常量池解析（2）
在parse_constant_pool()方法中呼叫parse_constant_pool_entries()方法對常量池中的各個項進行解析，方法的實現如下：
void ClassFileParser::parse_constant_pool_entries(int length, TRAPS) {
// Use a local copy of ClassFileStream. It helps the C++ compiler to optimize
// this function (_current can be allocated in a register, with scalar
// replacement of aggregates). The _current pointer is copied back to
// stream() when this function returns. DON'T call another method within
// this method that uses stream().
ClassFileStream* cfs0 = stream();
ClassFileStream cfs1 = *cfs0;
ClassFileStream* cfs = &cfs1;
Handle class_loader(THREAD, _loader_data->class_loader());
// Used for batching symbol allocations.
const char* names[SymbolTable::symbol_alloc_batch_size];
int lengths[SymbolTable::symbol_alloc_batch_size];
int indices[SymbolTable::symbol_alloc_batch_size];
unsigned int hashValues[SymbolTable::symbol_alloc_batch_size];
int names_count = 0;
// parsing Index 0 is unused
for (int index = 1; index < length; index++) {
// Each of the following case guarantees one more byte in the stream
// for the following tag or the access_flags following constant pool,
// so we don't need bounds-check for reading tag.
u1 tag = cfs->get_u1_fast();
switch (tag) {
case JVM_CONSTANT_Class :
{
cfs->guarantee_more(3, CHECK); // name_index, tag/access_flags
u2 name_index = cfs->get_u2_fast();
_cp->klass_index_at_put(index, name_index);
}
break;
case JVM_CONSTANT_Fieldref :
{
cfs->guarantee_more(5, CHECK); // class_index, name_and_type_index, tag/access_flags
u2 class_index = cfs->get_u2_fast();
u2 name_and_type_index = cfs->get_u2_fast();
_cp->field_at_put(index, class_index, name_and_type_index);
}
break;
case JVM_CONSTANT_Methodref :
{
cfs->guarantee_more(5, CHECK); // class_index, name_and_type_index, tag/access_flags
u2 class_index = cfs->get_u2_fast();
u2 name_and_type_index = cfs->get_u2_fast();
_cp->method_at_put(index, class_index, name_and_type_index);
}
break;
case JVM_CONSTANT_InterfaceMethodref :
{
cfs->guarantee_more(5, CHECK); // class_index, name_and_type_index, tag/access_flags
u2 class_index = cfs->get_u2_fast();
u2 name_and_type_index = cfs->get_u2_fast();
_cp->interface_method_at_put(index, class_index, name_and_type_index);
}
break;
case JVM_CONSTANT_String :
{
cfs->guarantee_more(3, CHECK); // string_index, tag/access_flags
u2 string_index = cfs->get_u2_fast();
_cp->string_index_at_put(index, string_index);
}
break;
case JVM_CONSTANT_MethodHandle :
case JVM_CONSTANT_MethodType :
if (tag == JVM_CONSTANT_MethodHandle) {
cfs->guarantee_more(4, CHECK); // ref_kind, method_index, tag/access_flags
u1 ref_kind = cfs->get_u1_fast();
u2 method_index = cfs->get_u2_fast();
_cp->method_handle_index_at_put(index, ref_kind, method_index);
} else if (tag == JVM_CONSTANT_MethodType) {
cfs->guarantee_more(3, CHECK); // signature_index, tag/access_flags
u2 signature_index = cfs->get_u2_fast();
_cp->method_type_index_at_put(index, signature_index);
} else {
ShouldNotReachHere();
}
break;
case JVM_CONSTANT_InvokeDynamic :
{
cfs->guarantee_more(5, CHECK); // bsm_index, nt, tag/access_flags
u2 bootstrap_specifier_index = cfs->get_u2_fast();
u2 name_and_type_index = cfs->get_u2_fast();
if (_max_bootstrap_specifier_index < (int) bootstrap_specifier_index)
_max_bootstrap_specifier_index = (int) bootstrap_specifier_index; // collect for later
_cp->invoke_dynamic_at_put(index, bootstrap_specifier_index, name_and_type_index);
}
break;
case JVM_CONSTANT_Integer :
{
cfs->guarantee_more(5, CHECK); // bytes, tag/access_flags
u4 bytes = cfs->get_u4_fast();
_cp->int_at_put(index, (jint) bytes);
}
break;
case JVM_CONSTANT_Float :
{
cfs->guarantee_more(5, CHECK); // bytes, tag/access_flags
u4 bytes = cfs->get_u4_fast();
_cp->float_at_put(index, *(jfloat*)&bytes);
}
break;
case JVM_CONSTANT_Long :
{
cfs->guarantee_more(9, CHECK); // bytes, tag/access_flags
u8 bytes = cfs->get_u8_fast();
_cp->long_at_put(index, bytes);
}
index++; // Skip entry following eigth-byte constant, see JVM book p. 98
break;
case JVM_CONSTANT_Double :
{
cfs->guarantee_more(9, CHECK); // bytes, tag/access_flags
u8 bytes = cfs->get_u8_fast();
_cp->double_at_put(index, *(jdouble*)&bytes);
}
index++; // Skip entry following eigth-byte constant, see JVM book p. 98
break;
case JVM_CONSTANT_NameAndType :
{
cfs->guarantee_more(5, CHECK); // name_index, signature_index, tag/access_flags
u2 name_index = cfs->get_u2_fast();
u2 signature_index = cfs->get_u2_fast();
_cp->name_and_type_at_put(index, name_index, signature_index);
}
break;
case JVM_CONSTANT_Utf8 :
{
cfs->guarantee_more(2, CHECK); // utf8_length
u2 utf8_length = cfs->get_u2_fast();
u1* utf8_buffer = cfs->get_u1_buffer();
assert(utf8_buffer != NULL, "null utf8 buffer");
// Got utf8 string, guarantee utf8_length+1 bytes, set stream position forward.
cfs->guarantee_more(utf8_length+1, CHECK); // utf8 string, tag/access_flags
cfs->skip_u1_fast(utf8_length);
if (EnableInvokeDynamic && has_cp_patch_at(index)) {
Handle patch = clear_cp_patch_at(index);
char* str = java_lang_String::as_utf8_string(patch());
// (could use java_lang_String::as_symbol instead, but might as well batch them)
utf8_buffer = (u1*) str;
utf8_length = (int) strlen(str);
}
unsigned int hash;
Symbol* result = SymbolTable::lookup_only((char*)utf8_buffer, utf8_length, hash);
if (result == NULL) {
names[names_count] = (char*)utf8_buffer;
lengths[names_count] = utf8_length;
indices[names_count] = index;
hashValues[names_count++] = hash;
if (names_count == SymbolTable::symbol_alloc_batch_size) {
SymbolTable::new_symbols(_loader_data, _cp, names_count, names, lengths, indices, hashValues, CHECK);
names_count = 0;
}
} else {
_cp->symbol_at_put(index, result);
}
}
break;
default:
classfile_parse_error("Unknown constant tag %u in class file %s", tag, CHECK);
break;
}
}
// Allocate the remaining symbols
if (names_count > 0) {
SymbolTable::new_symbols(_loader_data, _cp, names_count, names, lengths, indices, hashValues, CHECK);
}
cfs0->set_current(cfs1.current());
}
迴圈處理length個常量池項，不過第一個常量池項不需要處理，所以迴圈下標index的值初始化為1。
如果要了解各個常量池項的具體結構，程式碼的邏輯理解起來其實並不難。所有項的第一個位元組都是用來描述常量池元素型別，呼叫cfs->get_u1_fast()獲取元素型別後，就可以通過switch語句分情況進行處理。
1、JVM_CONSTANT_Class項的解析
JVM_CONSTANT_Class格式如下：
CONSTANT_Class_info {
u1 tag;
u2 name_index;
}
呼叫cfs->get_u2_fast()方法獲取name_index，然後呼叫_cp->klass_index_at_put()方法進行儲存。_cp的型別為ConstantPool*，ConstantPool類中的klass_index_at_put()方法的實現如下：
// For temporary use while constructing constant pool
void klass_index_at_put(int which, int name_index) {
tag_at_put(which, JVM_CONSTANT_ClassIndex);
*int_at_addr(which) = name_index;
}
void tag_at_put(int which, jbyte t) {
tags()->at_put(which, t);
}
jint* int_at_addr(int which) const {
assert(is_within_bounds(which), "index out of bounds");
return (jint*) &base()[which];
}
intptr_t* base() const {
return (intptr_t*) (
( (char*) this ) + sizeof(ConstantPool)
);
}
常量池項的下標與陣列的下標是相同的，也就是說，如果當前JVM_CONSTANT_Class儲存在常量池中的下標為1處，則也要儲存到tags陣列中下標為1的地方。同時要將名稱索引name_index儲存到ConstantPool中儲存資料區的對應位置上。
舉個例子如下：
#1 = Class #5 // TestClass ... #5 = Utf8 TestClass
假設JVM_CONSTANT_Class是常量池第一項，則解析完這一頂後的ConstantPool物件如下圖所示。
其中#0（表示常量池索引0）的值為0是因為在分配記憶體時會將其記憶體清零。
2、CONSTANT_Fieldref_info項的解析
格式如下：
CONSTANT_Fieldref_info {
u1 tag;
u2 class_index;
u2 name_and_type_index;
}
呼叫field_at_put()儲存class_index與name_and_type_index，方法的實現如下：
void field_at_put(int which, int class_index, int name_and_type_index) {
tag_at_put(which, JVM_CONSTANT_Fieldref);
*int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index;
}
name_and_type_index儲存在高16位，class_index儲存在低16位。
3、JVM_CONSTANT_Methodref項的解析
JVM_CONSTANT_Methodref項的格式如下：
CONSTANT_Methodref_info {
u1 tag;
u2 class_index;
u2 name_and_type_index;
}
按照格式讀取Class檔案，獲取到相關屬性值後呼叫ConstantPool的method_at_put()方法進行儲存，這個方法的實現如下：
void method_at_put(int which, int class_index, int name_and_type_index) {
tag_at_put(which, JVM_CONSTANT_Methodref);
*int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index;
}
由於ConstantPool資料區一個槽是一個指標型別的寬度，所以至少有32個位，又由於class_index與name_and_type_index屬性的型別為u2，這時候就可以使用高16位儲存name_and_type_index，低16位儲存class_index即可。
4、JVM_CONSTANT_InterfaceMethodref項的解析
格式如下：
CONSTANT_InterfaceMethodref_info {
u1 tag;
u2 class_index;
u2 name_and_type_index;
}
呼叫的interface_method_at_put()方法的實現如下：
void interface_method_at_put(int which, int class_index, int name_and_type_index) {
tag_at_put(which, JVM_CONSTANT_InterfaceMethodref);
*int_at_addr(which) = ((jint) name_and_type_index<<16) | class_index; // Not so nice
}
5、JVM_CONSTANT_String項的解析
格式如下：
CONSTANT_String_info {
u1 tag;
u2 string_index;
}
呼叫的string_index_at_put()方法的實現如下：
void string_index_at_put(int which, int string_index) {
tag_at_put(which, JVM_CONSTANT_StringIndex);
*int_at_addr(which) = string_index;
}
6、JVM_CONSTANT_MethodHandle項的解析
格式如下：
CONSTANT_MethodHandle_info {
u1 tag;
u1 reference_kind;
u2 reference_index;
}
呼叫的method_handle_index_at_put()方法的實現如下：
void method_handle_index_at_put(int which, int ref_kind, int ref_index) {
tag_at_put(which, JVM_CONSTANT_MethodHandle);
*int_at_addr(which) = ((jint) ref_index<<16) | ref_kind;
}
7、JVM_CONSTANT_MethodType項的解析
格式如下：
CONSTANT_MethodType_info {
u1 tag;
u2 descriptor_index;
}
呼叫的method_type_index_at_put()方法的實現如下：
void method_type_index_at_put(int which, int ref_index) {
tag_at_put(which, JVM_CONSTANT_MethodType);
*int_at_addr(which) = ref_index;
}
8、JVM_CONSTANT_InvokeDynamic項的解析
格式如下：
CONSTANT_InvokeDynamic_info {
u1 tag;
u2 bootstrap_method_attr_index;
u2 name_and_type_index;
}
呼叫的invoke_dynamic_at_put()方法的實現如下：
void invoke_dynamic_at_put(int which, int bootstrap_specifier_index, int name_and_type_index) {
tag_at_put(which, JVM_CONSTANT_InvokeDynamic);
*int_at_addr(which) = ((jint) name_and_type_index<<16) | bootstrap_specifier_index;
}
9、JVM_CONSTANT_Integer、JVM_CONSTANT_Float項的解析
格式如下：
CONSTANT_Integer_info {
u1 tag;
u4 bytes;
}
CONSTANT_Float_info {
u1 tag;
u4 bytes;
}
呼叫的方法分別為int_at_put()和float_at_put()方法，實現如下：
void int_at_put(int which, jint i) {
tag_at_put(which, JVM_CONSTANT_Integer);
*int_at_addr(which) = i;
}
void float_at_put(int which, jfloat f) {
tag_at_put(which, JVM_CONSTANT_Float);
*float_at_addr(which) = f;
}
10、JVM_CONSTANT_Long、JVM_CONSTANT_Double項的解析
格式如下：
CONSTANT_Long_info {
u1 tag;
u4 high_bytes;
u4 low_bytes;
}
CONSTANT_Double_info {
u1 tag;
u4 high_bytes;
u4 low_bytes;
}
呼叫的long_at_put()和double_at_put()方法的實現如下：
void long_at_put(int which, jlong l) {
tag_at_put(which, JVM_CONSTANT_Long);
// *long_at_addr(which) = l;
Bytes::put_native_u8((address)long_at_addr(which), *( (u8*) &l ));
}
void double_at_put(int which, jdouble d) {
tag_at_put(which, JVM_CONSTANT_Double);
// *double_at_addr(which) = d;
// u8 temp = *(u8*) &d;
Bytes::put_native_u8((address) double_at_addr(which), *((u8*) &d));
}
呼叫的Bytes::put_native_u8()方法的實現如下：
static inline void put_native_u8(address p, u8 x) { *(u8*)p = x; }
11、JVM_CONSTANT_NameAndType項的解析
格式如下：
CONSTANT_NameAndType_info {
u1 tag;
u2 name_index;
u2 descriptor_index;
}
呼叫的name_and_type_at_put()方法的實現如下：
void name_and_type_at_put(int which, int name_index, int signature_index) {
tag_at_put(which, JVM_CONSTANT_NameAndType);
*int_at_addr(which) = ((jint) signature_index<<16) | name_index; // Not so nice
}
12、JVM_CONSTANT_Utf8項的解析
格式如下：
CONSTANT_Utf8_info {
u1 tag;
u2 length;
u1 bytes[length];
}
在HotSpot虛擬機器中，字串通常都會表示為Symbol物件，這樣有利於使用符號表來儲存字串，對於2個相同的字串來說，完全可以使用同一個Symbol物件來表示。這樣就可以在ConstantPool資料區相應槽位上儲存指向Symbol的指標即可。
呼叫SymbolTable::lookup_only()方法從符號表中查詢對應的Symbol物件，如果查詢不到需要暫時將相關的資訊儲存到臨時的names、lengths、indices與hashValues陣列中，這樣就可以呼叫SymbolTable::new_symbols()進行批量新增Symbol物件來提高效率；如果找到對應的Symbol物件，則呼叫symbol_at_put()方法，實現如下：
void symbol_at_put(int which, Symbol* s) {
assert(s->refcount() != 0, "should have nonzero refcount");
tag_at_put(which, JVM_CONSTANT_Utf8);
*symbol_at_addr(which) = s;
}
Symbol** symbol_at_addr(int which) const {
assert(is_within_bounds(which), "index out of bounds");
return (Symbol**) &base()[which];
}
將指向Symbol物件的指標儲存到指定的位置。
如果Symbol物件表示的是類名稱，那麼後面是類連線後，相應索引位置上的值會更新為指向InstanceKlass例項的指標，後面會詳細介紹。
相關文章的連結如下：
1、 在Ubuntu 16.04上編譯OpenJDK8的原始碼
2、 除錯HotSpot原始碼
3、 HotSpot專案結構
4、 HotSpot的啟動過程
5、 HotSpot二分模型（1）
6、 HotSpot的類模型（2）
7、 HotSpot的類模型（3）
8、 HotSpot的類模型（4）
9、 HotSpot的物件模型（5）
10、HotSpot的物件模型（6）
11、操作控制代碼Handle（7）
12、控制代碼Handle的釋放（8）
13、類載入器
14、類的雙親委派機制
15、核心類的預裝載
16、Java主類的裝載
17、觸發類的裝載
18、類檔案介紹
19、檔案流
20、解析Class檔案
21、常量池解析（1）
作者持續維護的個人部落格classloading.com。
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參考：
（1）https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4
（2）《Java虛擬機器原理圖解》 1.2.2、Class檔案中的常量池詳解（上）
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