一、前言

先贴一份测试代码，大家可以先猜测一下，执行结果会是怎样的：

import java.util.concurrent.TimeUnit;

public class TestClassLoading {
    public static class A{
        static {
            System.out.println("class A init");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            new B();
        }

        public static void test() {
            System.out.println("aaa");
        }
    }

    public static class B{
        static {
            System.out.println("class B init");
            new A();
        }

        public static void test() {
            System.out.println("bbb");
        }
    }
    public static void main(String[] args) {
        new Thread(() -> A.test()).start();
        new Thread(() -> B.test()).start();
    }
}

不知道，你猜对了没有呢，实际的执行结果会是下面这样的：

二、原因分析

这里，一开始大家分析的是，和new有关系；但下面的代码和上面的结果完全一致，基本可以排除 new 的嫌疑：

public class TestClassLoadingNew {
    public static class A{
        static {
            System.out.println("class A init");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            B.test();
        }

        public static void test() {
            System.out.println("aaa");
        }
    }

    public static class B{
        static {
            System.out.println("class B init");
            A.test();
        }

        public static void test() {
            System.out.println("bbb");
        }
    }
    public static void main(String[] args) {
        new Thread(() -> A.test()).start();
        new Thread(() -> B.test()).start();
    }
}

这里，问题的根本原因，其实是：

classloader在初始化一个类的时候，会对当前类加锁，加锁后，再执行类的静态初始化块。

所以，上面会发生：

　　1、线程1：类A对class A加锁，加锁后，执行类的静态初始化块（在堆栈里体现为<clinit>函数），发现用到了class B，于是去加载B；

　　2、线程2：类B对class B加锁，加锁后，执行类的静态初始化块（在堆栈里体现为<clinit>函数），发现用到了class A，于是去加载A；

　　3、死锁发生。

　　有经验的同学，对于死锁是毫无畏惧的，因为我们有神器，jstack。 jstack 加上 -l 参数，即可打印出各个线程持有的锁的信息。（windows上直接jconsole就行，还能死锁检测）：

"Thread-1" #15 prio=5 os_prio=0 tid=0x000000002178a000 nid=0x2df8 in Object.wait() [0x0000000021f4e000]
   java.lang.Thread.State: RUNNABLE
        at com.dmtest.netty_learn.TestClassLoading$B.<clinit>(TestClassLoading.java:32)
        at com.dmtest.netty_learn.TestClassLoading.lambda$main$1(TestClassLoading.java:42)
        at com.dmtest.netty_learn.TestClassLoading$$Lambda$2/736709391.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
        - None

"Thread-0" #14 prio=5 os_prio=0 tid=0x0000000021787800 nid=0x2618 in Object.wait() [0x00000000213be000]
   java.lang.Thread.State: RUNNABLE
        at com.dmtest.netty_learn.TestClassLoading$A.<clinit>(TestClassLoading.java:21)
        at com.dmtest.netty_learn.TestClassLoading.lambda$main$0(TestClassLoading.java:41)
        at com.dmtest.netty_learn.TestClassLoading$$Lambda$1/611437735.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
        - None

这里，很奇怪的一个原因是，明明这两个线程发生了死锁，为什么没有显示呢？

因为，这是 jvm 内部加了锁，所以，jconsole、jstack都失效了。

三、一起深入JVM，探个究竟

1、单步跟踪

class 的加载都是由 classloader 来完成的，而且部分工作是在 jvm 层面完成，我们可以看到，在 java.lang.ClassLoader#defineClass1 的定义中：

以上几个方法都是本地方法。

其实际的实现在：/home/ckl/openjdk-jdk8u/jdk/src/share/native/java/lang/ClassLoader.c，

JNIEXPORT jclass JNICALL
Java_java_lang_ClassLoader_defineClass1(JNIEnv *env,
                                        jobject loader,
                                        jstring name,
                                        jbyteArray data,
                                        jint offset,
                                        jint length,
                                        jobject pd,
                                        jstring source)
{
    jbyte *body;
    char *utfName;
    jclass result = 0;
    char buf[128];
    char* utfSource;
    char sourceBuf[1024];

    if (data == NULL) {
        JNU_ThrowNullPointerException(env, 0);
        return 0;
    }

    /* Work around 4153825. malloc crashes on Solaris when passed a
     * negative size.
     */
    if (length < 0) {
        JNU_ThrowArrayIndexOutOfBoundsException(env, 0);
        return 0;
    }

    body = (jbyte *)malloc(length);

    if (body == 0) {
        JNU_ThrowOutOfMemoryError(env, 0);
        return 0;
    }

    (*env)->GetByteArrayRegion(env, data, offset, length, body);

    if ((*env)->ExceptionOccurred(env))
        goto free_body;

    if (name != NULL) {
        utfName = getUTF(env, name, buf, sizeof(buf));
        if (utfName == NULL) {
            goto free_body;
        }
        VerifyFixClassname(utfName);
    } else {
        utfName = NULL;
    }

    if (source != NULL) {
        utfSource = getUTF(env, source, sourceBuf, sizeof(sourceBuf));
        if (utfSource == NULL) {
            goto free_utfName;
        }
    } else {
        utfSource = NULL;
    }
    result = JVM_DefineClassWithSource(env, utfName, loader, body, length, pd, utfSource);

    if (utfSource && utfSource != sourceBuf)
        free(utfSource);

 free_utfName:
    if (utfName && utfName != buf)
        free(utfName);

 free_body:
    free(body);
    return result;
}

大家可以跟着标红的代码，我们一起大概看一下，这个方法的实现在/home/ckl/openjdk-jdk8u/hotspot/src/share/vm/prims/jvm.cpp 中，

JVM_ENTRY(jclass, JVM_DefineClassWithSource(JNIEnv *env, const char *name, jobject loader, const jbyte *buf, jsize len, jobject pd, const char *source))
  JVMWrapper2("JVM_DefineClassWithSource %s", name);

  return jvm_define_class_common(env, name, loader, buf, len, pd, source, true, THREAD);
JVM_END

jvm_define_class_common 的实现，还是在 jvm.cpp 中，

// common code for JVM_DefineClass() and JVM_DefineClassWithSource()
// and JVM_DefineClassWithSourceCond()
static jclass jvm_define_class_common(JNIEnv *env, const char *name,
                                      jobject loader, const jbyte *buf,
                                      jsize len, jobject pd, const char *source,
                                      jboolean verify, TRAPS) {
  if (source == NULL)  source = "__JVM_DefineClass__";

  assert(THREAD->is_Java_thread(), "must be a JavaThread");
  JavaThread* jt = (JavaThread*) THREAD;

  PerfClassTraceTime vmtimer(ClassLoader::perf_define_appclass_time(),
                             ClassLoader::perf_define_appclass_selftime(),
                             ClassLoader::perf_define_appclasses(),
                             jt->get_thread_stat()->perf_recursion_counts_addr(),
                             jt->get_thread_stat()->perf_timers_addr(),
                             PerfClassTraceTime::DEFINE_CLASS);

  if (UsePerfData) {
    ClassLoader::perf_app_classfile_bytes_read()->inc(len);
  }

  // Since exceptions can be thrown, class initialization can take place
  // if name is NULL no check for class name in .class stream has to be made.
  TempNewSymbol class_name = NULL;
  if (name != NULL) {
    const int str_len = (int)strlen(name);
    if (str_len > Symbol::max_length()) {
      // It's impossible to create this class;  the name cannot fit
      // into the constant pool.
      THROW_MSG_0(vmSymbols::java_lang_NoClassDefFoundError(), name);
    }
    class_name = SymbolTable::new_symbol(name, str_len, CHECK_NULL);
  }

  ResourceMark rm(THREAD);
  ClassFileStream st((u1*) buf, len, (char *)source);
  Handle class_loader (THREAD, JNIHandles::resolve(loader));
  if (UsePerfData) {
    is_lock_held_by_thread(class_loader,
                           ClassLoader::sync_JVMDefineClassLockFreeCounter(),
                           THREAD);
  }
  Handle protection_domain (THREAD, JNIHandles::resolve(pd));
  Klass* k = SystemDictionary::resolve_from_stream(class_name, class_loader,
                                                     protection_domain, &st,
                                                     verify != 0,
                                                     CHECK_NULL);

  if (TraceClassResolution && k != NULL) {
    trace_class_resolution(k);
  }

  return (jclass) JNIHandles::make_local(env, k->java_mirror());
}

resolve_from_stream 的实现在 SystemDictionary 类中，下面我们看下：

Klass* SystemDictionary::resolve_from_stream(Symbol* class_name,
                                             Handle class_loader,
                                             Handle protection_domain,
                                             ClassFileStream* st,
                                             bool verify,
                                             TRAPS) {

  // Classloaders that support parallelism, e.g. bootstrap classloader,
  // or all classloaders with UnsyncloadClass do not acquire lock here
  bool DoObjectLock = true;
  if (is_parallelCapable(class_loader)) {
    DoObjectLock = false;
  }

  ClassLoaderData* loader_data = register_loader(class_loader, CHECK_NULL);

  // Make sure we are synchronized on the class loader before we proceed
  Handle lockObject = compute_loader_lock_object(class_loader, THREAD);
  check_loader_lock_contention(lockObject, THREAD);
  ObjectLocker ol(lockObject, THREAD, DoObjectLock);

  TempNewSymbol parsed_name = NULL;

  // Parse the stream. Note that we do this even though this klass might
  // already be present in the SystemDictionary, otherwise we would not
  // throw potential ClassFormatErrors.
  //
  // Note: "name" is updated.

  instanceKlassHandle k = ClassFileParser(st).parseClassFile(class_name,
                                                             loader_data,
                                                             protection_domain,
                                                             parsed_name,
                                                             verify,
                                                             THREAD);

  const char* pkg = "java/";
  size_t pkglen = strlen(pkg);
  if (!HAS_PENDING_EXCEPTION &&
      !class_loader.is_null() &&
      parsed_name != NULL &&
      parsed_name->utf8_length() >= (int)pkglen &&
      !strncmp((const char*)parsed_name->bytes(), pkg, pkglen)) {
    // It is illegal to define classes in the "java." package from
    // JVM_DefineClass or jni_DefineClass unless you're the bootclassloader
    ResourceMark rm(THREAD);
    char* name = parsed_name->as_C_string();
    char* index = strrchr(name, '/');
    assert(index != NULL, "must be");
    *index = '\0'; // chop to just the package name
    while ((index = strchr(name, '/')) != NULL) {
      *index = '.'; // replace '/' with '.' in package name
    }
    const char* fmt = "Prohibited package name: %s";
    size_t len = strlen(fmt) + strlen(name);
    char* message = NEW_RESOURCE_ARRAY(char, len);
    jio_snprintf(message, len, fmt, name);
    Exceptions::_throw_msg(THREAD_AND_LOCATION,
      vmSymbols::java_lang_SecurityException(), message);
  }

  if (!HAS_PENDING_EXCEPTION) {
    assert(parsed_name != NULL, "Sanity");
    assert(class_name == NULL || class_name == parsed_name, "name mismatch");
    // Verification prevents us from creating names with dots in them, this
    // asserts that that's the case.
    assert(is_internal_format(parsed_name),
           "external class name format used internally");

    // Add class just loaded
    // If a class loader supports parallel classloading handle parallel define requests
    // find_or_define_instance_class may return a different InstanceKlass
    if (is_parallelCapable(class_loader)) {
      k = find_or_define_instance_class(class_name, class_loader, k, THREAD);
    } else {
      define_instance_class(k, THREAD);
    }
  }
96 
  return k();
}

上面的方法里，有几处值得注意的：

1：18-20行，进行了加锁，18行获取锁对象，这里是当前类加载器（从注释可以看出），20行就是加锁的语法

2：37-60行，这里是判断要加载的类的包名是否以 java 开头，以 java 开头的类是非法的，不能加载

3：第76行， define_instance_class(k, THREAD); 进行后续操作

接下来，我们看看 define_instance_class 的实现：

void SystemDictionary::define_instance_class(instanceKlassHandle k, TRAPS) {

  ClassLoaderData* loader_data = k->class_loader_data();
  Handle class_loader_h(THREAD, loader_data->class_loader());

  for (uintx it = 0; it < GCExpandToAllocateDelayMillis; it++){}

 // for bootstrap and other parallel classloaders don't acquire lock,
 // use placeholder token
 // If a parallelCapable class loader calls define_instance_class instead of
 // find_or_define_instance_class to get here, we have a timing
 // hole with systemDictionary updates and check_constraints
 if (!class_loader_h.is_null() && !is_parallelCapable(class_loader_h)) {
    assert(ObjectSynchronizer::current_thread_holds_lock((JavaThread*)THREAD,
         compute_loader_lock_object(class_loader_h, THREAD)),
         "define called without lock");
  }

  // Check class-loading constraints. Throw exception if violation is detected.
  // Grabs and releases SystemDictionary_lock
  // The check_constraints/find_class call and update_dictionary sequence
  // must be "atomic" for a specific class/classloader pair so we never
  // define two different instanceKlasses for that class/classloader pair.
  // Existing classloaders will call define_instance_class with the
  // classloader lock held
  // Parallel classloaders will call find_or_define_instance_class
  // which will require a token to perform the define class
  Symbol*  name_h = k->name();
  unsigned int d_hash = dictionary()->compute_hash(name_h, loader_data);
  int d_index = dictionary()->hash_to_index(d_hash);
  check_constraints(d_index, d_hash, k, class_loader_h, true, CHECK);

  // Register class just loaded with class loader (placed in Vector)
  // Note we do this before updating the dictionary, as this can
  // fail with an OutOfMemoryError (if it does, we will *not* put this
  // class in the dictionary and will not update the class hierarchy).
  // JVMTI FollowReferences needs to find the classes this way.
  if (k->class_loader() != NULL) {
    methodHandle m(THREAD, Universe::loader_addClass_method());
    JavaValue result(T_VOID);
    JavaCallArguments args(class_loader_h);
    args.push_oop(Handle(THREAD, k->java_mirror()));
    JavaCalls::call(&result, m, &args, CHECK);
  }

  // Add the new class. We need recompile lock during update of CHA.
  {
    unsigned int p_hash = placeholders()->compute_hash(name_h, loader_data);
    int p_index = placeholders()->hash_to_index(p_hash);

    MutexLocker mu_r(Compile_lock, THREAD);

    // Add to class hierarchy, initialize vtables, and do possible
    // deoptimizations.
    add_to_hierarchy(k, CHECK); // No exception, but can block

    // Add to systemDictionary - so other classes can see it.
    // Grabs and releases SystemDictionary_lock
    update_dictionary(d_index, d_hash, p_index, p_hash,
                      k, class_loader_h, THREAD);
  }
  k->eager_initialize(THREAD);

  // notify jvmti
  if (JvmtiExport::should_post_class_load()) {
      assert(THREAD->is_Java_thread(), "thread->is_Java_thread()");
      JvmtiExport::post_class_load((JavaThread *) THREAD, k());

  }

}

这里，由于我们的案例中，是class A 在初始化过程中出现死锁，所以我们关注第62行，eager_initialize：

void InstanceKlass::eager_initialize(Thread *thread) {
  if (!EagerInitialization) return;

  if (this->is_not_initialized()) {
    // abort if the the class has a class initializer
    if (this->class_initializer() != NULL) return;

    // abort if it is java.lang.Object (initialization is handled in genesis)
    Klass* super = this->super();
    if (super == NULL) return;

    // abort if the super class should be initialized
    if (!InstanceKlass::cast(super)->is_initialized()) return;

    // call body to expose the this pointer
    instanceKlassHandle this_oop(thread, this);
    eager_initialize_impl(this_oop);
  }
}

我们接着进入 eager_initialize_impl，该方法进入到了 InstanceKlass：

void InstanceKlass::eager_initialize_impl(instanceKlassHandle this_oop) {
  EXCEPTION_MARK;
  oop init_lock = this_oop->init_lock();
  ObjectLocker ol(init_lock, THREAD, init_lock != NULL);

  // abort if someone beat us to the initialization
  if (!this_oop->is_not_initialized()) return;  // note: not equivalent to is_initialized()

  ClassState old_state = this_oop->init_state();
  link_class_impl(this_oop, true, THREAD);
  if (HAS_PENDING_EXCEPTION) {
    CLEAR_PENDING_EXCEPTION;
    // Abort if linking the class throws an exception.

    // Use a test to avoid redundantly resetting the state if there's
    // no change.  Set_init_state() asserts that state changes make
    // progress, whereas here we might just be spinning in place.
    if( old_state != this_oop->_init_state )
      this_oop->set_init_state (old_state);
  } else {
    // linking successfull, mark class as initialized
    this_oop->set_init_state (fully_initialized);
    this_oop->fence_and_clear_init_lock();
    // trace
    if (TraceClassInitialization) {
      ResourceMark rm(THREAD);
      tty->print_cr("[Initialized %s without side effects]", this_oop->external_name());
    }
  }
}

这里，我们重点关注第3,4行：

1、第3行，获取初始化锁；

2、第4行，加锁

2、获取初始化锁并加锁

这里，我们首先获取锁的操作，

oop InstanceKlass::init_lock() const {
  // return the init lock from the mirror
  oop lock = java_lang_Class::init_lock(java_mirror());
  // Prevent reordering with any access of initialization state
  OrderAccess::loadload();
  assert((oop)lock != NULL || !is_not_initialized(), // initialized or in_error state
         "only fully initialized state can have a null lock");
  return lock;
}

其中，java_mirror() 方法就是返回 Klass 类中的以下字段：

// java/lang/Class instance mirroring this class
oop       _java_mirror;

再看 init_lock 方法：

oop java_lang_Class::init_lock(oop java_class) {
   assert(_init_lock_offset != 0, "must be set");
   return java_class->obj_field(_init_lock_offset);
 }

这里呢，应该就是获取我们传入的 java_class 中的某个字段，该字段就是充当 init_lock。（个人水平有限，还请指正）

下面为加锁操作的语句：

ObjectLocker ol(init_lock, THREAD, init_lock != NULL);

/ ObjectLocker enforced balanced locking and can never thrown an
// IllegalMonitorStateException. However, a pending exception may
// have to pass through, and we must also be able to deal with
// asynchronous exceptions. The caller is responsible for checking
// the threads pending exception if needed.
// doLock was added to support classloading with UnsyncloadClass which
// requires flag based choice of locking the classloader lock.
class ObjectLocker : public StackObj {
 private:
  Thread*   _thread;
  Handle    _obj;
  BasicLock _lock;
  bool      _dolock;   // default true
 public:
  ObjectLocker(Handle obj, Thread* thread, bool doLock = true);

// -----------------------------------------------------------------------------
// Internal VM locks on java objects
// standard constructor, allows locking failures
ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
  _dolock = doLock;
  _thread = thread;
  _obj = obj;

  if (_dolock) {
    TEVENT (ObjectLocker) ;

    ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
  }
}

接下来会进入到 synchronizer.cpp，

// -----------------------------------------------------------------------------
//  Fast Monitor Enter/Exit
// This the fast monitor enter. The interpreter and compiler use
// some assembly copies of this code. Make sure update those code
// if the following function is changed. The implementation is
// extremely sensitive to race condition. Be careful.

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 if (UseBiasedLocking) {
    if (!SafepointSynchronize::is_at_safepoint()) {
      BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
      if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
        return;
      }
    } else {
      assert(!attempt_rebias, "can not rebias toward VM thread");
      BiasedLocking::revoke_at_safepoint(obj);
    }
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 }

 slow_enter (obj, lock, THREAD) ;
}

上面会判断，是否使用偏向锁，如果不使用，则走 slow_enter 。

// -----------------------------------------------------------------------------
// Interpreter/Compiler Slow Case
// This routine is used to handle interpreter/compiler slow case
// We don't need to use fast path here, because it must have been
// failed in the interpreter/compiler code.
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
  markOop mark = obj->mark();
  assert(!mark->has_bias_pattern(), "should not see bias pattern here");

  if (mark->is_neutral()) {
    // Anticipate successful CAS -- the ST of the displaced mark must
    // be visible <= the ST performed by the CAS.
    lock->set_displaced_header(mark);
    if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
      TEVENT (slow_enter: release stacklock) ;
      return ;
    }
    // Fall through to inflate() ...
  } else
  if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
    assert(lock != mark->locker(), "must not re-lock the same lock");
    assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
    lock->set_displaced_header(NULL);
    return;
  }


  // The object header will never be displaced to this lock,
  // so it does not matter what the value is, except that it
  // must be non-zero to avoid looking like a re-entrant lock,
  // and must not look locked either.
  lock->set_displaced_header(markOopDesc::unused_mark());
  ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}

这里的代码结合注释，能大概看出来是，前面部分为轻量级锁，这里先不展开了，锁这块都可以单独写了。有兴趣的读者可以自行阅读。

四：如何解决类加载出现的死锁问题？

可以显式在主线程最开始用forName加载这些类的，这样类加载就变成在main线程中串行加载，问题得到解决：

public static void main(String[] args) throws ClassNotFoundException{
        
        Class.forName("com.**.**.A");
        Class.forName("com.**.**.B");

        new Thread(() -> A.test(), "thread-1").start();
        new Thread(() -> B.test(), "thread-2").start();
    }