1 /* 2 * Copyright (c) 1997, 2016, 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 "memory/allocation.hpp" 29 #include "utilities/exceptions.hpp" 30 #include "utilities/macros.hpp" 31 #include "utilities/utf8.hpp" 32 33 // A Symbol is a canonicalized string. 34 // All Symbols reside in global SymbolTable and are reference counted. 35 36 // Reference counting 37 // 38 // All Symbols are allocated and added to the SymbolTable. 39 // When a class is unloaded, the reference counts of the Symbol pointers in 40 // the ConstantPool and in InstanceKlass (see release_C_heap_structures) are 41 // decremented. When the reference count for a Symbol goes to 0, the garbage 42 // collector can free the Symbol and remove it from the SymbolTable. 43 // 44 // 0) Symbols need to be reference counted when a pointer to the Symbol is 45 // saved in persistent storage. This does not include the pointer 46 // in the SymbolTable bucket (the _literal field in HashtableEntry) 47 // that points to the Symbol. All other stores of a Symbol* 48 // to a field of a persistent variable (e.g., the _name filed in 49 // fieldDescriptor or _ptr in a CPSlot) is reference counted. 50 // 51 // 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for 52 // "name" and returns a pointer to F or finds a pre-existing Symbol F for 53 // "name" and returns a pointer to it. In both cases the reference count for F 54 // is incremented under the assumption that a pointer to F will be created from 55 // the return value. Thus the increment of the reference count is on the lookup 56 // and not on the assignment to the new Symbol*. That is 57 // Symbol* G = lookup() 58 // ^ increment on lookup() 59 // and not 60 // Symbol* G = lookup() 61 // ^ increment on assignmnet 62 // The reference count must be decremented manually when the copy of the 63 // pointer G is destroyed. 64 // 65 // 2) For a local Symbol* A that is a copy of an existing Symbol* B, the 66 // reference counting is elided when the scope of B is greater than the scope 67 // of A. For example, in the code fragment 68 // below "klass" is passed as a parameter to the method. Symbol* "kn" 69 // is a copy of the name in "klass". 70 // 71 // Symbol* kn = klass->name(); 72 // unsigned int d_hash = dictionary()->compute_hash(kn, class_loader); 73 // 74 // The scope of "klass" is greater than the scope of "kn" so the reference 75 // counting for "kn" is elided. 76 // 77 // Symbol* copied from ConstantPool entries are good candidates for reference 78 // counting elision. The ConstantPool entries for a class C exist until C is 79 // unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X, 80 // the Symbol* in the ConstantPool will in general out live X so the reference 81 // counting on X can be elided. 82 // 83 // For cases where the scope of A is not greater than the scope of B, 84 // the reference counting is explicitly done. See ciSymbol, 85 // ResolutionErrorEntry and ClassVerifier for examples. 86 // 87 // 3) When a Symbol K is created for temporary use, generally for substrings of 88 // an existing symbol or to create a new symbol, assign it to a 89 // TempNewSymbol. The SymbolTable methods new_symbol(), lookup() 90 // and probe() all potentially return a pointer to a new Symbol. 91 // The allocation (or lookup) of K increments the reference count for K 92 // and the destructor decrements the reference count. 93 // 94 // This cannot be inherited from ResourceObj because it cannot have a vtable. 95 // Since sometimes this is allocated from Metadata, pick a base allocation 96 // type without virtual functions. 97 class ClassLoaderData; 98 99 // Set _refcount to PERM_REFCOUNT to prevent the Symbol from being GC'ed. 100 #ifndef PERM_REFCOUNT 101 #define PERM_REFCOUNT -1 102 #endif 103 104 class Symbol : public MetaspaceObj { 105 friend class VMStructs; 106 friend class SymbolTable; 107 friend class MoveSymbols; 108 109 private: 110 ATOMIC_SHORT_PAIR( 111 volatile short _refcount, // needs atomic operation 112 unsigned short _length // number of UTF8 characters in the symbol (does not need atomic op) 113 ); 114 short _identity_hash; 115 jbyte _body[2]; 116 117 enum { 118 // max_symbol_length is constrained by type of _length 119 max_symbol_length = (1 << 16) -1 120 }; 121 122 static int size(int length) { 123 // minimum number of natural words needed to hold these bits (no non-heap version) 124 return (int)heap_word_size(sizeof(Symbol) + (length > 2 ? length - 2 : 0)); 125 } 126 127 void byte_at_put(int index, int value) { 128 assert(index >=0 && index < _length, "symbol index overflow"); 129 _body[index] = value; 130 } 131 132 Symbol(const u1* name, int length, int refcount); 133 void* operator new(size_t size, int len, TRAPS) throw(); 134 void* operator new(size_t size, int len, Arena* arena, TRAPS) throw(); 135 void* operator new(size_t size, int len, ClassLoaderData* loader_data, TRAPS) throw(); 136 137 void operator delete(void* p); 138 139 public: 140 // Low-level access (used with care, since not GC-safe) 141 const jbyte* base() const { return &_body[0]; } 142 143 int size() { return size(utf8_length()); } 144 145 // Returns the largest size symbol we can safely hold. 146 static int max_length() { return max_symbol_length; } 147 unsigned identity_hash() const { 148 unsigned addr_bits = (unsigned)((uintptr_t)this >> (LogMinObjAlignmentInBytes + 3)); 149 return ((unsigned)_identity_hash & 0xffff) | 150 ((addr_bits ^ (_length << 8) ^ (( _body[0] << 8) | _body[1])) << 16); 151 } 152 153 // For symbol table alternate hashing 154 unsigned int new_hash(juint seed); 155 156 // Reference counting. See comments above this class for when to use. 157 int refcount() const { return _refcount; } 158 void increment_refcount(); 159 void decrement_refcount(); 160 // Set _refcount non zero to avoid being reclaimed by GC. 161 void set_permanent() { 162 assert(LogTouchedMethods, "Should not be called with LogTouchedMethods off"); 163 if (_refcount != PERM_REFCOUNT) { 164 _refcount = PERM_REFCOUNT; 165 } 166 } 167 168 int byte_at(int index) const { 169 assert(index >=0 && index < _length, "symbol index overflow"); 170 return base()[index]; 171 } 172 173 const jbyte* bytes() const { return base(); } 174 175 int utf8_length() const { return _length; } 176 177 // Compares the symbol with a string. 178 bool equals(const char* str, int len) const; 179 bool equals(const char* str) const { return equals(str, (int) strlen(str)); } 180 181 // Tests if the symbol starts with the given prefix. 182 bool starts_with(const char* prefix, int len) const; 183 bool starts_with(const char* prefix) const { 184 return starts_with(prefix, (int) strlen(prefix)); 185 } 186 187 // Tests if the symbol ends with the given suffix. 188 bool ends_with(const char* suffix, int len) const; 189 bool ends_with(const char* suffix) const { 190 return ends_with(suffix, (int) strlen(suffix)); 191 } 192 193 // Returns index of the given substring in the symbol, starting 194 // at index 'i'. 195 // If the symbol doesn't contain the given substring, return -1. 196 int index_of_at(int i, const char* str, int len) const; 197 int index_of_at(int i, const char* str) const { 198 return index_of_at(i, str, (int) strlen(str)); 199 } 200 201 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg 202 // note that the ordering is not alfabetical 203 inline int fast_compare(const Symbol* other) const; 204 205 // Returns receiver converted to null-terminated UTF-8 string; string is 206 // allocated in resource area, or in the char buffer provided by caller. 207 char* as_C_string() const; 208 char* as_C_string(char* buf, int size) const; 209 // Use buf if needed buffer length is <= size. 210 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const; 211 212 // Returns an escaped form of a Java string. 213 char* as_quoted_ascii() const; 214 215 // Returns a null terminated utf8 string in a resource array 216 char* as_utf8() const { return as_C_string(); } 217 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const { 218 return as_C_string_flexible_buffer(t, buf, size); 219 } 220 221 jchar* as_unicode(int& length) const; 222 223 // Treating this symbol as a class name, returns the Java name for the class. 224 // String is allocated in resource area if buffer is not provided. 225 // See Klass::external_name() 226 const char* as_klass_external_name() const; 227 const char* as_klass_external_name(char* buf, int size) const; 228 229 // Printing 230 void print_symbol_on(outputStream* st = NULL) const; 231 void print_utf8_on(outputStream* st) const; 232 void print_on(outputStream* st) const; // First level print 233 void print_value_on(outputStream* st) const; // Second level print. 234 235 // printing on default output stream 236 void print() { print_on(tty); } 237 void print_value() { print_value_on(tty); } 238 239 #ifndef PRODUCT 240 // Empty constructor to create a dummy symbol object on stack 241 // only for getting its vtable pointer. 242 Symbol() { } 243 244 static int _total_count; 245 #endif 246 }; 247 248 // Note: this comparison is used for vtable sorting only; it doesn't matter 249 // what order it defines, as long as it is a total, time-invariant order 250 // Since Symbol*s are in C_HEAP, their relative order in memory never changes, 251 // so use address comparison for speed 252 int Symbol::fast_compare(const Symbol* other) const { 253 return (((uintptr_t)this < (uintptr_t)other) ? -1 254 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1); 255 } 256 #endif // SHARE_VM_OOPS_SYMBOL_HPP