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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.

Binary Releases

You can find published releases here:

Alternately, unofficial binary releases are also in maven central as:


API docs

Bugs and Patches

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

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The build uses sbt and the tests are written in Scala; however, the library itself is plain Java and the published jar has no Scala dependency.

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.

Merging config trees

Any two Config objects can be merged with an associative operation called withFallback, like merged = firstConfig.withFallback(secondConfig).

The withFallback operation is used inside the library to merge duplicate keys in the same file and to merge multiple files. ConfigFactory.load() uses it to stack system properties over application.conf over reference.conf.

You can also use withFallback to merge in some hardcoded values, or to "lift" a subtree up to the root of the configuration; say you have something like:


Then you could code something like:

Config devConfig = originalConfig

There are lots of ways to use withFallback.

JSON Superset

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

After processing a .conf file, the result is always just a JSON tree that you could have written (less conveniently) in JSON.

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.
  • += syntax to append elements to arrays, path += "/bin"

Examples of HOCON

All of these are valid 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.

Note that while you can write HOCON that looks a lot like a Java properties file (and many properties files will parse as HOCON), the details of escaping, whitespace handling, comments, and so forth are more like JSON. The spec (see in this directory) has some more detailed notes on this topic.

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} ]


Values on the same line are concatenated (for strings and arrays) or merged (for objects).

This is why unquoted strings work, here the number 42 and the string foo are concatenated into a string 42 foo:

key : 42 foo

When concatenating values into a string, leading and trailing whitespace is stripped but whitespace between values is kept.

Unquoted strings also support substitutions of course:

tasks-url : ${base-url}/tasks

A concatenation can refer to earlier values of the same field:

path : "/bin"
path : ${path}":/usr/bin"

Arrays can be concatenated as well:

path : [ "/bin" ]
path : ${path} [ "/usr/bin" ]

There is a shorthand for appending to arrays:

// equivalent to: path = ${?path} [ "/usr/bin" ]
path += "/usr/bin"

To prepend or insert into an array, there is no shorthand.

When objects are "concatenated," they are merged, so object concatenation is just a shorthand for defining the same object twice. The long way (mentioned earlier) is:

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

The concatenation-style shortcut is:

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

When concatenating objects and arrays, newlines are allowed inside each object or array, but not between them.

Non-newline whitespace is never a field or element separator. So [ 1 2 3 4 ] is an array with one unquoted string element "1 2 3 4". To get an array of four numbers you need either commas or newlines separating the numbers.

See the spec for full details on concatenation.

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.
  • including URLs (which would allow forcing file: when inside a classpath resource, among other things)


(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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