JAVA 懶載入

來自 main() 的呼叫，

// Simple lazy loader - not thread safe
HolderNaive holderNaive = new HolderNaive();
Heavy heavy = holderNaive.getHeavy();

Heavy.class

/**
 * 
 * Heavy objects are expensive to create.
 *
 */
public class Heavy {

  private static final Logger LOGGER = LoggerFactory.getLogger(Heavy.class);

  /**
   * Constructor
   */
  public Heavy() {
    LOGGER.info("Creating Heavy ...");
    try {
      Thread.sleep(1000);
    } catch (InterruptedException e) {
      LOGGER.error("Exception caught.", e);
    }
    LOGGER.info("... Heavy created");
  }
}

HolderNaive.class

/**
 * 
 * Simple implementation of the lazy loading idiom. However, this is not thread safe.
 *
 */
public class HolderNaive {

  private static final Logger LOGGER = LoggerFactory.getLogger(HolderNaive.class);

  private Heavy heavy;

  /**
   * Constructor
   */
  public HolderNaive() {
    LOGGER.info("HolderNaive created");
  }

  /**
   * Get heavy object
   */
  public Heavy getHeavy() {
    if (heavy == null) {
      heavy = new Heavy();
    }
    return heavy;
  }
}