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 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 during class initialization. \[[Bloch 05b|AA. Java References#Bloch 05b]\] 

h2. Noncompliant Code Example (background thread)

In this noncompliant code example, the {{static}} initializer starts 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 {{ConnectionFactory}} class, including {{dbConnection}}, has finished. 

{code:bgColor=#FFcccc}
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();
  }
}
{code}

 05b|AA. Java References#Bloch 05b]\] 

Similarly, it is inappropriate to start threads from constructors (see [CON14-J. Do not let the "this" reference escape during object construction] for more information). Creating timers that perform recurring tasks and starting them from within code responsible for initialization also creates liveness issues.


h2. Compliant Solution ({{static}} initializer, no background threads)

This compliant solution does not spawn any background threads from the {{static}} initializer. All fields are initialized in the main thread.

{code:bgColor=#ccccff}
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)
  }

  // ...
}

{code}


h2. Compliant Solution ({{ThreadLocal}})

This compliant solution initializes the database connection from a {{ThreadLocal}} object so that every thread can obtain its own instance of the connection.

{code:bgColor=#ccccff}
public final class ConnectionFactory {
  private static final ThreadLocal<Connection> connectionHolder
                       = new ThreadLocal<Connection>() {
     @Override public Connection initialValue() {
     try {
  try {
       Connection dbConnection =
        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();
  }
}
{code}

 


h2. Exceptions

*CON03:EX1*: It is permissible to start a background thread during class initialization provided that the background thread does not access any fields. For example, the {{ObjectPreserver}} class (based on \[[Patterns 02|AA. Java References#Patterns 02]\]) that is 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.

{code:bgColor=#ccccff}
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);
  }
}
{code} 

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 involves creating a background thread using the current instance of the class. The thread waits indefinitely by invoking {{

||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?|