Package org.apache.sis.util.collection

# Interface Cache.Handler<V>

Type Parameters:
  `V` - the type of value objects.

Enclosing class:
  Cache<K,V>

public static interface Cache.Handler<V>
The handler returned by `Cache​.lock(K)`, to be used for unlocking and storing the result. This handler should be used as below (the `try` … `finally` statements are important):

>     Value V = null;
>     Cache.Handler<V> handler = cache.lock(key);
>     try {
>         value = handler.peek();
>         if (value == null) {
>             value = createMyObject(key);
>         }
>     } finally {
>         handler.putAndUnlock(value);
>     }

See the `Cache` javadoc for a more complete example.

Since:
  0.3

Defined in the `sis-utility` module

- ## Method Summary

  Modifier and Type
  Method
  Description
  `V`
  `peek()`
  If the value is already in the cache, returns it.
  `void`
  `putAndUnlock(V result)`
  Stores the given value in the cache and release the lock.

- ## Method Details

  - ### peek

    V peek()
    If the value is already in the cache, returns it. Otherwise returns `null`. This method should be invoked after the `Handler` creation in case a value has been computed in another thread.
    Returns:
      the value from the cache, or `null` if none.

  - ### putAndUnlock

    void putAndUnlock(V result) throws IllegalStateException
    Stores the given value in the cache and release the lock. This method must be invoked in a `finally` block, no matter what the result is.
    Parameters:
      `result` - the result to store in the cache, or `null` for removing the entry from the cache. If an entry is removed, a new computation will be attempted the next time a handler is created for the same key.
    Throws:
      `IllegalStateException` - may be thrown if this method is not invoked in the pattern described in class javadoc, or if a key collision occurs.