一、前言
今天事不是很多，正好在Java交流群里，看到一个比较有意思的问题，于是花了点时间研究了一下，这里做个简单的分享。
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：
2
3 import java.util.concurrent.TimeUnit;
4
5
6 public class TestClassLoading {
7 public static class A{
8 static {
9 System.out.println("class A init");
10 try {
11 TimeUnit.SECONDS.sleep(1);
12 } catch (InterruptedException e) {
13 e.printStackTrace();
14 }
15 new B();
16 }
17
18 public static void test() {
19 System.out.println("aaa");
20 }
21 }
22
23 public static class B{
24 static {
25 System.out.println("class B init");
26 new A();
27 }
28
29
30 public static void test() {
31 System.out.println("bbb");
32 }
33 }
34 public static void main(String[] args) {
35 new Thread(() -> A.test()).start();
36 new Thread(() -> B.test()).start();
37 }
38 }
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：
1 public class TestClassLoadingNew {
2 public static class A{
3 static {
4 System.out.println("class A init");
5 try {
6 TimeUnit.SECONDS.sleep(1);
7 } catch (InterruptedException e) {
8 e.printStackTrace();
9 }
10 B.test();
11 }
12
13 public static void test() {
14 System.out.println("aaa");
15 }
16 }
17
18 public static class B{
19 static {
20 System.out.println("class B init");
21 A.test();
22 }
23
24
25 public static void test() {
26 System.out.println("bbb");
27 }
28 }
29 public static void main(String[] args) {
30 new Thread(() -> A.test()).start();
31 new Thread(() -> B.test()).start();
32 }
33 }
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，
1 JNIEXPORT jclass JNICALL
2 Java_java_lang_ClassLoader_defineClass1(JNIEnv *env,
3 jobject loader,
4 jstring name,
5 jbyteArray data,
6 jint offset,
7 jint length,
8 jobject pd,
9 jstring source)
10 {
11 jbyte *body;
12 char *utfName;
13 jclass result = 0;
14 char buf[128];
15 char* utfSource;
16 char sourceBuf[1024];
17
18 if (data == NULL) {
19 JNU_ThrowNullPointerException(env, 0);
20 return 0;
21 }
22
23 /* Work around 4153825. malloc crashes on Solaris when passed a
24 * negative size.
25 */
26 if (length < 0) {
27 JNU_ThrowArrayIndexOutOfBoundsException(env, 0);
28 return 0;
29 }
30
31 body = (jbyte *)malloc(length);
32
33 if (body == 0) {
34 JNU_ThrowOutOfMemoryError(env, 0);
35 return 0;
36 }
37
38 (*env)->GetByteArrayRegion(env, data, offset, length, body);
39
40 if ((*env)->ExceptionOccurred(env))
41 goto free_body;
42
43 if (name != NULL) {
44 utfName = getUTF(env, name, buf, sizeof(buf));
45 if (utfName == NULL) {
46 goto free_body;
47 }
48 VerifyFixClassname(utfName);
49 } else {
50 utfName = NULL;
51 }
52
53 if (source != NULL) {
54 utfSource = getUTF(env, source, sourceBuf, sizeof(sourceBuf));
55 if (utfSource == NULL) {
56 goto free_utfName;
57 }
58 } else {
59 utfSource = NULL;
60 }
61 result = JVM_DefineClassWithSource(env, utfName, loader, body, length, pd, utfSource);
62
63 if (utfSource && utfSource != sourceBuf)
64 free(utfSource);
65
66 free_utfName:
67 if (utfName && utfName != buf)
68 free(utfName);
69
70 free_body:
71 free(body);
72 return result;
73 }
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，
1 JVM_ENTRY(jclass, JVM_DefineClassWithSource(JNIEnv *env, const char *name, jobject loader, const jbyte *buf, jsize len, jobject pd, const char *source))
2 JVMWrapper2("JVM_DefineClassWithSource %s", name);
3
4 return jvm_define_class_common(env, name, loader, buf, len, pd, source, true, THREAD);
5 JVM_END
jvm_define_class_common 的实现，还是在 jvm.cpp 中，
1 // common code for JVM_DefineClass() and JVM_DefineClassWithSource()
2 // and JVM_DefineClassWithSourceCond()
3 static jclass jvm_define_class_common(JNIEnv *env, const char *name,
4 jobject loader, const jbyte *buf,
5 jsize len, jobject pd, const char *source,
6 jboolean verify, TRAPS) {
7 if (source == NULL) source = "__JVM_DefineClass__";
8
9 assert(THREAD->is_Java_thread(), "must be a JavaThread");
10 JavaThread* jt = (JavaThread*) THREAD;
11
12 PerfClassTraceTime vmtimer(ClassLoader::perf_define_appclass_time(),
13 ClassLoader::perf_define_appclass_selftime(),
14 ClassLoader::perf_define_appclasses(),
15 jt->get_thread_stat()->perf_recursion_counts_addr(),
16 jt->get_thread_stat()->perf_timers_addr(),
17 PerfClassTraceTime::DEFINE_CLASS);
18
19 if (UsePerfData) {
20 ClassLoader::perf_app_classfile_bytes_read()->inc(len);
21 }
22
23 // Since exceptions can be thrown, class initialization can take place
24 // if name is NULL no check for class name in .class stream has to be made.
25 TempNewSymbol class_name = NULL;
26 if (name != NULL) {
27 const int str_len = (int)strlen(name);
28 if (str_len > Symbol::max_length()) {
29 // It's impossible to create this class; the name cannot fit
30 // into the constant pool.
31 THROW_MSG_0(vmSymbols::java_lang_NoClassDefFoundError(), name);
32 }
33 class_name = SymbolTable::new_symbol(name, str_len, CHECK_NULL);
34 }
35
36 ResourceMark rm(THREAD);
37 ClassFileStream st((u1*) buf, len, (char *)source);
38 Handle class_loader (THREAD, JNIHandles::resolve(loader));
39 if (UsePerfData) {
40 is_lock_held_by_thread(class_loader,
41 ClassLoader::sync_JVMDefineClassLockFreeCounter(),
42 THREAD);
43 }
44 Handle protection_domain (THREAD, JNIHandles::resolve(pd));
45 Klass* k = SystemDictionary::resolve_from_stream(class_name, class_loader,
46 protection_domain, &st,
47 verify != 0,
48 CHECK_NULL);
49
50 if (TraceClassResolution && k != NULL) {
51 trace_class_resolution(k);
52 }
53
54 return (jclass) JNIHandles::make_local(env, k->java_mirror());
55 }
resolve_from_stream 的实现在 SystemDictionary 类中，下面我们看下：
1 Klass* SystemDictionary::resolve_from_stream(Symbol* class_name,
2 Handle class_loader,
3 Handle protection_domain,
4 ClassFileStream* st,
5 bool verify,
6 TRAPS) {
7
8 // Classloaders that support parallelism, e.g. bootstrap classloader,
9 // or all classloaders with UnsyncloadClass do not acquire lock here
10 bool DoObjectLock = true;
11 if (is_parallelCapable(class_loader)) {
12 DoObjectLock = false;
13 }
14
15 ClassLoaderData* loader_data = register_loader(class_loader, CHECK_NULL);
16
17 // Make sure we are synchronized on the class loader before we proceed
18 Handle lockObject = compute_loader_lock_object(class_loader, THREAD);
19 check_loader_lock_contention(lockObject, THREAD);
20 ObjectLocker ol(lockObject, THREAD, DoObjectLock);
21
22 TempNewSymbol parsed_name = NULL;
23
24 // Parse the stream. Note that we do this even though this klass might
25 // already be present in the SystemDictionary, otherwise we would not
26 // throw potential ClassFormatErrors.
27 //
28 // Note: "name" is updated.
29
30 instanceKlassHandle k = ClassFileParser(st).parseClassFile(class_name,
31 loader_data,
32 protection_domain,
33 parsed_name,
34 verify,
35 THREAD);
36
37 const char* pkg = "java/";
38 size_t pkglen = strlen(pkg);
39 if (!HAS_PENDING_EXCEPTION &&
40 !class_loader.is_null() &&
41 parsed_name != NULL &&
42 parsed_name->utf8_length() >= (int)pkglen &&
43 !strncmp((const char*)parsed_name->bytes(), pkg, pkglen)) {
44 // It is illegal to define classes in the "java." package from
45 // JVM_DefineClass or jni_DefineClass unless you're the bootclassloader
46 ResourceMark rm(THREAD);
47 char* name = parsed_name->as_C_string();
48 char* index = strrchr(name, '/');
49 assert(index != NULL, "must be");
50 *index = ' '; // chop to just the package name
51 while ((index = strchr(name, '/')) != NULL) {
52 *index = '.'; // replace '/' with '.' in package name
53 }
54 const char* fmt = "Prohibited package name: %s";
55 size_t len = strlen(fmt) + strlen(name);
56 char* message = NEW_RESOURCE_ARRAY(char, len);
57 jio_snprintf(message, len, fmt, name);
58 Exceptions::_throw_msg(THREAD_AND_LOCATION,
59 vmSymbols::java_lang_SecurityException(), message);
60 }
61
62 if (!HAS_PENDING_EXCEPTION) {
63 assert(parsed_name != NULL, "Sanity");
64 assert(class_name == NULL || class_name == parsed_name, "name mismatch");
65 // Verification prevents us from creating names with dots in them, this
66 // asserts that that's the case.
67 assert(is_internal_format(parsed_name),
68 "external class name format used internally");
69
70 // Add class just loaded
71 // If a class loader supports parallel classloading handle parallel define requests
72 // find_or_define_instance_class may return a different InstanceKlass
73 if (is_parallelCapable(class_loader)) {
74 k = find_or_define_instance_class(class_name, class_loader, k, THREAD);
75 } else {
76 define_instance_class(k, THREAD);
77 }
78 }
79 96
97 return k();
98 }
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：
1 void SystemDictionary::define_instance_class(instanceKlassHandle k, TRAPS) {
2
3 ClassLoaderData* loader_data = k->class_loader_data();
4 Handle class_loader_h(THREAD, loader_data->class_loader());
5
6 for (uintx it = 0; it < GCExpandToAllocateDelayMillis; it++){}
7
8 // for bootstrap and other parallel classloaders don't acquire lock,
9 // use placeholder token
10 // If a parallelCapable class loader calls define_instance_class instead of
11 // find_or_define_instance_class to get here, we have a timing
12 // hole with systemDictionary updates and check_constraints
13 if (!class_loader_h.is_null() && !is_parallelCapable(class_loader_h)) {
14 assert(ObjectSynchronizer::current_thread_holds_lock((JavaThread*)THREAD,
15 compute_loader_lock_object(class_loader_h, THREAD)),
16 "define called without lock");
17 }
18
19 // Check class-loading constraints. Throw exception if violation is detected.
20 // Grabs and releases SystemDictionary_lock
21 // The check_constraints/find_class call and update_dictionary sequence
22 // must be "atomic" for a specific class/classloader pair so we never
23 // define two different instanceKlasses for that class/classloader pair.
24 // Existing classloaders will call define_instance_class with the
25 // classloader lock held
26 // Parallel classloaders will call find_or_define_instance_class
27 // which will require a token to perform the define class
28 Symbol* name_h = k->name();
29 unsigned int d_hash = dictionary()->compute_hash(name_h, loader_data);
30 int d_index = dictionary()->hash_to_index(d_hash);
31 check_constraints(d_index, d_hash, k, class_loader_h, true, CHECK);
32
33 // Register class just loaded with class loader (placed in Vector)
34 // Note we do this before updating the dictionary, as this can
35 // fail with an OutOfMemoryError (if it does, we will *not* put this
36 // class in the dictionary and will not update the class hierarchy).
37 // JVMTI FollowReferences needs to find the classes this way.
38 if (k->class_loader() != NULL) {
39 methodHandle m(THREAD, Universe::loader_addClass_method());
40 JavaValue result(T_VOID);
41 JavaCallArguments args(class_loader_h);
42 args.push_oop(Handle(THREAD, k->java_mirror()));
43 JavaCalls::call(&result, m, &args, CHECK);
44 }
45
46 // Add the new class. We need recompile lock during update of CHA.
47 {
48 unsigned int p_hash = placeholders()->compute_hash(name_h, loader_data);
49 int p_index = placeholders()->hash_to_index(p_hash);
50
51 MutexLocker mu_r(Compile_lock, THREAD);
52
53 // Add to class hierarchy, initialize vtables, and do possible
54 // deoptimizations.
55 add_to_hierarchy(k, CHECK); // No exception, but can block
56
57 // Add to systemDictionary - so other classes can see it.
58 // Grabs and releases SystemDictionary_lock
59 update_dictionary(d_index, d_hash, p_index, p_hash,
60 k, class_loader_h, THREAD);
61 }
62 k->eager_initialize(THREAD);
63
64 // notify jvmti
65 if (JvmtiExport::should_post_class_load()) {
66 assert(THREAD->is_Java_thread(), "thread->is_Java_thread()");
67 JvmtiExport::post_class_load((JavaThread *) THREAD, k());
68
69 }
70
71 }
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：
1 void InstanceKlass::eager_initialize(Thread *thread) {
2 if (!EagerInitialization) return;
3
4 if (this->is_not_initialized()) {
5 // abort if the the class has a class initializer
6 if (this->class_initializer() != NULL) return;
7
8 // abort if it is java.lang.Object (initialization is handled in genesis)
9 Klass* super = this->super();
10 if (super == NULL) return;
11
12 // abort if the super class should be initialized
13 if (!InstanceKlass::cast(super)->is_initialized()) return;
14
15 // call body to expose the this pointer
16 instanceKlassHandle this_oop(thread, this);
17 eager_initialize_impl(this_oop);
18 }
19 }
我们接着进入 eager_initialize_impl，该方法进入到了 InstanceKlass：
1 void InstanceKlass::eager_initialize_impl(instanceKlassHandle this_oop) {
2 EXCEPTION_MARK;
3 oop init_lock = this_oop->init_lock();
4 ObjectLocker ol(init_lock, THREAD, init_lock != NULL);
5
6 // abort if someone beat us to the initialization
7 if (!this_oop->is_not_initialized()) return; // note: not equivalent to is_initialized()
8
9 ClassState old_state = this_oop->init_state();
10 link_class_impl(this_oop, true, THREAD);
11 if (HAS_PENDING_EXCEPTION) {
12 CLEAR_PENDING_EXCEPTION;
13 // Abort if linking the class throws an exception.
14
15 // Use a test to avoid redundantly resetting the state if there's
16 // no change. Set_init_state() asserts that state changes make
17 // progress, whereas here we might just be spinning in place.
18 if( old_state != this_oop->_init_state )
19 this_oop->set_init_state (old_state);
20 } else {
21 // linking successfull, mark class as initialized
22 this_oop->set_init_state (fully_initialized);
23 this_oop->fence_and_clear_init_lock();
24 // trace
25 if (TraceClassInitialization) {
26 ResourceMark rm(THREAD);
27 tty->print_cr("[Initialized %s without side effects]", this_oop->external_name());
28 }
29 }
30 }
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，
1 oop InstanceKlass::init_lock() const {
2 // return the init lock from the mirror
3 oop lock = java_lang_Class::init_lock(java_mirror());
4 // Prevent reordering with any access of initialization state
5 OrderAccess::loadload();
6 assert((oop)lock != NULL || !is_not_initialized(), // initialized or in_error state
7 "only fully initialized state can have a null lock");
8 return lock;
9 }
其中，java_mirror() 方法就是返回 Klass 类中的以下字段：
1 // java/lang/Class instance mirroring this class
2 oop _java_mirror;
再看 init_lock 方法：
1 oop java_lang_Class::init_lock(oop java_class) {
2 assert(_init_lock_offset != 0, "must be set");
3 return java_class->obj_field(_init_lock_offset);
4 }
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：
1 ObjectLocker ol(init_lock, THREAD, init_lock != NULL);
1 / ObjectLocker enforced balanced locking and can never thrown an
2 // IllegalMonitorStateException. However, a pending exception may
3 // have to pass through, and we must also be able to deal with
4 // asynchronous exceptions. The caller is responsible for checking
5 // the threads pending exception if needed.
6 // doLock was added to support classloading with UnsyncloadClass which
7 // requires flag based choice of locking the classloader lock.
8 class ObjectLocker : public StackObj {
9 private:
10 Thread* _thread;
11 Handle _obj;
12 BasicLock _lock;
13 bool _dolock; // default true
14 public:
15 ObjectLocker(Handle obj, Thread* thread, bool doLock = true);
1 // -----------------------------------------------------------------------------
2 // Internal VM locks on java objects
3 // standard constructor, allows locking failures
4 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
5 _dolock = doLock;
6 _thread = thread;
8 _obj = obj;
9
10 if (_dolock) {
11 TEVENT (ObjectLocker) ;
12
13 ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
14 }
15 }
接下来会进入到 synchronizer.cpp，
1 // -----------------------------------------------------------------------------
2 // Fast Monitor Enter/Exit
3 // This the fast monitor enter. The interpreter and compiler use
4 // some assembly copies of this code. Make sure update those code
5 // if the following function is changed. The implementation is
6 // extremely sensitive to race condition. Be careful.
7
8 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
9 if (UseBiasedLocking) {
10 if (!SafepointSynchronize::is_at_safepoint()) {
11 BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
12 if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
13 return;
14 }
15 } else {
16 assert(!attempt_rebias, "can not rebias toward VM thread");
17 BiasedLocking::revoke_at_safepoint(obj);
18 }
19 assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
20 }
21
22 slow_enter (obj, lock, THREAD) ;
23 }
上面会判断，是否使用偏向锁，如果不使用，则走 slow_enter 。
1 // -----------------------------------------------------------------------------
2 // Interpreter/Compiler Slow Case
3 // This routine is used to handle interpreter/compiler slow case
4 // We don't need to use fast path here, because it must have been
5 // failed in the interpreter/compiler code.
6 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
7 markOop mark = obj->mark();
8 assert(!mark->has_bias_pattern(), "should not see bias pattern here");
9
10 if (mark->is_neutral()) {
11 // Anticipate successful CAS -- the ST of the displaced mark must
12 // be visible <= the ST performed by the CAS.
13 lock->set_displaced_header(mark);
14 if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
15 TEVENT (slow_enter: release stacklock) ;
16 return ;
17 }
18 // Fall through to inflate() ...
19 } else
20 if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
21 assert(lock != mark->locker(), "must not re-lock the same lock");
22 assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
23 lock->set_displaced_header(NULL);
24 return;
25 }
26
34
35 // The object header will never be displaced to this lock,
36 // so it does not matter what the value is, except that it
37 // must be non-zero to avoid looking like a re-entrant lock,
38 // and must not look locked either.
39 lock->set_displaced_header(markOopDesc::unused_mark());
40 ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
41 }
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、总结
这里再说下结论吧，类初始化的过程，会对class加锁，再执行class的初始化，如果这时候发生了循环依赖，就会导致死锁。
如果有读者对上面的c++代码感兴趣，可以参考下面的文章，搭建调试环境：
源码编译OpenJdk 8，Netbeans调试Java原子类在JVM中的实现（Ubuntu 16.04）