NPE occurred when using EnableApolloConfig with Spring 3.1.1

[nobodyiam]

• I have checked the discussions
• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Describe the bug
When using @EnableApolloConfig with Spring 3.1.1, NullPointerException will occur.

To Reproduce
Steps to reproduce the behavior:

1. Mark a configuration class with @EnableApolloConfig

@Configuration
@EnableApolloConfig(value = "application", order = 10)
public class AppConfig {
  @Bean
  public NormalBean normalBean(@Value("${batch:100}") int batch) {
    NormalBean bean = new NormalBean();
    bean.setBatch(batch);
    return bean;
  }
}

2. Run the application with Spring version 3.1.1
3. NullPointerException will occur

Exception in thread "main" java.lang.NullPointerException
	at com.ctrip.framework.apollo.spring.spi.DefaultApolloConfigRegistrarHelper.resolveNamespaces(DefaultApolloConfigRegistrarHelper.java:52)
	at com.ctrip.framework.apollo.spring.spi.DefaultApolloConfigRegistrarHelper.registerBeanDefinitions(DefaultApolloConfigRegistrarHelper.java:29)
	at com.ctrip.framework.apollo.spring.annotation.ApolloConfigRegistrar.registerBeanDefinitions(ApolloConfigRegistrar.java:20)
	at org.springframework.context.annotation.ConfigurationClassParser.processImport(ConfigurationClassParser.java:331)
	at org.springframework.context.annotation.ConfigurationClassParser.doProcessConfigurationClass(ConfigurationClassParser.java:228)
	at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:149)
	at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:126)
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:263)
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:203)
	at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:617)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:446)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)
	at com.ctrip.framework.apollo.demo.spring.AnnotationApplication.main(AnnotationApplication.java:12)

Expected behavior
The application should work well

Screenshots
If applicable, add screenshots to help explain your problem.

Additional Details & Logs

• Version
• Error logs
• Configuration
• Platform and Operating System

[nobodyiam]

After some investigation, here is what I found now.

1. The ApolloConfigRegistrar and DefaultApolloConfigRegistrarHelper are marked EnvironmentAware in resolved embedded value in annotation @ApolloConfigChangeListener #3349
   see 6ce0932#diff-99cf529f565d3db2b0cb6d6c6a6ccdc92d0404cbb897cb61dcee90636bb12db1R24

2. We use the Environment to resolve the namespace placeholders
   see 6ce0932#diff-0bd5d05308645d08b006bdca711e6954b0305d1315e8b0d4625f2b68337261dbR52

3. However, ApolloConfigRegistrar and DefaultApolloConfigRegistrarHelper are both of type ImportBeanDefinitionRegistrar, which doesn't support EnvironmentAware until Spring 3.2.x
   see spring-projects/spring-framework@146a66f

4. So the environment variable will always be null if running with Spring versions prior to 3.2.x, which explains why npe occurred.