Abstract away FileIdGenerator implementations

athena-core/src/main/java/io/github/qubitpi/athena/application/AbstractBinderFactory.java

@@ -15,10 +15,8 @@
  */
 package io.github.qubitpi.athena.application;
 
-import io.github.qubitpi.athena.config.SystemConfig;
-import io.github.qubitpi.athena.config.SystemConfigFactory;
 import io.github.qubitpi.athena.file.identifier.FileIdGenerator;
-import io.github.qubitpi.athena.file.identifier.FileNameAndUploadedTimeBasedIdGenerator;
+import io.github.qubitpi.athena.file.identifier.FileIdGeneratorFactory;
 import io.github.qubitpi.athena.filestore.FileStore;
 import io.github.qubitpi.athena.metadata.MetaData;
 import io.github.qubitpi.athena.metastore.MetaStore;
@@ -28,31 +26,17 @@
 import org.glassfish.hk2.api.TypeLiteral;
 import org.glassfish.hk2.utilities.Binder;
 import org.glassfish.hk2.utilities.binding.AbstractBinder;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
 import graphql.schema.DataFetcher;
 import jakarta.validation.constraints.NotNull;
 
-import java.security.NoSuchAlgorithmException;
-
 /**
  * {@link AbstractBinderFactory} implements standard buildBinder functionality.
  * <p>
  * It is left to individual projects to subclass, providing {@link FileStore} and {@link MetaStore} classes, etc.
  */
 public abstract class AbstractBinderFactory implements BinderFactory {
 
-    private static final Logger LOG = LoggerFactory.getLogger(AbstractBinderFactory.class);
-    private static final SystemConfig SYSTEM_CONFIG = SystemConfigFactory.getInstance();
-
-    private static final String FILE_ID_HASHING_ALGORITHM_KEY = "file_id_hashing_algorithm";
-    private static final String FILE_ID_HASHING_ALGORITHM_DEFAULT = "MD5";
-
-    private static final String FILE_ID_HASHING_ALGORITHM = SYSTEM_CONFIG.getStringProperty(
-            SYSTEM_CONFIG.getPackageVariableName(FILE_ID_HASHING_ALGORITHM_KEY)
-    ).orElse(FILE_ID_HASHING_ALGORITHM_DEFAULT);
-
     @Override
     public Binder buildBinder() {
         return new AbstractBinder() {
@@ -126,16 +110,7 @@ public void afterRegistration(final ResourceConfig resourceConfig) {
      */
     @NotNull
     protected FileIdGenerator buildFileIdGenerator() {
-        try {
-            return FileNameAndUploadedTimeBasedIdGenerator.algorithm(FILE_ID_HASHING_ALGORITHM);
-        } catch (final NoSuchAlgorithmException exception) {
-            final String message = String.format(
-                    "'%s' is not a valid message digest algorithm name",
-                    FILE_ID_HASHING_ALGORITHM
-            );
-            LOG.error(message, exception);
-            throw new IllegalStateException(message, exception);
-        }
+        return FileIdGeneratorFactory.getInstance();
     }
 
     /**

athena-core/src/main/java/io/github/qubitpi/athena/file/identifier/FileIdGenerator.java

@@ -22,7 +22,8 @@
 /**
  * {@link FileIdGenerator} is responsible for generating unique identifiers as strings for any types of files.
  * <p>
- * To generate ID of arbitrary file, use {@link #apply(Object)}
+ * To generate ID of arbitrary file, use {@link #apply(Object)}. Please use {@link FileIdGeneratorFactory} to get
+ * instance of this type.
  * <p>
  * This is a functional interface and can therefore be used as the assignment target for a lambda expression or method
  * reference.

athena-core/src/main/java/io/github/qubitpi/athena/file/identifier/FileIdGeneratorFactory.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2024 Jiaqi Liu
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.github.qubitpi.athena.file.identifier;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import io.github.qubitpi.athena.application.AbstractBinderFactory;
+import io.github.qubitpi.athena.config.SystemConfig;
+import io.github.qubitpi.athena.config.SystemConfigFactory;
+import jakarta.validation.constraints.NotNull;
+import net.jcip.annotations.Immutable;
+import net.jcip.annotations.ThreadSafe;
+
+import java.security.NoSuchAlgorithmException;
+
+/**
+ * {@link FileIdGeneratorFactory} is an abstraction layer that hides the details of {@link FileIdGenerator} instance
+ * creation.
+ * <p>
+ * Please use {@link #getInstance()} always to get instance of {@link FileIdGenerator} whenever needed.
+ */
+@Immutable
+@ThreadSafe
+public class FileIdGeneratorFactory {
+
+    private static final Logger LOG = LoggerFactory.getLogger(AbstractBinderFactory.class);
+
+    private static final SystemConfig SYSTEM_CONFIG = SystemConfigFactory.getInstance();
+
+    private static final String FILE_ID_HASHING_ALGORITHM_KEY = "file_id_hashing_algorithm";
+    private static final String FILE_ID_HASHING_ALGORITHM_DEFAULT = "MD5";
+
+    private static final String FILE_ID_HASHING_ALGORITHM = SYSTEM_CONFIG.getStringProperty(
+            SYSTEM_CONFIG.getPackageVariableName(FILE_ID_HASHING_ALGORITHM_KEY)
+    ).orElse(FILE_ID_HASHING_ALGORITHM_DEFAULT);
+
+    private static FileIdGenerator instance = null;
+
+    /**
+     * Returns a cached and fully initialized {@link FileIdGenerator} object with the "MD5" as the default hashing
+     * algorithm.
+     * <p>
+     * One can override the hashing algorithm by setting a {@link SystemConfig config property} whose key is
+     * {@code file_id_hashing_algorithm}.
+     *
+     * @return the same instance
+     *
+     * @throws IllegalStateException if the particular cryptographic algorithm is requested but is not available in the
+     * environment.
+     */
+    @NotNull
+    public static FileIdGenerator getInstance() {
+        if (instance == null) {
+            try {
+                instance = FileNameAndUploadedTimeBasedIdGenerator.algorithm(FILE_ID_HASHING_ALGORITHM);
+            } catch (final NoSuchAlgorithmException exception) {
+                final String message = String.format(
+                        "'%s' is not a valid message digest algorithm name",
+                        FILE_ID_HASHING_ALGORITHM
+                );
+                LOG.error(message, exception);
+                throw new IllegalStateException(message, exception);
+            }
+        }
+
+        return instance;
+    }
+}

athena-core/src/main/java/io/github/qubitpi/athena/file/identifier/FileNameAndUploadedTimeBasedIdGenerator.java

@@ -38,7 +38,7 @@
  */
 @Immutable
 @ThreadSafe
-public class FileNameAndUploadedTimeBasedIdGenerator implements FileIdGenerator {
+class FileNameAndUploadedTimeBasedIdGenerator implements FileIdGenerator {
 
     private final MessageDigest messageDigest;
 

athena-core/src/test/groovy/io/github/qubitpi/athena/file/identifier/FileIdGeneratorFactorySpec.groovy

@@ -0,0 +1,26 @@
+/*
+ * Copyright Jiaqi Liu
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.github.qubitpi.athena.file.identifier
+
+import spock.lang.Specification
+
+class FileIdGeneratorFactorySpec extends Specification {
+
+    def "Factory produces singleton"() {
+        expect:
+        FileIdGeneratorFactory.getInstance() is FileIdGeneratorFactory.getInstance()
+    }
+}