HeapWord* CollectedHeap::common_mem_allocate_init(size_t size, bool is_noref, TRAPS) {
HeapWord* obj = common_mem_allocate_noinit(size, is_noref, CHECK_NULL);
init_obj(obj, size);
return obj;
}
6.common_mem_allocate_noinit()如下：
(1).若使用了本地线程分配缓冲TLAB，则会调用allocate_from_tlab()尝试从TLAB中分配内存
HeapWord* result = NULL;
if (UseTLAB) {
result = CollectedHeap::allocate_from_tlab(THREAD, size);
if (result != NULL) {
assert(!HAS_PENDING_EXCEPTION,
"Unexpected exception, will result in uninitialized storage");
return result;
}
}
(2).否则会调用堆的mem_allocate()尝试分配
bool gc_overhead_limit_was_exceeded = false;
result = Universe::heap()->mem_allocate(size,
is_noref,
false,
&gc_overhead_limit_was_exceeded);
(3).统计分配的字节数
if (result != NULL) {
//...
THREAD->incr_allocated_bytes(size * HeapWordSize);
return result;
}
(4).否则说明申请失败，若在申请过程中gc没有超时，则抛出OOM异常
if (!gc_overhead_limit_was_exceeded) {
// -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support
report_java_out_of_memory("Java heap space");
if (JvmtiExport::should_post_resource_exhausted()) {
JvmtiExport::post_resource_exhausted(
JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_JAVA_HEAP,
"Java heap space");
}
THROW_OOP_0(Universe::out_of_memory_error_java_heap());
7.对象内存分配后的初始化过程包括两部分，一个是init_obj()完成对对象内存空间的对齐和填充，一个是post_allocation_setup_obj()对堆上的oop对象进行初始化。
(1).init_obj()：
void CollectedHeap::init_obj(HeapWord* obj, size_t size) {
assert(obj != NULL, "cannot initialize NULL object");
const size_t hs = oopDesc::header_size();
assert(size >= hs, "unexpected object size");
((oop)obj)->set_klass_gap(0);
Copy::fill_to_aligned_words(obj + hs, size - hs);
}
hs就是对象头的大小，fill_to_aligned_words将对象空间除去对象头的部分做填0处理，该函数定义在/hotspot/src/share/vm/utilities/copy.h中，并转而调用pd_fill_to_aligned_words()。
pd_fill_to_aligned_words根据不同平台实现，以x86平台为例，该函数定义在/hotspot/src/cpu/x86/vm/copy_x86.h中：
static void pd_fill_to_words(HeapWord* tohw, size_t count, juint value) {
#ifdef AMD64
julong* to = (julong*) tohw;
julong v = ((julong) value << 32) | value;
while (count-- > 0) {
*to++ = v;
}
#else
juint* to = (juint*)tohw;
count *= HeapWordSize / BytesPerInt;
while (count-- > 0) {
*to++ = value;
}
#endif // AMD64
}
该函数的作用就是先将地址类型转换，然后把堆的字数转化为字节数，再对该段内存进行填值(value = 0)处理
(2).post_allocation_setup_obj()调用了post_allocation_setup_common()进行初始化工作，然后调用post_allocation_notify()通知JVMTI和dtrace
void CollectedHeap::post_allocation_setup_obj(KlassHandle klass,
HeapWord* obj,
size_t size) {
post_allocation_setup_common(klass, obj, size);
assert(Universe::is_bootstrapping() ||
!((oop)obj)->blueprint()->oop_is_array(), "must not be an array");
// notify jvmti and dtrace
post_allocation_notify(klass, (oop)obj);
}
post_allocation_setup_common()如下：
void CollectedHeap::post_allocation_setup_common(KlassHandle klass,
HeapWord* obj,
size_t size) {
post_allocation_setup_no_klass_install(klass, obj, size);
post_allocation_install_obj_klass(klass, oop(obj), (int) size);
}