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In this noncompliant code example, the statements in the static initializer start a background thread as part of class initialization. The background thread attempts to initialize a database connection but needs to wait until initialization of all members of the Lazy ConnectionFactory class, including dbConnection, has finished.

Code Block

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public final class LazyConnectionFactory {
  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();
  }
}

Wiki Markup

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fields are guaranteed to be fully constructed before they are made visible to other threads (see [CON26-J. Do not publish partially initialized objects] for more information).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 {{LazyConnectionFactory}} 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. \[[Bloch 05b|AA. Java References#Bloch 05b]\]

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Code Block

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public final class LazyConnectionFactory {
  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)
  }

  // ...
}

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Code Block

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public final class LazyConnectionFactory {
  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();
  }
}

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This is a singleton class (see CON23-J. Address the shortcomings of the Singleton design pattern for more information on how to defensively code singleton classes). The initialization creates involves creating a background thread using the current instance of the class. The thread waits indefinitely by invoking Object.wait(). Consequently, this object exists for the remainder of the JVM's lifetime. Because the object is managed by a daemon thread, the thread does not hinder a normal shutdown of the JVM.

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