share/classfile/moduleEntry.cpp

/*
 * Copyright (c) 2016, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "precompiled.hpp"
#include "jni.h"
#include "cds/archiveBuilder.hpp"
#include "cds/archiveUtils.hpp"
#include "cds/filemap.hpp"
#include "cds/heapShared.hpp"
#include "classfile/classLoader.hpp"
#include "classfile/classLoaderData.inline.hpp"
#include "classfile/javaClasses.inline.hpp"
#include "classfile/moduleEntry.hpp"
#include "logging/log.hpp"
#include "memory/resourceArea.hpp"
#include "memory/universe.hpp"
#include "oops/oopHandle.inline.hpp"
#include "oops/symbol.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/safepoint.hpp"
#include "utilities/events.hpp"
#include "utilities/growableArray.hpp"
#include "utilities/hashtable.inline.hpp"
#include "utilities/ostream.hpp"
#include "utilities/quickSort.hpp"
#include "utilities/resourceHash.hpp"

ModuleEntry* ModuleEntryTable::_javabase_module = NULL;

oop ModuleEntry::module() const { return _module.resolve(); }

void ModuleEntry::set_location(Symbol* location) {
  if (_location != NULL) {
    // _location symbol's refcounts are managed by ModuleEntry,
    // must decrement the old one before updating.
    _location->decrement_refcount();
  }

  _location = location;

  if (location != NULL) {
    location->increment_refcount();
    CDS_ONLY(if (UseSharedSpaces) {
        _shared_path_index = FileMapInfo::get_module_shared_path_index(location);
      });
  }
}

// Return true if the module's version should be displayed in error messages,
// logging, etc.
// Return false if the module's version is null, if it is unnamed, or if the
// module is not an upgradeable module.
// Detect if the module is not upgradeable by checking:
//     1. Module location is "jrt:/java." and its loader is boot or platform
//     2. Module location is "jrt:/jdk.", its loader is one of the builtin loaders
//        and its version is the same as module java.base's version
// The above check is imprecise but should work in almost all cases.
bool ModuleEntry::should_show_version() {
  if (version() == NULL || !is_named()) return false;

  if (location() != NULL) {
    ResourceMark rm;
    const char* loc = location()->as_C_string();
    ClassLoaderData* cld = loader_data();

    assert(!cld->has_class_mirror_holder(), "module's cld should have a ClassLoader holder not a Class holder");
    if ((cld->is_the_null_class_loader_data() || cld->is_platform_class_loader_data()) &&
        (strncmp(loc, "jrt:/java.", 10) == 0)) {
      return false;
    }
    if ((ModuleEntryTable::javabase_moduleEntry()->version()->fast_compare(version()) == 0) &&
        cld->is_permanent_class_loader_data() && (strncmp(loc, "jrt:/jdk.", 9) == 0)) {
      return false;
    }
  }
  return true;
}

void ModuleEntry::set_version(Symbol* version) {
  if (_version != NULL) {
    // _version symbol's refcounts are managed by ModuleEntry,
    // must decrement the old one before updating.
    _version->decrement_refcount();
  }

  _version = version;

  if (version != NULL) {
    version->increment_refcount();
  }
}

// Returns the shared ProtectionDomain
oop ModuleEntry::shared_protection_domain() {
  return _shared_pd.resolve();
}

// Set the shared ProtectionDomain atomically
void ModuleEntry::set_shared_protection_domain(ClassLoaderData *loader_data,
                                               Handle pd_h) {
  // Create a handle for the shared ProtectionDomain and save it atomically.
  // init_handle_locked checks if someone beats us setting the _shared_pd cache.
  loader_data->init_handle_locked(_shared_pd, pd_h);
}

// Returns true if this module can read module m
bool ModuleEntry::can_read(ModuleEntry* m) const {
  assert(m != NULL, "No module to lookup in this module's reads list");

  // Unnamed modules read everyone and all modules
  // read java.base.  If either of these conditions
  // hold, readability has been established.
  if (!this->is_named() ||
      (m == ModuleEntryTable::javabase_moduleEntry())) {
    return true;
  }

  MutexLocker m1(Module_lock);
  // This is a guard against possible race between agent threads that redefine
  // or retransform classes in this module. Only one of them is adding the
  // default read edges to the unnamed modules of the boot and app class loaders
  // with an upcall to jdk.internal.module.Modules.transformedByAgent.
  // At the same time, another thread can instrument the module classes by
  // injecting dependencies that require the default read edges for resolution.
  if (this->has_default_read_edges() && !m->is_named()) {
    ClassLoaderData* cld = m->loader_data();
    assert(!cld->has_class_mirror_holder(), "module's cld should have a ClassLoader holder not a Class holder");
    if (cld->is_the_null_class_loader_data() || cld->is_system_class_loader_data()) {
      return true; // default read edge
    }
  }
  if (!has_reads_list()) {
    return false;
  } else {
    return _reads->contains(m);
  }
}

// Add a new module to this module's reads list
void ModuleEntry::add_read(ModuleEntry* m) {
  // Unnamed module is special cased and can read all modules
  if (!is_named()) {
    return;
  }

  MutexLocker m1(Module_lock);
  if (m == NULL) {
    set_can_read_all_unnamed();
  } else {
    if (_reads == NULL) {
      // Lazily create a module's reads list
      _reads = new (ResourceObj::C_HEAP, mtModule) GrowableArray<ModuleEntry*>(MODULE_READS_SIZE, mtModule);
    }

    // Determine, based on this newly established read edge to module m,
    // if this module's read list should be walked at a GC safepoint.
    set_read_walk_required(m->loader_data());

    // Establish readability to module m
    _reads->append_if_missing(m);
  }
}

// If the module's loader, that a read edge is being established to, is
// not the same loader as this module's and is not one of the 3 builtin
// class loaders, then this module's reads list must be walked at GC
// safepoint. Modules have the same life cycle as their defining class
// loaders and should be removed if dead.
void ModuleEntry::set_read_walk_required(ClassLoaderData* m_loader_data) {
  assert(is_named(), "Cannot call set_read_walk_required on unnamed module");
  assert_locked_or_safepoint(Module_lock);
  if (!_must_walk_reads &&
      loader_data() != m_loader_data &&
      !m_loader_data->is_builtin_class_loader_data()) {
    _must_walk_reads = true;
    if (log_is_enabled(Trace, module)) {
      ResourceMark rm;
      log_trace(module)("ModuleEntry::set_read_walk_required(): module %s reads list must be walked",
                        (name() != NULL) ? name()->as_C_string() : UNNAMED_MODULE);
    }
  }
}

// Set whether the module is open, i.e. all its packages are unqualifiedly exported
void ModuleEntry::set_is_open(bool is_open) {
  assert_lock_strong(Module_lock);
  _is_open = is_open;
}

// Returns true if the module has a non-empty reads list. As such, the unnamed
// module will return false.
bool ModuleEntry::has_reads_list() const {
  assert_locked_or_safepoint(Module_lock);
  return ((_reads != NULL) && !_reads->is_empty());
}

// Purge dead module entries out of reads list.
void ModuleEntry::purge_reads() {
  assert_locked_or_safepoint(Module_lock);

  if (_must_walk_reads && has_reads_list()) {
    // This module's _must_walk_reads flag will be reset based
    // on the remaining live modules on the reads list.
    _must_walk_reads = false;

    if (log_is_enabled(Trace, module)) {
      ResourceMark rm;
      log_trace(module)("ModuleEntry::purge_reads(): module %s reads list being walked",
                        (name() != NULL) ? name()->as_C_string() : UNNAMED_MODULE);
    }

    // Go backwards because this removes entries that are dead.
    int len = _reads->length();
    for (int idx = len - 1; idx >= 0; idx--) {
      ModuleEntry* module_idx = _reads->at(idx);
      ClassLoaderData* cld_idx = module_idx->loader_data();
      if (cld_idx->is_unloading()) {
        _reads->delete_at(idx);
      } else {
        // Update the need to walk this module's reads based on live modules
        set_read_walk_required(cld_idx);
      }
    }
  }
}

void ModuleEntry::module_reads_do(ModuleClosure* f) {
  assert_locked_or_safepoint(Module_lock);
  assert(f != NULL, "invariant");

  if (has_reads_list()) {
    int reads_len = _reads->length();
    for (int i = 0; i < reads_len; ++i) {
      f->do_module(_reads->at(i));
    }
  }
}

void ModuleEntry::delete_reads() {
  delete _reads;
  _reads = NULL;
}

ModuleEntry* ModuleEntry::create_unnamed_module(ClassLoaderData* cld) {
  // The java.lang.Module for this loader's
  // corresponding unnamed module can be found in the java.lang.ClassLoader object.
  oop module = java_lang_ClassLoader::unnamedModule(cld->class_loader());

  // Ensure that the unnamed module was correctly set when the class loader was constructed.
  // Guarantee will cause a recognizable crash if the user code has circumvented calling the ClassLoader constructor.
  ResourceMark rm;
  guarantee(java_lang_Module::is_instance(module),
            "The unnamed module for ClassLoader %s, is null or not an instance of java.lang.Module. The class loader has not been initialized correctly.",
            cld->loader_name_and_id());

  ModuleEntry* unnamed_module = new_unnamed_module_entry(Handle(Thread::current(), module), cld);

  // Store pointer to the ModuleEntry in the unnamed module's java.lang.Module object.
  java_lang_Module::set_module_entry(module, unnamed_module);

  return unnamed_module;
}

ModuleEntry* ModuleEntry::create_boot_unnamed_module(ClassLoaderData* cld) {
  // For the boot loader, the java.lang.Module for the unnamed module
  // is not known until a call to JVM_SetBootLoaderUnnamedModule is made. At
  // this point initially create the ModuleEntry for the unnamed module.
  ModuleEntry* unnamed_module = new_unnamed_module_entry(Handle(), cld);
  assert(unnamed_module != NULL, "boot loader unnamed module should not be null");
  return unnamed_module;
}

// When creating an unnamed module, this is called without holding the Module_lock.
// This is okay because the unnamed module gets created before the ClassLoaderData
// is available to other threads.
ModuleEntry* ModuleEntry::new_unnamed_module_entry(Handle module_handle, ClassLoaderData* cld) {
  ModuleEntry* entry = NEW_C_HEAP_OBJ(ModuleEntry, mtModule);

  // Initialize everything BasicHashtable would
  entry->set_next(NULL);
  entry->set_hash(0);
  entry->set_literal(NULL);

  // Initialize fields specific to a ModuleEntry
  entry->init();

  // Unnamed modules can read all other unnamed modules.
  entry->set_can_read_all_unnamed();

  if (!module_handle.is_null()) {
    entry->set_module(cld->add_handle(module_handle));
  }

  entry->set_loader_data(cld);
  entry->_is_open = true;

  JFR_ONLY(INIT_ID(entry);)

  return entry;
}

void ModuleEntry::delete_unnamed_module() {
  // Do not need unlink_entry() since the unnamed module is not in the hashtable
  FREE_C_HEAP_OBJ(this);
}

ModuleEntryTable::ModuleEntryTable(int table_size)
  : Hashtable<Symbol*, mtModule>(table_size, sizeof(ModuleEntry))
{
}

ModuleEntryTable::~ModuleEntryTable() {
  // Walk through all buckets and all entries in each bucket,
  // freeing each entry.
  for (int i = 0; i < table_size(); ++i) {
    for (ModuleEntry* m = bucket(i); m != NULL;) {
      ModuleEntry* to_remove = m;
      // read next before freeing.
      m = m->next();

      ResourceMark rm;
      if (to_remove->name() != NULL) {
        log_info(module, unload)("unloading module %s", to_remove->name()->as_C_string());
      }
      log_debug(module)("ModuleEntryTable: deleting module: %s", to_remove->name() != NULL ?
                        to_remove->name()->as_C_string() : UNNAMED_MODULE);

      // Clean out the C heap allocated reads list first before freeing the entry
      to_remove->delete_reads();
      if (to_remove->name() != NULL) {
        to_remove->name()->decrement_refcount();
      }
      if (to_remove->version() != NULL) {
        to_remove->version()->decrement_refcount();
      }
      if (to_remove->location() != NULL) {
        to_remove->location()->decrement_refcount();
      }
      BasicHashtable<mtModule>::free_entry(to_remove);
    }
  }
  assert(number_of_entries() == 0, "should have removed all entries");
}

void ModuleEntry::set_loader_data(ClassLoaderData* cld) {
  assert(!cld->has_class_mirror_holder(), "Unexpected has_class_mirror_holder cld");
  _loader_data = cld;
}

#if INCLUDE_CDS_JAVA_HEAP
typedef ResourceHashtable<
  const ModuleEntry*,
  ModuleEntry*,
  primitive_hash<const ModuleEntry*>,
  primitive_equals<const ModuleEntry*>,
  557, // prime number
  ResourceObj::C_HEAP> ArchivedModuleEntries;
static ArchivedModuleEntries* _archive_modules_entries = NULL;

ModuleEntry* ModuleEntry::allocate_archived_entry() const {
  assert(is_named(), "unnamed packages/modules are not archived");
  ModuleEntry* archived_entry = (ModuleEntry*)ArchiveBuilder::rw_region_alloc(sizeof(ModuleEntry));
  memcpy((void*)archived_entry, (void*)this, sizeof(ModuleEntry));

  if (_archive_modules_entries == NULL) {
    _archive_modules_entries = new (ResourceObj::C_HEAP, mtClass)ArchivedModuleEntries();
  }
  assert(_archive_modules_entries->get(this) == NULL, "Each ModuleEntry must not be shared across ModuleEntryTables");
  _archive_modules_entries->put(this, archived_entry);

  return archived_entry;
}

ModuleEntry* ModuleEntry::get_archived_entry(ModuleEntry* orig_entry) {
  ModuleEntry** ptr = _archive_modules_entries->get(orig_entry);
  assert(ptr != NULL && *ptr != NULL, "must have been allocated");
  return *ptr;
}

// This function is used to archive ModuleEntry::_reads and PackageEntry::_qualified_exports.
// GrowableArray cannot be directly archived, as it needs to be expandable at runtime.
// Write it out as an Array, and convert it back to GrowableArray at runtime.
Array<ModuleEntry*>* ModuleEntry::write_growable_array(GrowableArray<ModuleEntry*>* array) {
  Array<ModuleEntry*>* archived_array = NULL;
  int length = (array == NULL) ? 0 : array->length();
  if (length > 0) {
    archived_array = ArchiveBuilder::new_ro_array<ModuleEntry*>(length);
    for (int i = 0; i < length; i++) {
      ModuleEntry* archived_entry = get_archived_entry(array->at(i));
      archived_array->at_put(i, archived_entry);
      ArchivePtrMarker::mark_pointer((address*)archived_array->adr_at(i));
    }
  }

  return archived_array;
}

GrowableArray<ModuleEntry*>* ModuleEntry::restore_growable_array(Array<ModuleEntry*>* archived_array) {
  GrowableArray<ModuleEntry*>* array = NULL;
  int length = (archived_array == NULL) ? 0 : archived_array->length();
  if (length > 0) {
    array = new (ResourceObj::C_HEAP, mtModule)GrowableArray<ModuleEntry*>(length, mtModule);
    for (int i = 0; i < length; i++) {
      ModuleEntry* archived_entry = archived_array->at(i);
      array->append(archived_entry);
    }
  }

  return array;
}

void ModuleEntry::iterate_symbols(MetaspaceClosure* closure) {
  closure->push(literal_addr()); // name
  closure->push(&_version);
  closure->push(&_location);
}

void ModuleEntry::init_as_archived_entry() {
  Array<ModuleEntry*>* archived_reads = write_growable_array(_reads);

  set_next(NULL);
  set_hash(0x0);        // re-init at runtime
  _loader_data = NULL;  // re-init at runtime
  _shared_path_index = FileMapInfo::get_module_shared_path_index(_location);
  if (literal() != NULL) {
    set_literal(ArchiveBuilder::get_relocated_symbol(literal()));
    ArchivePtrMarker::mark_pointer((address*)literal_addr());
  }
  _reads = (GrowableArray<ModuleEntry*>*)archived_reads;
  if (_version != NULL) {
    _version = ArchiveBuilder::get_relocated_symbol(_version);
  }
  if (_location != NULL) {
    _location = ArchiveBuilder::get_relocated_symbol(_location);
  }
  JFR_ONLY(set_trace_id(0));// re-init at runtime

  ArchivePtrMarker::mark_pointer((address*)&_reads);
  ArchivePtrMarker::mark_pointer((address*)&_version);
  ArchivePtrMarker::mark_pointer((address*)&_location);
}

void ModuleEntry::init_archived_oops() {
  assert(DumpSharedSpaces, "static dump only");
  oop module_obj = module();
  if (module_obj != NULL) {
    oop m = HeapShared::find_archived_heap_object(module_obj);
    assert(m != NULL, "sanity");
    _archived_module_index = HeapShared::append_root(m);
  }
  assert(shared_protection_domain() == NULL, "never set during -Xshare:dump");
  // Clear handles and restore at run time. Handles cannot be archived.
  OopHandle null_handle;
  _module = null_handle;
}

void ModuleEntry::load_from_archive(ClassLoaderData* loader_data) {
  set_loader_data(loader_data);
  _reads = restore_growable_array((Array<ModuleEntry*>*)_reads);
  JFR_ONLY(INIT_ID(this);)
}

void ModuleEntry::restore_archived_oops(ClassLoaderData* loader_data) {
  Handle module_handle(Thread::current(), HeapShared::get_root(_archived_module_index, /*clear=*/true));
  assert(module_handle.not_null(), "huh");
  set_module(loader_data->add_handle(module_handle));

  // This was cleared to zero during dump time -- we didn't save the value
  // because it may be affected by archive relocation.
  java_lang_Module::set_module_entry(module_handle(), this);

  if (loader_data->class_loader() != NULL) {
    java_lang_Module::set_loader(module_handle(), loader_data->class_loader());
  }
}

void ModuleEntry::clear_archived_oops() {
  HeapShared::clear_root(_archived_module_index);
}

static int compare_module_by_name(ModuleEntry* a, ModuleEntry* b) {
  assert(a == b || a->name() != b->name(), "no duplicated names");
  return a->name()->fast_compare(b->name());
}

void ModuleEntryTable::iterate_symbols(MetaspaceClosure* closure) {
  for (int i = 0; i < table_size(); ++i) {
    for (ModuleEntry* m = bucket(i); m != NULL; m = m->next()) {
      m->iterate_symbols(closure);
    }
  }
}

Array<ModuleEntry*>* ModuleEntryTable::allocate_archived_entries() {
  Array<ModuleEntry*>* archived_modules = ArchiveBuilder::new_rw_array<ModuleEntry*>(number_of_entries());
  int n = 0;
  for (int i = 0; i < table_size(); ++i) {
    for (ModuleEntry* m = bucket(i); m != NULL; m = m->next()) {
      archived_modules->at_put(n++, m);
    }
  }
  if (n > 1) {
    // Always allocate in the same order to produce deterministic archive.
    QuickSort::sort(archived_modules->data(), n, (_sort_Fn)compare_module_by_name, true);
  }
  for (int i = 0; i < n; i++) {
    archived_modules->at_put(i, archived_modules->at(i)->allocate_archived_entry());
    ArchivePtrMarker::mark_pointer((address*)archived_modules->adr_at(i));
  }
  return archived_modules;
}

void ModuleEntryTable::init_archived_entries(Array<ModuleEntry*>* archived_modules) {
  assert(DumpSharedSpaces, "dump time only");
  for (int i = 0; i < archived_modules->length(); i++) {
    ModuleEntry* archived_entry = archived_modules->at(i);
    archived_entry->init_as_archived_entry();
  }
}

void ModuleEntryTable::init_archived_oops(Array<ModuleEntry*>* archived_modules) {
  assert(DumpSharedSpaces, "dump time only");
  for (int i = 0; i < archived_modules->length(); i++) {
    ModuleEntry* archived_entry = archived_modules->at(i);
    archived_entry->init_archived_oops();
  }
}

void ModuleEntryTable::load_archived_entries(ClassLoaderData* loader_data,
                                             Array<ModuleEntry*>* archived_modules) {
  assert(UseSharedSpaces, "runtime only");

  for (int i = 0; i < archived_modules->length(); i++) {
    ModuleEntry* archived_entry = archived_modules->at(i);
    archived_entry->load_from_archive(loader_data);

    unsigned int hash = compute_hash(archived_entry->name());
    archived_entry->set_hash(hash);
    add_entry(hash_to_index(hash), archived_entry);
  }
}

void ModuleEntryTable::restore_archived_oops(ClassLoaderData* loader_data, Array<ModuleEntry*>* archived_modules) {
  assert(UseSharedSpaces, "runtime only");
  for (int i = 0; i < archived_modules->length(); i++) {
    ModuleEntry* archived_entry = archived_modules->at(i);
    archived_entry->restore_archived_oops(loader_data);
  }
}
#endif // INCLUDE_CDS_JAVA_HEAP

ModuleEntry* ModuleEntryTable::new_entry(unsigned int hash, Handle module_handle,
                                         bool is_open, Symbol* name,
                                         Symbol* version, Symbol* location,
                                         ClassLoaderData* loader_data) {
  assert(Module_lock->owned_by_self(), "should have the Module_lock");
  ModuleEntry* entry = (ModuleEntry*)Hashtable<Symbol*, mtModule>::new_entry(hash, name);

  // Initialize fields specific to a ModuleEntry
  entry->init();
  if (name != NULL) {
    name->increment_refcount();
  } else {
    // Unnamed modules can read all other unnamed modules.
    entry->set_can_read_all_unnamed();
  }

  if (!module_handle.is_null()) {
    entry->set_module(loader_data->add_handle(module_handle));
  }

  entry->set_loader_data(loader_data);
  entry->set_version(version);
  entry->set_location(location);
  entry->set_is_open(is_open);

  if (ClassLoader::is_in_patch_mod_entries(name)) {
    entry->set_is_patched();
    if (log_is_enabled(Trace, module, patch)) {
      ResourceMark rm;
      log_trace(module, patch)("Marked module %s as patched from --patch-module",
                               name != NULL ? name->as_C_string() : UNNAMED_MODULE);
    }
  }

  JFR_ONLY(INIT_ID(entry);)

  return entry;
}

void ModuleEntryTable::add_entry(int index, ModuleEntry* new_entry) {
  assert(Module_lock->owned_by_self(), "should have the Module_lock");
  Hashtable<Symbol*, mtModule>::add_entry(index, (HashtableEntry<Symbol*, mtModule>*)new_entry);
}

// Create an entry in the class loader's module_entry_table.  It is the
// caller's responsibility to ensure that the entry has not already been
// created.
ModuleEntry* ModuleEntryTable::locked_create_entry(Handle module_handle,
                                                   bool is_open,
                                                   Symbol* module_name,
                                                   Symbol* module_version,
                                                   Symbol* module_location,
                                                   ClassLoaderData* loader_data) {
  assert(module_name != NULL, "ModuleEntryTable locked_create_entry should never be called for unnamed module.");
  assert(Module_lock->owned_by_self(), "should have the Module_lock");
  assert(lookup_only(module_name) == NULL, "Module already exists");
  ModuleEntry* entry = new_entry(compute_hash(module_name), module_handle, is_open, module_name,
                                 module_version, module_location, loader_data);
  add_entry(index_for(module_name), entry);
  return entry;
}

// lookup_only by Symbol* to find a ModuleEntry.
ModuleEntry* ModuleEntryTable::lookup_only(Symbol* name) {
  assert(name != NULL, "name cannot be NULL");
  int index = index_for(name);
  for (ModuleEntry* m = bucket(index); m != NULL; m = m->next()) {
    if (m->name()->fast_compare(name) == 0) {
      return m;
    }
  }
  return NULL;
}

// Remove dead modules from all other alive modules' reads list.
// This should only occur at class unloading.
void ModuleEntryTable::purge_all_module_reads() {
  assert_locked_or_safepoint(Module_lock);
  for (int i = 0; i < table_size(); i++) {
    for (ModuleEntry* entry = bucket(i);
                      entry != NULL;
                      entry = entry->next()) {
      entry->purge_reads();
    }
  }
}

void ModuleEntryTable::finalize_javabase(Handle module_handle, Symbol* version, Symbol* location) {
  assert(Module_lock->owned_by_self(), "should have the Module_lock");
  ClassLoaderData* boot_loader_data = ClassLoaderData::the_null_class_loader_data();
  ModuleEntryTable* module_table = boot_loader_data->modules();

  assert(module_table != NULL, "boot loader's ModuleEntryTable not defined");

  if (module_handle.is_null()) {
    fatal("Unable to finalize module definition for " JAVA_BASE_NAME);
  }

  // Set java.lang.Module, version and location for java.base
  ModuleEntry* jb_module = javabase_moduleEntry();
  assert(jb_module != NULL, JAVA_BASE_NAME " ModuleEntry not defined");
  jb_module->set_version(version);
  jb_module->set_location(location);
  // Once java.base's ModuleEntry _module field is set with the known
  // java.lang.Module, java.base is considered "defined" to the VM.
  jb_module->set_module(boot_loader_data->add_handle(module_handle));

  // Store pointer to the ModuleEntry for java.base in the java.lang.Module object.
  java_lang_Module::set_module_entry(module_handle(), jb_module);
}

// Within java.lang.Class instances there is a java.lang.Module field that must
// be set with the defining module.  During startup, prior to java.base's definition,
// classes needing their module field set are added to the fixup_module_list.
// Their module field is set once java.base's java.lang.Module is known to the VM.
void ModuleEntryTable::patch_javabase_entries(Handle module_handle) {
  if (module_handle.is_null()) {
    fatal("Unable to patch the module field of classes loaded prior to "
          JAVA_BASE_NAME "'s definition, invalid java.lang.Module");
  }

  // Do the fixups for the basic primitive types
  java_lang_Class::set_module(Universe::int_mirror(), module_handle());
  java_lang_Class::set_module(Universe::float_mirror(), module_handle());
  java_lang_Class::set_module(Universe::double_mirror(), module_handle());
  java_lang_Class::set_module(Universe::byte_mirror(), module_handle());
  java_lang_Class::set_module(Universe::bool_mirror(), module_handle());
  java_lang_Class::set_module(Universe::char_mirror(), module_handle());
  java_lang_Class::set_module(Universe::long_mirror(), module_handle());
  java_lang_Class::set_module(Universe::short_mirror(), module_handle());
  java_lang_Class::set_module(Universe::void_mirror(), module_handle());

  // Do the fixups for classes that have already been created.
  GrowableArray <Klass*>* list = java_lang_Class::fixup_module_field_list();
  int list_length = list->length();
  for (int i = 0; i < list_length; i++) {
    Klass* k = list->at(i);
    assert(k->is_klass(), "List should only hold classes");
    java_lang_Class::fixup_module_field(k, module_handle);
    k->class_loader_data()->dec_keep_alive();
  }

  delete java_lang_Class::fixup_module_field_list();
  java_lang_Class::set_fixup_module_field_list(NULL);
}

void ModuleEntryTable::print(outputStream* st) {
  st->print_cr("Module Entry Table (table_size=%d, entries=%d)",
               table_size(), number_of_entries());
  for (int i = 0; i < table_size(); i++) {
    for (ModuleEntry* probe = bucket(i);
                              probe != NULL;
                              probe = probe->next()) {
      probe->print(st);
    }
  }
}

void ModuleEntry::print(outputStream* st) {
  ResourceMark rm;
  st->print_cr("entry " PTR_FORMAT " name %s module " PTR_FORMAT " loader %s version %s location %s strict %s next " PTR_FORMAT,
               p2i(this),
               name() == NULL ? UNNAMED_MODULE : name()->as_C_string(),
               p2i(module()),
               loader_data()->loader_name_and_id(),
               version() != NULL ? version()->as_C_string() : "NULL",
               location() != NULL ? location()->as_C_string() : "NULL",
               BOOL_TO_STR(!can_read_all_unnamed()), p2i(next()));
}

void ModuleEntryTable::verify() {
  verify_table<ModuleEntry>("Module Entry Table");
}

void ModuleEntry::verify() {
  guarantee(loader_data() != NULL, "A module entry must be associated with a loader.");
}