咱们都晓得，一个线程间接对应了一个Thread对象,在刚开始学习线程的时候咱们也晓得启动线程是通过start()办法,而并非run()办法。
那这是为什么呢？
如果你相熟Thread的代码的话,你应该晓得在这个类加载的时候会注册一些native办法
publicclass Thread implements Runnable { /* Make sure registerNatives is the first thing <clinit> does. */ private static native void registerNatives(); static { registerNatives(); }}复制代码一看到native我就想起了JNI,registerNatives()实际上就是java办法和C/C++的函数对应。在首次加载的时候就会注册这些native办法。Thread中有很多native办法，大家有趣味的能够去看看。
对于JNI办法的命名,咱们能够这样测试，咱们用java申明一个native办法，而后先应用javac编译源文件(比方javac main.java),而后在应用javah即可生成头文件(javah main),关上这个头文件你就晓得办法命名是如何的了
咱们在JVM源码中搜寻Java_java_lang_Thread_registerNatives能够看到registerNatives办法的具体实现
static JNINativeMethod methods[] = { {"start0", "()V", (void *)&JVM_StartThread}, {"stop0", "(" OBJ ")V", (void *)&JVM_StopThread}, {"isAlive", "()Z", (void *)&JVM_IsThreadAlive}, {"suspend0", "()V", (void *)&JVM_SuspendThread}, {"resume0", "()V", (void *)&JVM_ResumeThread}, {"setPriority0", "(I)V", (void *)&JVM_SetThreadPriority}, {"yield", "()V", (void *)&JVM_Yield}, {"sleep", "(J)V", (void *)&JVM_Sleep}, {"currentThread", "()" THD, (void *)&JVM_CurrentThread}, {"countStackFrames", "()I", (void *)&JVM_CountStackFrames}, {"interrupt0", "()V", (void *)&JVM_Interrupt}, {"isInterrupted", "(Z)Z", (void *)&JVM_IsInterrupted}, {"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock}, {"getThreads", "()[" THD, (void *)&JVM_GetAllThreads}, {"dumpThreads", "([" THD ")[[" STE, (void *)&JVM_DumpThreads}, {"setNativeName", "(" STR ")V", (void *)&JVM_SetNativeThreadName},};JNIEXPORT void JNICALLJava_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls){ (*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));}复制代码能够看到，在registerNatives函数中，注册了很多的native办法比方这里的start0()办法。
所有对JNI函数的调用都应用了env指针,该指针是对每一个本地办法的第一个参数。env指针是函数指针表的指针。咱们能够在docs.oracle.com/javase/8/do…中找到JNI API
在Thread.start()办法中，理论就是通过调用start0()办法来启动线程的。
public synchronized void start() { if (threadStatus != 0) throw new IllegalThreadStateException(); group.add(this); boolean started = false; try { // 次要调用了start0()这个native办法来启动线程 start0(); started = true; } finally { try { if (!started) { group.threadStartFailed(this); } } catch (Throwable ignore) { /* do nothing. If start0 threw a Throwable then it will be passed up the call stack */ } }}private native void start0();复制代码而JNINativeMethod这个数据结构定义如下:
typedef struct { char *name; char *signature; void *fnPtr;}复制代码因而start0()这个办法对应的本地函数是JVM_StartThread
{"start0", "()V", (void *)&JVM_StartThread}复制代码咱们接下来看JVM_StartThread的办法实现
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) JVMWrapper("JVM_StartThread"); JavaThread *native_thread = NULL; bool throw_illegal_thread_state = false; { // Ensure that the C++ Thread and OSThread structures aren't freed before // we operate. MutexLocker mu(Threads_lock); // 从JDK5开始，应用java.lang.Thread threadStatus来避免重新启动一个曾经启动的线程 if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) { throw_illegal_thread_state = true; } else { jlong size = java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread)); NOT_LP64(if (size > SIZE_MAX) size = SIZE_MAX;) size_t sz = size > 0 ? (size_t) size : 0; // 创立Java线程 native_thread = new JavaThread(&thread_entry, sz); if (native_thread->osthread() != NULL) { native_thread->prepare(jthread); } } } if (throw_illegal_thread_state) { THROW(vmSymbols::java_lang_IllegalThreadStateException()); } // 省略了局部代码 // 将线程状态设置为Runnable,示意能够被运行 Thread::start(native_thread);JVM_END复制代码下面代码次要做了三件事件
- 判断以后线程状态是否非法，不非法抛出IllegalThreadStateException
- 创立一个Java线程(咱们须要重点关注的)
- 将线程状态设置为Runnable
如果面试官当前再问你两次调用start()办法会怎么,你就大胆而动摇的回复说抛出IllegalThreadStateException。
在JavaThread构造函数中理论调用的是os::create_thread办法
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t req_stack_size) { OSThread* osthread = new OSThread(NULL, NULL); if (osthread == NULL) { return false; } osthread->set_thread_type(thr_type); 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); size_t stack_size = os::Posix::get_initial_stack_size(thr_type, req_stack_size); int status = pthread_attr_setstacksize(&attr, stack_size); ThreadState state; { pthread_t tid; // 创立线程 int ret = pthread_create(&tid, &attr, (void* (*)(void*)) thread_native_entry, thread); // 省略其余代码... } return true;}复制代码pthread_create函数作用是创立一个线程,它的第三个参数是线程运行函数的起始地址,第四个参数是运行函数参数
IEEE规范1003.1c中定义了线程的规范,它定义的线程包叫做Pthread,大部分UNIX零碎都反对这个规范。
咱们的thread_native_entry理论传入的是JavaThread这个对象,所以最终会调用JavaThread::run()(thread.cpp中)
void JavaThread::run() { ... thread_main_inner(); ...}void JavaThread::thread_main_inner() { ... this->entry_point()(this,this); ...}复制代码thread_main_inner函数中entry_point的返回值实际上是咱们在创立JavaThread的时候传入的第一个参数thread_entry。而thread_entry指针指向的函数如下:
static void thread_entry(JavaThread* thread, TRAPS) { HandleMark hm(THREAD); Handle obj(THREAD, thread->threadObj()); JavaValue result(T_VOID); JavaCalls::call_virtual(&result, obj, SystemDictionary::Thread_klass(), // run办法名称run vmSymbols::run_method_name(), // 办法签名()V vmSymbols::void_method_signature(), THREAD);}复制代码这样咱们就最终通过JavaCalls调用了run办法。
总结
new Thread只是创立了一个一般的Java对象,只有在调用了start()办法之后才会创立一个真正的线程,在JVM外部会在创立线程之后调用run()办法,执行相应的业务逻辑。
因为Java中线程最终还是和操作系统线程挂钩了的,所以线程资源是一个很重要的资源,为了复用咱们个别是通过线程池的形式来应用线程。
参考：《2020最新Java根底精讲视频教程和学习路线！》
链接：https://juejin.cn/post/685805...