Nacos logs do not print to nacos logs file but print into users' application logs.

[KomachiSion]

Describe the bug
When nacos-client run with Spring Cloud Alibaba. almost of nacos log will print into applications' log file but not into nacos log file we expected, which will make users confused because nacos client has async-thread to execute and print result in logs.

Expected behavior
Almost nacos-client logs should print into ${user.home}/logs/nacos/naming.log or ${user.home}/logs/nacos/config.log

Acutally behavior
Print into users' application business logs file.

How to Reproduce
Steps to reproduce the behavior:

1. Clone Spring cloud alibaba source code
2. Run with example nacos-config-example or nacos-discovery-spring-cloud-config-client-example
3. see ${user.home}/logs/nacos/naming.log or ${user.home}/logs/nacos/config.log, no logs or only little logs
4. See console, there are com.alibaba.nacos.client.config.XXX log info or com.alibaba.nacos.client.naming.XXX log info

Desktop (please complete the following information):

• OS: All
• Version All
• Module naming and config
• SDK spring-cloud-alibaba-nacos

Additional context

After researching, the reason is that spring will fire mutilple times ApplicationEnvironmentPreparedEvent event during init.
And each event will cause LogSystem clean and reload log configuration, Detail See AbstractLoggingSystem and its implementations like LogbackLoggingSystem or Log4j2LoggingSystem.

And nacos-client also will append its configuration into LogContext, But only once. If appending configuration after all event fired, It will work well. But if appending between two events, the nacos log configuration will be cleaned. So the log will print into users log file.

I think we should refactor the codes about nacos log load first. And then we should do changes in spring-boot-nacos and spring-cloud-alibaba-nacos to listener the ApplicationEnvironmentPreparedEvent and reload nacos log configuration.
Because nacos log config load should be a part of ApplicationEnvironmentPrepared.

But we should pay attention to the order, nacos log reload need after LoggingSystem reload.

[KomachiSion]

Issue Detai：

Troubleshooting

For normal startup and init

For trouble, problematic loading sequence and steps

Solution

Plan A

Do nothing in nacos-client, only Add listener for ContextRefreshedEvent and reload configuration

Advantage:

Easy to Implementation

Disadvantage:

Some logs during Init may also print into business log.

Plan B

Listen properties changes event of log framework like logback and log4j2

logback `LoggerContextListener` 
log4j2 `PropertyChangeListener` 

Advantage:

Whatever application framework user use. Only changes nacos-client.

Disadvantage:

Depend on log framework. If log has no event or listener extension. This plan can't works.

Plan C

Listen same spring events with Spring log system. And do reload after spring log system load finished.

Advantage:

Full meet the spring event meanings.

Disadvantage:

We need do changes all in nacos-client, nacos-spring-context, nacos-spring-boot and SCA-nacos.

[KomachiSion]

For now, We think Plan C is best. nacos-client need to do refactor to support API of reload log configurations.

[Maijh97]

@i will solve it@