1、SpringApplication正常结束

Spring Boot2.0为SpringApplication正常结束新引入了SpringApplicationRunListener的生命周期，即running(ConfigurableApplicationContext)，该方法在Spring应用上下文中已准备，并且CommandLineRunner和ApplicationRunner Bean均已执行完毕。EventPublishingRunListener作为Spring ApplicationRunner 唯一内建实现，本方法中仅简单地广播ApplicationReadyEvent事件：

	@Override
public void running(ConfigurableApplicationContext context) { 

   
	context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
}

不难看出当ApplicationReadyEvent事件触发后，SpringApplication的生命周期进入尾声，除非SpringApplicationRunListeners#running方法执行异常：

	public ConfigurableApplicationContext run(String... args) { 

   
	...
	try { 

   
		listeners.running(context);
	}
	catch (Throwable ex) { 

   
		handleRunFailure(context, ex, exceptionReporters, null);
		throw new IllegalStateException(ex);
	}
	return context;
}

换言之，开发人员有两种技术手段实现完成阶段的监听，一种为实现SpringApplicationRunListeners#running方法，另一种为实现ApplicationReadyEvent事件的SpringApplicationListener。

2、SpringApplication异常结束

SpringApplication异常结束就宣告Spring Boot应用运行失败。与正常流程类似，异常流程同样作为SpringApplication生命周期的一个环节，将在SpringApplicationRunListener#failed(ConfigurableApplicationContext, Throwable)方法。

2.1、Spring Boot异常处理

	private void handleRunFailure(ConfigurableApplicationContext context, Throwable exception,

Collection exceptionReporters, SpringApplicationRunListeners listeners) { 



try { 



try { 



handleExitCode(context, exception);

if (listeners != null) { 



listeners.failed(context, exception);

}

}

finally { 



reportFailure(exceptionReporters, exception);

if (context != null) { 



context.close();

}

}

}

catch (Exception ex) { 



logger.warn("Unable to close ApplicationContext", ex);

}

ReflectionUtils.rethrowRuntimeException(exception);

}

Spring Boot异常处理主要包含两部分：一是退出码处理，二是异常报告。退出码处理放在《Spring Boot应用退出》中讲解，这里这要分析异常报告。
在Spring Boot2.0中新增了一个SpringBootExceptionReporter接口，用于支持SpringApplication启动错误的自定义报告的回调接口。

public interface SpringBootExceptionReporter { 



boolean reportException(Throwable failure);

}

由于SpringBootExceptionReporter 集合在初始化过程中，明确地执行了getSpringFactoriesInstances(SpringBootExceptionReporter.class,new Class[] { ConfigurableApplicationContext.class }, context);语句，所以当自定义SpringBootExceptionReporter 时，必须用一个ConfigurableApplicationContext参数声明一个公共构造函数，比如Spring Boot2.x内建唯一实现FailureAnalyzers：

final class FailureAnalyzers implements SpringBootExceptionReporter { 



private static final Log logger = LogFactory.getLog(FailureAnalyzers.class);

private final ClassLoader classLoader;

private final List analyzers;

FailureAnalyzers(ConfigurableApplicationContext context) { 



this(context, null);

}

FailureAnalyzers(ConfigurableApplicationContext context, ClassLoader classLoader) { 



Assert.notNull(context, "Context must not be null");

this.classLoader = (classLoader != null) ? classLoader : context.getClassLoader();

this.analyzers = loadFailureAnalyzers(this.classLoader);

prepareFailureAnalyzers(this.analyzers, context);

}

...

}

可简单地认为FailureAnalyzers 是FailureAnalyzer的组合类，在其构造阶段通过Spring工厂加载机制初始化并排序FailureAnalyzer列表：

	private List loadFailureAnalyzers(ClassLoader classLoader) { 



List analyzerNames = SpringFactoriesLoader.loadFactoryNames(FailureAnalyzer.class, classLoader);

List analyzers = new ArrayList<>();

for (String analyzerName : analyzerNames) { 



try { 



Constructor constructor = ClassUtils.forName(analyzerName, classLoader).getDeclaredConstructor();

ReflectionUtils.makeAccessible(constructor);

analyzers.add((FailureAnalyzer) constructor.newInstance());

}

catch (Throwable ex) { 



logger.trace(LogMessage.format("Failed to load %s", analyzerName), ex);

}

}

AnnotationAwareOrderComparator.sort(analyzers);

return analyzers;

}

private void prepareFailureAnalyzers(List analyzers, ConfigurableApplicationContext context) { 



for (FailureAnalyzer analyzer : analyzers) { 



prepareAnalyzer(context, analyzer);

}

}

private void prepareAnalyzer(ConfigurableApplicationContext context, FailureAnalyzer analyzer) { 



if (analyzer instanceof BeanFactoryAware) { 



((BeanFactoryAware) analyzer).setBeanFactory(context.getBeanFactory());

}

if (analyzer instanceof EnvironmentAware) { 



((EnvironmentAware) analyzer).setEnvironment(context.getEnvironment());

}

}

加载后的FailureAnalyzer列表作为FailureAnalyzers#(Throwable, List)方法的参数，随着SpringApplication#(Collection, Throwable)方法调用执行：

	@Override

public boolean reportException(Throwable failure) { 



FailureAnalysis analysis = analyze(failure, this.analyzers);

return report(analysis, this.classLoader);

}

private FailureAnalysis analyze(Throwable failure, List analyzers) { 



for (FailureAnalyzer analyzer : analyzers) { 



try { 



FailureAnalysis analysis = analyzer.analyze(failure);

if (analysis != null) { 



return analysis;

}

}

catch (Throwable ex) { 



logger.debug(LogMessage.format("FailureAnalyzer %s failed", analyzer), ex);

}

}

return null;

}

private boolean report(FailureAnalysis analysis, ClassLoader classLoader) { 



List reporters = SpringFactoriesLoader.loadFactories(FailureAnalysisReporter.class,

classLoader);

if (analysis == null || reporters.isEmpty()) { 



return false;

}

for (FailureAnalysisReporter reporter : reporters) { 



reporter.report(analysis);

}

return true;

}

不难看出FailureAnalyzer仅分析故障，而故障报告则由FailureAnalysisReporter 对象负责。

2.2、错误分析报告器——FailureAnalysisReporter

同样地FailureAnalysisReporter也由Spring工厂加载机制初始化并排序。在Spring Boot框架中仅存在一个内建FailureAnalysisReporter的实现LoggingFailureAnalysisReporter。

public final class LoggingFailureAnalysisReporter implements FailureAnalysisReporter { 



private static final Log logger = LogFactory.getLog(LoggingFailureAnalysisReporter.class);

@Override

public void report(FailureAnalysis failureAnalysis) { 



if (logger.isDebugEnabled()) { 



logger.debug("Application failed to start due to an exception", failureAnalysis.getCause());

}

if (logger.isErrorEnabled()) { 



logger.error(buildMessage(failureAnalysis));

}

}

private String buildMessage(FailureAnalysis failureAnalysis) { 



StringBuilder builder = new StringBuilder();

builder.append(String.format("%n%n"));

builder.append(String.format("***************************%n"));

builder.append(String.format("APPLICATION FAILED TO START%n"));

builder.append(String.format("***************************%n%n"));

builder.append(String.format("Description:%n%n"));

builder.append(String.format("%s%n", failureAnalysis.getDescription()));

if (StringUtils.hasText(failureAnalysis.getAction())) { 



builder.append(String.format("%nAction:%n%n"));

builder.append(String.format("%s%n", failureAnalysis.getAction()));

}

return builder.toString();

}

}

与FailureAnalysisReporter不同的是，FailureAnalyzer的内建实现相当丰富，下面是org.springframework.boot:spring-boot-autoconfigure:2.3.0中的META-INF/spring.factories：

# Failure analyzers

org.springframework.boot.diagnostics.FailureAnalyzer=\

org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\

org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\

org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\

org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\

org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

其中NoSuchBeanDefinitionFailureAnalyzer和DataSourceBeanCreationFailureAnalyzer在Spring Boot中经常出现。