MicroProfile Config, which is part of the MicroProfile Specification, is the standardization for Java Enterprise and Microservices configuration.

Out of the box (i.e. mandatory for all implementations as defined by the specification) there are 3 ways to define your configuration:

  • System.getProperties()
  • System.getenv()
  • All META-INF/microprofile-config.properties on the classpath

The ordinal of these Config Sources determine the order in which the System will look for a certain property.

So if you have a Config property with the key of myservice.hostname, you will inject it in your code:

    @Inject @ConfigProperty(name = "myservice.hostname", defaultValue = "localhost")
    private String myServiceHostname;

The system will first see if there is a System property with the key myservice.hostname, if not it will try Environment variables, then all microprofile-config.property files on the classpath. If it could not find it anywhere, it will fallback to the defaultValue in the annotation.

Your own config source.

You can also provide your own config source(s) and define the load order for that source. The Config Api uses SPI to load all config sources, so it’s pretty easy to create your own.

For example, say we want a source that loads first (i.e. event before System properties) and we store that config value in memory, we can write a class that extends org.eclipse.microprofile.config.spi.ConfigSource:

    public class MemoryConfigSource implements ConfigSource {
        public static final String NAME = "MemoryConfigSource";
        private static final Map<String,String> PROPERTIES = new HashMap<>();

        public int getOrdinal() {
            return 900;

        public Map<String, String> getProperties() {
            return PROPERTIES;

        public String getValue(String key) {
                return PROPERTIES.get(key);
            return null;

        public String getName() {
            return NAME;

(see the full source here)

You also (as per SPI) register your implementation in META-INF/services by adding an entry in a file called org.eclipse.microprofile.config.spi.ConfigSource


(full example here)

Above is a fairly simple example, just keeping config values in a static map. You can then create a JAX-RS service (example) to add and remove values from this map.

But what if you want a more complex config source? One that itself needs configuration ?

Using MicroProfile Config to configure your own MicroProfile Config Source.

For example, if we want a Config source that finds the values in etcd, we also need to configure the etcd server details. The good news is we can use the Config Api for that !

However, Config Source implementations are not CDI Beans, so you can not @Inject the values. You also need to ignore yourself (i.e when configuring your source do not look at your source, else you will be in an endless loop)

To get the Config without CDI is very easy:

    Config config = ConfigProvider.getConfig();

(thanks to Mark Struberg, Rudy De Busscher and others on Twitter and the friendly MicroProfile Google Group for helping)

Now we just need to make sure to ignore our own config source, so in the getValue method:

    public String getValue(String key) {
        if (client == null && key.startsWith(KEY_PREFIX)) {
            // in case we are about to configure ourselves we simply ignore that key
            return null;

        ByteSequence bsKey = ByteSequence.fromString(key);
        CompletableFuture<GetResponse> getFuture = getClient().getKVClient().get(bsKey);
        try {
            GetResponse response = getFuture.get();
            String value = toString(response);
            return value;
        } catch (InterruptedException | ExecutionException ex) {
            log.log(Level.FINEST, "Can not get config value for [{0}] from etcd Config source: {1}", new Object[]{key, ex.getMessage()});
        return null;

(full example here)

Now I can define the server details of my etcd server with any of the other config source options.

Running the example.

I am running an example on Payara-micro (but it should work on any MicroProfile implementation).

Using maven:


(see the full pom.xml here)

If I uncomment javaCommandLineOptions I can change the etcd server host name, used in my etcd config source, to something else.

I can also use any of the other config sources to do this, for example, including a microprofile-config.properties in my example war file (like this example), or use my other custom config source and change this in memory.

Use it as a library.

You can also bundle all of this in a jar file to be used by any of your projects. I made the above available in maven central and github, so you can also use that directly.

Just add this to your pom.xml


And you have all of the above config sources.