问题

可重现的Demo代码：demo.zip
最近排查一个spring boot应用抛出hibernate.validator NoClassDefFoundError的问题，异常信息如下：

Caused by: java.lang.NoClassDefFoundError: Could not initialize class org.hibernate.validator.internal.engine.ConfigurationImpl
at org.hibernate.validator.HibernateValidator.createGenericConfiguration(HibernateValidator.java:33) ~[hibernate-validator-5.3.5.Final.jar:5.3.5.Final]
at javax.validation.Validation$GenericBootstrapImpl.configure(Validation.java:276) ~[validation-api-1.1.0.Final.jar:na]
at org.springframework.boot.validation.MessageInterpolatorFactory.getObject(MessageInterpolatorFactory.java:53) ~[spring-boot-1.5.3.RELEASE.jar:1.5.3.RELEASE]
at org.springframework.boot.autoconfigure.validation.DefaultValidatorConfiguration.defaultValidator(DefaultValidatorConfiguration.java:43) ~[spring-boot-autoconfigure-1.5.3.RELEASE.jar:1.5.3.RELEASE]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_112]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_112]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_112]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_112]
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ~[spring-beans-4.3.8.RELEASE.jar:4.3.8.RELEASE]
... 32 common frames omitted

这个错误信息外貌上是NoClassDefFoundError，可是实际上ConfigurationImpl这个类是在hibernate-validator-5.3.5.Final.jar里的，不该该呈现找不到类的环境。

那为什么应用里抛出这个NoClassDefFoundError ？

有履历的开拓人员从Could not initialize class 这个信息就可以知道，实际上是一个类在初始化时抛出的异常，好比static的静态代码块，可能static字段初始化的异常。

谁初始化了 org.hibernate.validator.internal.engine.ConfigurationImpl

可是当我们在HibernateValidator 这个类，建设ConfigurationImpl的代码块里打断点时，发明有两个线程触发了断点：

public class HibernateValidator implements ValidationProvider<HibernateValidatorConfiguration> {
	@Override
	public Configuration<?> createGenericConfiguration(BootstrapState state) {
		return new ConfigurationImpl( state );
	}

个中一个线程的挪用栈是：

Thread [background-preinit] (Class load: ConfigurationImpl)
	HibernateValidator.createGenericConfiguration(BootstrapState) line: 33
	Validation$GenericBootstrapImpl.configure() line: 276
	BackgroundPreinitializer$ValidationInitializer.run() line: 107
	BackgroundPreinitializer$1.runSafely(Runnable) line: 59
	BackgroundPreinitializer$1.run() line: 52
	Thread.run() line: 745

别的一个线程挪用栈是：

Thread [main] (Suspended (breakpoint at line 33 in HibernateValidator))
	owns: ConcurrentHashMap<K,V>  (id=52)
	owns: Object  (id=53)
	HibernateValidator.createGenericConfiguration(BootstrapState) line: 33
	Validation$GenericBootstrapImpl.configure() line: 276
	MessageInterpolatorFactory.getObject() line: 53
	DefaultValidatorConfiguration.defaultValidator() line: 43
	NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method]
	NativeMethodAccessorImpl.invoke(Object, Object[]) line: 62
	DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 43
	Method.invoke(Object, Object...) line: 498
	CglibSubclassingInstantiationStrategy(SimpleInstantiationStrategy).instantiate(RootBeanDefinition, String, BeanFactory, Object, Method, Object...) line: 162
	ConstructorResolver.instantiateUsingFactoryMethod(String, RootBeanDefinition, Object[]) line: 588
	DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).instantiateUsingFactoryMethod(String, RootBeanDefinition, Object[]) line: 1173

显然，这个线程的挪用栈是常见的spring的初始化进程。

BackgroundPreinitializer 做了什么

那么重点来看下 BackgroundPreinitializer 线程做了哪些工作：

@Order(LoggingApplicationListener.DEFAULT_ORDER + 1)
public class BackgroundPreinitializer
		implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {
	@Override
	public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
		try {
			Thread thread = new Thread(new Runnable() {
				@Override
				public void run() {
					runSafely(new MessageConverterInitializer());
					runSafely(new MBeanFactoryInitializer());
					runSafely(new ValidationInitializer());
					runSafely(new JacksonInitializer());
					runSafely(new ConversionServiceInitializer());
				}
				public void runSafely(Runnable runnable) {
					try {
						runnable.run();
					}
					catch (Throwable ex) {
						// Ignore
					}
				}
			}, "background-preinit");
			thread.start();
		}

可以看到BackgroundPreinitializer类是spring boot为了加快应用的初始化，软件开发，以一个独立的线程来加载hibernate validator这些组件。

这个 background-preinit 线程会吞掉所有的异常。
显然ConfigurationImpl 初始化的异常也被吞掉了，那么如何才气获取到最原始的信息？

获取到最原始的异常信息