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Configuration library for JVM languages.


  • implemented in plain Java with no dependencies
  • extensive test coverage
  • supports files in three formats: Java properties, JSON, and a human-friendly JSON superset
  • merges multiple files across all formats
  • can load from files, URLs, or classpath
  • good support for "nesting" (treat any subtree of the config the same as the whole config)
  • users can override the config with Java system properties, java
  • supports configuring an app, with its framework and libraries, all from a single file such as application.conf
  • parses duration and size settings, "512k" or "10 seconds"
  • converts types, so if you ask for a boolean and the value is the string "yes", or you ask for a float and the value is an int, it will figure it out.
  • JSON superset features:
    • comments
    • includes
    • substitutions ("foo" : ${bar}, "foo" : Hello ${who})
    • properties-like notation (a.b=c)
    • less noisy, more lenient syntax
    • substitute environment variables

This library limits itself to config files. If you want to load config from a database or something, you would need to write some custom code. The library has nice support for merging configurations so if you build one from a custom source it's easy to merge it in.


The license is Apache 2.0, see LICENSE-2.0.txt.

Bugs and Patches

Report bugs to the GitHub issue tracker. Send patches as pull requests on GitHub.

Along with any pull requests (or other means of contributing), please state that the contribution is your original work (or that you have the authority to license it) and that you license the work under the Apache 2.0 license.

Whether or not you state this explicitly, by submitting any copyrighted material via pull request, email, or other means you agree to license your the material under the Apache 2.0 license and warrant that you have the legal authority to do so.

API Example

Config conf = ConfigFactory.load();
int bar1 = conf.getInt("");
Config foo = conf.getConfig("foo");
int bar2 = foo.getInt("bar");

Longer Examples

See the examples in the examples/ directory.

You can run these from the sbt console with the commands project simple-app and then run.

In brief, as shown in the examples:

  • libraries should use a Config instance provided by the app, if any, and use ConfigFactory.load() if no special Config is provided. Libraries should put their defaults in a reference.conf on the classpath.
  • apps can create a Config however they want (ConfigFactory.load() is easiest and least-surprising), then provide it to their libraries. A Config can be created with the parser methods in ConfigFactory or built up from any file format or data source you like with the methods in ConfigValueFactory.

Standard behavior

The convenience method ConfigFactory.load() 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)
  • (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.

Libraries and frameworks should default to ConfigFactory.load() if the application does not provide a custom Config object. Libraries and frameworks should also allow the application to provide a custom Config object to be used instead of the default, in case the application needs multiple configurations in one JVM or wants to load extra config files from somewhere.

For applications using application.{conf,json,properties}, system properties can be used to force a different config source:

  • config.resource specifies a resource name - not a basename, i.e. application.conf not application
  • config.file specifies a filesystem path, again it should include the extension, not be a basename
  • config.url specifies a URL

These system properties specify a replacement for application.{conf,json,properties}, not an addition. They only affect apps using the default ConfigFactory.load() configuration. In the replacement config file, you can use include "application" to include the original default config file; after the include statement you could go on to override certain settings. (Caveat: right now includes are relative to the including entity, so include "application" would only work from another classpath resource, not from a file or URL.)

JSON Superset

Tentatively called "Human-Optimized Config Object Notation" or HOCON, also called .conf, see in this directory for more detail.

Features of HOCON

  • Comments, with # or //
  • Allow omitting the {} around a root object
  • Allow = as a synonym for :
  • Allow omitting the = or : before a { so foo { a : 42 }
  • Allow omitting commas as long as there's a newline
  • Allow trailing commas after last element in objects and arrays
  • Allow unquoted strings for keys and values
  • Unquoted keys can use dot-notation for nested objects, means foo { bar : 42 }
  • Duplicate keys are allowed; later values override earlier, except for object-valued keys where the two objects are merged recursively
  • include feature merges root object in another file into current object, so foo { include "bar.json" } merges keys in bar.json into the object foo
  • include with no file extension includes any of .conf, .json, .properties
  • substitutions foo : ${a.b} sets key foo to the same value as the b field in the a object
  • substitutions concatenate into unquoted strings, foo : the quick ${} jumped
  • substitutions fall back to environment variables if they don't resolve in the config itself, so ${HOME} would work as you expect. Also, most configs have system properties merged in so you could use ${user.home}.
  • substitutions normally cause an error if unresolved, but there is a syntax ${?a.b} to permit them to be missing.

Examples of HOCON

Start with valid JSON:

    "foo" : {
        "bar" : 10,
        "baz" : 12

Drop root braces:

"foo" : {
    "bar" : 10,
    "baz" : 12

Drop quotes:

foo : {
    bar : 10,
    baz : 12

Use = and omit it before {:

foo {
    bar = 10,
    baz = 12

Remove commas:

foo {
    bar = 10
    baz = 12

Use dotted notation for unquoted keys:

Put the dotted-notation fields on a single line:, foo.baz=12

The syntax is well-defined (including handling of whitespace and escaping). But it handles many reasonable ways you might want to format the file.

Uses of Substitutions

The ${} substitution feature lets you avoid cut-and-paste in some nice ways.

Factor out common values

This is the obvious use,

standard-timeout = 10ms
foo.timeout = ${standard-timeout}
bar.timeout = ${standard-timeout}


If you duplicate a field with an object value, then the objects are merged with last-one-wins. So:

foo = { a : 42, c : 5 }
foo = { b : 43, c : 6 }

means the same as:

foo = { a : 42, b : 43, c : 6 }

You can take advantage of this for "inheritance":

data-center-generic = { cluster-size = 6 }
data-center-east = ${data-center-generic}
data-center-east = { name = "east" }
data-center-west = ${data-center-generic}
data-center-west = { name = "west", cluster-size = 8 }

Using include statements you could split this across multiple files, too.

Optional system or env variable overrides

In default uses of the library, exact-match system properties already override the corresponding config properties. However, you can add your own overrides, or allow environment variables to override, using the ${?foo} substitution syntax.

basedir = "/whatever/whatever"
basedir = ${?FORCED_BASEDIR}

Here, the override field basedir = ${?FORCED_BASEDIR} simply vanishes if there's no value for FORCED_BASEDIR, but if you set an environment variable FORCED_BASEDIR for example, it would be used.

A natural extension of this idea is to support several different environment variable names or system property names, if you aren't sure which one will exist in the target environment.

Object fields and array elements with a ${?foo} substitution value just disappear if the substitution is not found:

// this array could have one or two elements
path = [ "a", ${?OPTIONAL_A} ]

Future Directions

Here are some features that might be nice to add.

  • "myapp.d directory": allow parsing a directory. All .json, .properties and .conf files should be loaded in a deterministic order based on their filename. If you include a file and it turns out to be a directory then it would be processed in this way.
  • some way to merge array types. One approach could be: searchPath=${searchPath} ["/usr/local/foo"], which involves two features: 1) substitutions referring to the key being assigned would have to look at that key's value later in the merge stack (rather than complaining about circularity); 2) arrays would have to be merged if a series of them appear after a key, similar to how strings are concatenated already. A simpler but much more limited approach would add += as an alternative to :/=, where += would append an array value to the array's previous value. (Note that regular = already merges object values, to avoid object merge you have to first set the object to a non-object such as null, then set a new object. For consistency, if there's "array concatenation" within one value, maybe objects should also be able to merge within one value.)


(For the curious.)

The three file formats each have advantages.

  • Java .properties:
    • Java standard, built in to JVM
    • Supported by many tools such as IDEs
  • JSON:
    • easy to generate programmatically
    • well-defined and standard
    • bad for human maintenance, with no way to write comments, and no mechanisms to avoid duplication of similar config sections
  • HOCON/.conf:
    • nice for humans to read, type, and maintain, with more lenient syntax
    • built-in tools to avoid cut-and-paste
    • ways to refer to the system environment, such as system properties and environment variables

The idea would be to use JSON if you're writing a script to spit out config, and use HOCON if you're maintaining config by hand. If you're doing both, then mix the two.

Two alternatives to HOCON syntax could be:

  • YAML is also a JSON superset and has a mechanism for adding custom types, so the include statements in HOCON could become a custom type tag like !include, and substitutions in HOCON could become a custom tag such as !subst, for example. The result is somewhat clunky to write, but would have the same in-memory representation as the HOCON approach.
  • Put a syntax inside JSON strings, so you might write something like "$include" : "filename" or allow "foo" : "${bar}". This is a way to tunnel new syntax through a JSON parser, but other than the implementation benefit (using a standard JSON parser), it doesn't really work. It's a bad syntax for human maintenance, and it's not valid JSON anymore because properly interpreting it requires treating some valid JSON strings as something other than plain strings. A better approach is to allow mixing true JSON files into the config but also support a nicer format.
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