1 /**
使此线程开始执行；Java虚拟机调用此线程的<code>run</code>方法。 2 * Causes this thread to begin execution; the Java Virtual Machine 3 * calls the <code>run</code> method of this thread. 4 * <p>
结果是两个线程同时运行：当前线程（从对<code>start</code>方法的调用返回）和另一个线程（执行其<code>run</code>方法）。 5 * The result is that two threads are running concurrently: the 6 * current thread (which returns from the call to the 7 * <code>start</code> method) and the other thread (which executes its 8 * <code>run</code> method). 9 * <p>
多次启动线程是不合法的。 10 * It is never legal to start a thread more than once.
特别是，线程一旦完成执行就不能重新启动。 11 * In particular, a thread may not be restarted once it has completed 12 * execution. 13 *
异常IllegalThreadStateException（如果线程已启动）。 14 * @exception IllegalThreadStateException if the thread was already 15 * started. 16 * @see #run() 17 * @see #stop() 18 */ 19 public synchronized void start() { 20 /** 21 * This method is not invoked for the main method thread or "system" 22 * group threads created/set up by the VM. Any new functionality added 23 * to this method in the future may have to also be added to the VM. 24 * 25 * A zero status value corresponds to state "NEW". 26 */
零状态值对应于状态“NEW”。 27 if (threadStatus != 0) 28 throw new IllegalThreadStateException(); 29
通知组此线程即将启动，以便可以将其添加到组的线程列表中，并且可以减少组的未启动计数。 30 /* Notify the group that this thread is about to be started 31 * so that it can be added to the group's list of threads 32 * and the group's unstarted count can be decremented. */ 33 group.add(this); 34 35 boolean started = false; 36 try { 37 start0(); 38 started = true; 39 } finally { 40 try { 41 if (!started) { 42 group.threadStartFailed(this); 43 } 44 } catch (Throwable ignore) {
什么都不做如果start0抛出了一个throwable，那么它将被传递到调用堆栈上 45 /* do nothing. If start0 threw a Throwable then 46 it will be passed up the call stack */ 47 } 48 } 49 } 50 51 private native void start0();
start0调用jvm的JVM_StartThread函数
1 JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) 2 3 JVMWrapper("JVM_StartThread"); 4 JavaThread *native_thread = NULL; 5 bool throw_illegal_thread_state = false; 6 { 7 MutexLocker mu(Threads_lock); 8 9 if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) { 10 11 throw_illegal_thread_state = true; 12 13 } else { 14 15 jlong size = 16 17 java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread)); 18 19 20 size_t sz = size > 0 ? (size_t) size : 0; 21 关注这里，创建新线程 22 native_thread = new JavaThread(&thread_entry, sz); 23 24 if (native_thread->osthread() != NULL) { 25 native_thread->prepare(jthread); 26 } 27 } 28 } 29 30 31 if (throw_illegal_thread_state) { 32 33 THROW(vmSymbols::java_lang_IllegalThreadStateException()); 34 35 } 36 37 assert(native_thread != NULL, "Starting null thread?"); 38 39 if (native_thread->osthread() == NULL) { 40 41 // No one should hold a reference to the 'native_thread'. 42 43 delete native_thread; 44 45 if (JvmtiExport::should_post_resource_exhausted()) { 46 47 JvmtiExport::post_resource_exhausted( 48 49 JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_THREADS, 50 51 "unable to create new native thread"); 52 53 } 54 55 THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(), 56 57 "unable to create new native thread"); 58 59 } 60 61 62 Thread::start(native_thread); 63 64 65 JVM_END
1 static void thread_entry(JavaThread* thread, TRAPS) { 2 3 HandleMark hm(THREAD); 4 5 Handle obj(THREAD, thread->threadObj()); 6 7 JavaValue result(T_VOID); 8
调用run方法
9 JavaCalls::call_virtual(&result, 10 11 obj, 12 13 KlassHandle(THREAD, SystemDictionary::Thread_klass()), 14 15 vmSymbols::run_method_name(), 16 17 vmSymbols::void_method_signature(), 18 19 THREAD); 20 21 }
总结：
1：JVM通过new JavaThread(&thread_entry, sz); 创建一个系统内核线程。
2：在内核线程的初始运行方法中，利用JavaCalls模块，调用java线程的run（）方法，开始java级别的线程执行。
