Merge pull request #153 from scalecube/feature/SystemEnvironmentVaria…

…blesConfigSource

Added SystemEnvironmentVariablesConfigSource

config/src/main/java/io/scalecube/config/source/SystemEnvironmentVariablesConfigSource.java

@@ -0,0 +1,61 @@
+package io.scalecube.config.source;
+
+import io.scalecube.config.ConfigProperty;
+import java.util.Map;
+import java.util.Objects;
+import java.util.TreeMap;
+import java.util.function.Consumer;
+
+public final class SystemEnvironmentVariablesConfigSource implements ConfigSource {
+
+  private Map<String, ConfigProperty> loadedConfig;
+
+  // from-to environment variables mappings
+  private final Map<String, String> mappings = new TreeMap<>();
+
+  public SystemEnvironmentVariablesConfigSource(Consumer<Mapper> consumer) {
+    consumer.accept(new MapperImpl());
+  }
+
+  @Override
+  public Map<String, ConfigProperty> loadConfig() {
+    if (loadedConfig != null) {
+      return loadedConfig;
+    }
+
+    Map<String, ConfigProperty> result = new TreeMap<>();
+
+    System.getenv()
+        .forEach(
+            (propName, propValue) -> {
+              String dst = mappings.get(propName);
+              if (dst != null) {
+                // store value as system property under mapped name
+                System.setProperty(dst, propValue);
+                result.put(dst, LoadedConfigProperty.forNameAndValue(dst, propValue));
+              } else {
+                result.put(propName, LoadedConfigProperty.forNameAndValue(propName, propValue));
+              }
+            });
+
+    return loadedConfig = result;
+  }
+
+  // Stores mapping between environemnt variable name and corresponding system property name.
+  interface Mapper {
+
+    Mapper map(String src, String dst);
+  }
+
+  // Inner implementation class
+  private class MapperImpl implements Mapper {
+
+    @Override
+    public Mapper map(String src, String dst) {
+      Objects.requireNonNull(src);
+      Objects.requireNonNull(dst);
+      mappings.put(src, dst);
+      return this;
+    }
+  }
+}