java - Why does HashMap.get(key) needs to be synchronized when change operations are synchronized? -

i use .get(...), .put(...) , .clear() operations multiple threads on 1 hashmap. .put(...) , .clear() inside synchronized block .get(...) not. can't imagine cause problems in other code i've seen .get() pretty synchronized.

relevant code get/put

object value = map.get(key); if(value == null) {   synchronized (map) {     value = map.get(key); // check again, might have been changed in between     if(value == null) {       map.put(key, new value(...));     }   } } 

and clear just:

synchronized (map) {   map.clear(); } 

the write operations invalidate caches because of synchronized , get(...) returns either null or instance. can't see go wrong or improve putting .get(...) operation synchronized(map) block.

here 1 simple scenario produce problem on unsynchronized get:

• thread starts get, computes hash bucket number, , gets pre-empted
• thread b calls clear(), smaller array of buckets gets allocated
• thread wakes up, , may run index-out-of-bounds exception

here more complex scenario:

• thread locks map update, , gets pre-empted
• thread b initiates get operation, computes hash bucket number, , gets pre-empted
• thread wakes up, , continues put, , realizes buckets need resizing
• thread allocates new buckets, copies old content them, , adds new item
• thread b wakes up, , continues search using old bucket index on new array of buckets.

at point, not going find right item, because in hash bucket @ different index. why get needs synchronized well.