java - why the following program can work under JLS spec on JVM memory model -



java - why the following program can work under JLS spec on JVM memory model -

on jls 3, 17.5 final field semantics's sec give-and-take part: http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.5

it said mys can equals "/tmp", can not understand this. 1 can give more explain? point illustration tells, mean if want share global.s between multi thread, need create final(if final, can not alter after construction) or need sync when read , write? or declare string array length 1 , final can changed , shared??

the original contents in jls:

consider next example. 1 thread (which shall refer thread 1) executes

global.s = "/tmp/usr".substring(4);

while thread (thread 2) executes

string mys = global.s; if (mys.equals("/tmp"))system.out.println(mys);

the explain in jls:

string objects intended immutable , string operations not perform synchronization. while string implementation not have info races, other code have info races involving utilize of strings, , memory model makes weak guarantees programs have info races. in particular, if fields of string class not final, possible (although unlikely) thread 2 see default value of 0 offset of string object, allowing compare equal "/tmp". later operation on string object might see right offset of 4, string object perceived beingness "/usr". many security features of java programming language depend upon strings beingness perceived immutable, if malicious code using info races pass string references between threads.

strings implemented using char[], offset, , count. substring method constructs new string object same char[] , new offset , count. based on execution semantics, possible new string partially initialized when thread 2 attempts access it. according the source, substring returns new string object constructed simple constructor:

644 // bundle private constructor shares value array speed. 645 string(int offset, int count, char value[]) { 646 this.value = value; 647 this.offset = offset; 648 this.count = count; 649 }

so without marking char[], offset, , count final in string class definition, thread 2 might see inconsistent values when accesses them. if happens, char[] set, offset , count might wrong. if offset still showing default of 0, you'd see whole string. of course, require amazing timing, specific reordering of instructions jit, , whole host of "luck" create happen.

java jvm

Comments

Post a Comment

Popular posts from this blog

iphone - Dismissing a UIAlertView -

iphone - Dismissing a UIAlertView - i set in app purchase app, , when user taps button, purchase started. basically, tap button, , depending on speed on net connection, waiting 10 seconds until new alert view comes asking if purchase product. user tap button multiple times since nil came up, , multiple purchase alert views come up. additionally, maybe seen user app bug. in end, problem. i want alert view come spinning wheel says "loading..." when users taps purchase button. problem is, how dismiss when new alert view comes asking user if want purchase product? if ([uialertview alloc] says: @"whatever apple's alert view says") { //dismiss "loading..." alert view here } i uncertainty work, input appreciated. thanks! you need have access alertview. can this. create alertview instance var in app delegate , when want show loading initialize instance var assign property , when want dismiss phone call [alertviewinstance d...
Read more

c# - Can ProtoBuf-Net deserialize to a flat class? -

c# - Can ProtoBuf-Net deserialize to a flat class? - protobuf-net uses nested protobuf constructs back upwards inheritance. however, can made force properties flat target class has same properties inherited "serialized" version? see test illustration below. needless result of flat namespace null both properties. possible solution: re-create info flat.b first on property property basis. note: not prefered option. using system; namespace hierarchy { using protobuf; [protocontract] public class { [protomember(1)] public string prop1 { get; set; } } [protocontract] public class b : { public b() { } [protomember(1)] public string prop2 { get; set; } public override string tostring() { homecoming "prop1=" + prop1 + ", prop2=" + prop2; } } } namespace flat { using protobuf; [protocontract] p...
Read more

javascript - Change element in each JQuery tab to dynamically generated colors -

javascript - Change element in each JQuery tab to dynamically generated colors - i'm using jquery ui tabs widget , have button add together additional tabs. i'm tying each tab markers on map coordinating randomly generated color using function gen_color() { homecoming '#'+(math.random()*0xffffff<<0).tostring(16); } so each tab has color corresponds number of markers of same color on map. i'm trying add together 15x15 block of color within tab's tab left of it's label. right i'm using tabtemplate alternative , trying alter when new tab added: $tabs = $("#tabs").tabs({ tabtemplate: "<li><span class='ui-icon-color' style='background-color:#000000;'></span><a href='#{href}'>#{label}</a> <span class='ui-icon ui-icon-close'>remove tab</span></li>", selected: 0, add: funct...
Read more