【源码】Thread相关native方法

Thread.registerNatives()

registerNatives()方法的作用是将本地方法注册JVM中，使得在JVM运行中可以调用到本地方法，如start0()。

//Thread.h
JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}
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},
};

start0()

当前线程调用start0()方法，使线程进入就绪状态，等待线程调度器的调度。从上面本地方法注册中可以看到start0()对应着JVM_StartThread方法

//jvm.cpp
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);//这里通过thread_entry回调了run()方法
      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) {
    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");
  }

  Thread::start(native_thread);

JVM_END

static void thread_entry(JavaThread* thread, TRAPS) {
  HandleMark hm(THREAD);
  Handle obj(THREAD, thread->threadObj());
  JavaValue result(T_VOID);
  JavaCalls::call_virtual(&result,
                          obj,
                          KlassHandle(THREAD, SystemDictionary::Thread_klass()),
                          vmSymbols::run_method_name(),//这里
                          vmSymbols::void_method_signature(),
                          THREAD);
}

//vmSymbols.hpp
template(run_method_name,                           "run") \
class vmSymbols: AllStatic {
  friend class vmIntrinsics;
  friend class VMStructs;
 public:
  // enum for figuring positions and size of array holding Symbol*s
  enum SID {
    NO_SID = 0,

    #define VM_SYMBOL_ENUM(name, string) VM_SYMBOL_ENUM_NAME(name),
    VM_SYMBOLS_DO(VM_SYMBOL_ENUM, VM_ALIAS_IGNORE)
    #undef VM_SYMBOL_ENUM

    SID_LIMIT,

    #define VM_ALIAS_ENUM(name, def) VM_SYMBOL_ENUM_NAME(name) = VM_SYMBOL_ENUM_NAME(def),
    VM_SYMBOLS_DO(VM_SYMBOL_IGNORE, VM_ALIAS_ENUM)
    #undef VM_ALIAS_ENUM

    FIRST_SID = NO_SID + 1
  };
  enum {
    log2_SID_LIMIT = 10         // checked by an assert at start-up
  };

 private:
  // The symbol array
  static Symbol* _symbols[];

  // Field signatures indexed by BasicType.
  static Symbol* _type_signatures[T_VOID+1];

 public:
  // Initialization
  static void initialize(TRAPS);
  // Accessing
  #define VM_SYMBOL_DECLARE(name, ignore)                 \
    static Symbol* name() {                               \
      return _symbols[VM_SYMBOL_ENUM_NAME(name)];         \
    }
  VM_SYMBOLS_DO(VM_SYMBOL_DECLARE, VM_SYMBOL_DECLARE)
  #undef VM_SYMBOL_DECLARE

  // Sharing support
  static void symbols_do(SymbolClosure* f);
  static void serialize(SerializeClosure* soc);

  static Symbol* type_signature(BasicType t) {
    assert((uint)t < T_VOID+1, "range check");
    assert(_type_signatures[t] != NULL, "domain check");
    return _type_signatures[t];
  }
  // inverse of type_signature; returns T_OBJECT if s is not recognized
  static BasicType signature_type(Symbol* s);

  static Symbol* symbol_at(SID id) {
    assert(id >= FIRST_SID && id < SID_LIMIT, "oob");
    assert(_symbols[id] != NULL, "init");
    return _symbols[id];
  }

  // Returns symbol's SID if one is assigned, else NO_SID.
  static SID find_sid(Symbol* symbol);
  static SID find_sid(const char* symbol_name);

#ifndef PRODUCT
  // No need for this in the product:
  static const char* name_for(SID sid);
#endif //PRODUCT
};

三、总结

通过代码可以清晰的看到Java线程是通过本地线程回调run()方法来实现的。

• Openjdk源码为openjdk-8u40-src-b25-10_feb_2015