1 一个小栗子

简单的写一个线程的小栗子：

public class ThreadTest extends Thread {@Overridepublic void run() {System.out.println("In run");}public static void main(String []argv) {(new ThreadTest()).start();}
}

2 start()方法源码分析

2.1 start()源码

查看java start()方法的源码：

    public synchronized void start() {/* This method is not invoked for the main method thread or "system"* group threads created/set up by the VM. Any new functionality added* to this method in the future may have to also be added to the VM. A zero status value corresponds to state "NEW".*/if (threadStatus != 0)throw new IllegalThreadStateException();/* Notify the group that this thread is about to be started* so that it can be added to the group's list of threads* and the group's unstarted count can be decremented. */group.add(this);boolean started = false;try {// 主要是这个函数start0();// started=true表示线程运行成功started = true;} finally {try {if (!started) {group.threadStartFailed(this);}} catch (Throwable ignore) {/* do nothing. If start0 threw a Throwable thenit will be passed up the call stack */}}}

2.2 start0()方法

	// start()方法调用start0()方法// native方法 调用底层C++方法private native void start0();

2.3 底层C++源码

更加底层的C++代码：

• thread.c (代码位置：openJDK\\src\\share\\native\\java\\lang)
• jvm.cpp (代码位置：openIDK\\hotspot\\src\\share\\vm\\prims)
• thread.cpp (代码位置：openJDK\\hotspot\\src\\share\\vm\\runtime)

代码——OpenJDK源码网址：http://openjdk.java.net/

2.3.1 thread.c

Thread.java对应thread.c源码

从.c的源码中可以看出start0()其实就是JVM_StartThread=>找到jvm.cpp

#include "jni.h"
#include "jvm.h"#include "java_lang_Thread.h"#define THD "Ljava/lang/Thread;"
#define OBJ "Ljava/lang/Object;"
#define STE "Ljava/lang/StackTraceElement;"
#define STR "Ljava/lang/String;"#define ARRAY_LENGTH(a) (sizeof(a)/sizeof(a[0]))static JNINativeMethod methods[] = {// 在这里{"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},
};#undef THD
#undef OBJ
#undef STE
#undef STRJNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{(*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}

2.3.2 jvm.cpp

jvm.cpp部分源码：
主要知道第一行JVM_StartThread方法和最后一行Thread::start(native_thread)启动了线程；

// JVM_StartThread方法
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))JVMWrapper("JVM_StartThread");JavaThread *native_thread = NULL;bool throw_illegal_thread_state = false;{MutexLocker mu(Threads_lock);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));size_t sz = size > 0 ? (size_t) size : 0;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());}assert(native_thread != NULL, "Starting null thread?");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");}#if INCLUDE_JFRif (JfrRecorder::is_recording() && EventThreadStart::is_enabled() &&EventThreadStart::is_stacktrace_enabled()) {JfrThreadLocal* tl = native_thread->jfr_thread_local();// skip Thread.start() and Thread.start0()tl->set_cached_stack_trace_id(JfrStackTraceRepository::record(thread, 2));}
#endif// 启动线程Thread::start(native_thread);

2.3.3 thread.cpp源码

最终是操作系统级别启动了start方法

void Thread::start(Thread* thread) {trace("start", thread);// Start is different from resume in that its safety is guaranteed by context or// being called from a Java method synchronized on the Thread object.if (!DisableStartThread) {// 初始化方法 if (thread->is_Java_thread()) {// Initialize the thread state to RUNNABLE before starting this thread.// Can not set it after the thread started because we do not know the// exact thread state at that time. It could be in MONITOR_WAIT or// in SLEEPING or some other state.java_lang_Thread::set_thread_status(((JavaThread*)thread)->threadObj(),java_lang_Thread::RUNNABLE);}// 最终是操作系统级别提高了start方法os::start_thread(thread);}
}