TSM02-J. Do not use background threads during class initialization

public final class ConnectionFactory {
  private static Connection dbConnection;
  // Other fields ...

  static {
    Thread dbInitializerThread = new Thread(new Runnable() {
        @Override public void run() {
          // Initialize the database connection
          try {
            dbConnection = DriverManager.getConnection("connection string");
          } catch (SQLException e) {
            dbConnection = null;
          }
        }
    });

    // Other initialization, for example, start other threads

    dbInitializerThread.start();
    try {
      dbInitializerThread.join();
    } catch (InterruptedException ie) {
      throw new AssertionError(ie);
    }
  }

  public static Connection getConnection() {
    if (dbConnection == null) {
      throw new IllegalStateException("Error initializing connection");
    }
    return dbConnection;
  }

  public static void main(String[] args) {
    // ...
    Connection connection = getConnection();
  }
}

public final class ConnectionFactory {
  private static Connection dbConnection;
  // Other fields ...

  static {
    // Initialize a database connection
    try {
      dbConnection = DriverManager.getConnection("connection string");
    } catch (SQLException e) {
      dbConnection = null;
    }
    // Other initialization (do not start any threads)
  }

  // ...
}

public final class ConnectionFactory {
  private static final ThreadLocal<Connection> connectionHolder
                       = new ThreadLocal<Connection>() {
   @Override public Connection initialValue() {
     try {
       Connection dbConnection =
           DriverManager.getConnection("connection string");
       return dbConnection;
     } catch (SQLException e) {
       return null;
     }
   }
 };

  // Other fields ...

  static {
    // Other initialization (do not start any threads)
  }

  public static Connection getConnection() {
    Connection connection = connectionHolder.get();
    if (connection == null) {
      throw new IllegalStateException("Error initializing connection");
    }
    return connection;
  }

  public static void main(String[] args) {
    // ...
    Connection connection = getConnection();
  }
}

public final class ObjectPreserver implements Runnable {
  private static final ObjectPreserver lifeLine = new ObjectPreserver();

  private ObjectPreserver() {
    Thread thread = new Thread(this);
    thread.setDaemon(true);
    thread.start(); // Keep this object alive
  }

  // Neither this class nor HashMap will be garbage-collected.
  // References from HashMap to other objects
  // will also exhibit this property
  private static final ConcurrentHashMap<Integer,Object> protectedMap
      = new ConcurrentHashMap<Integer,Object>();

  public synchronized void run() {
    try {
      wait();
    } catch (InterruptedException e) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }
  }

  // Objects passed to this method will be preserved until
  // the unpreserveObject() method is called
  public static void preserveObject(Object obj) {
    protectedMap.put(0, obj);
  }

  // Returns the same instance every time
  public static Object getObject() {
    return protectedMap.get(0);
  }

  // Unprotect the objects so that they can be garbage-collected
  public static void unpreserveObject() {
    protectedMap.remove(0);
  }
}