1 /*
   2  * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "gc/shared/oopStorage.inline.hpp"
  27 #include "logging/log.hpp"
  28 #include "memory/iterator.hpp"
  29 #include "oops/oop.inline.hpp"
  30 #include "runtime/handles.inline.hpp"
  31 #include "runtime/jniHandles.inline.hpp"
  32 #include "runtime/mutexLocker.hpp"
  33 #include "runtime/thread.inline.hpp"
  34 #include "trace/traceMacros.hpp"
  35 #include "utilities/align.hpp"
  36 #include "utilities/debug.hpp"
  37 #if INCLUDE_ALL_GCS
  38 #include "gc/g1/g1BarrierSet.hpp"
  39 #endif
  40 
  41 OopStorage* JNIHandles::_global_handles = NULL;
  42 OopStorage* JNIHandles::_weak_global_handles = NULL;
  43 
  44 
  45 jobject JNIHandles::make_local(oop obj) {
  46   if (obj == NULL) {
  47     return NULL;                // ignore null handles
  48   } else {
  49     Thread* thread = Thread::current();
  50     assert(oopDesc::is_oop(obj), "not an oop");
  51     assert(!current_thread_in_native(), "must not be in native");
  52     return thread->active_handles()->allocate_handle(obj);
  53   }
  54 }
  55 
  56 
  57 // optimized versions
  58 
  59 jobject JNIHandles::make_local(Thread* thread, oop obj) {
  60   if (obj == NULL) {
  61     return NULL;                // ignore null handles
  62   } else {
  63     assert(oopDesc::is_oop(obj), "not an oop");
  64     assert(thread->is_Java_thread(), "not a Java thread");
  65     assert(!current_thread_in_native(), "must not be in native");
  66     return thread->active_handles()->allocate_handle(obj);
  67   }
  68 }
  69 
  70 
  71 jobject JNIHandles::make_local(JNIEnv* env, oop obj) {
  72   if (obj == NULL) {
  73     return NULL;                // ignore null handles
  74   } else {
  75     JavaThread* thread = JavaThread::thread_from_jni_environment(env);
  76     assert(oopDesc::is_oop(obj), "not an oop");
  77     assert(!current_thread_in_native(), "must not be in native");
  78     return thread->active_handles()->allocate_handle(obj);
  79   }
  80 }
  81 
  82 
  83 static void report_handle_allocation_failure(AllocFailType alloc_failmode,
  84                                              const char* handle_kind) {
  85   if (alloc_failmode == AllocFailStrategy::EXIT_OOM) {
  86     // Fake size value, since we don't know the min allocation size here.
  87     vm_exit_out_of_memory(sizeof(oop), OOM_MALLOC_ERROR,
  88                           "Cannot create %s JNI handle", handle_kind);
  89   } else {
  90     assert(alloc_failmode == AllocFailStrategy::RETURN_NULL, "invariant");
  91   }
  92 }
  93 
  94 jobject JNIHandles::make_global(Handle obj, AllocFailType alloc_failmode) {
  95   assert(!Universe::heap()->is_gc_active(), "can't extend the root set during GC");
  96   assert(!current_thread_in_native(), "must not be in native");
  97   jobject res = NULL;
  98   if (!obj.is_null()) {
  99     // ignore null handles
 100     assert(oopDesc::is_oop(obj()), "not an oop");
 101     oop* ptr = _global_handles->allocate();
 102     // Return NULL on allocation failure.
 103     if (ptr != NULL) {
 104       *ptr = obj();
 105       res = reinterpret_cast<jobject>(ptr);
 106     } else {
 107       report_handle_allocation_failure(alloc_failmode, "global");
 108     }
 109   } else {
 110     CHECK_UNHANDLED_OOPS_ONLY(Thread::current()->clear_unhandled_oops());
 111   }
 112 
 113   return res;
 114 }
 115 
 116 
 117 jobject JNIHandles::make_weak_global(Handle obj, AllocFailType alloc_failmode) {
 118   assert(!Universe::heap()->is_gc_active(), "can't extend the root set during GC");
 119   assert(!current_thread_in_native(), "must not be in native");
 120   jobject res = NULL;
 121   if (!obj.is_null()) {
 122     // ignore null handles
 123     assert(oopDesc::is_oop(obj()), "not an oop");
 124     oop* ptr = _weak_global_handles->allocate();
 125     // Return NULL on allocation failure.
 126     if (ptr != NULL) {
 127       *ptr = obj();
 128       char* tptr = reinterpret_cast<char*>(ptr) + weak_tag_value;
 129       res = reinterpret_cast<jobject>(tptr);
 130     } else {
 131       report_handle_allocation_failure(alloc_failmode, "weak global");
 132     }
 133   } else {
 134     CHECK_UNHANDLED_OOPS_ONLY(Thread::current()->clear_unhandled_oops());
 135   }
 136   return res;
 137 }
 138 
 139 // Resolve some erroneous cases to NULL, rather than treating them as
 140 // possibly unchecked errors.  In particular, deleted handles are
 141 // treated as NULL (though a deleted and later reallocated handle
 142 // isn't detected).
 143 oop JNIHandles::resolve_external_guard(jobject handle) {
 144   oop result = NULL;
 145   if (handle != NULL) {
 146     result = resolve_impl<true /* external_guard */ >(handle);
 147   }
 148   return result;
 149 }
 150 
 151 oop JNIHandles::resolve_jweak(jweak handle) {
 152   assert(handle != NULL, "precondition");
 153   assert(is_jweak(handle), "precondition");
 154   oop result = jweak_ref(handle);
 155 #if INCLUDE_ALL_GCS
 156   if (result != NULL && UseG1GC) {
 157     G1BarrierSet::enqueue(result);
 158   }
 159 #endif // INCLUDE_ALL_GCS
 160   return result;
 161 }
 162 
 163 bool JNIHandles::is_global_weak_cleared(jweak handle) {
 164   assert(handle != NULL, "precondition");
 165   assert(is_jweak(handle), "not a weak handle");
 166   return jweak_ref(handle) == NULL;
 167 }
 168 
 169 void JNIHandles::destroy_global(jobject handle) {
 170   if (handle != NULL) {
 171     assert(!is_jweak(handle), "wrong method for detroying jweak");
 172     jobject_ref(handle) = NULL;
 173     _global_handles->release(&jobject_ref(handle));
 174   }
 175 }
 176 
 177 
 178 void JNIHandles::destroy_weak_global(jobject handle) {
 179   if (handle != NULL) {
 180     assert(is_jweak(handle), "JNI handle not jweak");
 181     jweak_ref(handle) = NULL;
 182     _weak_global_handles->release(&jweak_ref(handle));
 183   }
 184 }
 185 
 186 
 187 void JNIHandles::oops_do(OopClosure* f) {
 188   _global_handles->oops_do(f);
 189 }
 190 
 191 
 192 void JNIHandles::weak_oops_do(BoolObjectClosure* is_alive, OopClosure* f) {
 193   _weak_global_handles->weak_oops_do(is_alive, f);
 194 }
 195 
 196 
 197 void JNIHandles::weak_oops_do(OopClosure* f) {
 198   _weak_global_handles->weak_oops_do(f);
 199 }
 200 
 201 
 202 void JNIHandles::initialize() {
 203   _global_handles = new OopStorage("JNI Global",
 204                                    JNIGlobalAlloc_lock,
 205                                    JNIGlobalActive_lock);
 206   _weak_global_handles = new OopStorage("JNI Weak",
 207                                         JNIWeakAlloc_lock,
 208                                         JNIWeakActive_lock);
 209 }
 210 
 211 
 212 inline bool is_storage_handle(const OopStorage* storage, const oop* ptr) {
 213   return storage->allocation_status(ptr) == OopStorage::ALLOCATED_ENTRY;
 214 }
 215 
 216 
 217 jobjectRefType JNIHandles::handle_type(Thread* thread, jobject handle) {
 218   assert(handle != NULL, "precondition");
 219   jobjectRefType result = JNIInvalidRefType;
 220   if (is_jweak(handle)) {
 221     if (is_storage_handle(_weak_global_handles, &jweak_ref(handle))) {
 222       result = JNIWeakGlobalRefType;
 223     }
 224   } else {
 225     switch (_global_handles->allocation_status(&jobject_ref(handle))) {
 226     case OopStorage::ALLOCATED_ENTRY:
 227       result = JNIGlobalRefType;
 228       break;
 229 
 230     case OopStorage::UNALLOCATED_ENTRY:
 231       break;                    // Invalid global handle
 232 
 233     case OopStorage::INVALID_ENTRY:
 234       // Not in global storage.  Might be a local handle.
 235       if (is_local_handle(thread, handle) ||
 236           (thread->is_Java_thread() &&
 237            is_frame_handle((JavaThread*)thread, handle))) {
 238         result = JNILocalRefType;
 239       }
 240       break;
 241 
 242     default:
 243       ShouldNotReachHere();
 244     }
 245   }
 246   return result;
 247 }
 248 
 249 
 250 bool JNIHandles::is_local_handle(Thread* thread, jobject handle) {
 251   assert(handle != NULL, "precondition");
 252   JNIHandleBlock* block = thread->active_handles();
 253 
 254   // Look back past possible native calls to jni_PushLocalFrame.
 255   while (block != NULL) {
 256     if (block->chain_contains(handle)) {
 257       return true;
 258     }
 259     block = block->pop_frame_link();
 260   }
 261   return false;
 262 }
 263 
 264 
 265 // Determine if the handle is somewhere in the current thread's stack.
 266 // We easily can't isolate any particular stack frame the handle might
 267 // come from, so we'll check the whole stack.
 268 
 269 bool JNIHandles::is_frame_handle(JavaThread* thr, jobject handle) {
 270   assert(handle != NULL, "precondition");
 271   // If there is no java frame, then this must be top level code, such
 272   // as the java command executable, in which case, this type of handle
 273   // is not permitted.
 274   return (thr->has_last_Java_frame() &&
 275          (void*)handle < (void*)thr->stack_base() &&
 276          (void*)handle >= (void*)thr->last_Java_sp());
 277 }
 278 
 279 
 280 bool JNIHandles::is_global_handle(jobject handle) {
 281   assert(handle != NULL, "precondition");
 282   return !is_jweak(handle) && is_storage_handle(_global_handles, &jobject_ref(handle));
 283 }
 284 
 285 
 286 bool JNIHandles::is_weak_global_handle(jobject handle) {
 287   assert(handle != NULL, "precondition");
 288   return is_jweak(handle) && is_storage_handle(_weak_global_handles, &jweak_ref(handle));
 289 }
 290 
 291 size_t JNIHandles::global_handle_memory_usage() {
 292   return _global_handles->total_memory_usage();
 293 }
 294 
 295 size_t JNIHandles::weak_global_handle_memory_usage() {
 296   return _weak_global_handles->total_memory_usage();
 297 }
 298 
 299 
 300 // We assume this is called at a safepoint: no lock is needed.
 301 void JNIHandles::print_on(outputStream* st) {
 302   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
 303   assert(_global_handles != NULL && _weak_global_handles != NULL,
 304          "JNIHandles not initialized");
 305 
 306   st->print_cr("JNI global refs: " SIZE_FORMAT ", weak refs: " SIZE_FORMAT,
 307                _global_handles->allocation_count(),
 308                _weak_global_handles->allocation_count());
 309   st->cr();
 310   st->flush();
 311 }
 312 
 313 class VerifyJNIHandles: public OopClosure {
 314 public:
 315   virtual void do_oop(oop* root) {
 316     (*root)->verify();
 317   }
 318   virtual void do_oop(narrowOop* root) { ShouldNotReachHere(); }
 319 };
 320 
 321 void JNIHandles::verify() {
 322   VerifyJNIHandles verify_handle;
 323 
 324   oops_do(&verify_handle);
 325   weak_oops_do(&verify_handle);
 326 }
 327 
 328 // This method is implemented here to avoid circular includes between
 329 // jniHandles.hpp and thread.hpp.
 330 bool JNIHandles::current_thread_in_native() {
 331   Thread* thread = Thread::current();
 332   return (thread->is_Java_thread() &&
 333           JavaThread::current()->thread_state() == _thread_in_native);
 334 }
 335 
 336 
 337 void jni_handles_init() {
 338   JNIHandles::initialize();
 339 }
 340 
 341 
 342 int             JNIHandleBlock::_blocks_allocated     = 0;
 343 JNIHandleBlock* JNIHandleBlock::_block_free_list      = NULL;
 344 #ifndef PRODUCT
 345 JNIHandleBlock* JNIHandleBlock::_block_list           = NULL;
 346 #endif
 347 
 348 
 349 #ifdef ASSERT
 350 void JNIHandleBlock::zap() {
 351   // Zap block values
 352   _top = 0;
 353   for (int index = 0; index < block_size_in_oops; index++) {
 354     _handles[index] = NULL;
 355   }
 356 }
 357 #endif // ASSERT
 358 
 359 JNIHandleBlock* JNIHandleBlock::allocate_block(Thread* thread)  {
 360   assert(thread == NULL || thread == Thread::current(), "sanity check");
 361   JNIHandleBlock* block;
 362   // Check the thread-local free list for a block so we don't
 363   // have to acquire a mutex.
 364   if (thread != NULL && thread->free_handle_block() != NULL) {
 365     block = thread->free_handle_block();
 366     thread->set_free_handle_block(block->_next);
 367   }
 368   else {
 369     // locking with safepoint checking introduces a potential deadlock:
 370     // - we would hold JNIHandleBlockFreeList_lock and then Threads_lock
 371     // - another would hold Threads_lock (jni_AttachCurrentThread) and then
 372     //   JNIHandleBlockFreeList_lock (JNIHandleBlock::allocate_block)
 373     MutexLockerEx ml(JNIHandleBlockFreeList_lock,
 374                      Mutex::_no_safepoint_check_flag);
 375     if (_block_free_list == NULL) {
 376       // Allocate new block
 377       block = new JNIHandleBlock();
 378       _blocks_allocated++;
 379       block->zap();
 380       #ifndef PRODUCT
 381       // Link new block to list of all allocated blocks
 382       block->_block_list_link = _block_list;
 383       _block_list = block;
 384       #endif
 385     } else {
 386       // Get block from free list
 387       block = _block_free_list;
 388       _block_free_list = _block_free_list->_next;
 389     }
 390   }
 391   block->_top = 0;
 392   block->_next = NULL;
 393   block->_pop_frame_link = NULL;
 394   block->_planned_capacity = block_size_in_oops;
 395   // _last, _free_list & _allocate_before_rebuild initialized in allocate_handle
 396   debug_only(block->_last = NULL);
 397   debug_only(block->_free_list = NULL);
 398   debug_only(block->_allocate_before_rebuild = -1);
 399   return block;
 400 }
 401 
 402 
 403 void JNIHandleBlock::release_block(JNIHandleBlock* block, Thread* thread) {
 404   assert(thread == NULL || thread == Thread::current(), "sanity check");
 405   JNIHandleBlock* pop_frame_link = block->pop_frame_link();
 406   // Put returned block at the beginning of the thread-local free list.
 407   // Note that if thread == NULL, we use it as an implicit argument that
 408   // we _don't_ want the block to be kept on the free_handle_block.
 409   // See for instance JavaThread::exit().
 410   if (thread != NULL ) {
 411     block->zap();
 412     JNIHandleBlock* freelist = thread->free_handle_block();
 413     block->_pop_frame_link = NULL;
 414     thread->set_free_handle_block(block);
 415 
 416     // Add original freelist to end of chain
 417     if ( freelist != NULL ) {
 418       while ( block->_next != NULL ) block = block->_next;
 419       block->_next = freelist;
 420     }
 421     block = NULL;
 422   }
 423   if (block != NULL) {
 424     // Return blocks to free list
 425     // locking with safepoint checking introduces a potential deadlock:
 426     // - we would hold JNIHandleBlockFreeList_lock and then Threads_lock
 427     // - another would hold Threads_lock (jni_AttachCurrentThread) and then
 428     //   JNIHandleBlockFreeList_lock (JNIHandleBlock::allocate_block)
 429     MutexLockerEx ml(JNIHandleBlockFreeList_lock,
 430                      Mutex::_no_safepoint_check_flag);
 431     while (block != NULL) {
 432       block->zap();
 433       JNIHandleBlock* next = block->_next;
 434       block->_next = _block_free_list;
 435       _block_free_list = block;
 436       block = next;
 437     }
 438   }
 439   if (pop_frame_link != NULL) {
 440     // As a sanity check we release blocks pointed to by the pop_frame_link.
 441     // This should never happen (only if PopLocalFrame is not called the
 442     // correct number of times).
 443     release_block(pop_frame_link, thread);
 444   }
 445 }
 446 
 447 
 448 void JNIHandleBlock::oops_do(OopClosure* f) {
 449   JNIHandleBlock* current_chain = this;
 450   // Iterate over chain of blocks, followed by chains linked through the
 451   // pop frame links.
 452   while (current_chain != NULL) {
 453     for (JNIHandleBlock* current = current_chain; current != NULL;
 454          current = current->_next) {
 455       assert(current == current_chain || current->pop_frame_link() == NULL,
 456         "only blocks first in chain should have pop frame link set");
 457       for (int index = 0; index < current->_top; index++) {
 458         oop* root = &(current->_handles)[index];
 459         oop value = *root;
 460         // traverse heap pointers only, not deleted handles or free list
 461         // pointers
 462         if (value != NULL && Universe::heap()->is_in_reserved(value)) {
 463           f->do_oop(root);
 464         }
 465       }
 466       // the next handle block is valid only if current block is full
 467       if (current->_top < block_size_in_oops) {
 468         break;
 469       }
 470     }
 471     current_chain = current_chain->pop_frame_link();
 472   }
 473 }
 474 
 475 
 476 jobject JNIHandleBlock::allocate_handle(oop obj) {
 477   assert(Universe::heap()->is_in_reserved(obj), "sanity check");
 478   if (_top == 0) {
 479     // This is the first allocation or the initial block got zapped when
 480     // entering a native function. If we have any following blocks they are
 481     // not valid anymore.
 482     for (JNIHandleBlock* current = _next; current != NULL;
 483          current = current->_next) {
 484       assert(current->_last == NULL, "only first block should have _last set");
 485       assert(current->_free_list == NULL,
 486              "only first block should have _free_list set");
 487       if (current->_top == 0) {
 488         // All blocks after the first clear trailing block are already cleared.
 489 #ifdef ASSERT
 490         for (current = current->_next; current != NULL; current = current->_next) {
 491           assert(current->_top == 0, "trailing blocks must already be cleared");
 492         }
 493 #endif
 494         break;
 495       }
 496       current->_top = 0;
 497       current->zap();
 498     }
 499     // Clear initial block
 500     _free_list = NULL;
 501     _allocate_before_rebuild = 0;
 502     _last = this;
 503     zap();
 504   }
 505 
 506   // Try last block
 507   if (_last->_top < block_size_in_oops) {
 508     oop* handle = &(_last->_handles)[_last->_top++];
 509     *handle = obj;
 510     return (jobject) handle;
 511   }
 512 
 513   // Try free list
 514   if (_free_list != NULL) {
 515     oop* handle = _free_list;
 516     _free_list = (oop*) *_free_list;
 517     *handle = obj;
 518     return (jobject) handle;
 519   }
 520   // Check if unused block follow last
 521   if (_last->_next != NULL) {
 522     // update last and retry
 523     _last = _last->_next;
 524     return allocate_handle(obj);
 525   }
 526 
 527   // No space available, we have to rebuild free list or expand
 528   if (_allocate_before_rebuild == 0) {
 529       rebuild_free_list();        // updates _allocate_before_rebuild counter
 530   } else {
 531     // Append new block
 532     Thread* thread = Thread::current();
 533     Handle obj_handle(thread, obj);
 534     // This can block, so we need to preserve obj across call.
 535     _last->_next = JNIHandleBlock::allocate_block(thread);
 536     _last = _last->_next;
 537     _allocate_before_rebuild--;
 538     obj = obj_handle();
 539   }
 540   return allocate_handle(obj);  // retry
 541 }
 542 
 543 void JNIHandleBlock::rebuild_free_list() {
 544   assert(_allocate_before_rebuild == 0 && _free_list == NULL, "just checking");
 545   int free = 0;
 546   int blocks = 0;
 547   for (JNIHandleBlock* current = this; current != NULL; current = current->_next) {
 548     for (int index = 0; index < current->_top; index++) {
 549       oop* handle = &(current->_handles)[index];
 550       if (*handle == NULL) {
 551         // this handle was cleared out by a delete call, reuse it
 552         *handle = (oop) _free_list;
 553         _free_list = handle;
 554         free++;
 555       }
 556     }
 557     // we should not rebuild free list if there are unused handles at the end
 558     assert(current->_top == block_size_in_oops, "just checking");
 559     blocks++;
 560   }
 561   // Heuristic: if more than half of the handles are free we rebuild next time
 562   // as well, otherwise we append a corresponding number of new blocks before
 563   // attempting a free list rebuild again.
 564   int total = blocks * block_size_in_oops;
 565   int extra = total - 2*free;
 566   if (extra > 0) {
 567     // Not as many free handles as we would like - compute number of new blocks to append
 568     _allocate_before_rebuild = (extra + block_size_in_oops - 1) / block_size_in_oops;
 569   }
 570 }
 571 
 572 
 573 bool JNIHandleBlock::contains(jobject handle) const {
 574   return ((jobject)&_handles[0] <= handle && handle<(jobject)&_handles[_top]);
 575 }
 576 
 577 
 578 bool JNIHandleBlock::chain_contains(jobject handle) const {
 579   for (JNIHandleBlock* current = (JNIHandleBlock*) this; current != NULL; current = current->_next) {
 580     if (current->contains(handle)) {
 581       return true;
 582     }
 583   }
 584   return false;
 585 }
 586 
 587 
 588 size_t JNIHandleBlock::length() const {
 589   size_t result = 1;
 590   for (JNIHandleBlock* current = _next; current != NULL; current = current->_next) {
 591     result++;
 592   }
 593   return result;
 594 }
 595 
 596 class CountJNIHandleClosure: public OopClosure {
 597 private:
 598   int _count;
 599 public:
 600   CountJNIHandleClosure(): _count(0) {}
 601   virtual void do_oop(oop* ooph) { _count++; }
 602   virtual void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 603   int count() { return _count; }
 604 };
 605 
 606 const size_t JNIHandleBlock::get_number_of_live_handles() {
 607   CountJNIHandleClosure counter;
 608   oops_do(&counter);
 609   return counter.count();
 610 }
 611 
 612 // This method is not thread-safe, i.e., must be called while holding a lock on the
 613 // structure.
 614 size_t JNIHandleBlock::memory_usage() const {
 615   return length() * sizeof(JNIHandleBlock);
 616 }
 617 
 618 
 619 #ifndef PRODUCT
 620 
 621 bool JNIHandles::is_local_handle(jobject handle) {
 622   return JNIHandleBlock::any_contains(handle);
 623 }
 624 
 625 bool JNIHandleBlock::any_contains(jobject handle) {
 626   assert(handle != NULL, "precondition");
 627   for (JNIHandleBlock* current = _block_list; current != NULL; current = current->_block_list_link) {
 628     if (current->contains(handle)) {
 629       return true;
 630     }
 631   }
 632   return false;
 633 }
 634 
 635 void JNIHandleBlock::print_statistics() {
 636   int used_blocks = 0;
 637   int free_blocks = 0;
 638   int used_handles = 0;
 639   int free_handles = 0;
 640   JNIHandleBlock* block = _block_list;
 641   while (block != NULL) {
 642     if (block->_top > 0) {
 643       used_blocks++;
 644     } else {
 645       free_blocks++;
 646     }
 647     used_handles += block->_top;
 648     free_handles += (block_size_in_oops - block->_top);
 649     block = block->_block_list_link;
 650   }
 651   tty->print_cr("JNIHandleBlocks statistics");
 652   tty->print_cr("- blocks allocated: %d", used_blocks + free_blocks);
 653   tty->print_cr("- blocks in use:    %d", used_blocks);
 654   tty->print_cr("- blocks free:      %d", free_blocks);
 655   tty->print_cr("- handles in use:   %d", used_handles);
 656   tty->print_cr("- handles free:     %d", free_handles);
 657 }
 658 
 659 #endif