gc分为了多种回收器，在虚拟机启动时有默认参数或者输入参数来控制具体的gc回收器
thread#create_vm时进行参数加载

1、默认参数

具体的启动参数有哪些可以通过java命令java -XX:+PrintFlagsInitial查看
常用参数类型：
标准参数:-（如-version）
非标准:-X(如-Xmixed)
不稳定:-XX（如-XX:+UseG1GC）

这些参数在jvm内部处理一共有五种：
flags-file:文件配置参数，指定后-XX:参数前缀可以省略，多条换行即可
cmd_args：启动命令行传入的参数
vm_options_args：系统默认配置，通过解析jar或者jmod获取
java_options_args：jvm选项，可以覆盖java_tool_options_args
java_tool_options_args：jvm监控选项，如JPS，JMAP，JSTAT接收的参数，语法为-Djava.tool.option=value （如-Dcom.sun.management.jmxremote.port=8444）

在选择具体的gc之前，获取系统默认参数并且将vm option 解析出来进行替换

//arguments.cpp
jint Arguments::parse(const JavaVMInitArgs* initial_cmd_args) {
  assert(verify_special_jvm_flags(false), "deprecated and obsolete flag table inconsistent");
  JVMFlag::check_all_flag_declarations();

  // If flag "-XX:Flags=flags-file" is used it will be the first option to be processed.
  const char* hotspotrc = ".hotspotrc";
  bool settings_file_specified = false;
  bool needs_hotspotrc_warning = false;
  ScopedVMInitArgs initial_vm_options_args("");
  ScopedVMInitArgs initial_java_tool_options_args("env_var='JAVA_TOOL_OPTIONS'");
  ScopedVMInitArgs initial_java_options_args("env_var='_JAVA_OPTIONS'");

  // Pointers to current working set of containers
  JavaVMInitArgs* cur_cmd_args;
  JavaVMInitArgs* cur_vm_options_args;
  JavaVMInitArgs* cur_java_options_args;
  JavaVMInitArgs* cur_java_tool_options_args;

  // Containers for modified/expanded options
  ScopedVMInitArgs mod_cmd_args("cmd_line_args");
  ScopedVMInitArgs mod_vm_options_args("vm_options_args");
  ScopedVMInitArgs mod_java_tool_options_args("env_var='JAVA_TOOL_OPTIONS'");
  ScopedVMInitArgs mod_java_options_args("env_var='_JAVA_OPTIONS'");


  jint code =
      parse_java_tool_options_environment_variable(&initial_java_tool_options_args);
  if (code != JNI_OK) {
    return code;
  }

  code = parse_java_options_environment_variable(&initial_java_options_args);
  if (code != JNI_OK) {
    return code;
  }

  // Parse the options in the /java.base/jdk/internal/vm/options resource, if present
  //通过解析module文件中的配置获取vm options (jdk17源码中未找到这个文件，使用jmod查看java.base的包也没找到这个配置)
  char *vmoptions = ClassLoader::lookup_vm_options();
  if (vmoptions != NULL) {
    code = parse_options_buffer("vm options resource", vmoptions, strlen(vmoptions), &initial_vm_options_args);
    FREE_C_HEAP_ARRAY(char, vmoptions);
    if (code != JNI_OK) {
      return code;
    }
  }

  code = expand_vm_options_as_needed(initial_java_tool_options_args.get(),
                                     &mod_java_tool_options_args,
                                     &cur_java_tool_options_args);
  if (code != JNI_OK) {
    return code;
  }

  code = expand_vm_options_as_needed(initial_cmd_args,
                                     &mod_cmd_args,
                                     &cur_cmd_args);
  if (code != JNI_OK) {
    return code;
  }

  code = expand_vm_options_as_needed(initial_java_options_args.get(),
                                     &mod_java_options_args,
                                     &cur_java_options_args);
  if (code != JNI_OK) {
    return code;
  }

  code = expand_vm_options_as_needed(initial_vm_options_args.get(),
                                     &mod_vm_options_args,
                                     &cur_vm_options_args);
  if (code != JNI_OK) {
    return code;
  }

  const char* flags_file = Arguments::get_jvm_flags_file();
  settings_file_specified = (flags_file != NULL);

  if (IgnoreUnrecognizedVMOptions) {
    cur_cmd_args->ignoreUnrecognized = true;
    cur_java_tool_options_args->ignoreUnrecognized = true;
    cur_java_options_args->ignoreUnrecognized = true;
  }

  // Parse specified settings file
  if (settings_file_specified) {
    if (!process_settings_file(flags_file, true,
                               cur_cmd_args->ignoreUnrecognized)) {
      return JNI_EINVAL;
    }
  } else {
#ifdef ASSERT
    // Parse default .hotspotrc settings file
    if (!process_settings_file(".hotspotrc", false,
                               cur_cmd_args->ignoreUnrecognized)) {
      return JNI_EINVAL;
    }
#else
    struct stat buf;
    if (os::stat(hotspotrc, &buf) == 0) {
      needs_hotspotrc_warning = true;
    }
#endif
  }

  if (PrintVMOptions) {
    print_options(cur_java_tool_options_args);
    print_options(cur_cmd_args);
    print_options(cur_java_options_args);
  }

  // Parse JavaVMInitArgs structure passed in, as well as JAVA_TOOL_OPTIONS and _JAVA_OPTIONS
  jint result = parse_vm_init_args(cur_vm_options_args,
                                   cur_java_tool_options_args,
                                   cur_java_options_args,
                                   cur_cmd_args);

  if (result != JNI_OK) {
    return result;
  }

  //...

  apply_debugger_ergo();

  return JNI_OK;
}

2、选择gc

上一步初始化完成参数设置后在这里选择具体的gc
具体选择的地方在threads create_vm中

jint ergo_result = Arguments::apply_ergo();

jinit Arguments::apply_ergo(){
jint result = set_ergonomics_flags();
  if (result != JNI_OK) return result;

  // Set heap size based on available physical memory
  set_heap_size();
//选择gc后进行初始参数设置
  GCConfig::arguments()->initialize();
  //...
}
//调用了选择器
void GCConfig::initialize() {
  assert(_arguments == NULL, "Already initialized");
  _arguments = select_gc();
}

//选择对应的gc
GCArguments* GCConfig::select_gc() {
  //...
  // Exactly one GC selected
  FOR_EACH_INCLUDED_GC(gc) {
    if (gc->_flag) {
      return &gc->_arguments;
    }
  }
  fatal("Should have found the selected GC");
  return NULL;
}
//当前版本支持的gc以及参数,UseG1GC这第一个参数就是系统设置的gc
static const IncludedGC IncludedGCs[] = {
   EPSILONGC_ONLY_ARG(IncludedGC(UseEpsilonGC,       CollectedHeap::Epsilon,    epsilonArguments,    "epsilon gc"))
        G1GC_ONLY_ARG(IncludedGC(UseG1GC,            CollectedHeap::G1,         g1Arguments,         "g1 gc"))
  PARALLELGC_ONLY_ARG(IncludedGC(UseParallelGC,      CollectedHeap::Parallel,   parallelArguments,   "parallel gc"))
    SERIALGC_ONLY_ARG(IncludedGC(UseSerialGC,        CollectedHeap::Serial,     serialArguments,     "serial gc"))
SHENANDOAHGC_ONLY_ARG(IncludedGC(UseShenandoahGC,    CollectedHeap::Shenandoah, shenandoahArguments, "shenandoah gc"))
         ZGC_ONLY_ARG(IncludedGC(UseZGC,             CollectedHeap::Z,          zArguments,          "z gc"))
};

#define FOR_EACH_INCLUDED_GC(var)                                            \
  for (const IncludedGC* var = &IncludedGCs[0]; var < &IncludedGCs[ARRAY_SIZE(IncludedGCs)]; var++)

初始化选择了gc类型后调用到了同样在create_vm中的init_globals
在universe_init()中进行了gc初始化，完成后创建了SymbolTable，StringTable，ResolvedMethodTable

jint universe_init() {
  assert(!Universe::_fully_initialized, "called after initialize_vtables");
  guarantee(1 << LogHeapWordSize == sizeof(HeapWord),
         "LogHeapWordSize is incorrect.");
  guarantee(sizeof(oop) >= sizeof(HeapWord), "HeapWord larger than oop?");
  guarantee(sizeof(oop) % sizeof(HeapWord) == 0,
            "oop size is not not a multiple of HeapWord size");

  TraceTime timer("Genesis", TRACETIME_LOG(Info, startuptime));

  initialize_global_behaviours();

  GCLogPrecious::initialize();

  GCConfig::arguments()->initialize_heap_sizes();
 //看这里
  jint status = Universe::initialize_heap();
  if (status != JNI_OK) {
    return status;
  }

  Universe::initialize_tlab();

  Metaspace::global_initialize();

  // Initialize performance counters for metaspaces
  MetaspaceCounters::initialize_performance_counters();

  // Checks 'AfterMemoryInit' constraints.
  if (!JVMFlagLimit::check_all_constraints(JVMFlagConstraintPhase::AfterMemoryInit)) {
    return JNI_EINVAL;
  }

  // Create memory for metadata.  Must be after initializing heap for
  // DumpSharedSpaces.
  ClassLoaderData::init_null_class_loader_data();

  // We have a heap so create the Method* caches before
  // Metaspace::initialize_shared_spaces() tries to populate them.
  Universe::_finalizer_register_cache = new LatestMethodCache();
  Universe::_loader_addClass_cache    = new LatestMethodCache();
  Universe::_throw_illegal_access_error_cache = new LatestMethodCache();
  Universe::_throw_no_such_method_error_cache = new LatestMethodCache();
  Universe::_do_stack_walk_cache = new LatestMethodCache();

#if INCLUDE_CDS
  if (UseSharedSpaces) {
    // Read the data structures supporting the shared spaces (shared
    // system dictionary, symbol table, etc.).  After that, access to
    // the file (other than the mapped regions) is no longer needed, and
    // the file is closed. Closing the file does not affect the
    // currently mapped regions.
    MetaspaceShared::initialize_shared_spaces();
    StringTable::create_table();
  } else
#endif
  {
    SymbolTable::create_table();
    StringTable::create_table();
  }

#if INCLUDE_CDS
  if (Arguments::is_dumping_archive()) {
    MetaspaceShared::prepare_for_dumping();
  }
#endif

  if (strlen(VerifySubSet) > 0) {
    Universe::initialize_verify_flags();
  }

  ResolvedMethodTable::create_table();

  return JNI_OK;
}

initialize_heap根据参数找到对应的gc参数进行初始化

jint Universe::initialize_heap() {
  assert(_collectedHeap == NULL, "Heap already created");
  _collectedHeap = GCConfig::arguments()->create_heap();

  log_info(gc)("Using %s", _collectedHeap->name());
  return _collectedHeap->initialize();
}

_collectedHeap在g1时为G1CollectedHeap，在其方法initialize中，开启了gc线程
如果是serial gc则不会异步开启线程

jint G1CollectedHeap::initialize() {

  // Necessary to satisfy locking discipline assertions.

  MutexLocker x(Heap_lock);

  // While there are no constraints in the GC code that HeapWordSize
  // be any particular value, there are multiple other areas in the
  // system which believe this to be true (e.g. oop->object_size in some
  // cases incorrectly returns the size in wordSize units rather than
  // HeapWordSize).
  guarantee(HeapWordSize == wordSize, "HeapWordSize must equal wordSize");

  size_t init_byte_size = InitialHeapSize;
  size_t reserved_byte_size = G1Arguments::heap_reserved_size_bytes();

  // Ensure that the sizes are properly aligned.
  Universe::check_alignment(init_byte_size, HeapRegion::GrainBytes, "g1 heap");
  Universe::check_alignment(reserved_byte_size, HeapRegion::GrainBytes, "g1 heap");
  Universe::check_alignment(reserved_byte_size, HeapAlignment, "g1 heap");

  // Reserve the maximum.

  // When compressed oops are enabled, the preferred heap base
  // is calculated by subtracting the requested size from the
  // 32Gb boundary and using the result as the base address for
  // heap reservation. If the requested size is not aligned to
  // HeapRegion::GrainBytes (i.e. the alignment that is passed
  // into the ReservedHeapSpace constructor) then the actual
  // base of the reserved heap may end up differing from the
  // address that was requested (i.e. the preferred heap base).
  // If this happens then we could end up using a non-optimal
  // compressed oops mode.

  ReservedHeapSpace heap_rs = Universe::reserve_heap(reserved_byte_size,
                                                     HeapAlignment);

  initialize_reserved_region(heap_rs);

  // Create the barrier set for the entire reserved region.
  G1CardTable* ct = new G1CardTable(heap_rs.region());
  ct->initialize();
  G1BarrierSet* bs = new G1BarrierSet(ct);
  bs->initialize();
  assert(bs->is_a(BarrierSet::G1BarrierSet), "sanity");
  BarrierSet::set_barrier_set(bs);
  _card_table = ct;

  {
    G1SATBMarkQueueSet& satbqs = bs->satb_mark_queue_set();
    satbqs.set_process_completed_buffers_threshold(G1SATBProcessCompletedThreshold);
    satbqs.set_buffer_enqueue_threshold_percentage(G1SATBBufferEnqueueingThresholdPercent);
  }

  // Create the hot card cache.
  _hot_card_cache = new G1HotCardCache(this);

  // Create space mappers.
  size_t page_size = heap_rs.page_size();
  G1RegionToSpaceMapper* heap_storage =
    G1RegionToSpaceMapper::create_mapper(heap_rs,
                                         heap_rs.size(),
                                         page_size,
                                         HeapRegion::GrainBytes,
                                         1,
                                         mtJavaHeap);
  if(heap_storage == NULL) {
    vm_shutdown_during_initialization("Could not initialize G1 heap");
    return JNI_ERR;
  }

  os::trace_page_sizes("Heap",
                       MinHeapSize,
                       reserved_byte_size,
                       page_size,
                       heap_rs.base(),
                       heap_rs.size());
  heap_storage->set_mapping_changed_listener(&_listener);

  // Create storage for the BOT, card table, card counts table (hot card cache) and the bitmaps.
  G1RegionToSpaceMapper* bot_storage =
    create_aux_memory_mapper("Block Offset Table",
                             G1BlockOffsetTable::compute_size(heap_rs.size() / HeapWordSize),
                             G1BlockOffsetTable::heap_map_factor());

  G1RegionToSpaceMapper* cardtable_storage =
    create_aux_memory_mapper("Card Table",
                             G1CardTable::compute_size(heap_rs.size() / HeapWordSize),
                             G1CardTable::heap_map_factor());

  G1RegionToSpaceMapper* card_counts_storage =
    create_aux_memory_mapper("Card Counts Table",
                             G1CardCounts::compute_size(heap_rs.size() / HeapWordSize),
                             G1CardCounts::heap_map_factor());

  size_t bitmap_size = G1CMBitMap::compute_size(heap_rs.size());
  G1RegionToSpaceMapper* prev_bitmap_storage =
    create_aux_memory_mapper("Prev Bitmap", bitmap_size, G1CMBitMap::heap_map_factor());
  G1RegionToSpaceMapper* next_bitmap_storage =
    create_aux_memory_mapper("Next Bitmap", bitmap_size, G1CMBitMap::heap_map_factor());

  _hrm.initialize(heap_storage, prev_bitmap_storage, next_bitmap_storage, bot_storage, cardtable_storage, card_counts_storage);
  _card_table->initialize(cardtable_storage);

  // Do later initialization work for concurrent refinement.
  _hot_card_cache->initialize(card_counts_storage);

  // 6843694 - ensure that the maximum region index can fit
  // in the remembered set structures.
  const uint max_region_idx = (1U << (sizeof(RegionIdx_t)*BitsPerByte-1)) - 1;
  guarantee((max_reserved_regions() - 1) <= max_region_idx, "too many regions");

  // The G1FromCardCache reserves card with value 0 as "invalid", so the heap must not
  // start within the first card.
  guarantee(heap_rs.base() >= (char*)G1CardTable::card_size, "Java heap must not start within the first card.");
  G1FromCardCache::initialize(max_reserved_regions());
  // Also create a G1 rem set.
  _rem_set = new G1RemSet(this, _card_table, _hot_card_cache);
  _rem_set->initialize(max_reserved_regions());

  size_t max_cards_per_region = ((size_t)1 << (sizeof(CardIdx_t)*BitsPerByte-1)) - 1;
  guarantee(HeapRegion::CardsPerRegion > 0, "make sure it's initialized");
  guarantee(HeapRegion::CardsPerRegion < max_cards_per_region,
            "too many cards per region");

  FreeRegionList::set_unrealistically_long_length(max_regions() + 1);

  _bot = new G1BlockOffsetTable(reserved(), bot_storage);

  {
    size_t granularity = HeapRegion::GrainBytes;

    _region_attr.initialize(reserved(), granularity);
    _humongous_reclaim_candidates.initialize(reserved(), granularity);
  }
//gc线程
  _workers = new WorkGang("GC Thread", ParallelGCThreads,
                          true /* are_GC_task_threads */,
                          false /* are_ConcurrentGC_threads */);
  if (_workers == NULL) {
    return JNI_ENOMEM;
  }
  _workers->initialize_workers();

  _numa->set_region_info(HeapRegion::GrainBytes, page_size);

  // Create the G1ConcurrentMark data structure and thread.
  // (Must do this late, so that "max_[reserved_]regions" is defined.)
//gc标记线程
  _cm = new G1ConcurrentMark(this, prev_bitmap_storage, next_bitmap_storage);
  _cm_thread = _cm->cm_thread();

  // Now expand into the initial heap size.
  if (!expand(init_byte_size, _workers)) {
    vm_shutdown_during_initialization("Failed to allocate initial heap.");
    return JNI_ENOMEM;
  }

  // Perform any initialization actions delegated to the policy.
  policy()->init(this, &_collection_set);

  jint ecode = initialize_concurrent_refinement();
  if (ecode != JNI_OK) {
    return ecode;
  }

  ecode = initialize_service_thread();
  if (ecode != JNI_OK) {
    return ecode;
  }

  // Initialize and schedule sampling task on service thread.
  _rem_set->initialize_sampling_task(service_thread());

  // Create and schedule the periodic gc task on the service thread.
  _periodic_gc_task = new G1PeriodicGCTask("Periodic GC Task");
  _service_thread->register_task(_periodic_gc_task);

  {
    G1DirtyCardQueueSet& dcqs = G1BarrierSet::dirty_card_queue_set();
    dcqs.set_process_cards_threshold(concurrent_refine()->yellow_zone());
    dcqs.set_max_cards(concurrent_refine()->red_zone());
  }

  // Here we allocate the dummy HeapRegion that is required by the
  // G1AllocRegion class.
  HeapRegion* dummy_region = _hrm.get_dummy_region();

  // We'll re-use the same region whether the alloc region will
  // require BOT updates or not and, if it doesn't, then a non-young
  // region will complain that it cannot support allocations without
  // BOT updates. So we'll tag the dummy region as eden to avoid that.
  dummy_region->set_eden();
  // Make sure it's full.
  dummy_region->set_top(dummy_region->end());
  G1AllocRegion::setup(this, dummy_region);

  _allocator->init_mutator_alloc_regions();

  // Do create of the monitoring and management support so that
  // values in the heap have been properly initialized.
  _g1mm = new G1MonitoringSupport(this);

  _preserved_marks_set.init(ParallelGCThreads);

  _collection_set.initialize(max_reserved_regions());

  _regions_failed_evacuation = NEW_C_HEAP_ARRAY(volatile bool, max_regions(), mtGC);

  G1InitLogger::print();

  return JNI_OK;
}