#116: Initializing adapter registry on first use.

Author: redcatbear

pom.xml

@@ -4,7 +4,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.exasol</groupId>
   <artifactId>virtual-schema-common-java</artifactId>
-  <version>7.5.0</version>
+  <version>7.5.1</version>
   <name>Common module of Exasol Virtual Schemas Adapters</name>
   <description>This is one of the modules of Virtual Schemas Adapters. The libraries provided by this project are the foundation of the adapter development, i.e. adapters must be implemented on top of them.</description>
   <url>https://github.com/exasol/virtual-schema-common-java</url>

src/main/java/com/exasol/adapter/AdapterFactory.java

@@ -15,7 +15,7 @@ public interface AdapterFactory {
 
     /**
      * Create a new instance of the Virtual Schema Adapter
-     * 
+     *
      * @return new instance
      */
     public VirtualSchemaAdapter createAdapter();

src/main/java/com/exasol/adapter/AdapterRegistry.java

@@ -9,22 +9,24 @@
  */
 public final class AdapterRegistry {
     private static final Logger LOGGER = Logger.getLogger(AdapterRegistry.class.getName());
-    private static final AdapterRegistry instance = new AdapterRegistry();
+    private static AdapterRegistry instance;
     private final Map<String, AdapterFactory> registeredFactories = new HashMap<>();
 
     /**
      * Get the singleton instance of the {@link AdapterRegistry}
      *
      * @return singleton instance
      */
-    public static AdapterRegistry getInstance() {
+    public static final synchronized AdapterRegistry getInstance() {
+        if (instance == null) {
+            LOGGER.finer(() -> "Instanciating Virtual Schema Adapter registry and loading adapter factories.");
+            instance = new AdapterRegistry();
+            instance.loadAdapterFactories();
+        }
         return instance;
     }
 
-    /**
-     * Load adapter factories via the {@link ServiceLoader}
-     */
-    public void loadAdapterFactories() {
+    private void loadAdapterFactories() {
         final ServiceLoader<AdapterFactory> serviceLoader = ServiceLoader.load(AdapterFactory.class);
         final Iterator<AdapterFactory> factories = serviceLoader.iterator();
         while (factories.hasNext()) {
@@ -98,18 +100,22 @@ public void clear() {
      * @return description
      */
     public String describe() {
-        final StringBuilder builder = new StringBuilder("Currently registered Virtual Schema Adapter factories: ");
-        boolean first = true;
-        for (final String name : this.registeredFactories.keySet()) {
-            if (first) {
-                first = false;
-            } else {
-                builder.append(", ");
+        if (this.registeredFactories.isEmpty()) {
+            return "No Virtual Schema Adapter factories are currently reqistered.";
+        } else {
+            final StringBuilder builder = new StringBuilder("Currently registered Virtual Schema Adapter factories: ");
+            boolean first = true;
+            for (final String name : this.registeredFactories.keySet()) {
+                if (first) {
+                    first = false;
+                } else {
+                    builder.append(", ");
+                }
+                builder.append("\"");
+                builder.append(name);
+                builder.append("\"");
             }
-            builder.append("\"");
-            builder.append(name);
-            builder.append("\"");
+            return builder.toString();
         }
-        return builder.toString();
     }
 }

src/main/java/com/exasol/adapter/RequestDispatcher.java

@@ -28,10 +28,6 @@ public static synchronized RequestDispatcher getInstance() {
         return INSTANCE;
     }
 
-    private RequestDispatcher() {
-        AdapterRegistry.getInstance().loadAdapterFactories();
-    }
-
     /**
      * Main entry point for all Virtual Schema Adapter requests issued by the Exasol database.
      *
@@ -61,6 +57,11 @@ private String executeAdapterCall(final ExaMetadata metadata, final String rawRe
         }
     }
 
+    private VirtualSchemaAdapter findResponsibleAdapter(final AdapterRequest request) {
+        final String name = request.getAdapterName();
+        return AdapterRegistry.getInstance().getAdapterForName(name);
+    }
+
     private String processRequest(final AdapterRequest request, final VirtualSchemaAdapter adapter,
             final ExaMetadata metadata) throws AdapterException {
         final AdapterRequestType type = request.getType();
@@ -104,11 +105,6 @@ private void logRawRequest(final String rawRequest) {
         LOGGER.finer(() -> "Raw JSON request:\n" + rawRequest);
     }
 
-    private VirtualSchemaAdapter findResponsibleAdapter(final AdapterRequest request) {
-        final String name = request.getAdapterName();
-        return AdapterRegistry.getInstance().getAdapterForName(name);
-    }
-
     private String dispatchCreateVirtualSchemaRequestToAdapter(final AdapterRequest request,
             final VirtualSchemaAdapter adapter, final ExaMetadata metadata) throws AdapterException {
         final CreateVirtualSchemaResponse response = adapter.createVirtualSchema(metadata,

src/test/java/com/exasol/adapter/AdapterRegistryTest.java

@@ -44,7 +44,6 @@ void testDoesNotContainAdapterFactoryWithName() {
 
     @Test
     void testLoadFactories() {
-        this.registry.loadAdapterFactories();
         assertThat(this.registry.hasAdapterWithName("DUMMY"), equalTo(false));
     }
 
@@ -66,4 +65,9 @@ void testDescribe() {
         assertThat(this.registry.describe(),
                 equalTo("Currently registered Virtual Schema Adapter factories: \"One\", \"Two\""));
     }
+
+    @Test
+    void testDescribeWithNoFactories() {
+        assertThat(this.registry.describe(), equalTo("No Virtual Schema Adapter factories are currently reqistered."));
+    }
 }

src/test/java/com/exasol/adapter/DummyAdapterFactory.java

@@ -4,7 +4,6 @@
 import java.util.Set;
 
 public class DummyAdapterFactory implements AdapterFactory {
-
     @Override
     public Set<String> getSupportedAdapterNames() {
         final Set<String> names = new HashSet<>();