public static synchronized boolean shutdown() {
        if (instance != null) {
            instance.markShuttingDown();
            instance.interrupt();
            connectionManagers.clear();
            instance = null;
            return true;
        }
        return false;
    }
Shuts down the thread, allowing the class and instance to be collected.
<p/>
Since this is a daemon thread, its running will not prevent JVM shutdown.
It will, however, prevent this class from being unloaded or garbage
collected, in the context of a long-running application, until it is
interrupted. This method will stop the thread's execution and clear its
state. Any use of a service client will cause the thread to be restarted.

@return true if an actual shutdown has been made; false otherwise.
aws-java-sdk-core/src/main/java/com/amazonaws/http/IdleConnectionReaper.java