public Spider setEmptySleepTime(long emptySleepTime) {
        if(emptySleepTime<=0){
            throw new IllegalArgumentException("emptySleepTime should be more than zero!");
        }
        this.emptySleepTime = emptySleepTime;
        return this;
    }
Set wait time when no url is polled.<br><br>

@param emptySleepTime In MILLISECONDS.
@return this
webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java