SpringApplication_一个阶段结束

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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<SpringBootExceptionReporter> 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<FailureAnalyzer> 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<FailureAnalyzer> loadFailureAnalyzers(ClassLoader classLoader) { 
   
		List<String> analyzerNames = SpringFactoriesLoader.loadFactoryNames(FailureAnalyzer.class, classLoader);
		List<FailureAnalyzer> 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<FailureAnalyzer> 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<FailureAnalyzer>)方法的参数，随着SpringApplication#(Collection<SpringBootExceptionReporter>, 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<FailureAnalyzer> 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<FailureAnalysisReporter> 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。

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