Add a way to customize the SmallRyeConfigBuilder via ServiceLoader

implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilder.java

@@ -31,6 +31,7 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -643,6 +644,14 @@ public SmallRyeConfigBuilder setAddDiscoveredValidator(final boolean addDiscover
     public SmallRyeConfig build() {
         ConfigMappingProvider mappingProvider = mappingsBuilder.build();
         defaultValues.putAll(mappingProvider.getDefaultValues());
+
+        ServiceLoader<SmallRyeConfigBuilderCustomizer> customizers = ServiceLoader.load(SmallRyeConfigBuilderCustomizer.class,
+                classLoader);
+        customizers.stream()
+                .map(ServiceLoader.Provider::get)
+                .sorted(Comparator.comparingInt(SmallRyeConfigBuilderCustomizer::priority))
+                .forEach(customizer -> customizer.configBuilder(SmallRyeConfigBuilder.this));
+
         SmallRyeConfig config = new SmallRyeConfig(this);
         ConfigMappings.mapConfiguration(config, mappingProvider);
         return config;

implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilderCustomizer.java

@@ -0,0 +1,27 @@
+package io.smallrye.config;
+
+/**
+ * This {@code SmallRyeConfigBuilderCustomizer} allows to customize a {@link SmallRyeConfigBuilder}, used to create
+ * a {@link SmallRyeConfig} instance.
+ * <p>
+ * Instances of this interface will be discovered via the {@link java.util.ServiceLoader} mechanism and can be
+ * registered by providing a {@code META-INF/services/io.smallrye.config.SmallRyeConfigBuilderCustomizer} which
+ * contains the fully qualified class name of the custom {@link SmallRyeConfigBuilderCustomizer} implementation.
+ */
+public interface SmallRyeConfigBuilderCustomizer {
+    /**
+     * Customize the current {@link SmallRyeConfigBuilder}.
+     *
+     * @param builder the current {@link SmallRyeConfigBuilder}.
+     */
+    void configBuilder(SmallRyeConfigBuilder builder);
+
+    /**
+     * Returns the customizer priority. Customizers are sorted by ascending priority and executed in that order.
+     *
+     * @return the priority value.
+     */
+    default int priority() {
+        return 0;
+    }
+}

testsuite/extra/src/test/java/io/smallrye/config/test/builder/CustomConfigBuilder.java

@@ -0,0 +1,11 @@
+package io.smallrye.config.test.builder;
+
+import io.smallrye.config.SmallRyeConfigBuilder;
+import io.smallrye.config.SmallRyeConfigBuilderCustomizer;
+
+public class CustomConfigBuilder implements SmallRyeConfigBuilderCustomizer {
+    @Override
+    public void configBuilder(final SmallRyeConfigBuilder builder) {
+        builder.withDefaultValue("from.custom.builder", "1234");
+    }
+}

testsuite/extra/src/test/java/io/smallrye/config/test/builder/CustomOneConfigBuilder.java

@@ -0,0 +1,16 @@
+package io.smallrye.config.test.builder;
+
+import io.smallrye.config.SmallRyeConfigBuilder;
+import io.smallrye.config.SmallRyeConfigBuilderCustomizer;
+
+public class CustomOneConfigBuilder implements SmallRyeConfigBuilderCustomizer {
+    @Override
+    public void configBuilder(final SmallRyeConfigBuilder builder) {
+        builder.withDefaultValue("one", "one").addDefaultSources();
+    }
+
+    @Override
+    public int priority() {
+        return 1;
+    }
+}

testsuite/extra/src/test/java/io/smallrye/config/test/builder/CustomTwoConfigBuilder.java

@@ -0,0 +1,19 @@
+package io.smallrye.config.test.builder;
+
+import io.smallrye.config.SmallRyeConfigBuilder;
+import io.smallrye.config.SmallRyeConfigBuilderCustomizer;
+
+public class CustomTwoConfigBuilder implements SmallRyeConfigBuilderCustomizer {
+    @Override
+    public void configBuilder(final SmallRyeConfigBuilder builder) {
+        builder.withDefaultValue("one", "two");
+        if (builder.isAddDefaultSources()) {
+            builder.withDefaultValue("addDefaultSources", "true");
+        }
+    }
+
+    @Override
+    public int priority() {
+        return 2;
+    }
+}

testsuite/extra/src/test/java/io/smallrye/config/test/builder/SmallRyeConfigBuilderCustomizerTest.java

@@ -0,0 +1,80 @@
+package io.smallrye.config.test.builder;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Path;
+import java.util.stream.Stream;
+
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.asset.StringAsset;
+import org.jboss.shrinkwrap.api.exporter.ZipExporter;
+import org.jboss.shrinkwrap.api.spec.JavaArchive;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import io.smallrye.config.SmallRyeConfig;
+import io.smallrye.config.SmallRyeConfigBuilder;
+
+public class SmallRyeConfigBuilderCustomizerTest {
+    @Test
+    void builder(@TempDir Path tempDir) throws Exception {
+        JavaArchive serviceJar = ShrinkWrap
+                .create(JavaArchive.class, "service.jar")
+                .addAsManifestResource(new StringAsset("io.smallrye.config.test.builder.CustomConfigBuilder"),
+                        "services/io.smallrye.config.SmallRyeConfigBuilderCustomizer");
+
+        Path servidePath = tempDir.resolve("resources-one.jar");
+        serviceJar.as(ZipExporter.class).exportTo(servidePath.toFile());
+
+        ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
+
+        try (URLClassLoader urlClassLoader = urlClassLoader(contextClassLoader, "jar:" + servidePath.toUri() + "!/")) {
+            Thread.currentThread().setContextClassLoader(urlClassLoader);
+
+            SmallRyeConfig config = new SmallRyeConfigBuilder().build();
+
+            assertEquals("1234", config.getRawValue("from.custom.builder"));
+        } finally {
+            Thread.currentThread().setContextClassLoader(contextClassLoader);
+        }
+    }
+
+    @Test
+    void priority(@TempDir Path tempDir) throws Exception {
+        JavaArchive serviceJar = ShrinkWrap
+                .create(JavaArchive.class, "service.jar")
+                .addAsManifestResource(new StringAsset(
+                        "io.smallrye.config.test.builder.CustomOneConfigBuilder\n" +
+                                "io.smallrye.config.test.builder.CustomTwoConfigBuilder\n"),
+                        "services/io.smallrye.config.SmallRyeConfigBuilderCustomizer");
+
+        Path servidePath = tempDir.resolve("resources-one.jar");
+        serviceJar.as(ZipExporter.class).exportTo(servidePath.toFile());
+
+        ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
+
+        try (URLClassLoader urlClassLoader = urlClassLoader(contextClassLoader, "jar:" + servidePath.toUri() + "!/")) {
+            Thread.currentThread().setContextClassLoader(urlClassLoader);
+
+            SmallRyeConfig config = new SmallRyeConfigBuilder().build();
+
+            assertEquals("two", config.getRawValue("one"));
+            assertEquals("true", config.getRawValue("addDefaultSources"));
+        } finally {
+            Thread.currentThread().setContextClassLoader(contextClassLoader);
+        }
+    }
+
+    private static URLClassLoader urlClassLoader(ClassLoader parent, String... urls) {
+        return new URLClassLoader(Stream.of(urls).map(spec -> {
+            try {
+                return new URL(spec);
+            } catch (MalformedURLException e) {
+                throw new IllegalArgumentException(e);
+            }
+        }).toArray(URL[]::new), parent);
+    }
+}