src/main/java/com/alipay/remoting/Connection.java
public Object setAttributeIfAbsent(String key, Object value) {
        return attributes.putIfAbsent(key, value);
    }
public Object setAttributeIfAbsent(String key, Object value) {
        return attributes.putIfAbsent(key, value);
    }
set attribute if key absent.

@param key
@param value
@return