Depending upon the tenacity of the grasp of the reference object, Java language has defined four different kinds of reference types - Strong, Soft, Weak & Phantom.
A simple search leads you to good explanations on the topic. Particularly the article by Ethan, this SO discussion, the official Java docs from Oracle, and this article on reachability are useful.
The focus of this article is to explain the working of the example in KDGregory's post on Phantom References.
You must understand the call sequence shown above and the purpose of the participating classes. The key points are:
(i) PooledConnection: class extends Connection class.
(ii) Client Applications: only interact with instances of the wrapper PooledConnection class. Applications get hold of new connections via the getConnection() for the ConnectionPool.
(iii) ConnectionPool: has a fixed number of Connections (base).
(iv) IdentityHashMap: Strong references to currently assigned/ in use Connections are maintained in two IdentityHashMaps - ref2Cxt & cxt2Ref. The references in the two HashMaps is the PhantomReference to the newly created PooledConnection.
The code is able to handle the following cases with regard to releasing of connections:
Case 1: A well behaving client, which calls PooledConnection.close():
This in turn calls the ConnectionPool.releaseConnection(Connection), passing in the associated the Connection object, leading to returning of the associated Connection object to the Connection Pool.
Case 2: Misbehaving client, Crashed client, etc. which does NOT call PooledConnection.close():
Once the client application stops using the PooledConnection object, the PooledConnection object goes out of scope. At this stage the PhantomReference associated with the PooledConnection object (see wrappedConnection()), gets enqueued in the associated ReferenceQueue.
In the ConnectionPool.getConnection() method, the call to ReferenceQueue.remove(), returns the PhantomReference to the particular PooledConnection. Passing in this PhantomReference to ConnectionPool.releaseConnection(Reference), locates the associated Connection object from the _ref2Cxt, IdentityHashMap, & returns the Connection back to the Connection Pool.
This automatic reclaiming of the precious (yet orphaned) Connection is the crux of KD Gregory's Phantom Reference example.
A simple search leads you to good explanations on the topic. Particularly the article by Ethan, this SO discussion, the official Java docs from Oracle, and this article on reachability are useful.
The focus of this article is to explain the working of the example in KDGregory's post on Phantom References.
(i) PooledConnection: class extends Connection class.
(ii) Client Applications: only interact with instances of the wrapper PooledConnection class. Applications get hold of new connections via the getConnection() for the ConnectionPool.
(iii) ConnectionPool: has a fixed number of Connections (base).
(iv) IdentityHashMap: Strong references to currently assigned/ in use Connections are maintained in two IdentityHashMaps - ref2Cxt & cxt2Ref. The references in the two HashMaps is the PhantomReference to the newly created PooledConnection.
The code is able to handle the following cases with regard to releasing of connections:
Case 1: A well behaving client, which calls PooledConnection.close():
This in turn calls the ConnectionPool.releaseConnection(Connection), passing in the associated the Connection object, leading to returning of the associated Connection object to the Connection Pool.
Case 2: Misbehaving client, Crashed client, etc. which does NOT call PooledConnection.close():
Once the client application stops using the PooledConnection object, the PooledConnection object goes out of scope. At this stage the PhantomReference associated with the PooledConnection object (see wrappedConnection()), gets enqueued in the associated ReferenceQueue.
In the ConnectionPool.getConnection() method, the call to ReferenceQueue.remove(), returns the PhantomReference to the particular PooledConnection. Passing in this PhantomReference to ConnectionPool.releaseConnection(Reference), locates the associated Connection object from the _ref2Cxt, IdentityHashMap, & returns the Connection back to the Connection Pool.
This automatic reclaiming of the precious (yet orphaned) Connection is the crux of KD Gregory's Phantom Reference example.
