jvm源码解读--03 常量池的解析ConstantPool

先看bt栈

(gdb) bt
#0  ConstantPool::allocate (loader_data=0x7fe21802e868, length=87, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/oops/constantPool.cpp:47
#1  0x00007fe2206d0bbc in ClassFileParser::parse_constant_pool (this=0x7fe22229a010, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/classFileParser.cpp:331
#2  0x00007fe2206dce84 in ClassFileParser::parseClassFile (this=0x7fe22229a010, name=0x7fe21d4ad0e8, loader_data=0x7fe21802e868, protection_domain=..., host_klass=..., cp_patches=0x0, parsed_name=..., verify=false, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/classFileParser.cpp:3774
#3  0x00007fe2206eb9b5 in ClassFileParser::parseClassFile (this=0x7fe22229a010, name=0x7fe21d4ad0e8, loader_data=0x7fe21802e868, protection_domain=..., parsed_name=..., verify=false, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/classFileParser.hpp:468
#4  0x00007fe2206e97cb in ClassLoader::load_classfile (h_name=0x7fe21d4ad0e8, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/classLoader.cpp:931
#5  0x00007fe220d29903 in SystemDictionary::load_instance_class (class_name=0x7fe21d4ad0e8, class_loader=..., __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:1304
#6  0x00007fe220d27ced in SystemDictionary::resolve_instance_class_or_null (name=0x7fe21d4ad0e8, class_loader=..., protection_domain=..., __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:779
#7  0x00007fe220d266be in SystemDictionary::resolve_or_null (class_name=0x7fe21d4ad0e8, class_loader=..., protection_domain=..., __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:232
#8  0x00007fe220d2612f in SystemDictionary::resolve_or_fail (class_name=0x7fe21d4ad0e8, class_loader=..., protection_domain=..., throw_error=true, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:171
#9  0x00007fe220d26451 in SystemDictionary::resolve_or_fail (class_name=0x7fe21d4ad0e8, throw_error=true, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:212
#10 0x00007fe220d2b27b in SystemDictionary::initialize_wk_klass (id=SystemDictionary::Object_klass_knum, init_opt=0, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:1866
#11 0x00007fe220d2b39e in SystemDictionary::initialize_wk_klasses_until (limit_id=SystemDictionary::Cloneable_klass_knum, start_id=@0x7fe22229a9ec: SystemDictionary::Object_klass_knum, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:1882
#12 0x00007fe220d2eb86 in SystemDictionary::initialize_wk_klasses_through (end_id=SystemDictionary::Class_klass_knum, start_id=@0x7fe22229a9ec: SystemDictionary::Object_klass_knum, __the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.hpp:408
#13 0x00007fe220d2b4d0 in SystemDictionary::initialize_preloaded_classes (__the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:1901
#14 0x00007fe220d2b19d in SystemDictionary::initialize (__the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/classfile/systemDictionary.cpp:1843
#15 0x00007fe220d7c1d1 in Universe::genesis (__the_thread__=0x7fe21800b800) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/memory/universe.cpp:288
#16 0x00007fe220d7e439 in universe2_init () at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/memory/universe.cpp:991
#17 0x00007fe220917a5c in init_globals () at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/runtime/init.cpp:114
#18 0x00007fe220d5f756 in Threads::create_vm (args=0x7fe22229ae40, canTryAgain=0x7fe22229adff) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/runtime/thread.cpp:3424
#19 0x00007fe2209ca232 in JNI_CreateJavaVM (vm=0x7fe22229ae88, penv=0x7fe22229ae80, args=0x7fe22229ae40) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/hotspot/src/share/vm/prims/jni.cpp:5166
#20 0x00007fe221c67780 in InitializeJVM (pvm=0x7fe22229ae88, penv=0x7fe22229ae80, ifn=0x7fe22229ae90) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/jdk/src/share/bin/java.c:1145
#21 0x00007fe221c656f9 in JavaMain (_args=0x7ffe14eb62f0) at /home/atzhang/atzhang/openjdksource/openjdk8/openjdk/jdk/src/share/bin/java.c:371
#22 0x00007fe221e81ea5 in start_thread () from /lib64/libpthread.so.0
#23 0x00007fe22178a9fd in clone () from /lib64/libc.so.6

进入这个函数

ConstantPool*MetaWord(ClassLoaderData* loader_data, int length, TRAPS) {
  // Tags are RW but comment below applies to tags also.
  Array<u1>* tags = MetadataFactory::new_writeable_array<u1>(loader_data, length, 0, CHECK_NULL);
 
  int size = ConstantPool::size(length); //length=87
 
  // CDS considerations:
  // Allocate read-write but may be able to move to read-only at dumping time
  // if all the klasses are resolved.  The only other field that is writable is
  // the resolved_references array, which is recreated at startup time.
  // But that could be moved to InstanceKlass (although a pain to access from
  // assembly code).  Maybe it could be moved to the cpCache which is RW.
  return new (loader_data, size, false, MetaspaceObj::ConstantPoolType, THREAD) ConstantPool(tags);
}

上篇写了关于 tags实现的内容

现在解析一下size=的计算,

static int header_size()             { return sizeof(ConstantPool)/HeapWordSize; } //计算结果为11
  static int size(int length)          { return align_object_size(header_size() + length); } //11+87 =98个
 
inline bool is_object_aligned(intptr_t addr) {
  return addr == align_object_size(addr);//size=98

}

//在这个sizeof(ConstantPool) 计算出来的为1 不知道为什么,通过分析来得到大小

//查看分配好的常量池
(gdb) p * constant_pool
$13 = (ConstantPool) {
                    <Metadata> = {<MetaspaceObj> = {<No data fields>},
                                    _vptr.Metadata = 0x7ff4c424c070 <vtable for ConstantPool+16>,
                                    _valid = 0},
                    _tags = 0x7ff4c01900a8,
                    _cache = 0x0,
                    _pool_holder = 0x0,
                    _operands = 0x0,
                    _resolved_references = 0x0,
                    _reference_map = 0x0,
                    _flags = 0,
                    _length = 87,
                    _saved = {_resolved_reference_length = 0, _version = 0},
                    _lock = 0x7ff4bc06ec38}
(gdb) p sizeof(*constant_pool)
$15 = 88

这个通过对象来查看得到共有11项目,打印内存可以对应上

(gdb) p constant_pool
$1 = (ConstantPool *) 0x7f2563800108
(gdb) x/22x 0x7f2563800108
0x7f2563800108:    0x86b70070    0x00007f25    0x00000000    0x00000000
0x7f2563800118:    0x638000a8    0x00007f25    0x00000000    0x00000000
0x7f2563800128:    0x00000000    0x00000000    0x00000000    0x00000000
0x7f2563800138:    0x00000000    0x00000000    0x00000000    0x00000000
0x7f2563800148:    0x00000000    0x00000057    0x00000000    0x00000000
0x7f2563800158:    0x8006ec38    0x00007f25

需要特别说明的是: ConstantPool类里面有他的父类的,他的父类占用2个8字节空间,一个是虚表,一个是变量

接着

//进入重载new方法
void* MetaspaceObj::operator new(size_t size, ClassLoaderData* loader_data,
                                 size_t word_size, bool read_only,
                                 MetaspaceObj::Type type, TRAPS) throw() {
    // Klass has it's own operator new
    return Metaspace::allocate(loader_data, word_size, read_only,
                               type, CHECK_NULL);
}
//进入元空间进行内存分配
MetaWord* Metaspace::allocate(ClassLoaderData* loader_data, size_t word_size,
                              bool read_only, MetaspaceObj::Type type, TRAPS) {
    if (HAS_PENDING_EXCEPTION) {
        assert(false, "Should not allocate with exception pending");
        return NULL;  // caller does a CHECK_NULL too
    }
 
    assert(loader_data != NULL, "Should never pass around a NULL loader_data. "
                                "ClassLoaderData::the_null_class_loader_data() should have been used.");
 
    MetadataType mdtype = (type == MetaspaceObj::ClassType) ? ClassType : NonClassType;
 
    // Try to allocate metadata.
    MetaWord* result = loader_data->metaspace_non_null()->allocate(word_size, mdtype);
 
    if (result == NULL) {
        // Allocation failed.
        if (is_init_completed()) {
            // Only start a GC if the bootstrapping has completed.
 
            // Try to clean out some memory and retry.
            result = Universe::heap()->collector_policy()->satisfy_failed_metadata_allocation(
                    loader_data, word_size, mdtype);
        }
    }
 
    if (result == NULL) {
        report_metadata_oome(loader_data, word_size, mdtype, CHECK_NULL);
    }
 
    // Zero initialize.
    Copy::fill_to_aligned_words((HeapWord*)result, word_size, 0);
 
    return result;
}

接着

MetaWord* Metachunk::allocate(size_t word_size) {
    MetaWord* result = NULL;
    // If available, bump the pointer to allocate.
    if (free_word_size() >= word_size) {
        result = _top;
        _top = _top + word_size;
    }
    return result;
}

这里面设计一个指针加法,  _top + 8*word_size

可以看出,_top(0x7fd4b8800108) + 98 最终等于 0x7fd4b8800418,是因为指针加法要乘 指针类型大小即8字节

通过汇编验证

(gdb) x/15i  $pc
=> 0x7fdfce6ef8df <Metachunk::allocate(unsigned long)+63>:    mov    0x20(%rax),%rax
0x7fdfce6ef8e3 <Metachunk::allocate(unsigned long)+67>:    mov    -0x20(%rbp),%rdx
0x7fdfce6ef8e7 <Metachunk::allocate(unsigned long)+71>:    shl    $0x3,%rdx
0x7fdfce6ef8eb <Metachunk::allocate(unsigned long)+75>:    add    %rax,%rdx
 
(gdb) p *(long *) ($rax+0x20)
$15 = 140598841622792
        (gdb) p/x *(long *)($rax +0x20)
$16 = 0x7fdfb802c108
        (gdb) p/x *(long * )($rbp -0x20)
$17 = 0x62

接着

//看一个细节
 void MetaspaceAux::inc_used(Metaspace::MetadataType mdtype, size_t words) {
     // _allocated_used_words tracks allocations for
     // each piece of metadata.  Those allocations are
     // generally done concurrently by different application
     // threads so must be done atomically.
     Atomic::add_ptr(words, &_allocated_used_words[mdtype]);
 }
 这个
 static size_t _allocated_used_words[Metaspace:: MetadataTypeCount];
 
enum MetadataType {
    ClassType,
    NonClassType,
    MetadataTypeCount
};

具体为

(gdb) p mdtype
$9 = Metaspace::NonClassType
 
(gdb) p/x _allocated_used_words[mdtype]
$11 = 0x83
这里是02
那么整个过程由 0x21 变为了0x83 差值为0x62 十进制为98,和前面的98相同

这里就完成了constant_pool的创建