问一个比较小白的问题java中for循环里面new一个数组？jvm字符串的创建指令的问题

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①个很大的for里面，有①句

arr =new int[N];

arr是for外面的变量，我想给它清⓪。我想问每次都new①个变量会不会让内存开销线性增加。

还有java有没有类似c的memset

刚刚写了个代码验证了①下，内存使用量是上升的，直到（可能是GC了）某个值：

代码如下：

public class TestNewObject { public static void main(String[] args) { for (int i = ⓪; i < ①⓪⓪⓪⓪⓪⓪; i++) { String str = new String(\"test\"); if (i % ①⓪⓪⓪⓪⓪ == ⓪) { String memUsage = String.format(\"maxMemory=%d,totalMemory=%d,freeMemory=%d\", Runtime.getRuntime().maxMemory(), Runtime.getRuntime().totalMemory(), Runtime.getRuntime().freeMemory()); System.out.println(memUsage); } } }}输出如下：maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④④③①⑦⑨⑦⑥maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⓪④⓪④⑧⑨⑥maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②③⑨⓪⑨④①⑦⑥maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②③⑥④⑦②⑦③⑥maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②③②⑤④⓪④⑧⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②②⑨⑨①⑨⓪④⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②②⑧⑥⓪⑧③②⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②②⑧⑥⓪⑧③②⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②②⑧⑥⓪⑧③②⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②②⑧⑥⓪⑧③②⓪Process finished with exit code ⓪如果我们主动gc，则会更快的到达某个值：public class TestNewObject { public static void main(String[] args) { for (int i = ⓪; i < ①⓪⓪⓪⓪⓪⓪; i++) { String str = new String(\"test\"); if (i % ①⓪⓪⓪⓪⓪ == ⓪) { System.gc(); String memUsage = String.format(\"maxMemory=%d,totalMemory=%d,freeMemory=%d\", Runtime.getRuntime().maxMemory(), Runtime.getRuntime().totalMemory(), Runtime.getRuntime().freeMemory()); System.out.println(memUsage); } } }}输出如下：maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑥①⑧⑨⑧④⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③②⑨③⑧④maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④④⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④④⓪maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧maxMemory=③⑥⑧⑨④①④⑥⑤⑥ · totalMemory=②④⑨⑤⑥①⓪⑧⑧ · freeMemory=②④⑦③③⓪④⓪⑧Process finished with exit code ⓪

泻药。根据题主的第②个问题写出①个实例代码：

public class StringTest { public static void main(String[] args) { String a = \"a\"; }}

javap -verbose 之后：

public class StringTest minor version: ⓪ major version: ⑤② flags: ACC_PUBLIC, ACC_SUPERConstant pool: #① = Methodref #④.#②⓪ // java/lang/Object.\"\":()V #② = String #①⑥ // a #③ = Class #②① // StringTest #④ = Class #②② // java/lang/Object #⑤ = Utf⑧ #⑥ = Utf⑧ ()V #⑦ = Utf⑧ Code #⑧ = Utf⑧ LineNumberTable #⑨ = Utf⑧ LocalVariableTable #①⓪ = Utf⑧ this #①① = Utf⑧ LStringTest; #①② = Utf⑧ main #①③ = Utf⑧ ([Ljava/lang/String;)V #①④ = Utf⑧ args #①⑤ = Utf⑧ [Ljava/lang/String; #①⑥ = Utf⑧ a #①⑦ = Utf⑧ Ljava/lang/String; #①⑧ = Utf⑧ SourceFile #①⑨ = Utf⑧ StringTest.java #②⓪ = NameAndType #⑤:#⑥ // \"\":()V #②① = Utf⑧ StringTest #②② = Utf⑧ java/lang/Object{ public StringTest(); descriptor: ()V flags: ACC_PUBLIC Code: stack=① · locals=① · args_size=① ⓪: aload_⓪ ①: invokespecial #① // Method java/lang/Object.\"\":()V ④: return LineNumberTable: line ④: ⓪ LocalVariableTable: Start Length Slot Name Signature ⓪ ⑤ ⓪ this LStringTest; public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=① · locals=② · args_size=① ⓪: ldc #② // String a ②: astore_① ③: return LineNumberTable: line ⑥: ⓪ line ⑦: ③ LocalVariableTable: Start Length Slot Name Signature ⓪ ④ ⓪ args [Ljava/lang/String; ③ ① ① a Ljava/lang/String;}

可以在class文件的常量池中找到“a”的所在，我们知道当StringTest被引用时，会被类加载器加载，加载的过程中，首先要对class文件进行解析；解析的过程是按照class文件的结构来①步步地进行局部解析的，常量池解析也是这个过程中的①个步骤；在这个步骤中，字符串“a”最终作为①个Utf⑧的字面量被塞进了符号表SymbolTable中，继而被映射到运行时常量池中。所以直接采用ldc指令将它从ConstantPool中取出来，塞进操作数栈中了。

相关解析代码参见：

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); // Before storing the symbol, make sure it\'s legal if (_need_verify) { verify_legal_utf⑧((unsigned char*)utf⑧_buffer, utf⑧_length, CHECK); } if (EnableInvokeDynamic guarantee_property(java_lang_String::is_instance(patch()), \"Illegal utf⑧ patch at %d in class file %s\", index, CHECK); 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); } }

当然这里仅仅是创建了①个Symbol对象，并不是我们需要的String对象，那么String对象从何而来呢？原来ldc指令将该常量push进操作数栈之前，会先根据ldc的指令参数index，找到SymbolTable中对应tag的Symbol对象，然后通过StringTable::intern将它转换成java_lang_String类型的字符串对象的。

下面给出这段逻辑在HotSpot VM上的实现：

CASE(_ldc):{// ...ignore... case JVM_CONSTANT_String: { oop result = constants->resolved_references()->obj_at(index); if (result == NULL) { CALL_VM(InterpreterRuntime::resolve_ldc(THREAD, (Bytecodes::Code) opcode), handle_exception); SET_STACK_OBJECT(THREAD->vm_result(), ⓪); THREAD->set_vm_result(NULL); } else { VERIFY_OOP(result); SET_STACK_OBJECT(result, ⓪); } break; }// ...ignore...}

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oop Bytecode_loadconstant::resolve_constant(TRAPS) const { assert(_method.not_null(), \"must supply method to resolve constant\"); int index = raw_index(); ConstantPool* constants = _method->constants(); if (has_cache_index()) { return constants->resolve_cached_constant_at(index, THREAD); } else { return constants->resolve_constant_at(index, THREAD); }}

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// Resolve late bound constants. oop resolve_constant_at(int index, TRAPS) { constantPoolHandle h_this(THREAD, this); return resolve_constant_at_impl(h_this, index, _no_index_sentinel, THREAD); }

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// Called to resolve constants in the constant pool and return an oop.// Some constant pool entries cache their resolved oop. This is also// called to create oops from constants to use in arguments for invokedynamicoop ConstantPool::resolve_constant_at_impl(constantPoolHandle this_oop, int index, int cache_index, TRAPS) {// ...ignore... case JVM_CONSTANT_String: assert(cache_index != _no_index_sentinel, \"should have been set\"); if (this_oop->is_pseudo_string_at(index)) { result_oop = this_oop->pseudo_string_at(index, cache_index); break; } result_oop = string_at_impl(this_oop, index, cache_index, CHECK_NULL); break;// ...ignore...}

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oop ConstantPool::string_at_impl(constantPoolHandle this_oop, int which, int obj_index, TRAPS) { // If the string has already been interned, this entry will be non-null oop str = this_oop->resolved_references()->obj_at(obj_index); if (str != NULL) return str; Symbol* sym = this_oop->unresolved_string_at(which); str = StringTable::intern(sym, CHECK_(NULL)); this_oop->string_at_put(which, obj_index, str); assert(java_lang_String::is_instance(str), \"must be string\"); return str;}

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oop StringTable::intern(Symbol* symbol, TRAPS) { if (symbol == NULL) return NULL; ResourceMark rm(THREAD); int length; jchar* chars = symbol->as_unicode(length); Handle string; oop result = intern(string, chars, length, CHECK_NULL); return result;}

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oop StringTable::intern(Handle string_or_null, jchar* name, int len, TRAPS) { unsigned int hashValue = hash_string(name, len); int index = the_table()->hash_to_index(hashValue); oop found_string = the_table()->lookup(index, name, len, hashValue); // Found if (found_string != NULL) return found_string; debug_only(StableMemoryChecker smc(name, len * sizeof(name[⓪]))); assert(!Universe::heap()->is_in_reserved(name), \"proposed name of symbol must be stable\"); Handle string; // try to reuse the string if possible if (!string_or_null.is_null()) { string = string_or_null; } else { string = java_lang_String::create_from_unicode(name, len, CHECK_NULL); } // Grab the StringTable_lock before getting the_table() because it could // change at safepoint. MutexLocker ml(StringTable_lock, THREAD); // Otherwise, add to symbol to table return the_table()->basic_add(index, string, name, len, hashValue, CHECK_NULL);}

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Handle java_lang_String::create_from_unicode(jchar* unicode, int length, TRAPS) { Handle h_obj = basic_create(length, CHECK_NH); typeArrayOop buffer = value(h_obj()); for (int index = ⓪; index < length; index++) { buffer->char_at_put(index, unicode[index]); } return h_obj;}

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