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Starting and using background threads during class initialization can result in class initialization cycles and deadlock. For example, the main thread responsible for performing class initialization can block waiting for the background thread, which in turn will wait for the main thread to finish class initialization. This issue can arise, for example, when a database connection is established in a background thread during class initialization \[[Bloch 05b|AA. Java References#Bloch 05b]\]. 

public final class ConnectionFactory {
  private static Connection dbConnection;
  // Other fields ...
	  
  static {
    Thread dbInitializerThread = new Thread(new Runnable() {
      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();
  }
}

Statically initialized fields are guaranteed to be fully constructed before they are made visible to other threads (see [CON26-J. Do not publish partially initialized objects] for more information). Consequently, the background thread must wait for the main (or foreground) thread to finish initialization before it can proceed. However, the {{ConnectionFactory}} class's main thread invokes the {{join()}} method which waits for the background thread to finish. This interdependency causes a class initialization cycle that results in a deadlock situation \[[Bloch 05b|AA. Java References#Bloch 05b]\]. 

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>() {
      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();
  }
}

*CON03:EX1:* It is permissible to start a background thread during class initialization provided the thread does not access any fields. For example, the {{ObjectPreserver}} class (based on \[[Patterns 02|AA. Java References#Patterns 02]\]) shown below provides a mechanism for storing object references, which prevents an object from being garbage-collected, even if the object is not dereferenced in the future.

public final class ObjectPreserver implements Runnable {
  private static 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);
  }
}

\[[Bloch 05b|AA. Java References#Bloch 05b]\] 8. "Lazy Initialization"
\[[Patterns 02|AA. Java References#Patterns 02]\] Chapter 5, Creational Patterns, Singleton

||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|T|M|F|1269649993019|1269700561582|rcs_mgr|"Starting and using background threads during class initialization can result in class initialization cycles and deadlock. *For instance,* the main thread responsible for performing class initialization *may* block waiting for the background thread, which in turn will wait for the main thread to finish class initialization." ... see suggested words in bold...I am also generally unsure about the use of "can" vs. "may" because deadlocks are a "possibility" so perhaps "may" should be used?|