從 JVM 分析 hibernate-validator NoClassDefFoundError

最近排查一個spring boot應用拋出hibernate.validator NoClassDefFoundError的問題，異常信息如下：
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
這個錯誤信息表面上是 NoClassDefFoundError ，但是實際上 ConfigurationImpl這個類是在 hibernate-validator-5.3.5.Final.jar 裏的，不應該出現找不到類的情況。

那爲什麼應用裏拋出這個 NoClassDefFoundError ？

有經驗的開發人員從 Could not initialize class 這個信息就可以知道，實際上是一個類在初始化時拋出的異常，比如static的靜態代碼塊，或者static字段初始化的異常。

誰初始化了 org.hibernate.validator.internal.engine.ConfigurationImpl
但是當我們在 HibernateValidator 這個類，創建 ConfigurationImpl 的代碼塊裏打斷點時，發現有兩個線程觸發了斷點：

public class HibernateValidator implements ValidationProvider<HibernateValidatorConfiguration> {br/>@Override
public Configuration<?> createGenericConfiguration(BootstrapState state) {
return new ConfigurationImpl( state );
}
其中一個線程的調用棧是：

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
另外一個線程調用棧是：

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
顯然，這個線程的調用棧是常見的spring的初始化過程。

BackgroundPreinitializer 做了什麼
那麼重點來看下 BackgroundPreinitializer 線程做了哪些事情：

@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 類是spring boot爲了加速應用的初始化，以一個獨立的線程來加載hibernate validator這些組件。

這個 background-preinit 線程會吞掉所有的異常。

顯然 ConfigurationImpl 初始化的異常也被吞掉了，那麼如何才能獲取到最原始的信息？

獲取到最原始的異常信息
在 BackgroundPreinitializer 的 run() 函數裏打一個斷點（注意是 Suspend thread 類型, 不是 Suspend VM ），讓它先不要觸發 ConfigurationImpl 的加載，讓spring boot的正常流程去觸發 ConfigurationImpl 的加載，就可以知道具體的信息了。

那麼打出來的異常信息是：

Caused by: java.lang.NoSuchMethodError: org.jboss.logging.Logger.getMessageLogger(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Object;
at org.hibernate.validator.internal.util.logging.LoggerFactory.make(LoggerFactory.java:19) ~[hibernate-validator-5.3.5.Final.jar:5.3.5.Final]
at org.hibernate.validator.internal.util.Version.<clinit>(Version.java:22) ~[hibernate-validator-5.3.5.Final.jar:5.3.5.Final]
at org.hibernate.validator.internal.engine.ConfigurationImpl.<clinit>(ConfigurationImpl.java:71) ~[hibernate-validator-5.3.5.Final.jar:5.3.5.Final]
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]
那麼可以看出是 org.jboss.logging.Logger 這個類不兼容，少了 getMessageLogger(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Object 這個函數。

那麼檢查下應用的依賴，可以發現 org.jboss.logging.Logger 在 jboss-common-1.2.1.GA.jar 和 jboss-logging-3.3.1.Final.jar 裏都有。

顯然是 jboss-common-1.2.1.GA.jar 這個依賴過時了，需要排除掉。

總結異常的發生流程
應用依賴了 jboss-common-1.2.1.GA.jar ，它裏面的 org.jboss.logging.Logger太老

spring boot啓動時， BackgroundPreinitializer 裏的線程去嘗試加載 ConfigurationImpl ，然後觸發了 org.jboss.logging.Logger 的函數執行問題

BackgroundPreinitializer 吃掉了異常信息，jvm把 ConfigurationImpl 標記爲不可用的

spring boot正常的流程去加載 ConfigurationImpl ，jvm發現 ConfigurationImpl類是不可用，直接拋出 NoClassDefFoundError

Caused by: java.lang.NoClassDefFoundError: Could not initialize class org.hibernate.validator.internal.engine.ConfigurationImpl
深入JVM
爲什麼第二次嘗試加載 ConfigurationImpl 時，會直接拋出 java.lang.NoClassDefFoundError: Could not initialize class ？

下面用一段簡單的代碼來重現這個問題：

try {
org.hibernate.validator.internal.util.Version.touch();
} catch (Throwable e) {
e.printStackTrace();
}
System.in.read();

try {
org.hibernate.validator.internal.util.Version.touch();
} catch (Throwable e) {
e.printStackTrace();
}
使用HSDB來確定類的狀態
當拋出第一個異常時，嘗試用HSDB來看下這個類的狀態。

sudo java -classpath "$JAVA_HOME/lib/sa-jdi.jar" sun.jvm.hotspot.HSDB
然後在HSDB console裏查找到 Version 的地址信息

hsdb> class org.hibernate.validator.internal.util.Version
org/hibernate/validator/internal/util/Version @0x00000007c0060218
然後在 Inspector 查找到這個地址，發現 _init_state 是5。

再看下hotspot代碼，可以發現5對應的定義是 initialization_error ：

// /hotspot/src/share/vm/oops/instanceKlass.hpp
// See "The Java Virtual Machine Specification" section 2.16.2-5 for a detailed description
// of the class loading & initialization procedure, and the use of the states.
enum ClassState {
allocated, // allocated (but not yet linked)
loaded, // loaded and inserted in class hierarchy (but not linked yet)
linked, // successfully linked/verified (but not initialized yet)
being_initialized, // currently running class initializer
fully_initialized, // initialized (successfull final state)
initialization_error // error happened during initialization
};
JVM規範裏關於Initialization的內容
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html#jvms-5.5

從規範裏可以看到初始一個類/接口有12步，比較重要的兩步都用黑體標記出來了：

5: If the Class object for C is in an erroneous state, then initialization is not possible. Release LC and throw a NoClassDefFoundError.

11: Otherwise, the class or interface initialization method must have completed abruptly by throwing some exception E. If the class of E is not Error or one of its subclasses, then create a new instance of the class ExceptionInInitializerError with E as the argument, and use this object in place of E in the following step.

第一次嘗試加載Version類時
當第一次嘗試加載時，hotspot InterpreterRuntime在解析 invokestatic 指令時，嘗試加載 org.hibernate.validator.internal.util.Version 類， InstanceKlass 的 _init_state 先是標記爲 being_initialized ，然後當加載失敗時，被標記爲 initialization_error 。

對應 Initialization 的11步。

// hotspot/src/share/vm/oops/instanceKlass.cpp
// Step 10 and 11
Handle e(THREAD, PENDING_EXCEPTION);
CLEAR_PENDING_EXCEPTION;
// JVMTI has already reported the pending exception
// JVMTI internal flag reset is needed in order to report ExceptionInInitializerError
JvmtiExport::clear_detected_exception((JavaThread)THREAD);
{
EXCEPTION_MARK;
this_oop->set_initialization_state_and_notify(initialization_error, THREAD);
CLEAR_PENDING_EXCEPTION; // ignore any exception thrown, class initialization error is thrown below
// JVMTI has already reported the pending exception
// JVMTI internal flag reset is needed in order to report ExceptionInInitializerError
JvmtiExport::clear_detected_exception((JavaThread)THREAD);
}
DTRACE_CLASSINIT_PROBE_WAIT(error, InstanceKlass::cast(this_oop()), -1,wait);
if (e->is_a(SystemDictionary::Error_klass())) {
THROW_OOP(e());
} else {
JavaCallArguments args(e);
THROW_ARG(vmSymbols::java_lang_ExceptionInInitializerError(),
vmSymbols::throwable_void_signature(),
&args);
}
第二次嘗試加載Version類時
當第二次嘗試加載時，檢查 InstanceKlass 的 _init_state 是 initialization_error ，則直接拋出 NoClassDefFoundError: Could not initialize class .

對應 Initialization 的5步。

// hotspot/src/share/vm/oops/instanceKlass.cpp
void InstanceKlass::initialize_impl(instanceKlassHandle this_oop, TRAPS) {
// ...
// Step 5
if (this_oop->is_in_error_state()) {
DTRACE_CLASSINIT_PROBE_WAIT(erroneous, InstanceKlass::cast(this_oop()), -1,wait);
ResourceMark rm(THREAD);
const char desc = "Could not initialize class ";
const char className = this_oop->external_name();
size_t msglen = strlen(desc) + strlen(className) + 1;
char* message = NEW_RESOURCE_ARRAY(char, msglen);
if (NULL == message) {
// Out of memory: can't create detailed error message
THROW_MSG(vmSymbols::java_lang_NoClassDefFoundError(), className);
} else {
jio_snprintf(message, msglen, "%s%s", desc, className);
THROW_MSG(vmSymbols::java_lang_NoClassDefFoundError(), message);
}
}
總結
spring boot在 BackgroundPreinitializer 類裏用一個獨立的線程來加載validator，並吃掉了原始異常

第一次加載失敗的類，在jvm裏會被標記爲 initialization_error ，再次加載時會直接拋出 NoClassDefFoundError: Could not initialize class

當在代碼裏吞掉異常時要謹慎，否則排查問題帶來很大的困難