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PO
 

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  1. PO
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    <p>Using synchronized and Collections.synchronizedMap() just to guard the Map is redundant. The use cases for both of these items can vary which is why both are in your 'tool box'.</p> <p>If you need to access something in the Map, check it, then maybe put things back in as one operation, synchronized is what you should be using. If you are just getting and adding to the Map, Collections.synchronizedMap() should be sufficient.</p> <p>In theory you could try to interrupt the 'low priority' thread when accessed by a 'high priority' thread but being a hash map, it probably isn't doing much waiting that could be aborted.</p> <p>Is there something that the 'high priority' thread is doing that the 'low priority' thread isn't? Creating a copy of the Map might be something you need to do so that there is always a readable version, but it may get updates late. More information would be needed to tackle the problem from a different direction.</p> <p>Technically the simple answer to your question is no.</p>
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    1. POWhich thread is blocked during synchronized access of a map?
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    1. USpimaster
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      1. VTUpMod
    1. COI read that I should use both [here](http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Collections.html#synchronizedMap(java.util.Map)). I will be using an iterator with it in the low priority thread, and adding and removing items in the high priority thread. So do I only need to use `synchronized` when I'm iterating?
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      1. USmasonsbro
    2. COIn that case I wouldn't be mixing them so that it was obvious what was going on in the code. The synchronized here is very important (as pointed out) because the iterator can't start and release the lock for you. I would use just any old Map implementation, have some sort of lock object and go synchronize(lock) { map.get() }, same for put and iteration operations. If your low priority thread is doing the iteration, you may be able to check for interruptions, break out. Then continue later. A processing queue might be an alternative if you just need to process new items that come into the map.
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      1. USpimaster
    3. COYou are misinterpreting the Javadoc you link to. It warns that **if using `synrchronizedMap` you still need to synchronize iteration**. It doesn't say that you must **always use both**. In fact, using only `synchronized` blocks is enough and in fact preferred. If all your map accesses are within a `synchronized` block, the map itself being wrapped by `synchronizedMap` is redundant. Also, if mixing synchronizations, be **very careful** which locks you use.
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      1. USMarko Topolnik
 

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