How to reinitialize inside synchronized block in Java?

Question

I have a s3client which uses password with 60 days rotation policy. For that, we have a single s3client object being used but when the password rotates, all threads start giving exceptions. I want to catch and make sure only 1 of the threads reinitialize Client and others do nothing inside synchronized block. Can't figure out a way to do so.

private static AtomicBoolean initializingS3Client = new AtomicBoolean(false);

public static AmazonS3 reinitializeAndProvideS3Client() throws Exception {
    synchronized (initializingS3Client) {
        if (initializingS3Client.compareAndSet(false, true)) {
            s3Client.shutdown();
            s3Client = createAmazonS3Client();
            initializingS3Client.notifyAll();
        } else {
            initializingS3Client.wait();
        }
    }
    return s3Client;
}

Answer 1

You didn't share the code that does the actual error handling / trigger the re-initialize. I assume that you call the method on any occuring IOException. To avoid that eg temporal.network problems lead to thousands or millions of unwnated re-initializations I would suggest to specify a minimal time between two re-initializations. This helps also to make the code simpler:

public YourClass {
  ...
  private static final long MIN_TIME_BETWEEN_TWO_REINITIALIZES = 60 * 1_000; // 1 min
  privat static long lastReinitialize = 0; 

  public static AmazonS3 reinitializeAndProvideS3Client() throws Exception {
    synchronized (YourClass.class) {
      long now = System.currentTimeInMillis();
      if (now - lastReinitialize > MIN_TIME_BETWEEN_TWO_REINITIALIZES ) {
        s3Client.shutdown();
        s3Client = createAmazonS3Client();
        lastReinitialize = now;
      } 
      return s3Client;
    }
  }
}

I assume that the s3Client field is volatile and the s3Client implementation is thread-safe. If s3Client should not be volatile and access to it should already be synchronized by another object, then replace YourClass.class in the synchronized statement with that object.