Java 同步列表 for 循环
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Java synchronized list for loop
提问by user927476
Documentation on synchronizedList states that,
synchronizedList 上的文档指出，
It is imperative that the user manually synchronize on the returned list when iterating over it:
用户在迭代返回的列表时必须手动同步它：
List list = Collections.synchronizedList(new ArrayList());
...
synchronized(list) {
Iterator i = list.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
Failure to follow this advice may result in non-deterministic behavior.
不遵循此建议可能会导致不确定的行为。
This seems pretty clear, but I just wanted to confirm that a for each loop is prohibited. For example, I cannot do something like as follows right?
这看起来很清楚，但我只是想确认一个 for each 循环是被禁止的。例如，我不能做如下的事情对吗？
List<MyType> list = Collections.synchronizedList(new ArrayList(<MyType>));
...
synchronized(list){
for(MyType m : list){
foo(m);
m.doSomething();
}
}
回答by Jon Skeet
Yes, you can - your enhanced for loop is basically the same as your code which explicitly uses the iterator. It boils down to the same code - it's just calling iterator()and then alternating between next()and hasNext()calls.
是的，您可以 - 您增强的 for 循环与显式使用迭代器的代码基本相同。它归结为相同的代码 - 它只是调用iterator()然后在next()和hasNext()调用之间交替。
回答by Ryan Gross
You can do that. The foreach loop compiles to (nearly) the same bytecode as the while loop. The keys are:
你可以这样做。foreach 循环编译为（几乎）与 while 循环相同的字节码。关键是：
- You synchronize the block around the loop because the list may change while you are iterating over it.
- You use the list as the object that you are synchronizing on, since the implementation of this class locks on itself (through synchronized methods).
- 您在循环周围同步块，因为在迭代它时列表可能会发生变化。
- 您将列表用作同步对象，因为此类的实现锁定了自身（通过同步方法）。
回答by John B
If possible, you might want to consider using immutability rather than synchonization.
如果可能，您可能需要考虑使用不变性而不是同步。
回答by Mike Njong
Of course you can, the only problem I see here is a performance issue, if your method dosomething()or foo(m)are costly to execute, you will have a performance cost. The size of your collection is also important to take in account while looping in a synchronized block, due to the fact that, when a thread acquire the lock, while in the synchronized block, looping in a huge collection will push other threads to wait.
当然可以，我在这里看到的唯一问题是性能问题，如果您的方法dosomething()或foo(m)执行成本高昂，您将有性能成本。在同步块中循环时要考虑集合的大小也很重要，因为当线程获取锁时，而在同步块中，在巨大集合中循环将推动其他线程等待。
