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1. EnvironmentPostProcessor的作用
SpringBoot支持动态的读取文件，留下的扩展接 org.springframework.boot.env.EnvironmentPostProcessor 。这个接口是spring包下的，使用这个进行配置文件的集中管理，而不需要每个项目都去配置配置文件。这种方法也是springboot框架留下的一个扩展（可以自己去扩展），下面看一下源码：
@FunctionalInterface
public interface EnvironmentPostProcessor {
void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application);
}
源码很简单就一个方法。
2. EnvironmentPostProcessor在SpringBoot(v2.2.2)源码分析
源码分析主要分为以下几个步骤：
- 如何从spring.factories加载
- EnvironmentPostProcessor如何处理数据
2.1 如何从spring.factories加载
Spring框架加载 spring.factories 里面的数据主要是通过 SpringFactoriesLoader 来进行加载的，那么我们来看一下SpringBoot中如何加载EnvironmentPostProcessor：
public class ConfigFileApplicationListener implements EnvironmentPostProcessor, SmartApplicationListener, Ordered {
//省略代码
List<EnvironmentPostProcessor> loadPostProcessors() {
return SpringFactoriesLoader.loadFactories(EnvironmentPostProcessor.class, getClass().getClassLoader());
}
}
通过研究代码发现，主要是通过 ConfigFileApplicationListener 监听器中的 loadPostProcessors 方法来加载。
2.2 EnvironmentPostProcessor如何处理数据
通过上面的可以看出来主要是通过 ConfigFileApplicationListener 来进行加载的，那么接着来看一下如何处理数据。通过代码反推的方式来进行：
public class ConfigFileApplicationListener implements EnvironmentPostProcessor, SmartApplicationListener, Ordered {
//省略代码
//说明：从代码可以看出来实现了EnvironmentPostProcessor和SmartApplicationListener监听器
}
下面来看一下 loadPostProcessors 方法在 onApplicationEnvironmentPreparedEvent 中调用：
private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
//从spring.factories加载EnvironmentPostProcessor
List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
//添加当前的类实例
postProcessors.add(this);
//对EnvironmentPostProcessor进行排序--如果使用了Order
AnnotationAwareOrderComparator.sort(postProcessors);
for (EnvironmentPostProcessor postProcessor : postProcessors) {
//数据处理
postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());
}
}
通过上面的代码可以看出来，获取了spring.factories加载EnvironmentPostProcessor的实现，执行相关的EnvironmentPostProcessor的方法 postProcessEnvironment 。
那么分析了完成了如何进行调用 EnvironmentPostProcessor 。那调用如何触发的，前面可以看出来 ConfigFileApplicationListener 是监听器。 下面来看一下代码分析：
@Override
public void onApplicationEvent(ApplicationEvent event) {
if (event instanceof ApplicationEnvironmentPreparedEvent) {
onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
}
if (event instanceof ApplicationPreparedEvent) {
onApplicationPreparedEvent(event);
}
}
从上面的代码可以看出来，onApplicationEvent 中调用了私有方法 onApplicationEnvironmentPreparedEvent 。通过判断 ApplicationEvent 是否为 ApplicationEnvironmentPreparedEvent 去调用方法 onApplicationEnvironmentPreparedEvent 。
分析到这来就可以明白了，EnvironmentPostProcessor主要是通过Spring的事件驱动来出发调用的。
2.3 ConfigFileApplicationListener如何加载
在spring-boot代码模块中，可以看一下spring.factories文件中org.springframework.context.ApplicationListener选项。接下来就看看SpringBoot如何加载 ApplicationListener。通过代码分析可看一下SpringBoot运行的启动类 SpringApplication ，Spring的构造：
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//加载spring.factories中的ApplicationContextInitializer
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
////加载spring.factories中的ApplicationListener
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
通过上面可以看出来通过 SpringApplication#getSpringFactoriesInstances 方法来获取实例。设置到私有变量：private List<ApplicationListener<?>> listeners; 分析到这里已经看到了把 ApplicationListener 相关实现实例加载。那么怎么样调用呢？
在 SpringApplication#run 方法，中是SpringBoot运行的关键方法，在这个方法中有一个一段代码
SpringApplicationRunListeners listeners = getRunListeners(args);
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
return new SpringApplicationRunListeners(logger,
getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}
通过代码可以发现又从 spring.factories 中加载了 SpringApplicationRunListener 看一下文件中配置了哪些实现类：
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener
只有一个 EventPublishingRunListener 。下面来看一下这个类：
public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
private final SpringApplication application;
private final String[] args;
private final SimpleApplicationEventMulticaster initialMulticaster;
public EventPublishingRunListener(SpringApplication application, String[] args) {
this.application = application;
this.args = args;
this.initialMulticaster = new SimpleApplicationEventMulticaster();
for (ApplicationListener<?> listener : application.getListeners()) {
this.initialMulticaster.addApplicationListener(listener);
}
}
@Override
public int getOrder() {
return 0;
}
@Override
public void starting() {
this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
}
@Override
public void environmentPrepared(ConfigurableEnvironment environment) {
this.initialMulticaster
.multicastEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args, environment));
}
@Override
public void contextPrepared(ConfigurableApplicationContext context) {
this.initialMulticaster
.multicastEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
}
@Override
public void contextLoaded(ConfigurableApplicationContext context) {
for (ApplicationListener<?> listener : this.application.getListeners()) {
if (listener instanceof ApplicationContextAware) {
((ApplicationContextAware) listener).setApplicationContext(context);
}
context.addApplicationListener(listener);
}
this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
}
@Override
public void started(ConfigurableApplicationContext context) {
context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context));
}
@Override
public void running(ConfigurableApplicationContext context) {
context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
}
@Override
public void failed(ConfigurableApplicationContext context, Throwable exception) {
ApplicationFailedEvent event = new ApplicationFailedEvent(this.application, this.args, context, exception);
if (context != null && context.isActive()) {
// Listeners have been registered to the application context so we should
// use it at this point if we can
context.publishEvent(event);
}
else {
// An inactive context may not have a multicaster so we use our multicaster to
// call all of the context's listeners instead
if (context instanceof AbstractApplicationContext) {
for (ApplicationListener<?> listener : ((AbstractApplicationContext) context)
.getApplicationListeners()) {
this.initialMulticaster.addApplicationListener(listener);
}
}
this.initialMulticaster.setErrorHandler(new LoggingErrorHandler());
this.initialMulticaster.multicastEvent(event);
}
}
private static class LoggingErrorHandler implements ErrorHandler {
private static Log logger = LogFactory.getLog(EventPublishingRunListener.class);
@Override
public void handleError(Throwable throwable) {
logger.warn("Error calling ApplicationEventListener", throwable);
}
}
}
通过上面代码可以发现主要是发布Spring的事件：
- ApplicationStartingEvent
- ApplicationEnvironmentPreparedEvent
- ApplicationContextInitializedEvent
- ApplicationPreparedEvent
- ApplicationStartedEvent
- ApplicationReadyEvent
- ApplicationFailedEvent
这里面主要是七个事件，对应不同的状态。通过发送 ApplicationEnvironmentPreparedEvent 。通过加载组装为 SpringApplicationRunListeners 。根据不同的状态来调用 SpringApplicationRunListeners 中不同的方法。
最后来看一下在哪里广播发布 ApplicationEnvironmentPreparedEvent 事件：
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
configureEnvironment(environment, applicationArguments.getSourceArgs());
ConfigurationPropertySources.attach(environment);
//这里就是发布 ApplicationEnvironmentPreparedEvent
listeners.environmentPrepared(environment);
bindToSpringApplication(environment);
if (!this.isCustomEnvironment) {
environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
deduceEnvironmentClass());
}
ConfigurationPropertySources.attach(environment);
return environment;
}
通过发布 listeners.environmentPrepared(environment); 最后触发 EnvironmentPostProcessor#postProcessEnvironment 的方法。
3. 自定义步骤
-
自定义一个类实现EnvironmentPostProcessor接口
-
在资源文件 resources/META-INF下面创建一个spring.factories文件
-
spring.factories文件中添加如下
org.springframework.boot.env.EnvironmentPostProcessor=\ #自定义的类(包含包名) org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor官网的说明： EnvironmentPostProcessor
4. 总结
EnvironmentPostProcessor 的加载和Spring框架的加载是一样的，但是对于方法的调用是通过 Spring的事件发布来触发 EnvironmentPostProcessor 接口中的 postProcessEnvironment 方法。
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