编程中发生死锁问题的频率有多高？ [英] How commonly do deadlock issues occur in programming?
问题描述
我已经用多种语言编程，但是我不知道自己的代码中是否存在死锁。
I've programmed in a number of languages, but I am not aware of deadlocks in my code.
我认为这不会发生。
这种情况是否经常发生（在编程中，不是在数据库中），足以引起我的关注？
Does this happen frequently (in programming, not in the databases) enough that I should be concerned about it?
推荐答案
如果满足两个条件，则可能会出现死锁：您有无用的thead，并且它们争用多个资源。
Deadlocks could arise if two conditions are true: you have mutilple theads, and they contend for more than one resource.
您是否编写多线程代码？您可以通过启动自己的线程来显式地执行此操作，或者可以在看不见创建线程的框架中工作，因此您在多个线程中运行而在代码中看不到它。
Do you write multi-threaded code? You might do this explicitly by starting your own threads, or you might work in a framework where the threads are created out of your sight, and so you're running in more than one thread without you seeing that in your code.
一个例子：Java Servlet API。您编写一个servlet或JSP。您部署到应用服务器。有几个用户访问了您的网站，因此也访问了您的servlet。服务器可能每个用户都有一个线程。
An example: the Java Servlet API. You write a servlet or JSP. You deploy to the app server. Several users hit your web site, and hence your servlet. The server will likely have a thread per user.
现在考虑如果在处理请求时想要获取一些资源会发生什么情况：
Now consider what happens if in servicing the requests you want to aquire some resources:
if ( user Is Important ){
getResourceA();
}
getResourceB();
if (today is Thursday ) {
getResourceA();
}
// some more code
releaseResourceA();
releaseResoruceB();
在上面的人为示例中，请考虑当重要用户的请求到达时，星期四会发生什么，
In the contrived example above, think about what might happen on a Thursday when an important user's request arrives, and more or less simultaneously an unimportant user's request arrives.
重要用户的线程获得Resoruce A并想要B。不太重要的用户获得资源B并想要A。释放它们已经拥有的资源...死锁。
The important user's thread gets Resoruce A and wants B. The less important user gets resource B and wants A. Neither will let go of the resource that they already own ... deadlock.
如果您正在编写明确使用同步的代码，则实际上很容易发生。我最常见的情况是使用数据库时会发生这种情况，幸运的是数据库通常具有死锁检测功能，因此我们可以找出发生了什么错误。
This can actually happen quite easily if you are writing code that explicitly uses synchronization. Most commonly I see it happen when using databases, and fortunately databases usually have deadlock detection so we can find out what error we made.
防御死锁：
- 以明确的顺序获取资源。在上述示例中，如果总是在资源B之前获得资源A，则不会发生死锁。
- 如果可能的话，请使用超时机制，这样您就不必无限期地等待资源了。这将使您能够检测到争用并应用防御1。
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