ZOOKEEPER-5037: Add export and import commands to the CLI

[ckrumbein]

This PR adds two commands to the ZooKeeper CLI, export and import:

• "export <path> <filepath>" downloads the contents of a znode to an external file.
• "import <path> <filepath>" uploads the contents of an external file to a znode, replacing the znode's previous contents.

My team has been using both of these commands for some time. We are confident they are solid.

[PDavid]

Many thanks for this contribution. These commands work really nicely. 👍

Added some minor comments / ideas.

[PDavid]

Checkstyle identified some formatting problems in the build:

02:52:13  [INFO] There are 8 errors reported by Checkstyle 8.39 with checkstyle-strict.xml ruleset.
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ExportCommand.java:[28,1] (imports) ImportOrder: Extra separation in import group before 'java.io.IOException'
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ExportCommand.java:[28,1] (imports) ImportOrder: Import java.io.IOException appears after other imports that it should precede
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ExportCommand.java:[79,33] (whitespace) WhitespaceAround: '|' is not followed by whitespace.
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ExportCommand.java:[79,33] (whitespace) WhitespaceAround: '|' is not preceded with whitespace.
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ImportCommand.java:[28,1] (imports) ImportOrder: Extra separation in import group before 'java.io.IOException'
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ImportCommand.java:[28,1] (imports) ImportOrder: Import java.io.IOException appears after other imports that it should precede
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ImportCommand.java:[90,33] (whitespace) WhitespaceAround: '|' is not followed by whitespace.
02:52:13  [ERROR] src/main/java/org/apache/zookeeper/cli/ImportCommand.java:[90,33] (whitespace) WhitespaceAround: '|' is not preceded with whitespace.

https://ci-hadoop.apache.org/job/zookeeper-precommit-github-pr/job/PR-2375/1/console

[ckrumbein]

PDavid, thanks for your comments. I just incorporated your feedback.

[PDavid]

Many thanks for these improvements. Looks good to me. 👍

[ckrumbein]

Sorry, I need some help. This is my first Apache PR, and my changes have been approved. I want to merge this change, but I don't see any merge button. Is it because there are 3 workflows awaiting approval? If so, it says this workflow requires approval from a maintainer.

[anmolnar]

So, basically these commands export and import the data of a single znode to and from a file. Why don't you just add this functionality to get and set commands?

[ckrumbein]

So, basically these commands export and import the data of a single znode to and from a file. Why don't you just add this functionality to get and set commands?

I could have done it that way; after all, I created the export and import commands by copying the get and set commands. However, I was trying to follow the lead of the ZKUI app, which has separate commands for viewing a znode and downloading a znode to a file. It's really just a matter of preference.