• Overview

• Configuration Node Types

• Configuration Key

• In-memory Representation of Configuration

• Access by Key

• Access by General Navigation

• Detaching a Config Subtree

The config system represents configuration as a tree in memory. Many developers will choose to work directly with config values — values from the leaves in the tree — accessing them by their keys. You can also navigate explicitly among the nodes of the tree without using keys. This section describes what the tree looks like and how you can traverse it.

The config system represents configuration in memory using three types of nodes, each a different interface defined within the ConfigNode interface.

A node of any type can have a String value.

Each config tree in memory will have an object node as its root with child nodes as dictated by the source config data from which the config system built the tree.

config.get("key.does.not.exist")

Each config node (except the root) has a non-null key. Here is the formal definition of what keys can be:

config-key = *1( key-token *( "." key-token ) )
 key-token = *( unescaped / escaped )
 unescaped = %x00-2D / %x2F-7D / %x7F-10FFFF
           ; %x2E ('.') and %x7E ('~') are excluded from 'unescaped'
   escaped = "~" ( "0" / "1" )
           ; representing '~' and '.', respectively

The following example is in HOCON (human-optimized config object notation) format. The config system supports HOCON as an extension module.

app {
    greeting = "Hello"
    page-size = 20
    basic-range = [ -20, 20 ]
}
data {
    providers: [
        {
            name = "Provider1"
            class = "this.is.my.Provider1"
        },
        {
            name = "Provider2"
            class = "this.is.my.Provider2"
        }
    ]
}

The diagram below illustrates the in-memory tree for that configuration.

The Config object exposes methods to return the name, key, and type of the node.

For many applications, accessing configuration values by key will be the simplest approach. If you write the code with a specific configuration structure in mind, your code can retrieve the value from a specific configuration node very easily.

Your application can specify the entire navigation path as the key to a single get invocation, using dotted notation to separate the names of the nodes along the path. The code can navigate one level at a time using chained get invocations, each specifying one level of the path to the expected node. Or, you can mix the two styles.

All the following lines retrieve the same Config node.

assert config.get("") == config;
Config provName1 = config.get("data.providers.0.name"); 
Config provName2 = config.get("data.providers.0").get("name"); 
Config provName3 = config.get("data.providers").get("0.name");
Config provName4 = config.get("data").get("providers.0").get("name");
Config provName5 = config.get("data").get("providers").get("0").get("name"); 

• using a single key
• mixed style (composite key and single key)
• navigating one level with each get invocation

The Config.get(key) method always returns a Config object without throwing an exception. If the specified key does not exist the method returns a Config node of type MISSING. There are several ways your application can tell whether a given config value exists.

The config system throws a MissingValueException if the application tries to access the value of a missing node by invoking the ConfigValue.get() method.

Some applications might need to work with configuration without knowing its structure or key names ahead of time, and such applications can use various methods on the Config class to do this.

List<String> appNodeNames = config.get("app")
        .asNodeList() 
        .map(nodes -> { 
            return nodes
                    .stream()
                    .map(Config::name)
                    .sorted()
                    .toList();
        })
        .orElse(List.of()); 

assert appNodeNames.get(0).equals("basic-range"); 
assert appNodeNames.get(1).equals("greeting"); 
assert appNodeNames.get(2).equals("page-size"); 

• Get the ConfigValue with child Config instances.
• Map the node list to names using the Java Stream API (if present)
• Use an empty list if the "app" node does not exist
• Check that the list contains the expected child names: basic-range, greeting and page-size.

List<Config> providers = config.get("data.providers")
        .asNodeList().orElse(List.of()); 

assert providers.get(0).key().toString().equals("data.providers.0"); 
assert providers.get(1).key().toString().equals("data.providers.1"); 

• Get child nodes of the data.providers list node as a List of Config instances.
• Check that the list contains the expected child nodes with keys data.providers.0 and data.providers.1.

The traverse() method returns a stream of the nodes in the subtree that is rooted at the current configuration node. Depending on the structure of the loaded configuration the stream contains a mix of object, list or leaf value nodes.

config.get("data.providers")
        .traverse() 
        .forEach(node -> System.out.println(node.type() + " \t" + node.key())); 

• Visit the subtree rooted at the data.providers list node.
• Prints out following list of nodes (type and key):

OBJECT 	data.providers.0
VALUE 	data.providers.0.name
VALUE 	data.providers.0.class
OBJECT 	data.providers.1
VALUE 	data.providers.1.name
VALUE 	data.providers.1.class

The optional Predicate<Config> argument to the traverse methods allows the application to prune the traversal of a subtree at any point.

config.traverse(node -> !node.name().equals("data")) 
        .forEach(node -> System.out.println(node.type() + " \t" + node.key())); 

• Visit all root sub-nodes, excluding whole data tree structure but including others.
• Prints out following list of nodes (type and key):

OBJECT 	app
VALUE 	app.page-size
VALUE 	app.greeting
LIST 	app.basic-range
VALUE 	app.basic-range.0
VALUE 	app.basic-range.1

Sometimes it can be convenient to write part of your application to deal with configuration without it knowing if or where the relevant configuration is plugged into a larger config tree.

For example, the application.properties from the introduction section contains several settings prefixed with web such as web.page-size. Perhaps in another config source the same information might be stored as server.web.page-size:

server.web.page-size: 40
server.web.debug = true
server.web.ratio = 1.4

You might want to write the web portion of your app to work with a config subtree with keys that are independent of the subtree’s position in a larger tree. This would allow you to reuse the web portion of your application without change, regardless of which structure a config source used.

One easy way to do this is to detach a subtree from a larger config tree. When your application invokes the Config.detach method it gets back a copy of the config node but with no parent. The copy and the original node both point to the same objects for their child nodes (if any). The original node is unchanged.

// originalRoot is from the original example `.conf` file
// alternateRoot is from the alternate structure above

Config detachedFromOriginal = originalRoot.get("web").detach();
Config detachedFromAlternate = alternateRoot.get("server.web").detach();

assert originalRoot.get("web.debug").equals("true"); 
assert alternateRoot.get("server.web.debug").equals("true"); 

assert detachedFromOriginal.get("debug").equals("true"); 
assert detachedFromAlternate.get("debug").equals("true"); 

• Navigation depends on knowing the full structure of the config and so is different for the two cases.
• Detaching so the web node is the root can use the same key regardless of where the config subtree came from.