Make Credentials thread-safe during initial loading of credentials

Also add JavaDoc and add some more tests

src/main/java/org/dstadler/commons/util/Credentials.java

@@ -9,12 +9,35 @@
 import java.util.Properties;
 import java.util.logging.Logger;
 
+/**
+ * Helper class to fetch credentials from a file "credentials.properties".
+ *
+ * The file can be located in the current directory or in the parent directory (..)
+ * or in a directory "resources/".
+ */
 public class Credentials {
     private final static Logger log = LoggerFactory.make();
 
     private static Properties properties;
 
-    public static Properties loadCredentials() {
+	/**
+	 * Loads properties from a file "credentials.properties".
+	 *
+	 * Ensures data is replaced in a thread-safe way
+	 *
+	 * Failures to load the file are caught and reported
+	 * as severe logs, no IOException is thrown.
+	 *
+	 * An empty set of properties is returned if no credentials file can be found
+	 *
+	 * @return The properties loaded from the file or empty when no credentials are found..
+	 */
+    public synchronized static Properties loadCredentials() {
+		// are properties already initialized?
+		if (properties != null) {
+			return properties;
+		}
+
         properties = new Properties();
 
         File file = new File("credentials.properties");
@@ -30,6 +53,14 @@ public static Properties loadCredentials() {
         return properties;
     }
 
+	/**
+	 * Load properties from the given file.
+	 *
+	 * Failures to load the file are caught and reported
+	 * as severe logs, no IOException is thrown.
+	 *
+	 * @param file The text-file to read credentials.
+	 */
     protected static void loadProperties(File file) {
         try (FileInputStream fileStream = new FileInputStream(file)) {
             properties.load(fileStream);
@@ -43,10 +74,14 @@ protected static void loadProperties(File file) {
         }
     }
 
+	/**
+	 * Get the given credential or return null.
+	 *
+	 * @param key The credential to read.
+	 * @return The value found in the credentials or null if not found.
+	 */
     public static String getCredentialOrNull(String key) {
-        if (properties == null) {
-            loadCredentials();
-        }
+		loadCredentials();
 
         String result = properties.getProperty(key, null);
         if (result == null &&
@@ -57,6 +92,13 @@ public static String getCredentialOrNull(String key) {
         return result;
     }
 
+	/**
+	 * Get the given credential or throw an exception.
+	 *
+	 * @param key The credential to read.
+	 * @return The value found in the credentials.
+	 * @throws java.lang.IllegalStateException If no value is set for the given credential
+	 */
     public static String getCredentialOrFail(String key) {
     	String result = getCredentialOrNull(key);
     	if(result == null) {

src/test/java/org/dstadler/commons/util/CredentialsTest.java

@@ -3,13 +3,17 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 import java.io.File;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.apache.commons.io.FileUtils;
+import org.apache.commons.lang3.stream.IntStreams;
 import org.junit.Test;
 
 public class CredentialsTest {
@@ -29,8 +33,16 @@ public void testLoadProperties() throws IOException {
 		assertTrue(tempDir.delete());
 		assertTrue(tempDir.mkdirs());
 		try {
+			assertNull(Credentials.getCredentialOrNull("user"));
+			assertThrows(IllegalStateException.class,
+					() -> Credentials.getCredentialOrFail("user"));
+
 			Credentials.loadProperties(tempFile);
 
+			assertNull(Credentials.getCredentialOrNull("user"));
+			assertThrows(IllegalStateException.class,
+					() -> Credentials.getCredentialOrFail("user"));
+
 			FileUtils.writeStringToFile(tempFile, "user=abcd", "UTF-8");
 
 			Credentials.loadProperties(tempFile);
@@ -40,6 +52,15 @@ public void testLoadProperties() throws IOException {
 			assertNull(Credentials.getCredentialOrNull("pwd"));
 
 			Credentials.loadProperties(tempDir);
+
+			assertEquals("abcd", Credentials.getCredentialOrFail("user"));
+			assertEquals("abcd", Credentials.getCredentialOrNull("user"));
+
+			FileUtils.writeStringToFile(tempFile, "user=efgh", "UTF-8");
+			Credentials.loadProperties(tempFile);
+
+			assertEquals("efgh", Credentials.getCredentialOrFail("user"));
+			assertEquals("efgh", Credentials.getCredentialOrNull("user"));
 		} finally {
 			FileUtils.deleteQuietly(tempFile);
 			FileUtils.deleteQuietly(tempDir);
@@ -78,4 +99,40 @@ public void testLoadInvalidFile() {
 	public void testPrivateConstructor() throws Exception {
 		org.dstadler.commons.testing.PrivateConstructorCoverage.executePrivateConstructor(Credentials.class);
 	}
+
+	@Test
+	public void testParallel() throws Throwable {
+		AtomicReference<Throwable> exc = new AtomicReference<>();
+
+		// load credentials in multiple streams to verify multi-threading
+		IntStreams.range(100).asLongStream().
+				parallel().
+				forEach(value -> {
+					// populate the file at some point
+					if (value == 20) {
+						try {
+							File file = File.createTempFile("CredentialsTest", ".properties");
+							try {
+								FileUtils.writeStringToFile(file, """
+										user=ab
+										password=cdr
+										""",
+										StandardCharsets.UTF_8);
+								Credentials.loadProperties(file);
+							} finally {
+								assertTrue("Could not delete file " + file,
+										!file.exists() || file.delete());
+							}
+						} catch (Throwable e) {
+							exc.set(e);
+						}
+					}
+					Credentials.getCredentialOrNull("user");
+					Credentials.getCredentialOrNull("password");
+				});
+
+		if (exc.get() != null) {
+			throw exc.get();
+		}
+	}
 }