symbol.hpp revision 2062:3582bf76420e
1/* 2 * Copyright (c) 1997, 2009, 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 31// A Symbol is a canonicalized string. 32// All Symbols reside in global SymbolTable and are reference counted. 33 34// Reference counting 35// 36// All Symbols are allocated and added to the SymbolTable. 37// When a class is unloaded, the reference counts of the Symbol pointers in 38// the ConstantPool and in instanceKlass (see release_C_heap_structures) are 39// decremented. When the reference count for a Symbol goes to 0, the garbage 40// collector can free the Symbol and remove it from the SymbolTable. 41// 42// 0) Symbols need to be reference counted when a pointer to the Symbol is 43// saved in persistent storage. This does not include the pointer 44// in the SymbolTable bucket (the _literal field in HashtableEntry) 45// that points to the Symbol. All other stores of a Symbol* 46// to a field of a persistent variable (e.g., the _name filed in 47// FieldAccessInfo or _ptr in a CPSlot) is reference counted. 48// 49// 1) The lookup of a "name" in the SymbolTable either creates a Symbol F for 50// "name" and returns a pointer to F or finds a pre-existing Symbol F for 51// "name" and returns a pointer to it. In both cases the reference count for F 52// is incremented under the assumption that a pointer to F will be created from 53// the return value. Thus the increment of the reference count is on the lookup 54// and not on the assignment to the new Symbol*. That is 55// Symbol* G = lookup() 56// ^ increment on lookup() 57// and not 58// Symbol* G = lookup() 59// ^ increment on assignmnet 60// The reference count must be decremented manually when the copy of the 61// pointer G is destroyed. 62// 63// 2) For a local Symbol* A that is a copy of an existing Symbol* B, the 64// reference counting is elided when the scope of B is greater than the scope 65// of A. For example, in the code fragment 66// below "klass" is passed as a parameter to the method. Symbol* "kn" 67// is a copy of the name in "klass". 68// 69// Symbol* kn = klass->name(); 70// unsigned int d_hash = dictionary()->compute_hash(kn, class_loader); 71// 72// The scope of "klass" is greater than the scope of "kn" so the reference 73// counting for "kn" is elided. 74// 75// Symbol* copied from ConstantPool entries are good candidates for reference 76// counting elision. The ConstantPool entries for a class C exist until C is 77// unloaded. If a Symbol* is copied out of the ConstantPool into Symbol* X, 78// the Symbol* in the ConstantPool will in general out live X so the reference 79// counting on X can be elided. 80// 81// For cases where the scope of A is not greater than the scope of B, 82// the reference counting is explicitly done. See ciSymbol, 83// ResolutionErrorEntry and ClassVerifier for examples. 84// 85// 3) When a Symbol K is created for temporary use, generally for substrings of 86// an existing symbol or to create a new symbol, assign it to a 87// TempNewSymbol. The SymbolTable methods new_symbol(), lookup() 88// and probe() all potentially return a pointer to a new Symbol. 89// The allocation (or lookup) of K increments the reference count for K 90// and the destructor decrements the reference count. 91// 92// Another example of TempNewSymbol usage is parsed_name used in 93// ClassFileParser::parseClassFile() where parsed_name is used in the cleanup 94// after a failed attempt to load a class. Here parsed_name is a 95// TempNewSymbol (passed in as a parameter) so the reference count on its symbol 96// will be decremented when it goes out of scope. 97 98class Symbol : public CHeapObj { 99 friend class VMStructs; 100 friend class SymbolTable; 101 friend class MoveSymbols; 102 private: 103 volatile int _refcount; 104 int _identity_hash; 105 unsigned short _length; // number of UTF8 characters in the symbol 106 jbyte _body[1]; 107 108 enum { 109 // max_symbol_length is constrained by type of _length 110 max_symbol_length = (1 << 16) -1 111 }; 112 113 static int object_size(int length) { 114 size_t size = heap_word_size(sizeof(Symbol) + length); 115 return align_object_size(size); 116 } 117 118 void byte_at_put(int index, int value) { 119 assert(index >=0 && index < _length, "symbol index overflow"); 120 _body[index] = value; 121 } 122 123 Symbol(const u1* name, int length); 124 void* operator new(size_t size, int len); 125 126 public: 127 // Low-level access (used with care, since not GC-safe) 128 const jbyte* base() const { return &_body[0]; } 129 130 int object_size() { return object_size(utf8_length()); } 131 132 // Returns the largest size symbol we can safely hold. 133 static int max_length() { 134 return max_symbol_length; 135 } 136 137 int identity_hash() { 138 return _identity_hash; 139 } 140 141 // Reference counting. See comments above this class for when to use. 142 int refcount() const { return _refcount; } 143 void increment_refcount(); 144 void decrement_refcount(); 145 146 int byte_at(int index) const { 147 assert(index >=0 && index < _length, "symbol index overflow"); 148 return base()[index]; 149 } 150 151 const jbyte* bytes() const { return base(); } 152 153 int utf8_length() const { return _length; } 154 155 // Compares the symbol with a string. 156 bool equals(const char* str, int len) const; 157 bool equals(const char* str) const { return equals(str, (int) strlen(str)); } 158 159 // Tests if the symbol starts with the given prefix. 160 bool starts_with(const char* prefix, int len) const; 161 bool starts_with(const char* prefix) const { 162 return starts_with(prefix, (int) strlen(prefix)); 163 } 164 165 // Tests if the symbol starts with the given prefix. 166 int index_of_at(int i, const char* str, int len) const; 167 int index_of_at(int i, const char* str) const { 168 return index_of_at(i, str, (int) strlen(str)); 169 } 170 171 // Three-way compare for sorting; returns -1/0/1 if receiver is </==/> than arg 172 // note that the ordering is not alfabetical 173 inline int fast_compare(Symbol* other) const; 174 175 // Returns receiver converted to null-terminated UTF-8 string; string is 176 // allocated in resource area, or in the char buffer provided by caller. 177 char* as_C_string() const; 178 char* as_C_string(char* buf, int size) const; 179 // Use buf if needed buffer length is <= size. 180 char* as_C_string_flexible_buffer(Thread* t, char* buf, int size) const; 181 182 183 // Returns a null terminated utf8 string in a resource array 184 char* as_utf8() const { return as_C_string(); } 185 char* as_utf8_flexible_buffer(Thread* t, char* buf, int size) const { 186 return as_C_string_flexible_buffer(t, buf, size); 187 } 188 189 jchar* as_unicode(int& length) const; 190 191 // Treating this symbol as a class name, returns the Java name for the class. 192 // String is allocated in resource area if buffer is not provided. 193 // See Klass::external_name() 194 const char* as_klass_external_name() const; 195 const char* as_klass_external_name(char* buf, int size) const; 196 197 // Printing 198 void print_symbol_on(outputStream* st = NULL) const; 199 void print_on(outputStream* st) const; // First level print 200 void print_value_on(outputStream* st) const; // Second level print. 201 202 // printing on default output stream 203 void print() { print_on(tty); } 204 void print_value() { print_value_on(tty); } 205 206#ifndef PRODUCT 207 // Empty constructor to create a dummy symbol object on stack 208 // only for getting its vtable pointer. 209 Symbol() { } 210 211 static int _total_count; 212#endif 213}; 214 215// Note: this comparison is used for vtable sorting only; it doesn't matter 216// what order it defines, as long as it is a total, time-invariant order 217// Since Symbol*s are in C_HEAP, their relative order in memory never changes, 218// so use address comparison for speed 219int Symbol::fast_compare(Symbol* other) const { 220 return (((uintptr_t)this < (uintptr_t)other) ? -1 221 : ((uintptr_t)this == (uintptr_t) other) ? 0 : 1); 222} 223#endif // SHARE_VM_OOPS_SYMBOL_HPP 224