在SpringBoot项目启动过程中，当一些配置或者其他错误信息会有一些的规范的提示信息

***************************
APPLICATION FAILED TO START
***************************

Description:

Web server failed to start. Port 8080 was already in use.

Action:

Identify and stop the process that's listening on port 8080 or configure this application to listen on another port.

在SpringBoot 中其实现原理是什么，我们该如何自定义异常信息呢

1、SpringBoot异常处理的源码分析

在springboot启动的核心方法run中会加载所有的SpringBootExceptionReporter

	exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);

调用了getSpringFactoriesInstances方法

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
		ClassLoader classLoader = getClassLoader();
		// Use names and ensure unique to protect against duplicates
		Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
		List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
	}

其主要通过Spring的Factories机制来加载

public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
		//异常捕获中，向用户打印异常信息
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

在try catch中，catch会打印异常信息

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);
	}

private void reportFailure(Collection<SpringBootExceptionReporter> exceptionReporters, Throwable failure) {
		try {
			for (SpringBootExceptionReporter reporter : exceptionReporters) {
				if (reporter.reportException(failure)) {
					registerLoggedException(failure);
					return;
				}
			}
		}
		catch (Throwable ex) {
			// Continue with normal handling of the original failure
		}
		if (logger.isErrorEnabled()) {
			logger.error("Application run failed", failure);
			registerLoggedException(failure);
		}
	}

遍历exceptionReporters，打印日常信息 SpringBootExceptionReporter-

SpringBootExceptionReporter是一个回调接口，用于支持对SpringApplication启动错误的自定义报告。里面就一个报告启动失败的方法。 其实现类：org.springframework.boot.diagnostics.FailureAnalyzers 用于触发从spring.factories加载的FailureAnalyzer和FailureAnalysisReporter实例。

2、如何自定义异常信息

/**
 * <p>
 *
 * <p>
 *
 * @author: xuwd
 * @time: 2020/11/16 10:52
 */
public class WannaStopException extends RuntimeException {
}

public class StopFailureAnalyzer
        extends AbstractFailureAnalyzer<WannaStopException> {
    @Override
    protected FailureAnalysis analyze(Throwable rootFailure, WannaStopException cause) {
        for (StackTraceElement stackTraceElement : cause.getStackTrace()) {
            if (stackTraceElement.getClassName().equals("com.pigx.demo.Config21")) {
                return new FailureAnalysis("A想停止", "别要A了", cause);
            }
        }
        return null;
    }
}

接下来令他生效，通过上面分析可以可看出需要通过AutoConfigurationImportSelector，类似于自定义SpringBoot Starter AutoConfiguration的形式，我们需要在META-INF/spring.factories文件内进行定义，如下所示： juejin.cn/post/684490… 接着在合适的地方抛出WannaStopException 异常

总结 在springboot 启动过程中会先对异常信息进行补捕获，对进行日志格式处理的日志进行处理；其核心是通过SpringBootExceptionReporter回调及sping-spi bean的管理。