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https://github.com/lightbend/config

configuration library for JVM languages using HOCON files
https://github.com/lightbend/config

configuration-library hocon supported

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configuration library for JVM languages using HOCON files

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README 

        
# Configuration library for JVM languages.



[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.typesafe/config/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.typesafe/config)

[![Build Status](https://github.com/lightbend/config/actions/workflows/ci.yml/badge.svg)](https://github.com/lightbend/config/actions/workflows/ci.yml)



## Overview



 - implemented in plain Java with no dependencies

 - 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 -Dmyapp.foo.bar=10`

 - 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 (`logdir=${HOME}/logs`)

 - API based on immutable `Config` instances, for thread safety

   and easy reasoning about config transformations

 - extensive test coverage



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.



**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*



- [Essential Information](#essential-information)

  - [License](#license)

  - [Binary Releases](#binary-releases)

  - [Release Notes](#release-notes)

  - [API docs](#api-docs)

  - [Bugs and Patches](#bugs-and-patches)

  - [Build](#build)

- [Using the Library](#using-the-library)

  - [API Example](#api-example)

  - [Longer Examples](#longer-examples)

  - [Immutability](#immutability)

  - [Schemas and Validation](#schemas-and-validation)

  - [Standard behavior](#standard-behavior)

    - [Note about resolving substitutions in `reference.conf` and `application.conf`](#note-about-resolving-substitutions-in-referenceconf-and-applicationconf)

  - [Merging config trees](#merging-config-trees)

  - [How to handle defaults](#how-to-handle-defaults)

  - [Understanding `Config` and `ConfigObject`](#understanding-config-and-configobject)

  - [ConfigBeanFactory](#configbeanfactory)

- [Using HOCON, the JSON Superset](#using-hocon-the-json-superset)

  - [Features of HOCON](#features-of-hocon)

  - [Examples of HOCON](#examples-of-hocon)

  - [Uses of Substitutions](#uses-of-substitutions)

    - [Factor out common values](#factor-out-common-values)

    - [Inheritance](#inheritance)

    - [Optional system or env variable overrides](#optional-system-or-env-variable-overrides)

  - [Concatenation](#concatenation)

- [Miscellaneous Notes](#miscellaneous-notes)

  - [Debugging Your Configuration](#debugging-your-configuration)

  - [Supports Java 8 and Later](#supports-java-8-and-later)

  - [Rationale for Supported File Formats](#rationale-for-supported-file-formats)

  - [Other APIs (Wrappers, Ports and Utilities)](#other-apis-wrappers-ports-and-utilities)

    - [Guice integration](#guice-integration)

    - [Java (yep!) wrappers for the Java library](#java-yep-wrappers-for-the-java-library)

    - [Scala wrappers for the Java library](#scala-wrappers-for-the-java-library)

    - [Clojure wrappers for the Java library](#clojure-wrappers-for-the-java-library)

    - [Kotlin wrappers for the Java library](#kotlin-wrappers-for-the-java-library)

    - [Scala port](#scala-port)

    - [Ruby port](#ruby-port)

    - [Puppet module](#puppet-module)

    - [Python port](#python-port)

    - [C++ port](#c-port)

    - [JavaScript port](#javascript-port)

    - [C# port](#c-port-1)

    - [Go port](#go-port)

    - [Linting tool](#linting-tool)

    - [Online playground](#online-playground)



## Essential Information



### Binary Releases



Typesafe Config is compatible with Java 8 and above.



You can find published releases on Maven Central.



    

        com.typesafe

        config

        1.4.3

    



sbt dependency:



    libraryDependencies += "com.typesafe" % "config" % "1.4.3"



Link for direct download if you don't use a dependency manager:



 - https://repo1.maven.org/maven2/com/typesafe/config/



### Release Notes



Please see NEWS.md in this directory,

https://github.com/lightbend/config/blob/main/NEWS.md



### API docs



 - Online: https://lightbend.github.io/config/latest/api/

 - also published in jar form

 - consider reading this README first for an intro

 - for questions about the `.conf` file format, read

   [HOCON.md](https://github.com/lightbend/config/blob/main/HOCON.md)

   in this directory



### Bugs and Patches



**NOTE: Please read [Readme #Maintained-by](https://github.com/lightbend/config#Maintained-by) before spending time suggesting changes to this library.**



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

requests on GitHub.



Before we can accept pull requests, you will need to agree to the

Typesafe Contributor License Agreement online, using your GitHub

account - it takes 30 seconds.  You can do this at

https://www.lightbend.com/contribute/cla



Please see

[CONTRIBUTING](https://github.com/lightbend/config/blob/main/CONTRIBUTING.md)

for more including how to make a release.



### Build



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.



## Using the Library



### API Example

    import com.typesafe.config.ConfigFactory



    Config conf = ConfigFactory.load();

    int bar1 = conf.getInt("foo.bar");

    Config foo = conf.getConfig("foo");

    int bar2 = foo.getInt("bar");



### Longer Examples



See the examples in the `examples/` [directory](https://github.com/lightbend/config/tree/main/examples).



You can run these from the sbt console with the commands `project

config-simple-app-java` 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`.



### Immutability



Objects are immutable, so methods on `Config` which transform the

configuration return a new `Config`. Other types such as

`ConfigParseOptions`, `ConfigResolveOptions`, `ConfigObject`,

etc. are also immutable. See the

[API docs](https://lightbend.github.io/config/latest/api/) for

details of course.



### Schemas and Validation



There isn't a schema language or anything like that. However, two

suggested tools are:



 - use the

   [checkValid() method](https://lightbend.github.io/config/latest/api/com/typesafe/config/Config.html#checkValid-com.typesafe.config.Config-java.lang.String...-)

 - access your config through a Settings class with a field for

   each setting, and instantiate it on startup (immediately

   throwing an exception if any settings are missing)



In Scala, a Settings class might look like:



    class Settings(config: Config) {



        // validate vs. reference.conf

        config.checkValid(ConfigFactory.defaultReference(), "simple-lib")



        // non-lazy fields, we want all exceptions at construct time

        val foo = config.getString("simple-lib.foo")

        val bar = config.getInt("simple-lib.bar")

    }



See the examples/ directory for a full compilable program using

this pattern.



### 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)

  - `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`.



Libraries and frameworks should default to `ConfigFactory.load()`

if the application does not provide a custom `Config` object. This

way, libraries will see configuration from `application.conf` and

users can configure the whole app, with its libraries, in a single

`application.conf` file.



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.  The

library examples in `examples/` show how to accept a custom config

while defaulting to `ConfigFactory.load()`.



For applications using `application.{conf,json,properties}`,

system properties can be used to force a different config source

(e.g. from command line `-Dconfig.file=path/to/config-file`):



 - `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



 **Note**: you need to pass `-Dconfig.file=path/to/config-file` before the jar itself, e.g. `java -Dconfig.file=path/to/config-file.conf -jar path/to/jar-file.jar`. Same applies for `-Dconfig.resource=config-file.conf`



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.



If you set `config.resource`, `config.file`, or `config.url`

on-the-fly from inside your program (for example with

`System.setProperty()`), be aware that `ConfigFactory` has some

internal caches and may not see new values for system

properties. Use `ConfigFactory.invalidateCaches()` to force-reload

system properties.



#### Note about resolving substitutions in `reference.conf` and `application.conf`



The substitution syntax `${foo.bar}` will be resolved

twice. First, all the `reference.conf` files are merged and then

the result gets resolved. Second, all the `application.conf` are

layered over the unresolved `reference.conf` and the result of that

gets resolved again.



The implication of this is that the `reference.conf` stack has to

be self-contained; you can't leave an undefined value `${foo.bar}`

to be provided by `application.conf`. It is however possible to

override a variable that `reference.conf` refers to, as long as

`reference.conf` also defines that variable itself.



### 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:



    foo=42

    dev.foo=57

    prod.foo=10



Then you could code something like:



    Config devConfig = originalConfig

                         .getConfig("dev")

                         .withFallback(originalConfig)



There are lots of ways to use `withFallback`.



### How to handle defaults



Many other configuration APIs allow you to provide a default to

the getter methods, like this:



    boolean getBoolean(String path, boolean fallback)



Here, if the path has no setting, the fallback would be

returned. An API could also return `null` for unset values, so you

would check for `null`:



    // returns null on unset, check for null and fall back

    Boolean getBoolean(String path)



The methods on the `Config` interface do NOT do this, for two

major reasons:



 1. If you use a config setting in two places, the default

 fallback value gets cut-and-pasted and typically out of

 sync. This can result in Very Evil Bugs.

 2. If the getter returns `null` (or `None`, in Scala) then every

 time you get a setting you have to write handling code for

 `null`/`None` and that code will almost always just throw an

 exception. Perhaps more commonly, people forget to check for

 `null` at all, so missing settings result in

 `NullPointerException`.



For most situations, failure to have a setting is simply a bug to fix

(in either code or the deployment environment). Therefore, if a

setting is unset, by default the getters on the `Config` interface

throw an exception.



If you want to allow a setting to be missing from

`application.conf` in a particular case, then here are some

options:



 1. Set it in a `reference.conf` included in your library or

 application jar, so there's a default value.

 2. Use the `Config.hasPath()` method to check in advance whether

 the path exists (rather than checking for `null`/`None` after as

 you might in other APIs).

 3. Catch and handle `ConfigException.Missing`. NOTE: using an

 exception for control flow like this is much slower than using

 `Config.hasPath()`; the JVM has to do a lot of work to throw

 an exception.

 4. In your initialization code, generate a `Config` with your

 defaults in it (using something like `ConfigFactory.parseMap()`)

 then fold that default config into your loaded config using

 `withFallback()`, and use the combined config in your

 program. "Inlining" your reference config in the code like this

 is probably less convenient than using a `reference.conf` file,

 but there may be reasons to do it.

 5. Use `Config.root()` to get the `ConfigObject` for the

 `Config`; `ConfigObject` implements `java.util.Map` and

 the `get()` method on `Map` returns null for missing keys. See

 the API docs for more detail on `Config` vs. `ConfigObject`.

 6. Set the setting to `null` in `reference.conf`, then use

 `Config.getIsNull` and `Config.hasPathOrNull` to handle `null`

 in a special way while still throwing an exception if the setting

 is entirely absent.



The *recommended* path (for most cases, in most apps) is that you

require all settings to be present in either `reference.conf` or

`application.conf` and allow `ConfigException.Missing` to be

thrown if they are not. That's the design intent of the `Config`

API design.



Consider the "Settings class" pattern with `checkValid()` to

verify that you have all settings when you initialize the

app. See the [Schemas and Validation](#schemas-and-validation)

section of this README for more details on this pattern.



**If you do need a setting to be optional**: checking `hasPath()` in

advance should be the same amount of code (in Java) as checking

for `null` afterward, without the risk of `NullPointerException`

when you forget. In Scala, you could write an enrichment class

like this to use the idiomatic `Option` syntax:



```scala

implicit class RichConfig(val underlying: Config) extends AnyVal {

  def getOptionalBoolean(path: String): Option[Boolean] = if (underlying.hasPath(path)) {

     Some(underlying.getBoolean(path))

  } else {

     None

  }

}

```



Since this library is a Java library it doesn't come with that out

of the box, of course.



It is understood that sometimes defaults in code make sense. For

example, if your configuration lets users invent new sections, you

may not have all paths up front and may be unable to set up

defaults in `reference.conf` for dynamic paths. The design intent

of `Config` isn't to *prohibit* inline defaults, but simply to

recognize that it seems to be the 10% case (rather than the 90%

case). Even in cases where dynamic defaults are needed, you may

find that using `withFallback()` to build a complete

nothing-missing `Config` in one central place in your code keeps

things tidy.



Whatever you do, please remember not to cut-and-paste default

values into multiple places in your code. You have been warned!

:-)



### Understanding `Config` and `ConfigObject`



To read and modify configuration, you'll use the

[Config](https://lightbend.github.io/config/latest/api/com/typesafe/config/Config.html)

interface. A `Config` looks at a JSON-equivalent data structure as

a one-level map from paths to values. So if your JSON looks like

this:



```

  "foo" : {

    "bar" : 42

    "baz" : 43

  }

```



Using the `Config` interface, you could write

`conf.getInt("foo.bar")`. The `foo.bar` string is called a _path

expression_

([HOCON.md](https://github.com/lightbend/config/blob/main/HOCON.md)

has the syntax details for these expressions). Iterating over this

`Config`, you would get two entries; `"foo.bar" : 42` and

`"foo.baz" : 43`. When iterating a `Config` you will not find

nested `Config` (because everything gets flattened into one

level).



When looking at a JSON tree as a `Config`, `null` values are

treated as if they were missing. Iterating over a `Config` will

skip `null` values.



You can also look at a `Config` in the way most JSON APIs would,

through the

[ConfigObject](https://lightbend.github.io/config/latest/api/com/typesafe/config/ConfigObject.html)

interface. This interface represents an object node in the JSON

tree. `ConfigObject` instances come in multi-level trees, and the

keys do not have any syntax (they are just strings, not path

expressions). Iterating over the above example as a

`ConfigObject`, you would get one entry `"foo" : { "bar" : 42,

"baz" : 43 }`, where the value at `"foo"` is another nested

`ConfigObject`.



In `ConfigObject`, `null` values are visible (distinct from

missing values), just as they are in JSON.



`ConfigObject` is a subtype of [ConfigValue](https://lightbend.github.io/config/latest/api/com/typesafe/config/ConfigValue.html), where the other

subtypes are the other JSON types (list, string, number, boolean, null).



`Config` and `ConfigObject` are two ways to look at the same

internal data structure, and you can convert between them for free

using

[Config.root()](https://lightbend.github.io/config/latest/api/com/typesafe/config/Config.html#root--)

and

[ConfigObject.toConfig()](https://lightbend.github.io/config/latest/api/com/typesafe/config/ConfigObject.html#toConfig--).



### ConfigBeanFactory



As of version 1.3.0, if you have a Java object that follows

JavaBean conventions (zero-args constructor, getters and setters),

you can automatically initialize it from a `Config`.



Use

`ConfigBeanFactory.create(config.getConfig("subtree-that-matches-bean"),

MyBean.class)` to do this.



Creating a bean from a `Config` automatically validates that the

config matches the bean's implied schema. Bean fields can be

primitive types, typed lists such as `List`,

`java.time.Duration`, `ConfigMemorySize`, or even a raw `Config`,

`ConfigObject`, or `ConfigValue` (if you'd like to deal with a

particular value manually).



## Using HOCON, the JSON Superset



The JSON superset is called "Human-Optimized Config Object

Notation" or HOCON, and files use the suffix `.conf`.  See

[HOCON.md](https://github.com/lightbend/config/blob/main/HOCON.md)

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,

    `foo.bar=42` 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`

  - you can include files, URLs, or classpath resources; use

    `include url("http://example.com")` or `file()` or

    `classpath()` syntax to force the type, or use just `include

    "whatever"` to have the library do what you probably mean

    (Note: `url()`/`file()`/`classpath()` syntax is not supported

    in Play/Akka 2.0, only in later releases.)

  - 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 ${colors.fox} 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"`

  - multi-line strings with triple quotes as in Python or Scala



### 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:



    foo.bar=10

    foo.baz=12



Put the dotted-notation fields on a single line:



    foo.bar=10, 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 HOCON.md in this

directory) has some more detailed notes on this topic.



### Uses of Substitutions



The `${foo.bar}` 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}



#### Inheritance



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.



If you put two objects next to each other (close brace of the first

on the same line with open brace of the second), they are merged, so

a shorter way to write the above "inheritance" example would be:



    data-center-generic = { cluster-size = 6 }

    data-center-east = ${data-center-generic} { name = "east" }

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



#### 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} ]



By setting the JVM property `-Dconfig.override_with_env_vars=true`

it is possible to override any configuration value using environment

variables even if an explicit substitution is not specified.



The environment variable value will override any pre-existing value

and also any value provided as Java property.



With this option enabled only environment variables starting with

`CONFIG_FORCE_` are considered, and the name is mangled as follows:



  - the prefix `CONFIG_FORCE_` is stripped

  - single underscore(`_`) is converted into a dot(`.`)

  - double underscore(`__`) is converted into a dash(`-`)

  - triple underscore(`___`) is converted into a single underscore(`_`)



i.e. The environment variable `CONFIG_FORCE_a_b__c___d` set the

configuration key `a.b-c_d`



### Set array values outside configuration files



Setting the value of array items from java properties or environment

variables require specifying the index in the array for the value.

So, while in HOCON you can set multiple values into an array or

append to an array:



    ## HOCON

    items = ["a", "b"]

    items += "c"



Using java properties you specify the exact position:



    -Ditems.0="a" -Ditems.1="b"



as well as with environment variables:



    export CONFIG_FORCE_items_0=a

    export CONFIG_FORCE_items_1=b



### Concatenation



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.



Quoted or unquoted strings can also concatenate with substitutions of course:



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

    tasks-url : ${base-url}"tasks:colon-must-be-quoted"



Note: the `${}` syntax must be outside the quotes!



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.



Note: Play/Akka 2.0 have an earlier version that supports string

concatenation, but not object/array concatenation. `+=` does not

work in Play/Akka 2.0 either. Post-2.0 versions support these

features.



## Miscellaneous Notes



### Debugging Your Configuration



If you have trouble with your configuration, some useful tips.



 - Set the Java system property `-Dconfig.trace=loads` to get

   output on stderr describing each file that is loaded.

   Note: this feature is not included in the older version in

   Play/Akka 2.0.

 - Use `myConfig.root().render()` to get a `Config` as a

   string with comments showing where each value came from.

   This string can be printed out on console or logged to 

   a file etc.

 - If you see errors like

   `com.typesafe.config.ConfigException$Missing: No configuration setting found for key foo`,

   and you're sure that key is defined in your config file, they might appear e.g.

   when you're loading configuration from a thread that's not the JVM's main thread.

   Try passing the `ClassLoader` in manually - e.g. with `ConfigFactory.load(getClass().getClassLoader())`

   or setting the context class loader.

   If you don't pass one, Lightbend Config uses the calling thread's `contextClassLoader`, and in some cases,

   it may not have your configuration files in its classpath,

   so loading the config on that thread can yield unexpected, erroneous results.



### Supports Java 8 and Later



Currently the library is maintained against Java 8, but

version 1.2.1 and earlier will work with Java 6.



Please use 1.2.1 if you need Java 6 support, though some people

have expressed interest in a branch off of 1.3.x supporting

Java 7. If you want to work on that branch you might bring it up

on [chat](https://gitter.im/lightbend/config). We can release a

jar for Java 7 if someone(s) steps up to maintain the branch. The

main branch does not use Java 8 "gratuitously" but some APIs

that use Java 8 types will need to be removed.



### Rationale for Supported File Formats



(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.



### Other APIs (Wrappers, Ports and Utilities)



This may not be comprehensive - if you'd like to add mention of

your wrapper, just send a pull request for this README. We would

love to know what you're doing with this library or with the HOCON

format.



#### Guice integration

  * Typesafe Config Guice https://github.com/racc/typesafeconfig-guice



#### Java (yep!) wrappers for the Java library



  * tscfg https://github.com/carueda/tscfg



#### Scala wrappers for the Java library



  * Ficus https://github.com/iheartradio/ficus

  * configz https://github.com/arosien/configz

  * configs https://github.com/kxbmap/configs

  * config-annotation https://github.com/zhongl/config-annotation

  * PureConfig https://github.com/pureconfig/pureconfig

  * Simple Scala Config https://github.com/ElderResearch/ssc

  * konfig https://github.com/vpon/konfig

  * ScalaConfig https://github.com/andr83/scalaconfig

  * static-config https://github.com/Krever/static-config

  * validated-config https://github.com/carlpulley/validated-config

  * Cedi Config https://github.com/ccadllc/cedi-config

  * Cfg https://github.com/carueda/cfg

  * circe-config https://github.com/circe/circe-config

  * args4c https://github.com/aaronp/args4c



#### Clojure wrappers for the Java library



  * beamly-core.config https://github.com/beamly/beamly-core.config



#### Kotlin wrappers for the Java library



  * config4k https://github.com/config4k/config4k

  * hoplite https://github.com/sksamuel/hoplite



#### Scala port



  * SHocon https://github.com/akka-js/shocon (Supports Scala.js and Scala Native)

  * sconfig https://github.com/ekrich/sconfig (Supports JVM, Scala Native, and Scala.js)



#### Ruby port



   * https://github.com/puppetlabs/ruby-hocon



#### Puppet module



   * Manage your HOCON configuration files with Puppet!: https://forge.puppetlabs.com/puppetlabs/hocon



#### Python port



   * pyhocon https://github.com/chimpler/pyhocon



#### C++ port



   * https://github.com/puppetlabs/cpp-hocon



#### JavaScript port



  * https://github.com/yellowblood/hocon-js (missing features, under development)



#### C# port



  * https://github.com/akkadotnet/HOCON



#### Rust port



  * https://github.com/mockersf/hocon.rs



#### Go port



  * https://github.com/gurkankaymak/hocon

 

#### Erlang port

  * https://github.com/emqx/hocon



#### Linting tool



   * A web based linting tool http://www.hoconlint.com/



#### Online playground



   * https://hocon-playground.avelier.dev/



# Maintenance notes



## License



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



## Maintained by 



The "Typesafe Config" library is an important foundation to how Akka and other JVM libraries manage configuration. We at [Lightbend](https://lightbend.com) consider the functionality of this library as feature complete. We will make sure "Typesafe Config" keeps up with future JVM versions, but will rarely make any other changes.



We are thankful for all the work [@havocp](https://github.com/havocp) has put into creating the library initially and supporting its users over many more years, even after leaving Lightbend.