Wrap CacheKeyGenerator MessageDigest in ThreadLocal (#2829)

Co-authored-by: Simone Gerevini <sgerevini@atrify.com>

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/factory/cache/keygenerator/CacheKeyGenerator.java

@@ -31,6 +31,7 @@ import org.springframework.http.server.reactive.ServerHttpRequest;
 /**
  * @author Marta Medio
  * @author Ignacio Lozano
+ * @author Simone Gerevini
  */
 public class CacheKeyGenerator {
 
@@ -38,19 +39,21 @@ public class CacheKeyGenerator {
 
 	private static final byte[] KEY_SEPARATOR_BYTES = KEY_SEPARATOR.getBytes();
 
-	private final MessageDigest messageDigest;
+	private final ThreadLocal<MessageDigest> messageDigest;
 
 	/* for testing */ static final List<KeyValueGenerator> DEFAULT_KEY_VALUE_GENERATORS = List.of(
 			new UriKeyValueGenerator(), new HeaderKeyValueGenerator(HttpHeaders.AUTHORIZATION, KEY_SEPARATOR),
 			new CookiesKeyValueGenerator(KEY_SEPARATOR));
 
 	public CacheKeyGenerator() {
-		try {
-			messageDigest = MessageDigest.getInstance("MD5");
-		}
-		catch (NoSuchAlgorithmException e) {
-			throw new RuntimeException("Error creating CacheKeyGenerator", e);
-		}
+		messageDigest = ThreadLocal.withInitial(() -> {
+			try {
+				return MessageDigest.getInstance("MD5");
+			}
+			catch (NoSuchAlgorithmException e) {
+				throw new RuntimeException("Error creating CacheKeyGenerator", e);
+			}
+		});
 	}
 
 	public String generateMetadataKey(ServerHttpRequest request, String... varyHeaders) {
@@ -63,7 +66,7 @@ public class CacheKeyGenerator {
 
 	public String generateKey(ServerHttpRequest request, List<String> varyHeaders) {
 		byte[] rawKey = generateRawKey(request, varyHeaders);
-		byte[] digest = messageDigest.digest(rawKey);
+		byte[] digest = messageDigest.get().digest(rawKey);
 
 		return Base64.getEncoder().encodeToString(digest);
 	}

spring-cloud-gateway-server/src/test/java/org/springframework/cloud/gateway/filter/factory/cache/CacheKeyGeneratorTest.java

@@ -16,7 +16,11 @@
 
 package org.springframework.cloud.gateway.filter.factory.cache;
 
+import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
 
 import org.junit.jupiter.api.Test;
 
@@ -30,6 +34,7 @@ import static org.springframework.http.HttpHeaders.AUTHORIZATION;
 
 /**
  * @author Ignacio Lozano
+ * @author Simone Gerevini
  */
 class CacheKeyGeneratorTest {
 
@@ -201,4 +206,49 @@ class CacheKeyGeneratorTest {
 		assertThat(keyWithoutVary).isEqualTo(keyWithFirstVary);
 	}
 
+	@Test
+	public void shouldNotFailWhenRunningInParallel() throws InterruptedException {
+		MockServerHttpRequest request = MockServerHttpRequest.get("http://this").build();
+		int numberOfThreads = 100;
+
+		List<Exception> exceptions = executeInParallel(Executors.newFixedThreadPool(numberOfThreads), numberOfThreads,
+				() -> cacheKeyGenerator.generateKey(request));
+
+		assertThat(exceptions.size()).isEqualTo(0);
+	}
+
+	private List<Exception> executeInParallel(Executor executor, int nThreads, Runnable action)
+			throws InterruptedException {
+		CountDownLatch ready = new CountDownLatch(nThreads);
+		CountDownLatch start = new CountDownLatch(1);
+		CountDownLatch done = new CountDownLatch(nThreads);
+		List<Exception> exceptions = new ArrayList<>(nThreads);
+
+		for (int i = 0; i < nThreads; i++) {
+			executor.execute(() -> {
+				ready.countDown();
+				try {
+					start.await();
+					action.run();
+
+				}
+				catch (InterruptedException e) {
+					Thread.currentThread().interrupt();
+				}
+				catch (RuntimeException e) {
+					exceptions.add(e);
+				}
+				finally {
+					done.countDown();
+				}
+			});
+		}
+
+		ready.await();
+		start.countDown();
+		done.await();
+
+		return exceptions;
+	}
+
 }