Starting and using background threads during class initialization can result in class initialization cycles and eventually, deadlock. This is because 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. This issue can arise, for example, when a database connection is established in a background thread while class initialization is in progress. \[[Bloch 05b|AA. Java References#Bloch 05b]\] 

public class Lazy {
  private static int number;
  static {
    Thread t = new Thread(new Runnable() {
      public void run() {
        // Initialize, for example, a database connection
        Lazy.this.number = 42;
      }
    });
    
    t.start();
    try {
      t.join();
    } catch(InterruptedException ie) {
        throw new AssertionError(ie);
    }
    // Other initialization
  }
  public static void main(String[] args) {
    System.out.println(number);
  }
}

The code in the {{static}} block is responsible for initialization, and starts a background thread. The background thread attempts to assign to the {{number}} but needs to wait before initialization of the {{Lazy}} class has finished. Remember that statically-initialized fields are guaranteed to be fully constructed before becoming visible to other threads (see [CON26-J. Do not publish partially initialized objects] for more info). Consequently the background thread must wait for the foreground thread to finish initialization before it may proceed. However, the {{Lazy}} 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 class Lazy {
  private static int number;
  
  static {
    // Initialize, for example, a database connection
    this.number = 42;
  }
  
  public static void main(String[] args) {
    System.out.println(number);
  }
}

public class Lazy {
  private static boolean flag;

  private static final ThreadLocal<Connection> connectionHolder
    = new ThreadLocal<Connection>() {
      public Connection initialValue() {
        try {
          Connection conn = DriverManager.getConnection("connectionstring");
          flag = true;
          return conn;
        } catch (SQLException e) {
          flag = false;
          return null;
        }
      }
    };
    
  public static Connection getConnection() {
    return connectionHolder.get();
  }

  public static void main(String[] args) {
    Connection conn = getConnection();
    System.out.println(flag);
  }
}

*CON03:EX1*: The {{ObjectPreserver}} class (based on \[[Patterns 02|AA. Java References#Patterns 02]\]) is shown below: This class provides a mechanism for storing object references, which prevents an object from being garbage-collected, even if the remaining program no longer maintains a reference to the object.

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 HashSet 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) { /* Forward to handler */ }
  }

  // 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