如何实现Object类(hashCode和内部字段等方法)？

Question

我很好奇Object类是如何实现的.例如

1. 方法hashCode()或wait()
2. 内部状态如何表示.例如,一个内部锁或用于存储调用对象的wait()的线程的数据结构.

为了找到这些,我已经下载了一个OpenJDK源并开始深入挖掘.首先,我遇到的是\ openjdksrc\jdk\src\share \native\java\lang\Object.c文件,其中包含其他:

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};
JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls,
                            methods, sizeof(methods)/sizeof(methods[0]));
}

JNIEXPORT jclass JNICALL
Java_java_lang_Object_getClass(JNIEnv *env, jobject this)
{
    if (this == NULL) {
        JNU_ThrowNullPointerException(env, NULL);
        return 0;
    } else {
        return (*env)->GetObjectClass(env, this);
    }
}

据我所知,methods []数组定义了Object方法的本机实现之间的映射.例如,Object的hashCode()被映射到JVM_IHashCode函数.JVM_IHashCode在\ openjdksrc\hotspot\src\share\vm\prims\jvm.cpp中实现.这是我的第一个问题.为什么这已经是VM本身的一部分(它已在\ openjdksrc\hotspot\src\share\vm中定义)？ 但是让我们转到JVM_IHashCode的代码:

JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_IHashCode");
  // as implemented in the classic virtual machine; return 0 if object is NULL
  return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ;
JVM_END

为什么如果object为null我们返回0？我想应该抛出一个NPE.否则,从\ openjdksrc\hotspot\src\share\vm\runtime\synchronizer.cpp调用FastHashCode,并最终在某个时间点调用get_next_hash来计算实际值. 一旦计算出来,问题就在于它存储的位置？

    intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {

...CUT...

      ObjectMonitor* monitor = NULL;
      markOop temp, test;
      intptr_t hash;
      markOop mark = ReadStableMark (obj);

...CUT...

      if (mark->is_neutral()) {
        hash = mark->hash();              // this is a normal header
        if (hash) {                       // if it has hash, just return it
          return hash;
        }
        hash = get_next_hash(Self, obj);  // allocate a new hash code
        temp = mark->copy_set_hash(hash); // merge the hash code into header
        // use (machine word version) atomic operation to install the hash
        test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
        if (test == mark) {
          return hash;
        }
        // If atomic operation failed, we must inflate the header
        // into heavy weight monitor. We could add more code here
        // for fast path, but it does not worth the complexity.
      } 
...CUT...
      return hash;
    }

所以oop类/ struct(？)有一个markOop类/ struct(？),其中存储了哈希值.Funilly我找不到这些类/结构.我能找到的只有:

class oopDesc {
  friend class VMStructs;
 private:
  volatile markOop  _mark;
...CUT...

在\ openjdksrc\hotspot\src\share\vm\oops\oop.hpp中,它似乎在私有字段中有markOop.但那么真正的"oop"在其余的代码中引用了什么？哪里可以找到markOop定义？我找到了一个相应的:

class markOopDesc: public oopDesc 
...CUT...

在\ openjdksrc \热点的\ src \共享\虚拟机\哎呀\ markOop.hpp但它是唯一的全枚举的,并不能找到地方可以存储的散列值的字段.如果有人能回答我至少一部分问题,我将非常感激.谢谢!

Answer 1

Java对象的哈希码一旦计算出来就存储在对象头中。

http://www.javamex.com/tutorials/memory/object_memory_usage.shtml

http://hunmr.blogspot.com/2012/08/java-performance-tunning.html

来自 hotspot/src/share/vm/oops/markOop.hpp

// The markOop describes the header of an object.
//
// Note that the mark is not a real oop but just a word.
// It is placed in the oop hierarchy for historical reasons.
//
// Bit-format of an object header (most significant first, big endian layout below):
//
//  32 bits:
//  --------
//             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
//             JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
//             size:32 ------------------------------------------>| (CMS free block)
//             PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//