feat(java): synchronize getAccessToken for threwad safety

Author: dgawande12

README.md

@@ -187,7 +187,7 @@ The `ConfidentialClient` refreshes access tokens proactively before their actual
 Default behaviour:
 - A 30 second (30,000 ms) proactive offset is applied automatically.
 - Calls to `getAccessToken()` (or `getAccessToken(false)`) reuse the cached token while it is still considered valid under this adjusted expiry.
-- `getAccessToken(true)` forces a fresh token unless one was very recently refreshed (within 5 seconds) to avoid unnecessary duplicate requests.
+- `getAccessToken(true)` forces a fresh token unless one was very recently refreshed (within 5 seconds) to avoid unnecessary duplicate requests from concurrent threads.
 
 You can override the proactive offset by configuring it in `RequestOptions`:
 

src/main/java/com/factset/sdk/utils/authentication/ConfidentialClient.java

@@ -194,7 +194,7 @@ protected ConfidentialClient(final Configuration config, final TokenRequestBuild
      * @throws AccessTokenException If it can't make a successful request or parse the TokenRequest.
      * @throws SigningJwsException  If the signing of the JWS fails.
      */
-    public String getAccessToken(boolean forceRefresh) throws AccessTokenException, SigningJwsException {
+    public synchronized String getAccessToken(boolean forceRefresh) throws AccessTokenException, SigningJwsException {
         if (this.isCachedTokenValid()) {
             if (!forceRefresh) {
                 LOGGER.info("Retrieved access token which expires in: {} seconds", TimeUnit.MILLISECONDS.toSeconds(this.accessTokenExpireTime - System.currentTimeMillis()));

src/test/java/com/factset/sdk/utils/authentication/ConfidentialClientTest.java

@@ -326,6 +326,31 @@ void getAccessTokenForceRefreshThenCachedReturnsCorrectTokens() throws Exception
         verify(harness.httpRequestMock, times(1)).send();
     }
 
+    @Test
+    void getAccessTokenTwoDifferentThreadsSimultaneouslyOnlyFetchesOnce() throws Exception {
+        TestHarness harness = createClientWithTokens(899, "threadedToken");
+
+        Runnable task = () -> {
+            String token;
+            try {
+                token = harness.client.getAccessToken();
+            } catch (AccessTokenException | SigningJwsException e) {
+                throw new RuntimeException(e);
+            }
+            assertEquals("threadedToken", token);
+        };
+
+        Thread thread1 = new Thread(task);
+        Thread thread2 = new Thread(task);
+
+        thread1.start();
+        thread2.start();
+
+        thread1.join();
+        thread2.join();
+
+        verify(harness.httpRequestMock, times(1)).send();
+    }
 
     @Test
     void forceRefreshWithinGracePeriodReturnsCachedToken() throws Exception {