小さな栗 1個
スレッドの小さな栗を書くだけです:
public class ThreadTest extends Thread {
@Override
public void run() {
System.out.println("In run");
}
public static void main(String []argv) {
(new ThreadTest()).start();
}
}
2 start()メソッドのソースコード解析
2.1 start() ソースコード
Java start() メソッドのソース コードを表示します。
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 then
it will be passed up the call stack */
}
}
}
2.2 start0() メソッド
// start()方法调用start0()方法
// native方法 调用底层C++方法
private native void start0();
2.3 基礎となる C++ ソース コード
さらに低レベルの C++ コード:
- thread.c (コードの場所: openJDK\src\share\native\java\lang)
- jvm.cpp (コードの場所: openIDK\hotspot\src\share\vm\prims)
- thread.cpp (コードの場所: openJDK\hotspot\src\share\vm\runtime)
コード - OpenJDK ソース URL: http://openjdk.java.net/
2.3.1 thread.c
Thread.java は thread.c ソース コードに対応します
.c のソース コードから、start0() が実際には JVM_StartThread=>find jvm.cpp であることがわかります。
#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 STR
JNIEXPORT 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 のソース コードの一部:
主に、JVM_StartThread メソッドの最初の行と Thread::start(native_thread) の最後の行がスレッドを開始することがわかります。
// 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_JFR
if (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 ソースコード
最後に、start メソッドがオペレーティング システム レベルで開始されます。
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);
}
}