java：函数在hashmap.put（key，function（））中是线程安全的吗？

Question

I want to put a value returned from a function into ConcurrentHashmap like that

private static ConcurrentHashMap<String, Object> map = new ConcurrentHashMap<String, Object>();

public void process() {
    map.put(key, function());
}

public Object function() {
    return anyObject;
}

May I know the function (eg function()) is thread-safe or not? Please advise if there is any tutorial Thanks.

Answer 1

call itself in not thread-safe, only put operation atomic.

Instead you can use guava

ConcurrentMap<Key, Graph> graphs = new MapMaker()
   .concurrencyLevel(4)
   .weakKeys()
   .maximumSize(10000)
   .expireAfterWrite(10, TimeUnit.MINUTES)
   .makeComputingMap(
       new Function<Key, Graph>() {
         public Graph apply(Key key) {
           return createExpensiveGraph(key);
         }
       });

Please notice makeComputingMap() call

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/MapMaker.html

Answer 2

You could use the Read / Write Locks in Java.

Example:

class X {
  ReadWriteLock rw;

  public void read() throws InterruptedException { 
    rw.readLock().acquire();
    try {
      // ... do the read
    } finally {
      rw.readlock().release()
    }
 }

 public void write() throws InterruptedException { 
     rw.writeLock().acquire();
     try {
       // ... do the write
     } finally {
       rw.writelock().release()
     }
   }
 }

Ref:

http://www.coderanch.com/t/232685/threads/java/Concurrent-access-HashMap http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/ReadWriteLock.html http://tutorials.jenkov.com/java-concurrency/read-write-locks.html