config/HOCON.md

HOCON (Human-Optimized Config Object Notation)

This is an informal spec, but hopefully it's clear.

Goals / Background

The primary goal is: keep the semantics (tree structure; set of types; encoding/escaping) from JSON, but make it more convenient as a human-editable config file format.

The following features are desirable, to support human usage:

• less noisy / less pedantic syntax
• ability to refer to another part of the configuration (set a value to another value)
• import/include another configuration file into the current file
• a mapping to a flat properties list such as Java's System properties
• ability to get values from environment variables
• ability to write comments

Implementation-wise, the format should have these properties:

• a JSON superset, that is, all valid JSON should be valid and should result in the same in-memory data that a JSON parser would have produced.
• be deterministic; the format is flexible, but it is not heuristic. It should be clear what's invalid and invalid files should generate errors.
• require minimal look-ahead; should be able to tokenize the file by looking at only the next three characters. (right now, the only reason to look at three is to find "//" comments; otherwise you can parse looking at two.)

HOCON is significantly harder to specify and to parse than JSON. Think of it as moving the work from the person maintaining the config file to the computer program.

Definitions

• a key is a string JSON would have to the left of : and a value is anything JSON would have to the right of :. i.e. the two halves of an object field.

• a value is any "value" as defined in the JSON spec, plus unquoted strings and substitutions as defined in this spec.

• a simple value is any value excluding an object or array value.

• a field is a key, any separator such as ':', and a value.

• references to a file ("the file being parsed") can be understood to mean any byte stream being parsed, not just literal files in a filesystem.

Syntax

Much of this is defined with reference to JSON; you can find the JSON spec at http://json.org/ of course.

Unchanged from JSON

• files must be valid UTF-8
• quoted strings are in the same format as JSON strings
• values have possible types: string, number, object, array, boolean, null
• allowed number formats matches JSON; as in JSON, some possible floating-point values are not represented, such as NaN

Comments

Anything between // or # and the next newline is considered a comment and ignored, unless the // or # is inside a quoted string.

Omit root braces

JSON documents must have an array or object at the root. Empty files are invalid documents, as are files containing only a non-array non-object value such as a string.

In HOCON, if the file does not begin with a square bracket or curly brace, it is parsed as if it were enclosed with {} curly braces.

A HOCON file is invalid if it omits the opening { but still has a closing }; the curly braces must be balanced.

Key-value separator

The = character can be used anywhere JSON allows :, i.e. to separate keys from values.

If a key is followed by {, the : or = may be omitted. So "foo" {} means "foo" : {}"

Commas

Values in arrays, and fields in objects, need not have a comma between them as long as they have at least one ASCII newline (\n, decimal value 10) between them.

The last element in an array or last field in an object may be followed by a single comma. This extra comma is ignored.

• [1,2,3,] and [1,2,3] are the same array.
• [1\n2\n3] and [1,2,3] are the same array.
• [1,2,3,,] is invalid because it has two trailing commas.
• [,1,2,3] is invalid because it has an initial comma.
• [1,,2,3] is invalid because it has two commas in a row.
• these same comma rules apply to fields in objects.

Whitespace

The JSON spec simply says "whitespace"; in HOCON whitespace is defined as follows:

• any Unicode space separator (Zs category), line separator (Zl category), or paragraph separator (Zp category), including nonbreaking spaces (such as 0x00A0, 0x2007, and 0x202F).
• tab (\t 0x0009), newline ('\n' 0x000A), vertical tab ('\v' 0x000B), form feed (\f' 0x000C), carriage return ('\r' 0x000D), file separator (0x001C), group separator (0x001D), record separator (0x001E), unit separator (0x001F).

In Java, the isWhitespace() method covers these characters with the exception of nonbreaking spaces.

While all Unicode separators should be treated as whitespace, in this spec "newline" refers only and specifically to ASCII newline 0x000A.

Duplicate keys

The JSON spec does not clarify how duplicate keys in the same object should be handled. In HOCON, duplicate keys that appear later override those that appear earlier, unless both values are objects. If both values are objects, then the objects are merged.

Note: this would make HOCON a non-superset of JSON if you assume that JSON requires duplicate keys to have a behavior. The assumption here is that duplicate keys are invalid JSON.

To merge objects:

• add fields present in only one of the two objects to the merged object.
• for non-object-valued fields present in both objects, the field found in the second object must be used.
• for object-valued fields present in both objects, the object values should be recursively merged according to these same rules.

Object merge can be prevented by setting the key to another value first. This is because merging is always done two values at a time; if you set a key to an object, a non-object, then an object, first the non-object falls back to the object (non-object always wins), and then the object falls back to the non-object (no merging, object is the new value). So the two objects never see each other.

These two are equivalent:

{
    "foo" : { "a" : 42 },
    "foo" : { "b" : 43 }
}

{
    "foo" : { "a" : 42, "b" : 43 }
}

And these two are equivalent:

{
    "foo" : { "a" : 42 },
    "foo" : null,
    "foo" : { "b" : 43 }
}

{
    "foo" : { "b" : 43 }
}

The intermediate setting of "foo" to null prevents the object merge.

Unquoted strings

A sequence of characters outside of a quoted string is a string value if:

• it does not contain "forbidden characters" '$', '"', '{', '}', '[', ']', ':', '=', ',', '+', '#', '' (backslash), or whitespace.
• it does not contain the two-character string "//" (which starts a comment)
• its initial characters do not parse as true, false, null, or a number.

Unquoted strings are used literally, they do not support any kind of escaping. Quoted strings may always be used as an alternative when you need to write a character that is not permitted in an unquoted string.

truefoo parses as the boolean token true followed by the unquoted string foo. However, footrue parses as the unquoted string footrue. Similarly, 10.0bar is the number 10.0 then the unquoted string bar but bar10.0 is the unquoted string bar10.0.

In general, once an unquoted string begins, it continues until a forbidden character or the two-character string "//" is encountered. Embedded (non-initial) booleans, nulls, and numbers are not recognized as such, they are part of the string.

An unquoted string may not begin with the digits 0-9 or with a hyphen (-, 0x002D) because those are valid characters to begin a JSON number. The initial number character, plus any valid-in-JSON number characters that follow it, must be parsed as a number value. Again, these characters are not special inside an unquoted string; they only trigger number parsing if they appear initially.

Note that quoted JSON strings may not contain control characters (control characters include some whitespace characters, such as newline). This rule is from the JSON spec. However, unquoted strings have no restriction on control characters, other than the ones listed as "forbidden characters" above.

Value concatenation

The value of an object field or an array element may consist of multiple values which are concatenated into one string.

Only simple values participate in value concatenation. Recall that a simple value is any value other than arrays and objects.

As long as simple values are separated only by non-newline whitespace, the whitespace between them is preserved and the values, along with the whitespace, are concatenated into a string.

Value concatenations never span a newline, or a character that is not part of a simple value.

A value concatenation may appear in any place that a string may appear, including object keys, object values, and array elements.

Whenever a value would appear in JSON, a HOCON parser instead collects multiple values (including the whitespace between them) and concatenates those values into a string.

Whitespace before the first and after the last simple value must be discarded. Only whitespace between simple values must be preserved.

So for example foo bar baz parses as three unquoted strings, and the three are value-concatenated into one string. The inner whitespace is kept and the leading and trailing whitespace is trimmed. The equivalent string, written in quoted form, would be "foo bar baz".

Value concatenation foo bar (two unquoted strings with whitespace) and quoted string "foo bar" would result in the same in-memory representation, seven characters.

For purposes of value concatenation, non-string values are converted to strings as follows (strings shown as quoted strings):

• true and false become the strings "true" and "false".
• null becomes the string "null".
• quoted and unquoted strings are themselves.
• numbers should be kept as they were originally written in the file. For example, if you parse 1e5 then you might render it alternatively as 1E5 with capital E, or just 100000. For purposes of value concatenation, it should be rendered as it was written in the file.
• a substitution is replaced with its value which is then converted to a string as above, except that a substitution which evaluates to null becomes the empty string "".
• it is invalid for arrays or objects to appear in a value concatenation.

A single value is never converted to a string. That is, it would be wrong to value concatenate true by itself; that should be parsed as a boolean-typed value. Only true foo (true with another simple value on the same line) should be parsed as a value concatenation and converted to a string.

Path expressions

Path expressions are used to write out a path through the object graph. They appear in two places; in substitutions, like ${foo.bar}, and as the keys in objects like { foo.bar : 42 }.

Path expressions are syntactically identical to a value concatenation, except that they may not contain substitutions. This means that you can't nest substitutions inside other substitutions, and you can't have substitutions in keys.

When concatenating the path expression, any . characters outside quoted strings are understood as path separators, while inside quoted strings . has no special meaning. So foo.bar."hello.world" would be a path with three elements, looking up key foo, key bar, then key hello.world.

The main tricky point is that . characters in numbers do count as a path separator. When dealing with a number as part of a path expression, it's essential to retain the original string representation of the number as it appeared in the file (rather than converting it back to a string with a generic number-to-string library function).

• 10.0foo is a number then unquoted string foo and should be the two-element path with 10 and 0foo as the elements.
• foo10.0 is an unquoted string with a . in it, so this would be a two-element path with foo10 and 0 as the elements.
• foo"10.0" is an unquoted then a quoted string which are concatenated, so this is a single-element path.

Unlike value concatenations, path expressions are always converted to a string, even if they are just a single value.

If you have an array or element value consisting of the single value true, it's a value concatenation and retains its character as a boolean value.

If you have a path expression (in a key or substitution) then it must always be converted to a string, so true becomes the string that would be quoted as "true".

If a path element is an empty string, it must always be quoted. That is, a."".b is a valid path with three elements, and the middle element is an empty string. But a..b is invalid and should generate an error. Following the same rule, a path that starts or ends with a . is invalid and should generate an error.

Paths as keys

If a key is a path expression with multiple elements, it is expanded to create an object for each path element other than the last. The last path element, combined with the value, becomes a field in the most-nested object.

In other words:

foo.bar : 42

is equivalent to:

foo { bar : 42 }

and:

foo.bar.baz : 42

is equivalent to:

foo { bar { baz : 42 } }

and so on. These values are merged in the usual way; which implies that:

a.x : 42, a.y : 43

is equivalent to:

a { x : 42, y : 43 }

Because path expressions work like value concatenations, you can have whitespace in keys:

a b c : 42

is equivalent to:

"a b c" : 42

Because path expressions are always converted to strings, even single values that would normally have another type become strings.

• true : 42 is "true" : 42
• 3.14 : 42 is "3.14" : 42

As a special rule, the unquoted string include may not begin a path expression in a key, because it has a special interpretation (see below).

Substitutions

Substitutions are a way of referring to other parts of the configuration tree.

For substitutions which are not found in the configuration tree, implementations may try to resolve them by looking at system environment variables, Java system properties, or other external sources of configuration.

The syntax is ${pathexpression} where the pathexpression is a path expression as described above. This path expression has the same syntax that you could use for an object key.

Substitutions are not parsed inside quoted strings. To get a string containing a substitution, you must use value concatenation with the substitution in the unquoted portion:

key : ${animal.favorite} is my favorite animal

Or you could quote the non-substitution portion:

key : ${animal.favorite}" is my favorite animal"

Substitutions are resolved by looking up the path in the configuration. The path begins with the root configuration object, i.e. it is "absolute" rather than "relative."

Substitution processing is performed as the last parsing step, so a substitution can look forward in the configuration. If a configuration consists of multiple files, it may even end up retrieving a value from another file. If a key has been specified more than once, the substitution will always evaluate to its latest-assigned value (the merged object or the last non-object value that was set).

If a substitutions does not match any value present in the configuration, implementations may look up that substitution in one or more external sources, such as a Java system property or an environment variable. (More detail on this in a later section.)

If a configuration sets a value to null then it should not be looked up in the external source. Unfortunately there is no way to "undo" this in a later configuration file; if you have { "HOME" : null } in a root object, then ${HOME} will never look at the environment variable. There is no equivalent to JavaScript's delete operation in other words.

If a substitution does not match any value present in the configuration and is not resolved by an external source, it is evaluated to null.

Substitutions are only allowed in object field values and array elements (value concatenations), they are not allowed in keys or nested inside other substitutions (path expressions).

A substitution is replaced with any value type (number, object, string, array, true, false, null). If the substitution is the only part of a value, then the type is preserved. Otherwise, it is value-concatenated to form a string. There is one special rule:

• null is converted to an empty string, not the string null.

Because missing substitutions are evaluated to null, either missing or explicitly-set-to-null substitutions become an empty string when concatenated.

Circular substitutions are invalid and should generate an error.

Implementations must take care, however, to allow objects to refer to paths within themselves. For example, this must work:

bar : { foo : 42,
        baz : ${bar.foo}
      }

Here, if an implementation resolved all substitutions in bar as part of resolving the substitution ${bar.foo}, there would be a cycle. The implementation must only resolve the foo field in bar, rather than recursing the entire bar object.

Includes

Include syntax

An include statement consists of the unquoted string include and a single quoted string immediately following it. An include statement can appear in place of an object field.

If the unquoted string include appears at the start of a path expression where an object key would be expected, then it is not interpreted as a path expression or a key.

Instead, the next value must be a quoted string. The quoted string is interpreted as a filename or resource name to be included.

Together, the unquoted include and the quoted string substitute for an object field syntactically, and are separated from the following object fields or includes by the usual comma (and as usual the comma may be omitted if there's a newline).

If an unquoted include at the start of a key is followed by anything other than a single quoted string, it is invalid and an error should be generated.

There can be any amount of whitespace, including newlines, between the unquoted include and the quoted string.

Value concatenation is NOT performed on the "argument" to include. The argument must be a single quoted string. No substitutions are allowed, and the argument may not be an unquoted string or any other kind of value.

Unquoted include has no special meaning if it is not the start of a key's path expression.

It may appear later in the key:

# this is valid
{ foo include : 42 }
# equivalent to
{ "foo include" : 42 }

It may appear as an object or array value:

{ foo : include } # value is the string "include"
[ include ]       # array of one string "include"

You can quote "include" if you want a key that starts with the word "include", only unquoted include is special:

{ "include" : 42 }

Include semantics: merging

An including file contains the include statement and an included file is the one specified in the include statement. (They need not be regular files on a filesystem, but assume they are for the moment.)

An included file must contain an object, not an array. This is significant because both JSON and HOCON allow arrays as root values in a document.

If an included file contains an array as the root value, it is invalid and an error should be generated.

The included file should be parsed, producing a root object. The keys from the root object are conceptually substituted for the include statement in the including file.

• If a key in the included object occurred prior to 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.
• If the including file repeats a key from an earlier-included object, the including file's value would override or merge with the one from the included file.

Include semantics: substitution

Recall that substitution happens as a final step, after parsing. It should be done for the entire app's configuration, not for single files in isolation.

Therefore, if an included file contains substitutions, they must be "fixed up" to be relative to the app's configuration root.

Say for example that the root configuration is this:

{ a : { include "foo.conf" } }

And "foo.conf" might look like this:

{ x : 10, y : ${x} }

If you parsed "foo.conf" in isolation, then ${x} would evaluate to 10, the value at the path x. If you include "foo.conf" in an object at key a, however, then it must be fixed up to be ${a.x} rather than ${x}.

Say that the root configuration redefines a.x, like this:

{
    a : { include "foo.conf" }
    a : { x : 42 }
}

Then the ${x} in "foo.conf", which has been fixed up to ${a.x}, would evaluate to 42 rather than to 10. Substitution happens after parsing the whole configuration.

Include semantics: missing files

If an included file does not exist, the include statement should be silently ignored (as if the included file contained only an empty object).

Include semantics: file formats and extensions

Implementations may support including files in other formats. Those formats must be compatible with the JSON type system, or have some documented mapping to JSON's type system.

If an implementation supports multiple formats, then the extension may be omitted from the name of included files:

include "foo"

If a filename has no extension, the implementation should treat it as a basename and try loading the file with all known extensions.

If the file exists with multiple extensions, they should all be loaded and merged together.

Files in HOCON format should be parsed last. Files in JSON format should be parsed next-to-last.

In short, include "foo" might be equivalent to:

include "foo.properties"
include "foo.json"
include "foo.conf"

Include semantics: locating resources

Conceptually speaking, the quoted string in an include statement identifies a file or other resource "adjacent to" the one being parsed and of the same type as the one being parsed. The meaning of "adjacent to", and the string itself, has to be specified separately for each kind of resource.

Implementations may vary in the kinds of resources they support including.

For plain files on the filesystem:

• if the included file is an absolute path then it should be kept absolute and loaded as such.
• if the included file is a relative path, then it should be located relative to the directory containing the including file. The current working directory of the process parsing a file must NOT be used when interpreting included paths.

For resources located on the Java classpath:

• included resources are looked up by calling getResource() on the same class or class loader used to look up the including resource.
• if the included resource name is absolute (starts with '/') then it should be passed to getResource() as-is.
• if the included resource name does not start with '/' then it should have the "directory" of the including resource. prepended to it, before passing it to getResource(). If the including resource is not absolute (no '/') and has no "parent directory" (is just a single path element), then the included relative resource name should be left as-is.
• it would be wrong to use getResource() to get a URL and then locate the included name relative to that URL, because a class loader is not required to have a one-to-one mapping between paths in its URLs and the paths it handles in getResource(). In other words, the "adjacent to" computation should be done on the resource name not on the resource's URL.

URLs:

• for both filesystem files and Java resources, if the included name is a URL (begins with a protocol), it would be reasonable behavior to try to load the URL rather than treating the name as a filename or resource name.
• for files loaded from a URL, "adjacent to" should be based on parsing the URL's path component, replacing the last path element with the included name.

Implementations need not support files, Java resources, or URLs; and they need not support particular URL protocols. However, if they do support them they should do so as described above.

API Recommendations

Implementations of HOCON ideally follow certain conventions and work in a predictable way.

Automatic type conversions

If an application asks for a value with a particular type, the implementation should attempt to convert types as follows:

• number to string: convert the number into a string representation that would be a valid number in JSON.
• boolean to string: should become the string "true" or "false"
• string to number: parse the number with the JSON rules
• string to boolean: the strings "true", "yes", "on", "false", "no", "off" should be converted to boolean values. It's tempting to support a long list of other ways to write a boolean, but for interoperability and keeping it simple, it's recommended to stick to these six.
• string to null: the string "null" should be converted to a null value if the application specifically asks for a null value, though there's probably no reason an app would do this.

The following type conversions should NOT be performed:

• null to anything: If the application asks for a specific type and finds null instead, that should usually result in an error.
• object to anything
• array to anything
• anything to object
• anything to array

Converting objects and arrays to and from strings is tempting, but in practical situations raises thorny issues of quoting and double-escaping.

Units format

Implementations may wish to support interpreting a value with some family of units, such as time units or memory size units: 10ms or 512K. HOCON does not have an extensible type system and there is no way to add a "duration" type. However, for example, if an application asks for milliseconds, the implementation can try to interpret a value as a milliseconds value.

If an API supports this, for each family of units it should define a default unit in the family. For example, the family of duration units might default to milliseconds (see below for details on durations). The implementation should then interpret values as follows:

• if the value is a number, it is taken to be a number in the default unit.

• if the value is a string, it is taken to be:

  • optional whitespace
    • a number
    • optional whitespace
    • an optional unit name consisting only of letters (letters are the Unicode L* categories, Java isLetter())
    • optional whitespace

  If a string value has no unit name, then it should be interpreted with the default unit, as if it were a number. If a string value has a unit name, that name of course specifies the value's interpretation.

Duration format

Implementations may wish to support a getMilliseconds() (and similar for other time units).

This can use the general "units format" described above; bare numbers are taken to be in milliseconds already, while strings are parsed as a number plus an optional unit string.

The supported unit strings for duration are case sensitive and must be lowercase. Exactly these strings are supported:

• ns, nanosecond, nanoseconds
• us, microsecond, microseconds
• ms, millisecond, milliseconds
• s, second, seconds
• m, minute, minutes
• h, hour, hours
• d, day, days

Size in bytes format

Implementations may wish to support a getBytes() returning a size in bytes.

This can use the general "units format" described above; bare numbers are taken to be in bytes already, while strings are parsed as a number plus an optional unit string.

The one-letter unit strings may be uppercase (note: duration units are always lowercase, so this convention is specific to size units).

There is an unfortunate nightmare with size-in-bytes units, that they may be in powers or two or powers of ten. The approach defined by standards bodies appears to differ from common usage, such that following the standard leads to people being confused. Worse, common usage varies based on whether people are talking about RAM or disk sizes, and various existing operating systems and apps do all kinds of different things. See http://en.wikipedia.org/wiki/Binary_prefix#Deviation_between_powers_of_1024_and_powers_of_1000 for examples. It appears impossible to sort this out without causing confusion for someone sometime.

For single bytes, exactly these strings are supported:

• B, b, byte, bytes

For powers of ten, exactly these strings are supported:

• kB, kilobyte, kilobytes
• MB, megabyte, megabytes
• GB, gigabyte, gigabytes
• TB, terabyte, terabytes
• PB, petabyte, petabytes
• EB, exabyte, exabytes
• ZB, zettabyte, zettabytes
• YB, yottabyte, yottabytes

For powers of two, exactly these strings are supported:

• K, k, Ki, KiB, kibibyte, kibibytes
• M, m, Mi, MiB, mebibyte, mebibytes
• G, g, Gi, GiB, gibibyte, gibibytes
• T, t, Ti, TiB, tebibyte, tebibytes
• P, p, Pi, PiB, pebibyte, pebibytes
• E, e, Ei, EiB, exbibyte, exbibytes
• Z, z, Zi, ZiB, zebibyte, zebibytes
• Y, y, Yi, YiB, yobibyte, yobibytes

It's very unclear which units the single-character abbreviations ("128K") should go with; some precedents such as java -Xmx 2G and the GNU tools such as ls map these to powers of two, so this spec copies that. You can certainly find examples of mapping these to powers of ten, though. If you don't like ambiguity, don't use the single-letter abbreviations.

Config object merging and file merging

It may be useful to offer a method to merge two objects. If such a method is provided, it should work as if the two objects were duplicate values for the same key in the same file. (See the section earlier on duplicate key handling.)

As with duplicate keys, an intermediate non-object value "hides" earlier object values. So say you merge three objects in this order:

• { a : { x : 1 } } (first priority)
• { a : 42 } (fallback)
• { a : { y : 2 } } (another fallback)

The result would be { a : { x : 1 } }. The two objects are not merged because they are not "adjacent"; the merging is done in pairs, and when 42 is paired with { y : 2 }, 42 simply wins and loses all information about what it overrode.

But if you re-ordered like this:

• { a : { x : 1 } } (first priority)
• { a : { y : 2 } } (fallback)
• { a : 42 } (another fallback)

Now the result would be { a : { x : 1, y : 2 } } because the two objects are adjacent.

This rule for merging objects loaded from different files is exactly the same behavior as for merging duplicate fields in the same file. All merging works the same way.

Needless to say, normally it's well-defined whether a config setting is supposed to be a number or an object. This kind of weird pathology where the two are mixed should not be happening.

The one place where it matters, though, is that it allows you to "clear" an object and start over by setting it to null and then setting it back to a new object. So this behavior gives people a way to get rid of default fallback values they don't want.

Java properties mapping

It may be useful to merge Java properties data with data loaded from JSON or HOCON. See the Java properties spec here: http://download.oracle.com/javase/7/docs/api/java/util/Properties.html#load%28java.io.Reader%29

Java properties parse as a one-level map from string keys to string values.

To convert to HOCON, first split each key on the . character, keeping any empty strings (including leading and trailing empty strings). Note that this is very different from parsing a path expression.

The key split on . is a series of path elements. So the properties key with just . is a path with two elements, both of them an empty string. a. is a path with two elements, a and empty string. (Java's String.split() does NOT do what you want for this.)

It is impossible to represent a key with a . in it in a properties file. If a JSON/HOCON key has a . in it, which is possible if the key is quoted, then there is no way to refer to it as a Java property. It is not recommended to name HOCON keys with a . in them, since it would be confusing at best in any case.

Once you have a path for each value, construct a tree of JSON-style objects with the string value of each property located at that value's path.

Values from properties files are always strings, even if they could be parsed as some other type. Implementations should do type conversion if an app asks for an integer, as described in an earlier section.

When Java loads a properties file, unfortunately it does not preserve the order of the file. As a result, there is an intractable case where a single key needs to refer to both a parent object and a string value. For example, say the Java properties file has:

a=hello
a.b=world

In this case, a needs to be both an object and a string value. The object must always win in this case... the "object wins" rule throws out at most one value (the string) while "string wins" would throw out all values in the object. Unfortunately, when properties files are mapped to the JSON structure, there is no way to access these strings that conflict with objects.

The usual rule in HOCON would be that the later assignment in the file wins, rather than "object wins"; but implementing that for Java properties would require implementing a custom Java properties parser, which is surely not worth it.

Root paths

By convention, a given application or library has a "root path." Most commonly the root path has a single path element - "akka" for example. But it could have multiple.

Conventional config file names and property names are derived from the root path.

If an API looks like load(rootPath) then it would return an object conceptually "at" the root path, not an object containing the root path.

Conventional configuration file names for JVM apps

To get config file names, join the elements of the root path with a hyphen, then add appropriate filename extensions.

If the root path is foo.bar (two elements, foo and bar), then the configuration files should be searched for under the following resource names on the classpath:

• /foo-bar.conf
• /foo-bar.json
• /foo-bar.properties
• /foo-bar-reference.conf
• /foo-bar-reference.json
• /foo-bar-reference.properties

The .json and .properties files are examples, different implementations may support different file types. The "reference" files are intended to contain defaults and be shipped with the library or application being configured.

Note that the configuration files are absolute resource paths, not relative to the package. So you would call klass.getResource("/foo-bar.conf") not klass.getResource("foo-bar.conf").

Conventional override by system properties

For an application's config, Java System properties override HOCON found in the configuration file. This supports specifying config options on the command line.

Those system properties which begin with an application's root path should override the configuration for that application.

For example, say your config is for root path "akka" then your config key "foo" would go with -Dakka.foo=10. When loading your config, any system properties starting with akka. would be merged into the config.

Substitution fallback to system properties

Recall that if a substitution is not present (not even set to null) within a configuration tree, implementations may search for it from external sources. One such source could be Java system properties.

To find a value for substitution, Java applications should look at system properties directly, without the root path namespace. Remember that namespaced system properties were already used as overrides.

${user.home} would first look for a user.home in the configuration tree (which has a scoped system property like akka.user.home merged in!).

If no value for ${user.home} exists in the configuration, the implementation would look at system property user.home without the akka. prefix.

The unprefixed system properties are not merged in to the configuration tree; if you iterate over your configuration, they should not be in there. They are only used as a fallback when evaluating substitutions.

The effect is to allow using generic system properties like user.home and also to allow overriding those per-app. So if someone wants to set their home directory for all apps, they set the user.home system property. If they then want to force a particular home directory only for Akka, they could set akka.user.home instead.

Substitution fallback to environment variables

Substitutions not found in the configuration may also fall back to environment variables. In Java, fallback should be to system properties first and environment variables second.

It's recommended that HOCON keys always use lowercase, because environment variables generally are capitalized. This avoids naming collisions between environment variables and configuration properties. (While on Windows getenv() is generally not case-sensitive, the lookup will be case sensitive all the way until the env variable fallback lookup is reached.)

An application can explicitly block looking up a substitution in the environment by setting a value in the configuration, with the same name as the environment variable. You could set HOME : null in your root object to avoid expanding ${HOME} from the environment, for example.

Environment variables are interpreted as follows:

• present and set to empty string: treated as not present
• System.getenv throws SecurityException: treated as not present
• encoding is handled by Java (System.getenv already returns a Unicode string)
• environment variables always become a string value, though if an app asks for another type automatic type conversion would kick in

Open issues

• should a few more special characters be banned from unquoted strings, to allow future extensions?