java.util.concurrent.Semaphore

java.util.concurrent.Semaphore

If we wants to make programmatically Connection management, connection may be Database or TCP Socket or JMS or other. Instead of establishing connection every time for every SQL command, we can cache the connection for future use. Once the usage of connection usage is completed then we can take connection back to pool. For instance, We have established 20 connections, but 50 SQL commands has to be run in the given point of time. These 20 connections has to be alotted to first 20 SQL commands to run and whenever SQL command is completed then connection will be used for rest of SQL commands.

We have to maintain a mark for each connection that whether that connection is get used or not. Two methods has to be used to acquire and release the resource. Everything perfect, who will take care of filtering and ordering the thread entry, how thread state managed. For these questions Semaphore is the right answer, here is the code snippet

 class ConnectionPool {
   private static final int MAX_AVAILABLE = 100;
   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
 
   public Object getConnection() throws InterruptedException {
      available.acquire(); 
     return getNextAvailableItem();
   }
 
   public void releaseConnection(Object x) {
     if (markAsUnused(x))
       available.release();
   }
 
   protected Object[] connections = ... whatever kinds of connections being managed
   protected boolean[] used = new boolean[MAX_AVAILABLE];
 
   protected synchronized Object getNextAvailableItem() {
     for (int i = 0; i < MAX_AVAILABLE; ++i) {
       if (!used[i]) {
          used[i] = true;
          return connections[i];
       }
     }
     return null; // not reached
   }
 
   protected synchronized boolean markAsUnused(Object item) {
     for (int i = 0; i < MAX_AVAILABLE; ++i) {
       if (item == connections[i]) {
          if (used[i]) {
            used[i] = false;
            return true;
          } else
            return false;
       }
     }
     return false;
   }
 
 }