package eu.fbk.knowledgestore;
   
   import java.util.List;
   
   import javax.annotation.Nullable;
   
   import com.google.common.collect.Lists;
   
   import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  public abstract class AbstractKnowledgeStore implements KnowledgeStore {
  
      private static final Logger LOGGER = LoggerFactory.getLogger(AbstractKnowledgeStore.class);
  
      private final List<Session> sessions;
  
      private boolean closed;
  
      protected AbstractKnowledgeStore() {
          this.sessions = Lists.newLinkedList();
          this.closed = false;
      }
  
      @Override
      public final synchronized Session newSession() throws IllegalStateException {
          return newSession(null, null);
      }
  
      @Override
      public final synchronized Session newSession(@Nullable final String username,
              @Nullable final String password) throws IllegalStateException {
          synchronized (this.sessions) {
              evictClosedSessions();
              final Session session = doNewSession(username, password);
              this.sessions.add(session);
              return session;
          }
      }
  
      @Override
      public final synchronized void close() {
          if (!this.closed) {
              for (final Session session : this.sessions) {
                  try {
                      session.close();
                  } catch (final Throwable ex) {
                      LOGGER.error("Error closing session: " + ex.getMessage(), ex);
                  }
              }
              try {
                  doClose();
              } finally {
                  this.closed = true;
              }
          }
      }
  
      @Override
      public String toString() {
          return getClass().getSimpleName() + "[" + (this.closed ? "closed" : "open") + ", "
                  + this.sessions.size() + " sessions]";
      }
  
      @Override
      public final synchronized boolean isClosed() {
          return this.closed;
      }
  
      protected final void checkNotClosed() {
          if (this.closed) {
              throw new IllegalStateException("KnowledgeStore has been closed");
          }
      }
  
      protected final void evictClosedSessions() {
          synchronized (this.sessions) {
              for (int i = this.sessions.size() - 1; i >= 0; --i) {
                  if (this.sessions.get(i).isClosed()) {
                      this.sessions.remove(i);
                  }
              }
          }
      }
  
      protected abstract Session doNewSession(@Nullable String username, @Nullable String password);
  
      protected void doClose() {
      }
  
  }