[SPARK-3599]Avoid loading properties file frequently

[WangTaoTheTonic]

https://issues.apache.org/jira/browse/SPARK-3599

[SparkQA]

QA tests have started for PR 2454 at commit 2a79f26.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2454 at commit 2a79f26.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • sealed trait Matrix extends Serializable
  • class SparseMatrix(
  • sealed trait Vector extends Serializable

[vanzin]

LGTM. Could you fix the typo in the PR title (loaing)?

[WangTaoTheTonic]

@vanzin Sorry for that. Fixed.

[SparkQA]

QA tests have started for PR 2454 at commit 7dca036.

• This patch merges cleanly.

[SparkQA]

QA tests have started for PR 2454 at commit 3681182.

• This patch merges cleanly.

[WangTaoTheTonic]

I think it is better using lazy val for readability(putting all elements of defaultSparkProperties into value properties is more comfortable than conversely) and not using clone to avoid shallow copy( I did not do deep research about clone methods in scala and take it same as in Java. Please point it out if not).

[SparkQA]

QA tests have finished for PR 2454 at commit 7dca036.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

QA tests have finished for PR 2454 at commit 3681182.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[pwendell]

it's canonical to not use new here.

i.e.

val properties = HashMap[String, String]()

[pwendell]

I don't think the overhead of reading the file is significant here, but agree it's nice to avoid extra print statements. lazy val's like this are a little brittle because they make the access order sensitive. I.e. if someone tries to reference this earlier in the program it will break in a weird way.

Nonetheless think this is fine to have. It's a small enough code segment here. I'll merge it and make the minor style fix suggested here.