在操作系统中，有两种不同的方法提供线程支持：用户层的用户线程，或内核层的内核线程。
其中用户线程在内核之上支持，并在用户层通过线程库来实现。不需要用户态/核心态切换，速度快。操作系统内核不知道多线程的存在，因此一个线程阻塞将使得整个进程（包括它的所有线程）阻塞。由于这里的处理器时间片分配是以进程为基本单位，所以每个线程执行的时间相对减少。
内核线程由操作系统直接支持。由操作系统内核创建、调度和管理。内核维护进程及线程的上下文信息以及线程切换。一个内核线程由于I/O操作而阻塞，不会影响其它线程的运行。
Java线程的实现是怎样的呢？我们通过SUN Java 6的源码了解其在Windows和Linux下的实现。
在Windows下的实现，os_win32.cpp中
// Allocate and initialize a new OSThread
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
unsigned thread_id;
// Allocate the OSThread object
OSThread* osthread = new OSThread(NULL, NULL);
if (osthread == NULL) {
return false;
}
// Initial state is ALLOCATED but not INITIALIZED
{
MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag);
osthread->set_state(ALLOCATED);
}
// Initialize support for Java interrupts
HANDLE interrupt_event = CreateEvent(NULL, true, false, NULL);
if (interrupt_event == NULL) {
delete osthread;
return NULL;
}
osthread->set_interrupt_event(interrupt_event);
osthread->set_interrupted(false);
thread->set_osthread(osthread);
if (stack_size == 0) {
switch (thr_type) {
case os::java_thread:
// Java threads use ThreadStackSize which default value can be changed with the flag -Xss
if (JavaThread::stack_size_at_create() > 0)
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;
}
}
// Create the Win32 thread
//
// Contrary to what MSDN document says, "stack_size" in _beginthreadex()
// does not specify stack size. Instead, it specifies the size of
// initially committed space. The stack size is determined by
// PE header in the executable. If the committed "stack_size" is larger
// than default value in the PE header, the stack is rounded up to the
// nearest multiple of 1MB. For example if the launcher has default
// stack size of 320k, specifying any size less than 320k does not
// affect the actual stack size at all, it only affects the initial
// commitment. On the other hand, specifying 'stack_size' larger than
// default value may cause significant increase in memory usage, because
// not only the stack space will be rounded up to MB, but also the
// entire space is committed upfront.
//
// Finally Windows XP added a new flag 'STACK_SIZE_PARAM_IS_A_RESERVATION'
// for CreateThread() that can treat 'stack_size' as stack size. However we
// are not supposed to call CreateThread() directly according to MSDN
// document because JVM uses C runtime library. The good news is that the
// flag appears to work with _beginthredex() as well.
#ifndef STACK_SIZE_PARAM_IS_A_RESERVATION
#define STACK_SIZE_PARAM_IS_A_RESERVATION (0x10000)
#endif
HANDLE thread_handle =
(HANDLE)_beginthreadex(NULL,
(unsigned)stack_size,
(unsigned (__stdcall *)(void*)) java_start,
thread,
CREATE_SUSPENDED | STACK_SIZE_PARAM_IS_A_RESERVATION,
&thread_id);
if (thread_handle == NULL) {
// perhaps STACK_SIZE_PARAM_IS_A_RESERVATION is not supported, try again
// without the flag.
thread_handle =
(HANDLE)_beginthreadex(NULL,
(unsigned)stack_size,
(unsigned (__stdcall *)(void*)) java_start,
thread,
CREATE_SUSPENDED,
&thread_id);
}
if (thread_handle == NULL) {
// Need to clean up stuff we've allocated so far
CloseHandle(osthread->interrupt_event());
thread->set_osthread(NULL);
delete osthread;
return NULL;
}
Atomic::inc_ptr((intptr_t*)&os::win32::_os_thread_count);
// Store info on the Win32 thread into the OSThread
osthread->set_thread_handle(thread_handle);
osthread->set_thread_id(thread_id);
// Initial thread state is INITIALIZED, not SUSPENDED
{
MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag);
osthread->set_state(INITIALIZED);
}
// The thread is returned suspended (in state INITIALIZED), and is started higher up in the call chain
return true;
}
可以看出，SUN JVM在Windows下的实现，使用_beginthreadex来创建线程，注释中也说明了为什么不用“Window编程书籍推荐使用”的CreateThread函数。由此看出，Java线程在Window下的实现是使用内核线程。
而在Linux下又是怎样的呢？
在os_linux.cpp文件中的代码摘录如下：
# include <pthread.h>
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
assert(thread->osthread() == NULL, "caller responsible");
// Allocate the OSThread object
OSThread* osthread = new OSThread(NULL, NULL);
if (osthread == NULL) {
return false;
}
// set the correct thread state
osthread->set_thread_type(thr_type);
// Initial state is ALLOCATED but not INITIALIZED
osthread->set_state(ALLOCATED);
thread->set_osthread(osthread);
// init thread attributes
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
// 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
if (JavaThread::stack_size_at_create() > 0) 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.
}
// glibc guard page
pthread_attr_setguardsize(&attr, os::Linux::default_guard_size(thr_type));
ThreadState state;
{
// 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();
return false;
}
// 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;
return false;
}
// The thread is returned suspended (in state INITIALIZED),
// and is started higher up in the call chain
assert(state == INITIALIZED, "race condition");
return true;
}
Java在Linux下的线程的创建，使用了pthread线程库，而pthread就是一个用户线程库，因此结论是，Java在Linux下是使用用户线程实现的。