java线程启动代码
上次写了个《Thread的调用》过于简单，在这以javaThread的启动过程做一个详细分析。先从java/lang/Thread.java看起
public synchronized void start() {
.....
start0(); // start0为本地方法 private native void start0();
.....
}
上面start0的具体实现是jvm.cpp中的JVM_StartThread。
......
native_thread = new JavaThread(&thread_entry, sz);
if (native_thread->osthread() != NULL) {
native_thread->prepare(jthread);
}
.....
Thread::start(native_thread);
上面传入函数指针thread_entry，thread_entry就是调用Thread.java中的run方法，这就是java程序中只需要new Thread().start，虚拟机就调用run方法的原因。
static void thread_entry(JavaThread* thread, TRAPS) {
HandleMark hm(THREAD);
Handle obj(THREAD, thread->threadObj());
JavaValue result(T_VOID);
JavaCalls::call_virtual(&result,
obj,
KlassHandle(THREAD, SystemDictionary::thread_klass()),
vmSymbolHandles::run_method_name(), //调用用户编写thread类run方法
vmSymbolHandles::void_method_signature(),
THREAD);
}
再看new JavaThread(...)这步。
JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) : Thread() {
.........
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);
}
到了os::create_thread这步。
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
OSThread* osthread = new OSThread(NULL, NULL);
thread->set_osthread(osthread);
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);
//上面创建线程的时候，线程不自动运行，而是堵塞，直到上面的Thread::start(native_thread)才开始运行
.............
}
再看启动函数java_start。
static unsigned __stdcall java_start(Thread* thread) {
__try {
thread->run(); //注意：传入的是javaThread，所以在这调用javaThread::run
} __except(topLevelExceptionFilter(
(_EXCEPTION_POINTERS*)_exception_info())) {
}
}
查看
void JavaThread::run() {
.......
thread_main_inner();//在里面真正调用用户写的java类run方法
}
void JavaThread::thread_main_inner() {
this->entry_point()(this, this); //调用传入thread_entry
//退出
this->exit(false);
delete this;
}
最后看一下
void Thread::start(Thread* thread) {
........
os::start_thread(thread); //运行创建时堵塞的线程
}
void os::start_thread(Thread* thread) {
.........
OSThread* osthread = thread->osthread();
osthread->set_state(RUNNABLE);
pd_start_thread(thread);
}
void os::pd_start_thread(Thread* thread) {
//真正恢复操作系统线程运行的代码
DWORD ret = ResumeThread(thread->osthread()->thread_handle());
}
由上面可以看到每个继承java.lang.Thread的类，调用start之后，都和一个操作系统的线程进行绑定。
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