源码分析：Java堆的创建

虚拟机在内存中申请一片区域，由虚拟机自动管理，用来满足应用程序对象分配的空间需求，即堆空间。

由于程序运行的局部特性，程序创建的大多数对象都具有非常短的生命周期，而程序也会创建一些生命周期特别长的对象。简单的复制收集器无论对象的生命周期是长是短，都会进行复制操作。而生命周期较长的对象在多次垃圾回收期间内并不会被回收，这就使得这些对象被来回复制而使得算法性能大大下降。

分代收集把堆分为多个子堆，分别用来存放不同寿命的对象。新生对象空间的将经历最频繁的垃圾回收，而对于经历了若干次垃圾收集后仍然存活的对象，将成长为成熟对象，并移动到成熟对象的子堆中，而对老生代子堆的垃圾回收就不会像新生对象子堆那么频繁。

HotSpot的堆空间分为新生代(YoungGen)和老年代(OldGen，此外还有位于非堆空间的永久代，但在Java8中将移除永久代)，新生代又分为Eden区和2个Survivor区(From/To)用以进行复制收集垃圾对象。对Java堆和对象的分析将从Java堆的创建开始，然后分析Java对象的分配与垃圾回收。

一、堆的实现方式

//定义在/hotspot/src/share/vm/memory/universe.cpp中 if (UseParallelGC) { #ifndef SERIALGC     Universe::_collectedHeap = new ParallelScavengeHeap(); #else  // SERIALGC     fatal("UseParallelGC not supported in java kernel vm."); #endif // SERIALGC    }

2.若虚拟机配置UseG1GC，那么将选择堆类型为G1CollectedHeap，垃圾收集策略将使用专用的G1CollectorPolicy(垃圾优先收集)策略

else if (UseG1GC) { #ifndef SERIALGC  G1CollectorPolicy* g1p = new G1CollectorPolicy_BestRegionsFirst();  G1CollectedHeap* g1h = new G1CollectedHeap(g1p);  Universe::_collectedHeap = g1h; #else  // SERIALGC  fatal("UseG1GC not supported in java kernel vm."); #endif // SERIALGC   } 

3.否则，虚拟机将使用GenCollectedHeap(分代收集堆)

Universe::_collectedHeap = new GenCollectedHeap(gc_policy);

各个堆实现类的类关系如下：

对于默认情况下的堆实现，还要根据配置选择垃圾回收策略gc_policy来构造一个GenCollectedHeap，这里根据虚拟机配置选择不同的GC策略：(1).若虚拟机配置UseSerialGC，那么将使用MarkSweepPolicy(标记-清除)策略

GenCollectorPolicy *gc_policy;      if (UseSerialGC) {       gc_policy = new MarkSweepPolicy();     }

(2).若虚拟机配置UseConcMarkSweepGC和UseAdaptiveSizePolicy，那么将使用ASConcurrentMarkSweepPolicy(自适应并发标记-清除)策略，若没有指定UseAdaptiveSizePolicy，虚拟机将默认使用ConcurrentMarkSweepPolicy(并发标记-清除)策略

else if (UseConcMarkSweepGC) { #ifndef SERIALGC       if (UseAdaptiveSizePolicy) {         gc_policy = new ASConcurrentMarkSweepPolicy();       } else {         gc_policy = new ConcurrentMarkSweepPolicy();       }

(3).若没有进行配置，虚拟机将默认使用MarkSweepPolicy策略

else { // default old generation       gc_policy = new MarkSweepPolicy();     }

如下表所示：

其中垃圾回收策略类的关系如下图：

4.接下来是相应实现的堆的初始化

jint status = Universe::heap()->initialize();   if (status != JNI_OK) {     return status;   }

addr = <narrow_oop_base> + <narrow_oop> << 3 + <field_offset>

if ((uint64_t)Universe::heap()->reserved_region().end() > OopEncodingHeapMax) {       // Can't reserve heap below 32Gb.       Universe::set_narrow_oop_base(Universe::heap()->base() - os::vm_page_size());       Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes); }

(2).否则设置基础地址为0

else {       Universe::set_narrow_oop_base(0);       //...       }

若heap的地址空间的最大地址大于NarrowOopHeapMax(4GB，小于32GB)，则设置偏移为LogMinObjAlignmentInBytes(默认为3)，即使用零基压缩技术，否则设置偏移为0，即直接使用压缩对象指针进行寻址

if((uint64_t)Universe::heap()->reserved_region().end() > NarrowOopHeapMax) {         // Can't reserve heap below 4Gb.         Universe::set_narrow_oop_shift(LogMinObjAlignmentInBytes);       } else {         Universe::set_narrow_oop_shift(0);

二、堆的初始化：分代实现方式

接下来分析特定堆的初始化过程，这里以GenCollectedHeap和MarkSweepPolicy为例：

GenCollectedHeap的构造函数中使用传入的策略作为_gen_policy(代策略)。以MarkSweepPolicy为例，看看其构造函数：

//定义在/hotspot/src/share/vm/memory/collectorPolicy.cpp中 MarkSweepPolicy::MarkSweepPolicy() {   initialize_all(); }

MarkSweepPolicy的构造函数调用了initialize_all()来完成策略的初始化，initialize_all()是父类GenCollectorPolicy()的虚函数，它调用了三个子初始化虚函数，这三个子初始化过程由GenCollectorPolicy的子类实现。 其中initialize_flags()初始化了永久代的一些大小配置参数，initialize_size_info()设置了Java堆大小的相关参数，initialize_generations()根据用户参数，配置各内存代的管理器。

//hotspot/src/share/vm/memory/collectorPolicy.hpp中 virtual void initialize_all() {     initialize_flags();     initialize_size_info();     initialize_generations();   }

下面通过initialize_generations()来看看各代有哪些实现方式：1.若配置了UseParNewGC，并且并行GC线程数大于1，那么新生代就会使用ParNew实现

//永久代初始化   _generations = new GenerationSpecPtr[number_of_generations()];   //...    if (UseParNewGC && ParallelGCThreads > 0) {     _generations[0] = new GenerationSpec(Generation::ParNew, _initial_gen0_size, _max_gen0_size);   }

2.默认新生代使用DefNew实现

else {     _generations[0] = new GenerationSpec(Generation::DefNew, _initial_gen0_size, _max_gen0_size);   }

3.老年代固定使用MarkSweepCompact实现

_generations[1] = new GenerationSpec(Generation::MarkSweepCompact, _initial_gen1_size, _max_gen1_size);

(其中DefNew、ParNew、MarkSweepCompact等均为Generation的枚举集合Name的成员，描述了可能实现的各种代实现类型)MarkSweepPolicy、ConcurrentMarkSweepPolicy、ASConcurrentMarkSweepPolicy对各代的实现综合如下表所示： 

三、堆的初始化：堆内存空间分配

//定义在/hotspot/src/share/vm/memory/genCollectedHeap.cpp中 jint GenCollectedHeap::initialize() {   //...   _n_gens = gen_policy()->number_of_generations();

根据GenCollectedHeap的定义可以看到，GenCollectedHeap最多支持10个分代

enum SomeConstants {     max_gens = 10   };  //...  private:   int _n_gens;   Generation* _gens[max_gens];

其实并不需要这么多分代，MarkSweepPolicy、ConcurrentMarkSweepPolicy、ASConcurrentMarkSweepPolicy(ConcurrentMarkSweepPolicy的子类)均有着共同的祖先类TwoGenerationCollectorPolicy，其分代只有2代，即新生代和老年代。

2.每代的大小是基于GenGrain大小对齐的

// The heap must be at least as aligned as generations.   size_t alignment = Generation::GenGrain;

GenGrain定义在/hotspot/src/share/vm/memory/generation.h中，在非ARM平台中是2^16字节，即64KB大小3.获取各分代的管理器指针数组和永久代的管理器指针，并对齐各代的大小到64KB

PermanentGenerationSpec *perm_gen_spec =  collector_policy()->permanent_generation(); // Make sure the sizes are all aligned. for (i = 0; i < _n_gens; i++) {   _gen_specs[i]->align(alignment); } perm_gen_spec->align(alignment); 

GenerationSpec的align()定义在/hotspot/src/share/vm/memory/generationSpec.h，使初始和最大大小值向上对齐至64KB的倍数

// Alignment   void align(size_t alignment) {     set_init_size(align_size_up(init_size(), alignment));     set_max_size(align_size_up(max_size(), alignment));   }

4.调用allocate()为堆分配空间，其起始地址为heap_address

char* heap_address;   size_t total_reserved = 0;   int n_covered_regions = 0;   ReservedSpace heap_rs(0);    heap_address = allocate(alignment, perm_gen_spec, &total_reserved,                           &n_covered_regions, &heap_rs);

5.初始分配所得的空间将被封装在_reserved(CollectedHeap的MemRegion成员)中

_reserved = MemRegion((HeapWord*)heap_rs.base(),                         (HeapWord*)(heap_rs.base() + heap_rs.size()));

调整实际的堆大小为去掉永久代的misc_data和misc_code的空间，并创建一个覆盖整个空间的数组，数组每个字节对应于堆的512字节，用于遍历新生代和老年代空间

_reserved.set_word_size(0);   _reserved.set_start((HeapWord*)heap_rs.base());   size_t actual_heap_size = heap_rs.size() - perm_gen_spec->misc_data_size()                                            - perm_gen_spec->misc_code_size();   _reserved.set_end((HeapWord*)(heap_rs.base() + actual_heap_size));    _rem_set = collector_policy()->create_rem_set(_reserved, n_covered_regions);   set_barrier_set(rem_set()->bs());

7.调用heap_rs的的first_part()，依次为新生代和老年代分配空间并调用各代管理器的init()将其初始化为Generation空间，最后为永久代分配空间和进行初始化

_gch = this;    for (i = 0; i < _n_gens; i++) {     ReservedSpace this_rs = heap_rs.first_part(_gen_specs[i]->max_size(),                                               UseSharedSpaces, UseSharedSpaces);     _gens[i] = _gen_specs[i]->init(this_rs, i, rem_set());     heap_rs = heap_rs.last_part(_gen_specs[i]->max_size());   }   _perm_gen = perm_gen_spec->init(heap_rs, PermSize, rem_set());

四、内存空间申请实现

char* GenCollectedHeap::allocate(size_t alignment,  PermanentGenerationSpec* perm_gen_spec,  size_t* _total_reserved,  int* _n_covered_regions,  ReservedSpace* heap_rs){ //... // Now figure out the total size. size_t total_reserved = 0; int n_covered_regions = 0; const size_t pageSize = UseLargePages ?     os::large_page_size() : os::vm_page_size(); for (int i = 0; i < _n_gens; i++) {   total_reserved += _gen_specs[i]->max_size();   if (total_reserved < _gen_specs[i]->max_size()) {     vm_exit_during_initialization(overflow_msg);   }   n_covered_regions += _gen_specs[i]->n_covered_regions(); } 

加上永久代空间的大小和块数

total_reserved += perm_gen_spec->max_size(); if (total_reserved < perm_gen_spec->max_size()) {     vm_exit_during_initialization(overflow_msg);   }   n_covered_regions += perm_gen_spec->n_covered_regions();

(2).加上永久代的misc_data和misc_code的空间大小(数据区和代码区)，但其实并不是堆的一部分

size_t s = perm_gen_spec->misc_data_size() + perm_gen_spec->misc_code_size();    total_reserved += s;

(3).如果配置了UseLargePages，那么将向上将申请的内存空间大小对齐至页

if (UseLargePages) {     assert(total_reserved != 0, "total_reserved cannot be 0");     total_reserved = round_to(total_reserved, os::large_page_size());     if (total_reserved < os::large_page_size()) {       vm_exit_during_initialization(overflow_msg);     }   }

(4).对象地址压缩的内容根据UnscaledNarrowOop(直接使用压缩指针)选取合适的堆起始地址，并尝试在该地址上分配内存

if (UseCompressedOops) {       heap_address = Universe::preferred_heap_base(total_reserved, Universe::UnscaledNarrowOop);       *_total_reserved = total_reserved;       *_n_covered_regions = n_covered_regions;       *heap_rs = ReservedHeapSpace(total_reserved, alignment,                                    UseLargePages, heap_address);

若不能再该地址进行分配内存，则尝试使用ZereBasedNarrowOop(零基压缩)尝试在更高的地址空间上进行分配

if (heap_address != NULL && !heap_rs->is_reserved()) {         // Failed to reserve at specified address - the requested memory         // region is taken already, for example, by 'java' launcher.         // Try again to reserver heap higher.         heap_address = Universe::preferred_heap_base(total_reserved, Universe::ZeroBasedNarrowOop);         *heap_rs = ReservedHeapSpace(total_reserved, alignment,                                      UseLargePages, heap_address);

若仍然失败，则使用普通的指针压缩技术在其他地址上进行分配

if (heap_address != NULL && !heap_rs->is_reserved()) {  // Failed to reserve at specified address again - give up.  heap_address = Universe::preferred_heap_base(total_reserved, Universe::HeapBasedNarrowOop);  assert(heap_address == NULL, "");  *heap_rs = ReservedHeapSpace(total_reserved, alignment,          UseLargePages, heap_address);   } } 

2.调用ReservedHeapSpace的构造函数进行内存空间的申请

*_total_reserved = total_reserved;   *_n_covered_regions = n_covered_regions;   *heap_rs = ReservedHeapSpace(total_reserved, alignment,                                UseLargePages, heap_address);    return heap_address;

在构造函数中并没有发现对内存空间进行申请，那么继续看父类ReservedSpace的构造函数

ReservedSpace::ReservedSpace(size_t size, size_t alignment,                                bool large,                                char* requested_address,                                const size_t noaccess_prefix) {     initialize(size+noaccess_prefix, alignment, large, requested_address,  noaccess_prefix, false);   }

3.initialize()的实现如下：(1).如果目标操作系统不支持large_page_memory，那么将进行特殊处理，此外，对指针压缩处理还需要对请求分配的内存空间大小进行调整

if (requested_address != 0) {       requested_address -= noaccess_prefix; // adjust requested address       assert(requested_address != NULL, "huge noaccess prefix?");     }

(2).对于上述特殊情况，会调用reserve_memory_special()进行内存空间的申请，并若申请成功会进行空间大小的对齐验证

if (special) {    //向操作系统申请指定大小的内存，并映射到用户指定的内存空间中   base = os::reserve_memory_special(size, requested_address, executable);    if (base != NULL) {     if (failed_to_reserve_as_requested(base, requested_address, size, true)) {       // OS ignored requested address. Try different address.       return;     }     // Check alignment constraints     assert((uintptr_t) base % alignment == 0, "Large pages returned a non-aligned address");     _special = true; 

(3).若配置了UseSharedSpace或UseCompressedOops，那么堆将在指定地址进行申请，就会调用attempt_reserve_memory_at()进行申请，否则，调用reserve_memory()进行申请

if (requested_address != 0) {     base = os::attempt_reserve_memory_at(size, requested_address);      if (failed_to_reserve_as_requested(base, requested_address, size, false)) {       // OS ignored requested address. Try different address.       base = NULL;     }   } else {     base = os::reserve_memory(size, NULL, alignment);   } 

(4).若分配成功，还需要对分配的起始地址进行对齐验证。若没有对齐，则会进行手工调整。 调整的方法为尝试申请一块size+alignment大小的空间，若成功则向上对齐所得的内存空间的起始地址(失败则无法对齐，直接返回)，并以此为起始地址重新申请一块size大小的空间，这块size大小的空间必然包含于size+alignment大小的空间内，以此达到对齐地址的目的。

// Check alignment constraints   if ((((size_t)base + noaccess_prefix) & (alignment - 1)) != 0) {     // Base not aligned, retry     if (!os::release_memory(base, size)) fatal("os::release_memory failed");     // Reserve size large enough to do manual alignment and     // increase size to a multiple of the desired alignment     size = align_size_up(size, alignment);     size_t extra_size = size + alignment;     do {       char* extra_base = os::reserve_memory(extra_size, NULL, alignment);       if (extra_base == NULL) return;        // Do manual alignement       base = (char*) align_size_up((uintptr_t) extra_base, alignment);       assert(base >= extra_base, "just checking");       // Re-reserve the region at the aligned base address.       os::release_memory(extra_base, extra_size);       base = os::reserve_memory(size, base);     } while (base == NULL); 

最后，在地址空间均已分配完毕，GenCollectedHeap的initialize()中为各代划分了各自的内存空间范围，就会调用各代的GenerationSpec的init()函数完成各代的初始化。

switch (name()) {     case PermGen::MarkSweepCompact:       return new CompactingPermGen(perm_rs, shared_rs, init_size, remset, this);  #ifndef SERIALGC     case PermGen::MarkSweep:       guarantee(false, "NYI");       return NULL;      case PermGen::ConcurrentMarkSweep: {       assert(UseConcMarkSweepGC, "UseConcMarkSweepGC should be set");       CardTableRS* ctrs = remset->as_CardTableRS();       if (ctrs == NULL) {         vm_exit_during_initialization("RemSet/generation incompatibility.");       }       // XXXPERM       return new CMSPermGen(perm_rs, init_size, ctrs,                    (FreeBlockDictionary::DictionaryChoice)CMSDictionaryChoice);     } #endif // SERIALGC     default:       guarantee(false, "unrecognized GenerationName");       return NULL;   }

各分代实现类的类关系如下：

归纳堆初始化的流程图如下：