// We cannot hold the Threads_lock when we throw an exception, // due to rank ordering issues. Example: we might need to grab the // Heap_lock while we construct the exception. bool throw_illegal_thread_state = false;
// We must release the Threads_lock before we can post a jvmti event // in Thread::start. { // Ensure that the C++ Thread and OSThread structures aren't freed before // we operate. MutexLocker mu(Threads_lock);
// Since JDK 5 the java.lang.Thread threadStatus is used to prevent // re-starting an already started thread, so we should usually find // that the JavaThread is null. However for a JNI attached thread // there is a small window between the Thread object being created // (with its JavaThread set) and the update to its threadStatus, so we // have to check for this if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) { throw_illegal_thread_state = true; } else { // We could also check the stillborn flag to see if this thread was already stopped, but // for historical reasons we let the thread detect that itself when it starts running
jlong size = java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread)); // Allocate the C++ Thread structure and create the native thread. The // stack size retrieved from java is signed, but the constructor takes // size_t (an unsigned type), so avoid passing negative values which would // result in really large stacks. size_t sz = size > 0 ? (size_t) size : 0; native_thread = newJavaThread(&thread_entry, sz);
// At this point it may be possible that no osthread was created for the // JavaThread due to lack of memory. Check for this situation and throw // an exception if necessary. Eventually we may want to change this so // that we only grab the lock if the thread was created successfully - // then we can also do this check and throw the exception in the // JavaThread constructor. if (native_thread->osthread() != NULL) { // Note: the current thread is not being used within "prepare". native_thread->prepare(jthread); } } }
if (throw_illegal_thread_state) { THROW(vmSymbols::java_lang_IllegalThreadStateException()); }
if (native_thread->osthread() == NULL) { // No one should hold a reference to the 'native_thread'. delete native_thread; if (JvmtiExport::should_post_resource_exhausted()) { JvmtiExport::post_resource_exhausted( JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_THREADS, "unable to create new native thread"); } THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(), "unable to create new native thread"); }
Thread::start(native_thread);
JVM_END
可以注意到有native_thread->osthread() == NULL 这个判断，如果osthread为空了会抛出OutOfMemoryError 核心点在于native_thread = new JavaThread(&thread_entry, sz); 创建了一个新的线程
JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) : Thread() #if INCLUDE_ALL_GCS , _satb_mark_queue(&_satb_mark_queue_set), _dirty_card_queue(&_dirty_card_queue_set) #endif// INCLUDE_ALL_GCS { if (TraceThreadEvents) { tty->print_cr("creating thread %p", this); } initialize(); _jni_attach_state = _not_attaching_via_jni; set_entry_point(entry_point); // Create the native thread itself. // %note runtime_23 os::ThreadType thr_type = os::java_thread; thr_type = entry_point == &compiler_thread_entry ? os::compiler_thread : os::java_thread; os::create_thread(this, thr_type, stack_sz); // The _osthread may be NULL here because we ran out of memory (too many threads active). // We need to throw and OutOfMemoryError - however we cannot do this here because the caller // may hold a lock and all locks must be unlocked before throwing the exception (throwing // the exception consists of creating the exception object & initializing it, initialization // will leave the VM via a JavaCall and then all locks must be unlocked). // // The thread is still suspended when we reach here. Thread must be explicit started // by creator! Furthermore, the thread must also explicitly be added to the Threads list // by calling Threads:add. The reason why this is not done here, is because the thread // object must be fully initialized (take a look at JVM_Start) }
// stack size if (os::Linux::supports_variable_stack_size()) { // calculate stack size if it's not specified by caller if (stack_size == 0) { stack_size = os::Linux::default_stack_size(thr_type);
switch (thr_type) { case os::java_thread: // Java threads use ThreadStackSize which default value can be // changed with the flag -Xss assert (JavaThread::stack_size_at_create() > 0, "this should be set"); stack_size = JavaThread::stack_size_at_create(); break; case os::compiler_thread: if (CompilerThreadStackSize > 0) { stack_size = (size_t)(CompilerThreadStackSize * K); break; } // else fall through: // use VMThreadStackSize if CompilerThreadStackSize is not defined case os::vm_thread: case os::pgc_thread: case os::cgc_thread: case os::watcher_thread: if (VMThreadStackSize > 0) stack_size = (size_t)(VMThreadStackSize * K); break; } }
stack_size = MAX2(stack_size, os::Linux::min_stack_allowed); pthread_attr_setstacksize(&attr, stack_size); } else { // let pthread_create() pick the default value. }
{ // Serialize thread creation if we are running with fixed stack LinuxThreads bool lock = os::Linux::is_LinuxThreads() && !os::Linux::is_floating_stack(); if (lock) { os::Linux::createThread_lock()->lock_without_safepoint_check(); }
pthread_t tid; int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
pthread_attr_destroy(&attr);
if (ret != 0) { if (PrintMiscellaneous && (Verbose || WizardMode)) { perror("pthread_create()"); } // Need to clean up stuff we've allocated so far thread->set_osthread(NULL); delete osthread; if (lock) os::Linux::createThread_lock()->unlock(); returnfalse; }
// Store pthread info into the OSThread osthread->set_pthread_id(tid);
// Wait until child thread is either initialized or aborted { Monitor* sync_with_child = osthread->startThread_lock(); MutexLockerEx ml(sync_with_child, Mutex::_no_safepoint_check_flag); while ((state = osthread->get_state()) == ALLOCATED) { sync_with_child->wait(Mutex::_no_safepoint_check_flag); } }
if (lock) { os::Linux::createThread_lock()->unlock(); } }
// Aborted due to thread limit being reached if (state == ZOMBIE) { thread->set_osthread(NULL); delete osthread; returnfalse; }
// The thread is returned suspended (in state INITIALIZED), // and is started higher up in the call chain assert(state == INITIALIZED, "race condition"); returntrue; }