Is javascript guaranteed to be single-threaded?
来源:互联网 发布:淘宝客户运营平台 编辑:程序博客网 时间:2024/05/18 02:46
That's a good question. I'd love to say “yes”. I can't.
JavaScript is usually considered to have a single thread of execution visible to scripts(*), so that when your inline script, event listener or timeout is entered, you remain completely in control until you return from the end of your block or function.
(*: ignoring the question of whether browsers really implement their JS engines using one OS-thread, or whether other limited threads-of-execution are introduced by WebWorkers.)
However, in reality this isn't quite true, in sneaky nasty ways.
The most common case is immediate events. Browsers will fire these right away when your code does something to cause them:
<textarea id="log" rows="20" cols="40"></textarea><input id="inp"><script type="text/javascript"> var l= document.getElementById('log'); var i= document.getElementById('inp'); i.onblur= function() { l.value+= 'blur\n'; }; setTimeout(function() { l.value+= 'log in\n'; l.focus(); l.value+= 'log out\n'; }, 100); i.focus();</script>
Results in log in, blur, log out
on all except IE. These events don't just fire because you called focus()
directly, they could happen because you called alert()
, or opened a pop-up window, or anything else that moves the focus.
This can also result in other events. For example add an i.onchange
listener and type something in the input before the focus()
call unfocuses it, and the log order is log in, change, blur, log out
, except in Opera where it's log in, blur, log out, change
and IE where it's (even less explicably) log in, change, log out, blur
.
Similarly calling click()
on an element that provides it calls the onclick
handler immediately in all browsers (at least this is consistent!).
(I'm using the direct on...
event handler properties here, but the same happens with addEventListener
and attachEvent
.)
There's also a bunch of circumstances in which events can fire whilst your code is threaded in, despite you having done nothing to provoke it. An example:
<textarea id="log" rows="20" cols="40"></textarea><button id="act">alert</button><script type="text/javascript"> var l= document.getElementById('log'); document.getElementById('act').onclick= function() { l.value+= 'alert in\n'; alert('alert!'); l.value+= 'alert out\n'; }; window.onresize= function() { l.value+= 'resize\n'; };</script>
Hit alert
and you'll get a modal dialogue box. No more script executes until you dismiss that dialogue, yes? Nope. Resize the main window and you will get alert in, resize, alert out
in the textarea.
You might think it's impossible to resize a window whilst a modal dialogue box is up, but not so: in Linux, you can resize the window as much as you like; on Windows it's not so easy, but you can do it by changing the screen resolution from a larger to a smaller one where the window doesn't fit, causing it to get resized.
You might think, well, it's only resize
(and probably a few more like scroll
) that can fire when the user doesn't have active interaction with the browser because script is threaded. And for single windows you might be right. But that all goes to pot as soon as you're doing cross-window scripting. For all browsers other than Safari, which blocks all windows/tabs/frames when any one of them is busy, you can interact with a document from the code of another document, running in a separate thread of execution and causing any related event handlers to fire.
Places where events that you can cause to be generated can be raised whilst script is still threaded:
when the modal popups (
alert
,confirm
,prompt
) are open, in all browsers but Opera;during
showModalDialog
on browsers that support it;the “A script on this page may be busy...” dialogue box, even if you choose to let the script continue to run, allows events like resize and blur to fire and be handled even whilst the script is in the middle of a busy-loop, except in Opera.
a while ago for me, in IE with the Sun Java Plugin, calling any method on an applet could allow events to fire and script to be re-entered. This was always a timing-sensitive bug, and it's possible Sun have fixed it since (I certainly hope so).
probably more. It's been a while since I tested this and browsers have gained complexity since.
In summary, JavaScript appears to most users, most of the time, to have a strict event-driven single thread of execution. In reality, it has no such thing. It is not clear how much of this is simply a bug and how much deliberate design, but if you're writing complex applications, especially cross-window/frame-scripting ones, there is every chance it could bite you — and in intermittent, hard-to-debug ways.
If the worst comes to the worst, you can solve concurrency problems by indirecting all event responses. When an event comes in, drop it in a queue and deal with the queue in order later, in a setInterval
function. If you are writing a framework that you intend to be used by complex applications, doing this could be a good move. postMessage
will also hopefully soothe the pain of cross-document scripting in the future.
- Is javascript guaranteed to be single-threaded?
- Apache is running a threaded MPM, but your php module is not compiled to be threadsafe. you need to
- 解决【Apache is running a threaded MPM, but your php module is not compiled to be threadsafe.】
- The connected J-Link is defective,Proper operation cannot be guaranteed.......
- 第一章 Single Threaded Execution
- Single Threaded Execution Pattern
- Multi-threaded design patterns -- Single Threaded Execution
- [STAThread]即 single-threaded apartment
- as-is and to-be
- Code is to be happy
- 线程之旅--Single Threaded Execution Pattern
- 多线程模式之-single Threaded Execution Pattern
- 【译】Why JavaScript Is and Will Continue to Be the First Choice of Programmers
- Introduction to Multi-threaded Code
- TO BE OR NOT TO BE THAT IS AQUESTION
- To be or Not to be that is the question
- to be or not to be, that is a question...
- To be or not to be that is a question
- 数据压缩与信息熵
- threadid=3: reacting to signal 3 threadid=3: spin on suspend #1 threadid=19 (pcf=0)
- Android下划线
- java中set和get的基本使用
- 广东程序员在加利福尼亚
- Is javascript guaranteed to be single-threaded?
- HDU-2564-词组缩写
- 第十九周项目一(二):动态链表体验二(输出链表中是否有值为x的结点)
- iOS 富文本类库RTLabel
- iostat来对linux硬盘IO性能进行了解
- Cocos架构和目录结构
- ios项目代码编写规范
- linux ssh免密码登录
- 黑马程序员--------单例设计模式