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最新评论
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xwv:
能对你有启发和帮助就好
为什么Android的Adapter中,bindview被调用了多次 -
lipei.98:
哈哈 谢谢你的博客,解决了我一个头痛的问题。
为什么Android的Adapter中,bindview被调用了多次
在Java code实践中,需要使用Weak References。对这个东东不了解,故Google了一下,找到了下面这个web,用以作为入门。下面对其全文转一下。
Some time ago I was interviewing candidates for a Senior Java Engineer position. Among the many questions I asked was "What can you tell me about weak references?" I wasn't expecting a detailed technical treatise on the subject. I would probably have been satisfied with "Umm... don't they have something to do with garbage collection?" I was instead surprised to find that out of twenty-odd engineers, all of whom had at least five years of Java experience and good qualifications, only two of them even knew that weak references existed, and only one of those two had actual useful knowledge about them. I even explained a bit about them, to see if I got an "Oh yeah" from anybody -- nope. I'm not sure why this knowledge is (evidently) uncommon, as weak references are a massively useful feature which have been around since Java 1.2 was released, over seven years ago.
Now, I'm not suggesting you need to be a weak reference expert to qualify as a decent Java engineer. But I humbly submit that you should at least know what they are -- otherwise how will you know when you should be using them? Since they seem to be a little-known feature, here is a brief overview of what weak references are, how to use them, and when to use them.
Strong references
First I need to start with a refresher on strong references . A strong reference is an ordinary Java reference, the kind you use every day. For example, the code:
StringBuffer buffer = new StringBuffer();
creates a new StringBuffer()
and stores a strong reference to it in the variable buffer
.
Yes, yes, this is kiddie stuff, but bear with me. The important part
about strong references -- the part that makes them "strong" -- is how
they interact with the garbage collector. Specifically, if an object is
reachable via a chain of strong references (strongly reachable), it is
not eligible for garbage collection. As you don't want the garbage
collector destroying objects you're working on, this is normally exactly
what you want.
When strong references are too strong
It's not uncommon for an application to use classes that it can't reasonably extend. The class might simply be marked final
,
or it could be something more complicated, such as an interface
returned by a factory method backed by an unknown (and possibly even
unknowable) number of concrete implementations. Suppose you have to use
a class Widget
and, for whatever reason, it isn't possible or practical to extend Widget
to add new functionality.
What happens when you need to keep track of extra information about
the object? In this case, suppose we find ourselves needing to keep
track of each Widget's
serial number, but the Widget
class doesn't actually have a serial number property -- and because Widget
isn't extensible, we can't add one. No problem at all, that's what HashMaps
are for:
serialNumberMap.put(widget, widgetSerialNumber);
This might look okay on the surface, but the strong reference to widget
will almost certainly cause problems. We have to know (with 100% certainty) when a particular Widget's
serial number is no longer needed, so we can remove its entry from the
map. Otherwise we're going to have a memory leak (if we don't remove Widgets
when we should) or we're going to inexplicably find ourselves missing serial numbers (if we remove Widgets
that we're still using). If these problems sound familiar, they
should: they are exactly the problems that users of
non-garbage-collected languages face when trying to manage memory, and
we're not supposed to have to worry about this in a more civilized
language like Java.
Another common problem with strong references is caching, particular with very large structures like images. Suppose you have an application which has to work with user-supplied images, like the web site design tool I work on. Naturally you want to cache these images, because loading them from disk is very expensive and you want to avoid the possibility of having two copies of the (potentially gigantic) image in memory at once.
Because an image cache is supposed to prevent us from reloading images when we don't absolutely need to, you will quickly realize that the cache should always contain a reference to any image which is already in memory. With ordinary strong references, though, that reference itself will force the image to remain in memory, which requires you (just as above) to somehow determine when the image is no longer needed in memory and remove it from the cache, so that it becomes eligible for garbage collection. Once again you are forced to duplicate the behavior of the garbage collector and manually determine whether or not an object should be in memory.
Weak references
A weak reference , simply put, is a reference that isn't strong enough to force an object to remain in memory. Weak references allow you to leverage the garbage collector's ability to determine reachability for you, so you don't have to do it yourself. You create a weak reference like this:
WeakReference<Widget> weakWidget = new WeakReference<Widget>(widget);
and then elsewhere in the code you can use weakWidget.get()
to get the actual Widget
object. Of course the weak reference isn't strong enough to prevent
garbage collection, so you may find (if there are no strong references
to the widget) that weakWidget.get()
suddenly starts returning null
.
To solve the "widget serial number" problem above, the easiest thing to do is use the built-in WeakHashMap
class. WeakHashMap
works exactly like HashMap
, except that the keys (not
the values!) are referred to using weak references. If a WeakHashMap
key becomes garbage, its entry is removed automatically. This avoids
the pitfalls I described and requires no changes other than the switch
from HashMap
to a WeakHashMap
. If you're following the standard convention of referring to your maps via the Map
interface, no other code needs to even be aware of the change.
Reference queues
Once a WeakReference
starts returning null
, the object it pointed to has become garbage and the WeakReference
WeakHashMap
, for example, has to remove such defunct entries to avoid holding onto an ever-increasing number of dead WeakReferences
.
object is pretty much useless. This generally means that some sort of cleanup is required;
The ReferenceQueue
class makes it easy to keep track of dead references. If you pass a ReferenceQueue
into a weak reference's constructor, the reference object will be
automatically inserted into the reference queue when the object to which
it pointed becomes garbage. You can then, at some regular interval,
process the ReferenceQueue
and perform whatever cleanup is needed for dead references.
Different degrees of weakness
Up to this point I've just been referring to "weak references", but there are actually four different degrees of reference strength: strong, soft, weak, and phantom, in order from strongest to weakest. We've already discussed strong and weak references, so let's take a look at the other two.
Soft references
A soft reference
is exactly like a weak reference, except that
it is less eager to throw away the object to which it refers. An
object which is only weakly reachable (the strongest references to it
are WeakReferences
) will be discarded
at the next garbage collection cycle, but an object which is softly
reachable will generally stick around for a while.
SoftReferences
aren't required
to behave any differently than WeakReferences
,
but in practice softly reachable objects are generally retained as long
as memory is in plentiful supply. This makes them an excellent
foundation for a cache, such as the image cache described above, since
you can let the garbage collector worry about both how reachable the
objects are (a strongly reachable object will never
be removed from the cache) and how badly it needs the memory they are consuming.
Phantom references
A phantom reference
is quite different than either SoftReference
or WeakReference
. Its grip on its object is so tenuous that you can't even retrieve the object -- its get()
method always returns null
. The only use for such a reference is keeping track of when it gets enqueued into a ReferenceQueue
, as at that point you know the object to which it pointed is dead. How is that different from WeakReference
, though?
The difference is in exactly when the enqueuing happens. WeakReferences
are enqueued as soon as the object to which they point becomes weakly reachable. This is before
finalization or garbage collection has actually happened; in theory the object could even be "resurrected" by an unorthodox finalize()
method, but the WeakReference
would remain dead. PhantomReferences
are enqueued only when the object is physically removed from memory, and the get()
method always returns null
specifically to prevent you from being able to "resurrect" an almost-dead object.
What good are PhantomReferences
?
I'm only aware of two serious cases for them: first, they allow you to
determine exactly when an object was removed from memory. They are in
fact the only
way to determine that. This isn't generally that
useful, but might come in handy in certain very specific circumstances
like manipulating large images: if you know for sure that an image
should be garbage collected, you can wait until it actually is before
attempting to load the next image, and therefore make the dreaded OutOfMemoryError
less likely.
Second, PhantomReferences
avoid a fundamental problem with finalization: finalize()
methods can "resurrect" objects by creating new strong references to
them. So what, you say? Well, the problem is that an object which
overrides finalize()
must now be
determined to be garbage in at least two separate garbage collection
cycles in order to be collected. When the first cycle determines that
it is garbage, it becomes eligible for finalization. Because of the
(slim, but unfortunately real) possibility that the object was
"resurrected" during finalization, the garbage collector has to run
again before the object can actually be removed. And because
finalization might not have happened in a timely fashion, an arbitrary
number of garbage collection cycles might have happened while the object
was waiting for finalization. This can mean serious delays in actually
cleaning up garbage objects, and is why you can get OutOfMemoryErrors
even when most of the heap is garbage.
With PhantomReference
, this situation is impossible -- when a PhantomReference
is enqueued, there is absolutely no way to get a pointer to the
now-dead object (which is good, because it isn't in memory any longer).
Because PhantomReference
cannot be
used to resurrect an object, the object can be instantly cleaned up
during the first garbage collection cycle in which it is found to be
phantomly reachable. You can then dispose whatever resources you need
to at your convenience.
Arguably, the finalize()
method should never have been provided in the first place. PhantomReferences
are definitely safer and more efficient to use, and eliminating finalize()
would have made parts of the VM considerably simpler. But, they're also more work to implement, so I confess to still using finalize()
most of the time. The good news is that at least you have a choice.
Conclusion
I'm sure some of you are grumbling by now, as I'm talking about an API which is nearly a decade old and haven't said anything which hasn't been said before. While that's certainly true, in my experience many Java programmers really don't know very much (if anything) about weak references, and I felt that a refresher course was needed. Hopefully you at least learned a little something from this review.
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2013-01-25 12:31 1248在PHP项目中有一个方案要采用函数parse_i ... -
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2012-12-18 15:50 3478最近需要在服 ... -
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2012-11-29 10:11 0资料: 1.http://www.p ... -
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2012-11-26 15:19 0关于Perl中的模块。 如何查看已安 ... -
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