每当在SO上出现关于Java同步的问题时,有些人都急于指出应该避免同步(这一点)。相反,他们声称,对私有引用的锁定是首选的。

其中一些原因是:

一些邪恶的代码可能会偷你的锁(这个很流行,也有一个“意外”变体) 同一类中的所有同步方法都使用完全相同的锁,这降低了吞吐量 你(不必要地)暴露了太多信息

包括我在内的其他人认为,synchronized(this)是一个被大量使用的习语(在Java库中也是如此),是安全的,而且很容易理解。它不应该被避免,因为你有一个错误,你不知道你的多线程程序中发生了什么。换句话说:如果它是适用的,那么就使用它。

我感兴趣的是看到一些现实世界的例子(没有foobar的东西),避免锁定在这是可取的,当同步(这)也会做的工作。

因此:您应该总是避免synchronized(this),并将其替换为私有引用上的锁吗?


一些进一步的信息(随着答案的更新):

we are talking about instance synchronization both implicit (synchronized methods) and explicit form of synchronized(this) are considered if you quote Bloch or other authorities on the subject, don't leave out the parts you don't like (e.g. Effective Java, item on Thread Safety: Typically it is the lock on the instance itself, but there are exceptions.) if you need granularity in your locking other than synchronized(this) provides, then synchronized(this) is not applicable so that's not the issue


当前回答

当您使用synchronized(this)时,您正在使用类实例作为锁本身。这意味着当线程1获得锁时,线程2应该等待。

假设有以下代码:

public void method1() {
    // do something ...
    synchronized(this) {
        a ++;      
    }
    // ................
}


public void method2() {
    // do something ...
    synchronized(this) {
        b ++;      
    }
    // ................
}

方法一修改变量a,方法二修改变量b,应避免两个线程同时修改同一个变量。但是当thread1修改a和thread2修改b时,它可以在没有任何竞争条件的情况下执行。

不幸的是,上面的代码不允许这样做,因为我们对锁使用了相同的引用;这意味着线程即使没有处于竞争状态也应该等待,显然代码牺牲了程序的并发性。

解决方案是对两个不同的变量使用两个不同的锁:

public class Test {

    private Object lockA = new Object();
    private Object lockB = new Object();

    public void method1() {
        // do something ...
        synchronized(lockA) {
            a ++;      
        }
        // ................
    }


    public void method2() {
        // do something ...
        synchronized(lockB) {
            b ++;      
        }
        // ................
    }

}

上面的例子使用了更细粒度的锁(2个锁而不是一个锁(分别针对变量a和变量b的lockA和lockB),结果允许更好的并发性,另一方面,它变得比第一个例子更复杂……

其他回答

虽然我同意不要盲目地遵守教条规则,但“偷锁”的场景对你来说是不是很古怪?一个线程确实可以从你的对象“外部”获得锁(synchronized(theObject){…}),阻塞其他线程等待同步实例方法。

如果您不相信恶意代码,请考虑这些代码可能来自第三方(例如,如果您开发了某种应用程序服务器)。

“意外”版本似乎不太可能,但就像他们说的那样,“让一些东西不受白痴的影响,就会有人发明一个更好的白痴”。

所以我同意“这取决于这个班级做什么”的观点。


编辑以下eljenso的前3条评论:

我从来没有遇到过偷锁的问题,但这里有一个想象的场景:

假设您的系统是一个servlet容器,我们考虑的对象是ServletContext实现。它的getAttribute方法必须是线程安全的,因为上下文属性是共享数据;所以你声明它是同步的。让我们再想象一下,您提供了一个基于容器实现的公共托管服务。

我是您的客户,并在您的站点上部署我的“好”servlet。我的代码碰巧包含对getAttribute的调用。

黑客伪装成另一个客户,在您的站点上部署恶意servlet。它在init方法中包含以下代码:

synchronized (this.getServletConfig().getServletContext()) {
   while (true) {}
}

假设我们共享相同的servlet上下文(只要两个servlet位于同一个虚拟主机上,规范就允许),那么我对getAttribute的调用将永远锁定。黑客已经在我的servlet上实现了DoS。

如果getAttribute在私有锁上同步,则这种攻击是不可能的,因为第三方代码无法获得此锁。

我承认这个例子是人为设计的,对servlet容器如何工作的看法过于简单,但恕我直言,它证明了这一点。

因此,我将基于安全性考虑做出设计选择:我是否能够完全控制访问实例的代码?线程无限期地持有实例锁的后果是什么?

锁可以用于可见性,也可以用于保护一些数据不受可能导致竞争的并发修改的影响。

当您需要将基本类型操作设置为原子类型时,可以使用AtomicInteger之类的选项。

但是假设你有两个整数,它们像x和y坐标一样彼此相关,它们彼此相关,应该以原子的方式改变。然后使用相同的锁来保护它们。

锁应该只保护彼此相关的状态。不多不少。如果在每个方法中都使用synchronized(this),那么即使类的状态是不相关的,即使更新不相关的状态,所有线程也将面临争用。

class Point{
   private int x;
   private int y;

   public Point(int x, int y){
       this.x = x;
       this.y = y;
   }

   //mutating methods should be guarded by same lock
   public synchronized void changeCoordinates(int x, int y){
       this.x = x;
       this.y = y;
   }
}

在上面的例子中,我只有一个方法同时改变x和y,而不是两个不同的方法,因为x和y是相关的,如果我给了两个不同的方法分别改变x和y,那么它就不会是线程安全的。

这个例子只是为了演示它的实现方式,而不一定是这样。最好的方法是让它成为IMMUTABLE。

现在,与Point例子相反的是,@Andreas已经提供了一个TwoCounters的例子,其中状态被两个不同的锁保护,因为状态彼此不相关。

使用不同的锁来保护不相关的状态的过程称为锁剥离或锁分裂

简单的回答:您必须了解其中的区别,并根据代码做出选择。

Long answer: In general I would rather try to avoid synchronize(this) to reduce contention but private locks add complexity you have to be aware of. So use the right synchronization for the right job. If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic. (That said: just using synchronize(this) does not automatically make your class fully thread-safe.) This is a not an easy topic but once you get used to it, the answer whether to use synchronize(this) or not comes naturally.

在c#和Java阵营中似乎有不同的共识。我看到的大多数Java代码使用:

// apply mutex to this instance
synchronized(this) {
    // do work here
}

而大多数c#代码选择了更安全的:

// instance level lock object
private readonly object _syncObj = new object();

...

// apply mutex to private instance level field (a System.Object usually)
lock(_syncObj)
{
    // do work here
}

c#语言当然更安全。如前所述,不能从实例外部对锁进行恶意/意外访问。Java代码也有这种风险,但随着时间的推移,Java社区似乎倾向于稍微不那么安全,但稍微更简洁的版本。

这并不是对Java的挖苦,只是我在这两种语言上工作的经验的反映。

I think there is a good explanation on why each of these are vital techniques under your belt in a book called Java Concurrency In Practice by Brian Goetz. He makes one point very clear - you must use the same lock "EVERYWHERE" to protect the state of your object. Synchronised method and synchronising on an object often go hand in hand. E.g. Vector synchronises all its methods. If you have a handle to a vector object and are going to do "put if absent" then merely Vector synchronising its own individual methods isn't going to protect you from corruption of state. You need to synchronise using synchronised (vectorHandle). This will result in the SAME lock being acquired by every thread which has a handle to the vector and will protect overall state of the vector. This is called client side locking. We do know as a matter of fact vector does synchronised (this) / synchronises all its methods and hence synchronising on the object vectorHandle will result in proper synchronisation of vector objects state. Its foolish to believe that you are thread safe just because you are using a thread safe collection. This is precisely the reason ConcurrentHashMap explicitly introduced putIfAbsent method - to make such operations atomic.

总之

Synchronising at method level allows client side locking. If you have a private lock object - it makes client side locking impossible. This is fine if you know that your class doesn't have "put if absent" type of functionality. If you are designing a library - then synchronising on this or synchronising the method is often wiser. Because you are rarely in a position to decide how your class is going to be used. Had Vector used a private lock object - it would have been impossible to get "put if absent" right. The client code will never gain a handle to the private lock thus breaking the fundamental rule of using the EXACT SAME LOCK to protect its state. Synchronising on this or synchronised methods do have a problem as others have pointed out - someone could get a lock and never release it. All other threads would keep waiting for the lock to be released. So know what you are doing and adopt the one that's correct. Someone argued that having a private lock object gives you better granularity - e.g. if two operations are unrelated - they could be guarded by different locks resulting in better throughput. But this i think is design smell and not code smell - if two operations are completely unrelated why are they part of the SAME class? Why should a class club unrelated functionalities at all? May be a utility class? Hmmmm - some util providing string manipulation and calendar date formatting through the same instance?? ... doesn't make any sense to me at least!!