1 /* 2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_VM_OOPS_SYMBOL_HPP 26 #define SHARE_VM_OOPS_SYMBOL_HPP 27 28 #include "utilities/utf8.hpp" 29 #include "memory/allocation.hpp" 30 #include "runtime/atomic.inline.hpp" 31 32 // A Symbol is a canonicalized string. 33 // All Symbols reside in global SymbolTable and are reference counted. 34 35 // Reference counting 36 // 37 // All Symbols are allocated and added to the SymbolTable. 38 // When a class is unloaded, the reference counts of the Symbol pointers in 39 // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are 40 // decremented. When the reference count for a Symbol goes to 0, the garbage 41 // collector can free the Symbol and remove it from the SymbolTable. 42 // 43 // 0) Symbols need to be reference counted when a pointer to the Symbol is 44 // saved in persistent storage. This does not include the pointer 45 // in the SymbolTable bucket (the _literal field in HashtableEntry) 46 // that points to the Symbol. All other stores of a Symbol* 47 // to a field of a persistent variable (e.g., the _name filed in 48 // FieldAccessInfo or _ptr in a CPSlot) is reference counted. 49 // 50 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for 51 // "name" and returns a pointer to F or finds a pre-existing Symbol F for 52 // "name" and returns a pointer to it. In both cases the reference count for F 53 // is incremented under the assumption that a pointer to F will be created from 54 // the return value. Thus the increment of the reference count is on the lookup 55 // and not on the assignment to the new Symbol*. That is 56 // Symbol* G = lookup() 57 // ^ increment on lookup() 58 // and not 59 // Symbol* G = lookup() 60 // ^ increment on assignmnet 61 // The reference count must be decremented manually when the copy of the 62 // pointer G is destroyed. 63 // 64 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the 65 // reference counting is elided when the scope of B is greater than the scope 66 // of A. For example, in the code fragment 67 // below "klass" is passed as a parameter to the method. Symbol* "kn" 68 // is a copy of the name in "klass". 69 // 70 // Symbol* kn = klass->name(); 71 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader); 72 // 73 // The scope of "klass" is greater than the scope of "kn" so the reference 74 // counting for "kn" is elided. 75 // 76 // Symbol* copied from ConstantPool entries are good candidates for reference 77 // counting elision. The ConstantPool entries for a class C exist until C is 78 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X, 79 // the Symbol* in the ConstantPool will in general out live X so the reference 80 // counting on X can be elided. 81 // 82 // For cases where the scope of A is not greater than the scope of B, 83 // the reference counting is explicitly done. See ciSymbol, 84 // ResolutionErrorEntry and ClassVerifier for examples. 85 // 86 // 3) When a Symbol K is created for temporary use, generally for substrings of 87 // an existing symbol or to create a new symbol, assign it to a 88 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup() 89 // and probe() all potentially return a pointer to a new Symbol. 90 // The allocation (or lookup) of K increments the reference count for K 91 // and the destructor decrements the reference count. 92 // 93 // Another example of TempNewSymbol usage is parsed_name used in 94 // ClassFileParser::parseClassFile() where parsed_name is used in the cleanup 95 // after a failed attempt to load a class. Here parsed_name is a 96 // TempNewSymbol (passed in as a parameter) so the reference count on its symbol 97 // will be decremented when it goes out of scope. 98 99 100 // This cannot be inherited from ResourceObj because it cannot have a vtable. 101 // Since sometimes this is allocated from Metadata, pick a base allocation 102 // type without virtual functions. 103 class ClassLoaderData; 104 105 class Symbol : public MetaspaceObj { 106 friend class VMStructs; 107 friend class SymbolTable; 108 friend class MoveSymbols; 109 private: 110 volatile int _refcount; 111 int _identity_hash; 112 unsigned short _length; // number of UTF8 characters in the symbol 113 jbyte _body[1]; 114 115 enum { 116 // max_symbol_length is constrained by type of _length 117 max_symbol_length = (1 << 16) -1 118 }; 119 120 static int size(int length) { 121 size_t sz = heap_word_size(sizeof(Symbol) + (length > 0 ? length - 1 : 0)); 122 return align_object_size(sz); 123 } 124 125 void byte_at_put(int index, int value) { 126 assert(index >=0 && index < _length, "symbol index overflow"); 127 _body[index] = value; 128 } 129 130 Symbol(const u1* name, int length, int refcount); 131 void* operator new(size_t size, int len, TRAPS); 132 void* operator new(size_t size, int len, Arena* arena, TRAPS); 133 void* operator new(size_t size, int len, ClassLoaderData* loader_data, TRAPS); 134 135 void operator delete(void* p); 136 137 public: 138 // Low-level access (used with care, since not GC-safe) 139 const jbyte* base() const { return &_body[0]; } 140 141 int size() { return size(utf8_length()); } 142 143 // Returns the largest size symbol we can safely hold. 144 static int max_length() { return max_symbol_length; } 145 146 int identity_hash() { return _identity_hash; } 147 148 // For symbol table alternate hashing 149 unsigned int new_hash(jint seed); 150 151 // Reference counting. See comments above this class for when to use. 152 int refcount() const { return _refcount; } 153 inline void increment_refcount(); 154 inline void decrement_refcount(); 155 156 int byte_at(int index) const { 157 assert(index >=0 && index < _length, "symbol index overflow"); 158 return base()[index]; 159 } 160 161 const jbyte* bytes() const { return base(); } 162 163 int utf8_length() const { return _length; } 164 165 // Compares the symbol with a string. 166 bool equals(const char* str, int len) const; 167 bool equals(const char* str) const { return equals(str, (int) strlen(str)); } 168 169 // Tests if the symbol starts with the given prefix. 170 bool starts_with(const char* prefix, int len) const; 171 bool starts_with(const char* prefix) const { 172 return starts_with(prefix, (int) strlen(prefix)); 173 } 174 175 // Tests if the symbol starts with the given prefix. 176 int index_of_at(int i, const char* str, int len) const; 177 int index_of_at(int i, const char* str) const { 178 return index_of_at(i, str, (int) strlen(str)); 179 } 180 181 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg 182 // note that the ordering is not alfabetical 183 inline int fast_compare(Symbol* other) const; 184 185 // Returns receiver converted to null-terminated UTF-8 string; string is 186 // allocated in resource area, or in the char buffer provided by caller. 187 char* as_C_string() const; 188 char* as_C_string(char* buf, int size) const; 189 // Use buf if needed buffer length is <= size. 190 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const; 191 192 // Returns an escaped form of a Java string. 193 char* as_quoted_ascii() const; 194 195 // Returns a null terminated utf8 string in a resource array 196 char* as_utf8() const { return as_C_string(); } 197 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const { 198 return as_C_string_flexible_buffer(t, buf, size); 199 } 200 201 jchar* as_unicode(int& length) const; 202 203 // Treating this symbol as a class name, returns the Java name for the class. 204 // String is allocated in resource area if buffer is not provided. 205 // See Klass::external_name() 206 const char* as_klass_external_name() const; 207 const char* as_klass_external_name(char* buf, int size) const; 208 209 // Printing 210 void print_symbol_on(outputStream* st = NULL) const; 211 void print_on(outputStream* st) const; // First level print 212 void print_value_on(outputStream* st) const; // Second level print. 213 214 // printing on default output stream 215 void print() { print_on(tty); } 216 void print_value() { print_value_on(tty); } 217 218 #ifndef PRODUCT 219 // Empty constructor to create a dummy symbol object on stack 220 // only for getting its vtable pointer. 221 Symbol() { } 222 223 static int _total_count; 224 #endif 225 }; 226 227 // Note: this comparison is used for vtable sorting only; it doesn't matter 228 // what order it defines, as long as it is a total, time-invariant order 229 // Since Symbol*s are in C_HEAP, their relative order in memory never changes, 230 // so use address comparison for speed 231 int Symbol::fast_compare(Symbol* other) const { 232 return (((uintptr_t)this < (uintptr_t)other) ? -1 233 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1); 234 } 235 236 inline void Symbol::increment_refcount() { 237 // Only increment the refcount if positive. If negative either 238 // overflow has occurred or it is a permanent symbol in a read only 239 // shared archive. 240 if (_refcount >= 0) { 241 Atomic::inc(&_refcount); 242 NOT_PRODUCT(Atomic::inc(&_total_count);) 243 } 244 } 245 246 inline void Symbol::decrement_refcount() { 247 if (_refcount >= 0) { 248 Atomic::dec(&_refcount); 249 #ifdef ASSERT 250 if (_refcount < 0) { 251 print(); 252 assert(false, "reference count underflow for symbol"); 253 } 254 #endif 255 } 256 } 257 #endif // SHARE_VM_OOPS_SYMBOL_HPP