[jira] Commented: (HADOOP-3360) DataNode should be marked as final to prevent subclassing

["Raghu Angadi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-3360?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12594929#action_12594929 ] 

Raghu Angadi commented on HADOOP-3360:
--------------------------------------

> Consider splitting the constructor from the start() operation.
I think this can be done with simple changes. An option is to split {{startDataNode()}} into {{initDataNode()}} and {{startDataNode()}} that starts the threads. The constructor would call initDataNode, and makeInstance() would invoke startDataNode() after constructing a DataNode. 

> DataNode should be marked as final to prevent subclassing
> ---------------------------------------------------------
>
>                 Key: HADOOP-3360
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3360
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.16.3
>            Reporter: Steve Loughran
>            Priority: Minor
>
> Reviewing the DataNode core, it starts a thread in its constructor calling back in to the Run() method. This is generally perceived as very dangerous, as if DataNode were ever subclassed, the subclass would start to be invoked in the run() method before its own constructor had finished working.
> 1. Consider splitting the constructor from the start() operation.
> 2. If this cannot be changed, mark DataNode as final so nobody can subclass it.  Though if the latter were done, it would be convenient to have a method to let external management components poll for the health of the node, and to pick up reasons for the node shutting down.

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