HSEARCH-3827 Add a ConfigurationPropertySource for system properties

engine/src/main/java/org/hibernate/search/engine/cfg/impl/SystemConfigurationPropertySource.java

@@ -0,0 +1,57 @@
+/*
+ * Hibernate Search, full-text search for your domain model
+ *
+ * License: GNU Lesser General Public License (LGPL), version 2.1 or later
+ * See the lgpl.txt file in the root directory or <http://www.gnu.org/licenses/lgpl-2.1.html>.
+ */
+package org.hibernate.search.engine.cfg.impl;
+
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import org.hibernate.search.engine.cfg.spi.AllAwareConfigurationPropertySource;
+
+public class SystemConfigurationPropertySource implements AllAwareConfigurationPropertySource {
+
+	private static final SystemConfigurationPropertySource INSTANCE = new SystemConfigurationPropertySource();
+
+	public static AllAwareConfigurationPropertySource get() {
+		return INSTANCE;
+	}
+
+	private SystemConfigurationPropertySource() {
+	}
+
+	@Override
+	public Optional<?> get(String key) {
+		Object value = System.getProperty( key );
+		return Optional.ofNullable( value );
+	}
+
+	@Override
+	public Optional<String> resolve(String key) {
+		return Optional.of( key );
+	}
+
+	@Override
+	public Set<String> resolveAll(String prefix) {
+		Set<String> prefixedPropertyKeys = new HashSet<>();
+		for ( Map.Entry<Object, Object> entry : System.getProperties().entrySet() ) {
+			Object key = entry.getKey();
+			if ( key instanceof String ) {
+				String stringKey = (String) key;
+				if ( stringKey.startsWith( prefix ) ) {
+					prefixedPropertyKeys.add( stringKey );
+				}
+			}
+		}
+		return prefixedPropertyKeys;
+	}
+
+	@Override
+	public String toString() {
+		return getClass().getSimpleName();
+	}
+}

engine/src/main/java/org/hibernate/search/engine/cfg/spi/ConfigurationPropertySource.java

@@ -15,6 +15,7 @@
 import org.hibernate.search.engine.cfg.impl.MaskedConfigurationPropertySource;
 import org.hibernate.search.engine.cfg.impl.OverriddenConfigurationPropertySource;
 import org.hibernate.search.engine.cfg.impl.PrefixedConfigurationPropertySource;
+import org.hibernate.search.engine.cfg.impl.SystemConfigurationPropertySource;
 
 /**
  * A source of property values for Hibernate Search.
@@ -97,6 +98,13 @@ static AllAwareConfigurationPropertySource fromMap(Map<String, ?> map) {
 		return new MapConfigurationPropertySource( map );
 	}
 
+	/**
+	 * @return A source containing the system properties ({@link System#getProperty(String)}).
+	 */
+	static AllAwareConfigurationPropertySource system() {
+		return SystemConfigurationPropertySource.get();
+	}
+
 	/**
 	 * @return A source without any property.
 	 */