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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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