ArrayList对象在内存中占用24个字节，并且在栈上分配。

##java内存分布jol查看工具的查询结果

java.util.ArrayList object internals:
OFF  SZ                 TYPE DESCRIPTION               VALUE
  0   8                      (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4                      (object header: class)    0x00032490
 12   4                  int AbstractList.modCount     0
 16   4                  int ArrayList.size            0
 20   4   java.lang.Object[] ArrayList.elementData 

##实验代码

import java.util.ArrayList;
import java.util.List;

public class OutOfMemoryTest {

    public static void main(String[] args) {
        List<MiBigObject> miBigObjects = new ArrayList<>(260000);
        System.out.println(miBigObjects);
    }

}

ThreadLocalAllocBuffer::allocate 方法确实负责从线程本地分配缓冲区（TLAB）中分配内存。这个方法尝试在当前线程的 TLAB 中分配指定大小的内存块。

分析代码

1. 获取当前顶部指针：

   • HeapWord* obj = top(); 获取当前 TLAB 的顶部指针。

2. 检查空间是否足够：

   • if (pointer_delta(end(), obj) >= size) 检查从当前顶部指针到 TLAB 结束的剩余空间是否足够容纳请求的大小。

3. 分配内存：

   • 如果空间足够，方法将顶部指针 top 向前移动 size 个 HeapWord，并返回原来的顶部指针 obj 作为分配的内存块的起始地址。

4. 断言检查：

   • 在 ASSERT 宏定义的情况下，代码会将分配的内存块（除了对象头部）填充为一个特殊的值 badHeapWordVal，以确保并发 GC 线程不会错误地解析这块内存。

5. 更新顶部指针：

   • set_top(obj + size); 更新 TLAB 的顶部指针。

6. 返回分配的内存：

   • 如果分配成功，返回 obj；如果失败，返回 NULL。

计算分配的字节数

• size 参数表示要分配的 HeapWord 数量。
• 每个 HeapWord 的大小通常为 8 字节（在 64 位系统中）。

因此，如果 size = 3，则分配的内存大小为： 3 HeapWords×8 bytes/HeapWord=24 bytes3 HeapWords×8 bytes/HeapWord=24 bytes

总结

代码展示了如何从线程本地分配缓冲区中分配内存。在这个例子中，请求分配的大小为 3 个 HeapWord，即 24 字节。这种分配方式是为了减少分配时的锁争用，提高内存分配效率。

这段代码是在 TLAB 上分配了 24 字节的内存。这种分配是在 Java 堆上进行的，而不是在栈上。TLAB 是 Java 堆的一部分，用于提高对象分配的效率。

##C++源码 

HeapWord* MemAllocator::mem_allocate(Allocation& allocation) const {
  if (UseTLAB) {
    HeapWord* result = allocate_inside_tlab(allocation);
    if (result != NULL) {
      return result;
    }
  }

  return allocate_outside_tlab(allocation);
}

HeapWord* MemAllocator::allocate_inside_tlab(Allocation& allocation) const {
  assert(UseTLAB, "should use UseTLAB");

  // Try allocating from an existing TLAB.
  HeapWord* mem = _thread->tlab().allocate(_word_size);
  if (mem != NULL) {
    return mem;
  }

  // Try refilling the TLAB and allocating the object in it.
  return allocate_inside_tlab_slow(allocation);
}

inline HeapWord* ThreadLocalAllocBuffer::allocate(size_t size) {
  invariants();
  HeapWord* obj = top();
  if (pointer_delta(end(), obj) >= size) {
    // successful thread-local allocation
#ifdef ASSERT
    // Skip mangling the space corresponding to the object header to
    // ensure that the returned space is not considered parsable by
    // any concurrent GC thread.
    size_t hdr_size = oopDesc::header_size();
    Copy::fill_to_words(obj + hdr_size, size - hdr_size, badHeapWordVal);
#endif // ASSERT
    // This addition is safe because we know that top is
    // at least size below end, so the add can't wrap.
    set_top(obj + size);

    invariants();
    return obj;
  }
  return NULL;
}

##gdb调试堆栈 3个 word  。一个word=8个字节。3*8=24字节。

#0  ThreadLocalAllocBuffer::allocate (this=0x7ffff0028a30, size=3) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/gc/shared/threadLocalAllocBuffer.inline.hpp:40
#1  0x00007ffff677a0a4 in MemAllocator::allocate_inside_tlab (this=0x7ffff7bfce30, allocation=...)
    at /home/yym/openjdk17/jdk17-master/src/hotspot/share/gc/shared/memAllocator.cpp:274
#2  0x00007ffff677a3bf in MemAllocator::mem_allocate (this=0x7ffff7bfce30, allocation=...) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/gc/shared/memAllocator.cpp:352
#3  0x00007ffff677a428 in MemAllocator::allocate (this=0x7ffff7bfce30) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/gc/shared/memAllocator.cpp:365
#4  0x00007ffff6351347 in CollectedHeap::obj_allocate (this=0x7ffff00453d0, klass=0x100054468, size=3, __the_thread__=0x7ffff0028920)
    at /home/yym/openjdk17/jdk17-master/src/hotspot/share/gc/shared/collectedHeap.inline.hpp:36
#5  0x00007ffff6344ef4 in InstanceKlass::allocate_instance (this=0x100054468, __the_thread__=0x7ffff0028920)
    at /home/yym/openjdk17/jdk17-master/src/hotspot/share/oops/instanceKlass.cpp:1385
#6  0x00007ffff63687d9 in InterpreterRuntime::_new (current=0x7ffff0028920, pool=0x7fffd9b70330, index=35)
    at /home/yym/openjdk17/jdk17-master/src/hotspot/share/interpreter/interpreterRuntime.cpp:243
#7  0x00007fffe1023ad2 in ?? ()
#8  0x00007fffe1023a46 in ?? ()
#9  0x00000000fff37f50 in ?? ()
#10 0x00007ffff7bfcfb0 in ?? ()
#11 0x00007fffd9b73d4d in ?? ()
#12 0x00007ffff7bfd038 in ?? ()
#13 0x00007fffd9b7af60 in ?? ()
#14 0x0000000000000000 in ?? ()