Support property-defined batch cluster configurations

[bbottema]

Simple Java Mail currently has one property-backed connection-pool / cluster configuration active for batch-module sending:

simplejavamail.defaults.connectionpool.clusterkey.uuid=...
simplejavamail.defaults.connectionpool.coresize=...
simplejavamail.defaults.connectionpool.maxsize=...
simplejavamail.defaults.connectionpool.claimtimeout.millis=...
simplejavamail.defaults.connectionpool.expireafter.millis=...
simplejavamail.defaults.connectionpool.loadbalancing.strategy=...

That works for a single default cluster, but it does not model applications that are configured entirely through properties and need multiple batch clusters with different pool behavior.

Concept to investigate:

• keep the existing properties as global/default fallback behavior
• add a collection-style property model keyed by cluster key or cluster alias
• allow cluster-specific core size, max size, claim timeout, expiry, and load-balancing settings
• keep the Java API and properties/defaults governance aligned

This is related to #565, but separate from the Java-side cluster configuration fix: the Java API can configure multiple clusters directly, while property-only applications need a richer ConfigLoader model.

[bbottema]

Java API support for separate cluster defaults is handled under #565. This issue tracks the follow-up for property-only applications that need to describe multiple batch clusters from configuration alone.

Possible property shape, to be designed:

simplejavamail.defaults.connectionpool.clusters.orders.clusterkey.uuid=...
simplejavamail.defaults.connectionpool.clusters.orders.coresize=0
simplejavamail.defaults.connectionpool.clusters.orders.maxsize=2
simplejavamail.defaults.connectionpool.clusters.orders.claimtimeout.millis=30000
simplejavamail.defaults.connectionpool.clusters.orders.expireafter.millis=600000
simplejavamail.defaults.connectionpool.clusters.orders.loadbalancing.strategy=ROUND_ROBIN

simplejavamail.defaults.connectionpool.clusters.bulk.clusterkey.uuid=...
simplejavamail.defaults.connectionpool.clusters.bulk.coresize=0
simplejavamail.defaults.connectionpool.clusters.bulk.maxsize=8
simplejavamail.defaults.connectionpool.clusters.bulk.loadbalancing.strategy=RANDOM_ACCESS

The exact names are still open; the goal is keeping property configuration aligned with the Java API while retaining the existing single-cluster properties as defaults/fallbacks.

[bbottema]

Implemented in 30e135f.

This adds property-defined batch cluster configs on top of the regular global connection-pool defaults. The global properties remain the fallback; a matching cluster entry overrides only the values it declares.

Example:

simplejavamail.defaults.connectionpool.coresize=1
simplejavamail.defaults.connectionpool.maxsize=4
simplejavamail.defaults.connectionpool.loadbalancing.strategy=ROUND_ROBIN

simplejavamail.defaults.connectionpool.clusters.bulk.clusterkey.uuid=00000000-0000-0000-0000-000000000202
simplejavamail.defaults.connectionpool.clusters.bulk.coresize=0
simplejavamail.defaults.connectionpool.clusters.bulk.maxsize=8
simplejavamail.defaults.connectionpool.clusters.bulk.loadbalancing.strategy=RANDOM_ACCESS

A direct UUID-keyed form is also supported when no readable alias is needed:

simplejavamail.defaults.connectionpool.clusters.00000000-0000-0000-0000-000000000202.maxsize=8

[bbottema]

Final follow-up after checking Spring support:

• Plain Spring and Spring Boot configuration both pass the dynamic simplejavamail.defaults.connectionpool.clusters.* namespace into ConfigLoader.
• The 9.0.0 README release note now mentions property-file and Spring-configured clustered sending.
• A website TODO was added locally to document usage examples for alias-based cluster properties, direct UUID-keyed properties, and Spring application.properties.

Related work:

• Different corePoolSize for different Mailer instances (with using batch module) #565 added the Java API side for cluster-specific pool defaults.
• Use clustered-object-pool cluster-specific configuration simple-java-mail/smtp-connection-pool#8 updated SMTP pool support for cluster-specific configuration.
• Support cluster-specific pool configuration clustered-object-pool#6 added the upstream clustered pool support this builds on.

[bbottema]

Released in Simple Java Mail v9.0.0, which is now available from Maven Central:

https://repo1.maven.org/maven2/org/simplejavamail/simple-java-mail/9.0.0/

Release notes:
https://github.com/bbottema/simple-java-mail/releases/tag/9.0.0