1. A method of memory overload management for a Web application server system, wherein the Web application server system comprises multiple Java machines (JVMs) for processing sessions, the method comprising: determining one or more replica shards for which replacement shall be performed; determining one or more target JVMs for storing a corresponding replica shard set including at least one replica shard from the one or more replica shards; and for each target JVM, performing the following: judging whether the free memory of the target JVM is adequate for storing the corresponding replica shard set; if the judging result is negative, performing the following: causing the target JVM to suspend the creation of session until the free memory of the target JVM becomes adequate for storing the corresponding replica shard set; storing the corresponding replica shard set to the target JVM; and causing the target JVM to resume the creation of session.