java系列 - Thread

疑问

Java的线程是如何创建的,是直接调用OS的API,还是有自己的“抽象线程”?

java线程是映射到操作系统的内核线程上的

跟踪Thread.start()

java

// Thread.java
public synchronized void start() {
    group.add(this);
    ...
    start0();
    ...
}

private native void start0();

native方法hotspot源码

// openjdk\jdk\src\share\native\java\lang\Thread.c
static JNINativeMethod methods[] = {
    {"start0",           "()V",        (void *)&JVM_StartThread},
   ...
};


// openjdk\hotspot\src\share\vm\prims\jvm.cpp
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
  JVMWrapper("JVM_StartThread");
  JavaThread *native_thread = NULL;
  ...
  native_thread = new JavaThread(&thread_entry, sz); // 重点
  ...
  Thread::start(native_thread); // 重点
JVM_END



// openjdk\hotspot\src\share\vm\runtime\thread.cpp
JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) :
  Thread() {
   ...
   os::create_thread(this, thr_type, stack_sz);
   // 可以看出java线程是映射到操作系统的内核线程上的
   ...
}

void Thread::start(Thread* thread) {
    ...
    os::start_thread(thread);
  }
}


// 在hotspot\src\os目录下可以看到windows, linux, solaris和posix的实现,先检查linux\vm\os_linux.cpp

bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
  ...
  int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);  // linux中调用的pthread,POSIX中的api
  ...
}