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logo YConf

Simple, elegant configuration.

Maven release Total alerts Gradle build codecov


YConf is a mapping layer for a one-way conversion of structured documents (XML, JSON, YAML, etc.) into object graphs, specifically optimised for configuration scenarios. It is not a general-purpose object serialisation framework; instead, it's like an ORM for configuration artifacts.

YConf currently supports YAML using the SnakeYAML parser and JSON using Gson. Other document formats and parsers are relatively trivial to add.

Why not use <insert your favourite parser here>

Parsers such as SnakeYAML, Jackson, Gson, Genson, XStream et al. already support bidirectional object serialisation. And they also support custom (de)serialisers. So why the middleman?

YConf was designed to provide a parser-agnostic object mapper. With YConf you can switch from one document format to another, and your application is none the wiser. And importantly, your hand-crafted mapping code will continue to work regardless of the underlying library.

YConf standardises type mappings. Your team might support multiple projects with varying configuration needs — different formats, parsers and mappings. Rather than learning the intricacies of every parser, YConf provides a uniform mapping language that works across parsers and document formats.

Parametrised Configuration

That's all well and good, but is an abstraction layer really worth it? So here goes. Decoupling yourself from the parser frees you from its limitations. For instance, can your parser do this?

- type: Stochatic
  lookback: ${env.STO_LOOKBACK}
  kPeriod: ${env.STO_K}
  overbought: ${env.STO_OVERBOUGHT}
  oversold: ${1 - env.STO_OVERBOUGHT}

Look carefully. We've bootstrapped a stochastic oscillator, but the lookback and kPeriod coefficients aren't supplied in the config file; instead, they're taken from the environment variables STO_LOOKBACK and STO_K. And the fun doesn't end there. The overbought and oversold signals aren't just sourced from env. The oversold value is actually derived from the overbought value. So if overbought is set to 80% (0.8), then oversold would get 20% (0.2).

This isn't something that YAML supports natively, but the yconf-juel plugin does this for us effortlessly, using the Unified Expression Language (EL) to evaluate arbitrary expressions in the config. And it will work equally well with JSON, without changing a line of code.

  "indicators": [
      "type": "Stochastic",
      "lookback": "${env.STO_LOOKBACK}",
      "kPeriod": "${env.STO_K}",
      "overbought": "${env.STO_OVERBOUGHT}",
      "oversold": "${1 - env.STO_OVERBOUGHT}"

Linking Files

We've all seen jumbo sized configuration files that grow out of control. The example below is YConf's solution to this problem.

- ${'indicators/stochastic.yaml')}
- ${'indicators/macd.yaml')}
- ${'indicators/bollinger.yaml')}

In our example, indicators/stochastic.yaml is a simple YAML snippet containing nothing but the Stochastic Oscillator configuration.

type: Stochatic
lookback: ${env.STO_LOOKBACK}
kPeriod: ${env.STO_K}
overbought: ${env.STO_OVERBOUGHT}
oversold: ${1 - env.STO_OVERBOUGHT}

Getting Started

Getting YConf

Just add the following snippet to your build file. Replace the version placeholder x.y.z in the snippet with the version shown on the Download badge at the top of this README.

api 'com.obsidiandynamics.yconf:yconf-core:x.y.z'
api 'com.obsidiandynamics.yconf:<module>:x.y.z'

You need to add yconf-core and at least one other module, depending on the desired parser. The following is a list of available modules on JCenter.

Module name Description
yconf-core The core YConf library. Required for all deployments.
yconf-snakeyaml Snakeyaml (version 1.20) plugin for parsing YAML documents.
yconf-gson Gson (version 2.8.2) plugin, for parsing JSON documents.
yconf-juel JUEL (version 2.2.7) plugin, supporting the Unified Expression Language (EL).

We're going to stick to YAML for our examples. Our sample Gradle dependencies resemble the following:

compile 'com.obsidiandynamics.yconf:yconf-core:0.3.0'
compile 'com.obsidiandynamics.yconf:yconf-juel:0.3.0'
compile 'com.obsidiandynamics.yconf:yconf-snakeyaml:0.3.0'

Field injection

Assume the following YAML file:

aString: hello
aNumber: ${3 + 0.14}
  aBool: true
  - a
  - b
  - c

And the following Java class structure:

public class Top {
  public static class Inner {
    boolean aBool;
    String[] anArray;
  String aString = "some default";
  double aNumber;
  Inner inner;

All it takes is the following to map from the document to the object model, storing the result in a variable named top:

final Top top = new MappingContext()
    .withParser(new SnakeyamlParser())
    .withDomTransform(new JuelTransform())
    .fromStream(new FileInputStream("sample-basic.yaml"))

Note: we use .withParser() to specify the document parser. If using JSON, invoke .withParser(new GsonParser()). We've also used JUEL for our DOM transform, which automatically evaluates EL expressions present in the document.

The result of calling .fromStream() is a YObject instance, which encapsulates the root document object model (DOM) — essentially an object graph derived from the underlying file. This isn't yet what we need. So the last call in the chain is .map(), which does the actual work — mapping the DOM to the given Class type.

The aString field in our example provides a default value. So if the document omits a value for aString, the default assignment will remain. This is really convenient when your configuration has sensible defaults. Beware of one gotcha: if the document provides a value, but that value is null, this is treated as the absence of a value. So if null happens to be a valid value in your scenario, it would also have to be the default value.

For the above example to work we've had to do a few things:

  • Annotate the mapped classes with @Y;
  • Annotate the mapped fields with @YInject; and
  • Ensure that the classes have a public no-arg constructor.

The @YInject annotation has two optional fields:

  • name — The name of the attribute in the document that will be mapped to the annotated field or parameter. If omitted, the name will be inferred from the annotated field. If the annotation is applied to a constructor parameter, the name must be set explicitly.
  • type — The Class type of the mapped object. If omitted, the type will be inferred from the annotated field or parameter.

Constructor injection

Suppose you can't annotate fields and/or provide a no-arg constructor. Perhaps you are inheriting from a base class over which you have no control. The following is an alternative that uses constructor injection:

public class Top {
  public static class Inner {
    final boolean aBool;
    final String[] anArray;

    Inner(@YInject(name="aBool") boolean aBool, 
          @YInject(name="anArray") String[] anArray) {
      this.aBool = aBool;
      this.anArray = anArray;
  final String aString;
  final double aNumber;
  final Inner inner;

  Top(@YInject(name="aString") String aString,
      @YInject(name="aNumber") double aNumber, 
      @YInject(name="inner") Inner inner) {
    this.aString = aString;
    this.aNumber = aNumber;
    this.inner = inner;

Note: When using constructor injection, the name attribute of @YInject is mandatory, as parameter names (unlike fields) cannot be inferred at runtime.

Constructor injection does not mandate a public no-arg constructor. In fact, it doesn't even require that your constructor is public. It does, however, require that the injected constructor is fully specified in terms of @YInject annotations. That is, each of the parameters must be annotated, or the constructor will not be used. At this stage, no behaviour is prescribed for partially annotated constructors or multiple constructors with @YInject annotations. This may be supported in future versions.

Hybrid injection

This is basically constructor injection, topped off with field injection — for any annotated fields that weren't set by the constructor. The latter takes place automatically, immediately after object instantiation.

Custom Mappings

The earlier examples assume that the configuration corresponds, more or less, to the resulting object graph. It's also assumed that you have some control over the underlying classes, at least to add the appropriate annotations. Sometimes this isn't the case.

Type mapping 101

We need to dissect some of the underlying mechanisms before we go any further. At the heart of YConf there are three main classes:

  • MappingContext — Holds contextual data about the current mapping session, as well as settings — a registry of type mappers and DOM transforms. When you need to change YConf's behaviour, this is the class you turn to.
  • YObject — A wrapper around a section of the underlying document object model (DOM) which, in turn, is the raw output of the parser. If you can visualise the entire DOM as a tree that will be mapped to the root of your resulting object graph, a YObject will house a subtree that corresponds to the current point in the graph where the mapper is currently operating.
  • TypeMapper — An interface specifying how a YObject is mapped to an output object. This is YConf's main extension point — allowing you to specify custom mapping behaviour.

Built-in mappers


This mapper is by default applied to everything of type Object, as well as to any types that are not explicitly added to the type mapper registry. In other words, if you are trying to map to an output of an unknown type, a RuntimeMapper is what gets used. So when wouldn't you know the target type?

One word: polymorphism. If the target or parameter is a subclass (or sub-interface) of the concrete object, then the target type is virtually useless to the mapper. What it needs is the concrete type, and this can only come from the configuration document. The preferred way to specify the concrete type is to state its fully-qualified class name in a special type attribute, as illustrated in the example below.

- type: com.acme.Dog
  name: Dingo
  breed: Labrador
- type: com.acme.Bird
  name: Olly
  wingspan: 13.47

The animals field can be an Object[] or an Animal[] (assuming Dog and Bird extend Animal). It really doesn't matter, as YConf will always consult the type attribute when no mapper is defined for the target (base) type. If, for some reason, you can't use the name type in your configuration (perhaps type is already taken to mean something else), the name attribute can be overridden as follows:

new MappingContext()
.withRuntimeMapper(new RuntimeMapper().withTypeAttribute("anotherAttribute"))

If all your types come from the same base package, you can do one better. The RuntimeMapper has a withTypeFormatter() method, allowing you to alter the value of the type attribute. This can be used to perform any manipulation on the type name; for example, to prefix the type with a base package:

new MappingContext()
.withRuntimeMapper(new RuntimeMapper().withTypeAttribute("_type").withTypeFormatter("com.acme."::concat))

The above setting can now be used with the following document:

- _type: Dog
  name: Dingo
  breed: Labrador
- _type: Bird
  name: Olly
  wingspan: 13.47


This mapper was used in our initial examples, to reflectively populate with fields and parameters annotated with @YInject. It is also the default mapper used where an class is annotated with @Y, where no explicit TypeMapper class is specified.


Coercing is the process of 'forcing' one type to another (not to be confused with casting), and is normally used with scalar values. A CoercingMapper performs an optional conversion by first comparing the type of the original value in the DOM with the target type, passing the value unchanged if the target type is assignable from the original. Otherwise, if the types are incompatible, coercion will occur by first reducing the original to a String (by calling its toString() method) and then invoking a supplied CoercingMapper.StringConverter implementation, taking in a String value and outputting a subclass of the target type. (Note: null objects are always passed through uncoerced.)

Coercion is typically used where the original type is somewhat similar to the target type, but cannot be converted through a conventional cast or a(n) (un)boxing operation. For example, a string literal containing a sequence of digits appears to be a number, but isn't. In this case, coercion will run the original string through Long::parseLong (or another parser, as appropriate) to get the desired outcome.

Type boxing is another area where YConf uses coercion. For example, an object's properties are typically represented using a Map<String, Object> in the DOM. For number types, document parsers typically output the wider of the possible forms (e.g. long in place of int, double over float, etc.). Because values in a map must be of a reference type, a narrowing type cast cannot be used if the target (primitive or reference) type is narrower than the original reference type.

Finally, we can use coercion to translate strings to a more complex type. For example, a string containing a fully qualified class name can be coerced to a Class type. You can easily add your own coercions, by supplying a lambda that takes in a String and outputs the target type. The example below demonstrates this technique using the URL class (supplying the URL(String) constructor).

new MappingContext().withMapper(URL.class, new CoercingMapper(URL.class, URL::new))...

Writing your own

Now we get to the crux of it. You need a way to create an arbitrary object graph that bears little resemblance to the underlying configuration file.

Suppose we have the following file, containing a list of named server endpoints:

- name: Health check
  protocol: http
  host: localhost
  port: 8080
  path: /health

- name: Message broker
  protocol: ws
  path: /broker

- name: Service discovery
  protocol: https

We'd like to translate this to a class that contains a map of endpoint names to their URIs:

public final class WebConfig {
  final Map<String, URI> servers;

Furthermore, we'd like to ensure that the endpoint names are unique, throwing an error if a duplicate name is encountered. We also want the port and path fields to be optional.

Start by implementing a TypeMapper for the root object.

public final class Mapper implements TypeMapper {
  @Override public Object map(YObject y, Class<?> type) {
    final Map<String, URI> servers = new HashMap<>();
    for (YObject server : y.asList()) {
      final String name = server.mapAttribute("name", String.class);
      if (servers.containsKey(name)) {
        throw new MappingException("Duplicate server name " + name);
      final String protocol = server.mapAttribute("protocol", String.class);
      final String host = server.mapAttribute("host", String.class);
      final Integer port = server.mapAttribute("port", Integer.class);
      final String path = server.mapAttribute("path", String.class);
      final URI uri;
      try {
        uri = new URI(protocol, null, host, port != null ? port : -1, path, null, null);
      } catch (URISyntaxException e) {
        throw new MappingException("Error parsing URI", e);
      servers.put(name, uri);
    return new WebConfig(servers);

This is a simple class with one functional method — taking in a DOM fragment and the desired type (which we already know to be WebConfig) and outputting a WebConfig instance. But before going further, let's pause for a minute to consider how YConf actually works.

Behind the scenes, all structured document parsers really deal with just three kinds of elements — scalars, lists and maps. (And where this mightn't be the case, it's relatively straightforward to map the parser's output to a scalar, list or a map.) The scalar is usually either a primitive type or its boxed equivalent, and sometimes simple types such as a Date might also be supported natively by the parser. A list is typically an ArrayList<?>. A map tends to be a LinkedHashMap<?, ?> (to preserve insertion order).

Part of YConf's value-add is the assurance that the underlying DOM is either a scalar, a List<YObject>, or a Map<String, YObject> — irrespective of the native data types that the backing parser might emit. As far as scalars go, the value will be of the widest type and may also come pre-converted, if the parser was under instruction to do so.

Now that we know the fundamentals, let's take another look at the original document. To us it's now just a list of maps.

The configuration is an array at the top level, so our mapper calls the asList() method of the given DOM, which returns a List<YObject> — listing each item in the array. Each of the elements is a Map, so we use the mapAttribute() method to convert the value of an attribute to a specific target type.

Notice the use of recursion? The mapAttribute() method doesn't simply return the raw value; it actually uses the underlying MappingContext (embedded within the YObject instance) to initiate a deeper query into the DOM which, in turn, will invoke another mapper as required. Also, mapAttribute("name", String.class) is just another way of saying asMap().get("name").map(String.class).

After extracting the name, we check that our staging servers map doesn't already contain an identically-named entry. If it does, we'll throw a MappingException — an unchecked exception that bubble up to our ultimate caller. The rest of the code is fairly trivial; we map the remaining attributes onto local variables, which are then used to construct a URI. The requirement to support optional port and path fields is accommodated by the use of reference types (Integer in place of an int); the mapper will return null if the requested attribute value isn't present in the document, or is explicitly set to null.

Note: You might be wondering — why does the map() method need the type parameter, given that our mapper implementation already knows which type it should be dealing with? What else could it be? The reason is that although our custom mapper is very specific, there are other mappers (such as the CoercingMapper) that are generic — designed to deal with a variety of types. So knowing the type at runtime may occasionally be necessary.

Registering the mapper

The next step is to register our mapper implementation with the MappingContext. There are three ways this can be done: using the @Y annotation, registering directly with the MappingContext instance, or in-line via the @YInject annotation.

Register with @Y

The example below shows the annotation approach.

public final class WebConfig {
  public static final class Mapper implements TypeMapper {
    @Override public Object map(YObject y, Class<?> type) {
      // the mapper implementation, omitted for brevity
  final Map<String, URI> servers;

  WebConfig(Map<String, URI> servers) {
    this.servers = servers;

We've embedded the mapper into the config class for convenience, and have referenced it from @Y. Simple.

Note: Classes referenced from @Y must be bean-instantiable. That is, they must be public, have a public no-arg constructor, and must not have an encapsulating instance. The latter is easy to overlook when using a nested class; make sure you're declaring your mapper with the static modifier if nesting within another type.

Register with MappingContext

Alternatively, if you don't want to (or unable to, for whatever reason) add an annotation to your class, the following snippet registers the type mapper directly with the context.

new MappingContext().withMapper(WebConfig.class, new WebConfig.Mapper())...

Registering in-line with @YInject

In come cases we might not have access to MappingContext and are also unable to @Y-annotate the target type. YConf supports in-line type mapper specification using the mapper attribute of @YInject.

In the following example, we'd like to parametrise our configuration with an SLF4J logger, but we're unable to annotate SLF4J's LoggerFactory directly (since this class is part of another project). Also, suppose the configuration is loaded by a framework outside of our direct control, so we need to use @YInject to specify a custom mapper that fashions a logger for a given name.

public final class LogConfig {
  private Logger logger;
  public Logger getLogger() {
    return logger;

The mapper implementation is shown below.

public final class LoggerMapper implements TypeMapper {
  public Object map(YObject y, Class<?> type) {
    return LoggerFactory.getLogger(;

The configuration file looks like this:

logger: org.myproject.MyLogger

Parsing JSON

Our previous examples have focused on YAML. To parse JSON instead, you need to include the yconf-gson module into your build script in place of yconf-snakeyaml. You also need to build your MappingContext using .withParser(new GsonParser()).

Number types

There's one gotcha with Gson. By default, it coerces all typeless numbers to a double on the pretense that JSON doesn't support integral types. This will result in storing double values in Lists and Maps, unless you use a custom TypeMapper and specify an explicit component type.

So by default GsonParser will transform non-decimal doubles back to ints or longs. If this isn't what you want (i.e. you'd rather stick with doubles), Gson's default behaviour can be unmasked with .withParser(new GsonParser().withFixDoubles(false)).

Note: Gson's default behaviour with respect to numbers results in a lossy decoding, as the 52-bit mantissa in an IEEE 754 double cannot accommodate the full 64-bit range of a long. We suspect the same behaviour may be present in other parsers. If you need to store numbers larger than +/- 252 in a JSON config, we suggest encoding the value in a string and decoding it with a custom type. The same technique should also be used for handling arbitrary-precision decimal numbers, such as currencies.

Writing Parsers

YConf is built on a modular design, with each non-core component residing in a separate build module. If you'd like to contribute with a new parser, we ask that you please keep to this convention, as it improves testability and minimises transitive dependency conflicts (you only import what you need into your project).

A Parser is simple functional interface, accepting a Reader and outputting a complete DOM graph.

public interface Parser {
  Object load(Reader reader) throws IOException;

Most off-the-shelf parsers already have a method akin to this. So in most cases, it's simply a matter of wrapping the underlying parser.

Sometimes you need to massage the resulting DOM into what YConf expects. Remember, each node in the object graph needs to be either a scalar (a primitive or its boxed type), a List<Object> or a Map<String, Object>, where the Object element/value is either a scalar or again a List/Map.

Another thing to watch out for is number types. See for an example on how YConf's Gson plugin recursively transforms non-decimal doubles to ints and longs in an object graph.

More on EL


By convention, all YConf plugins are configured in-line using a fluent API. The following example attaches a JuelTransform to a MappingContext, having first registered one new variable and one new function.

new MappingContext()
.withDomTransform(new JuelTransform()
                  .withVariable("pi", Math.PI)
                  .withFunction("math", "round", Math.class.getMethod("round", double.class)));

The withVariable() method accepts a variable value for a given name. In our example, the value of Pi can be accessed with the expression ${pi}.

The withFunction() method accepts a static method and, optionally, a namespace (the first argument). In our example, we can round a floating-point number with the expression ${math:round(pi)}, resulting in the long 3. Without the namespace, the expression would be just ${round(pi)}.

Built-in variables


The env variable is a map of all environment variables visible to the application when JuelTransform is instantiated. On the author's machine, calling ${env.USER} produces the string emil.


The session variable captures the current mapping context. At present it has a single method named link(), which you would have seen earlier. Calling ${} will load the contents of filename. The format of the linked file must be compatible with the parser that is registered in MappingContext. In other words, you can't link a YAML file from a JSON document.

Built-in functions

mandatory(Object, String)

Returns the value of the first argument, providing it's not null. Otherwise, it will throw a com.obsidiandynamics.yconf.MissingValueException with the message given in the second argument. This is useful for enforcing the presence of a value during application bootstrapping, particularly if the value is sourced externally (for example, from env).


Wraps the value of the argument in a com.obsidiandynamics.yconf.Secret object. Calling Secret.toString() returns the hard-coded value <masked>. It can be used to encapsulate a configuration parameter that isn't intended for general knowledge.

To unwrap the secret, simply call the static Secret.unmask(Object) method, passing in the secret object. The unmask() method is fairly flexible, and can be called with a non-Secret, returning the given object's toString() representation. It's also null-safe; if a null is passed in, a null is returned.

Secret was invented to solve a common problem. Lot's of applications log their configured values during startup. Since logging a secret is undesirable, applications will typically skip over the secret. This works if the application has a priori knowledge of which parameters are secret. But the application might not know this in advance (say, if you're building a framework). In this case, the correct thing to do is to assume that all strings are secret, and explicitly resolve them with Secret.unmask() prior to use. Logging the values directly (without unmasking them first) is harmless — it will just print the string <masked>.


Simple, elegant bootstrapping and configuration. YConf is like an ORM for config files.







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