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，異常信息以下：

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
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這個錯誤信息表面上是 NoClassDefFoundError ，可是實際上 ConfigurationImpl 這個類是在 hibernate-validator-5.3.5.Final.jar 裏的，不該該出現找不到類的狀況。

那爲何應用裏拋出這個 NoClassDefFoundError ？

有經驗的開發人員從 Could not initialize class 這個信息就能夠知道，其實是一個類在初始化時拋出的異常，好比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

可是當咱們在 HibernateValidator 這個類，建立 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> {
    @Override
    public Configuration<?> createGenericConfiguration(BootstrapState state) {
        return new ConfigurationImpl( state );
    }
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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
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另一個線程調用棧是：

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
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顯然，這個線程的調用棧是常見的spring的初始化過程。

BackgroundPreinitializer 作了什麼

那麼重點來看下 BackgroundPreinitializer 線程作了哪些事情：

@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();
        }
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能夠看到 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 線程會吞掉全部的異常。

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，就能夠知道具體的信息了。

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]
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那麼能夠看出是 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，能夠發現 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 這個依賴過期了，須要排除掉。

總結異常的發生流程

1. 應用依賴了 jboss-common-1.2.1.GA.jar ，它裏面的 org.jboss.logging.Logger 太老

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

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

4. 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
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深刻JVM

爲何第二次嘗試加載 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();
}
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使用HSDB來肯定類的狀態

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
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而後在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
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而後在 Inspector 查找到這個地址，發現 _init_state 是5。

再看下hotspot代碼，能夠發現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
};
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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

從規範裏能夠看到初始一個類/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 ，而後當加載失敗時，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);
}
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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); } } 複製代碼

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，不然排查問題帶來很大的困難

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