Just like logback-classic does for normal logs.

At the first glance the base class (AsyncAppenderBase) looks like it can be used with access logs - it is not an abstract one and nothing hints it should not. But that class misses a call to event object's prepareForDeferredProcessing method so is not good for Jetty that can recycle the request objects.

The implementation is straightforward:

import ch.qos.logback.access.spi.IAccessEvent;
import ch.qos.logback.core.AsyncAppenderBase;

public class AsyncAccessAppender extends AsyncAppenderBase<IAccessEvent> {
    @Override
    protected void preprocess(final IAccessEvent eventObject) {
        eventObject.prepareForDeferredProcessing();
    }
}