, mebibytes
2 ^ 30 G, g, Gi, GiB, gibibyte, gibibytes
2 ^ 40 T, t, Ti, TiB, tebibyte, tebibytes
2 ^ 50 P, p, Pi, PiB, pebibyte, pebibytes
2 ^ 60 E, e, Ei, EiB, exbibyte, exbibytes
2 ^ 70 Z, z, Zi, ZiB, zebibyte, zebibytes
2 ^ 80 Y, y, Yi, YiB, yobibyte, yobibytes

There is other unit format(s), like Period. For more accurate and up-to-date documentation custom types, see units format in the HOCON spec.

IDE Support

If you use HOCON files (*.conf) in IntelliJ IDEA, you may want to use HOCON Plugin. Its main features are syntax highlighting, brace matching and code folding; code formatting; breadcrumbs; copy reference; move statement up/down; quick documentation; auto-completion; etc. See more detail in the IntelliJ Plugin page or the README of their source code.


In this article, I shared with you the basic features of Typesafe Config: its structure, loading mechanism, substitution, include, unit format support and IDE support. The source code of this article is available on GitHub. Interested to know more? You can subscribe to the feed of my blog, follow me on Twitter or GitHub. Hope you enjoy this article, see you the next time!