设计模式：factory-method

设计模式源码 https://github.com/iluwatar/java-design-patterns

基本概念

FactoryMethod是一种创建性模式,它定义了一个创建对象的接口,但是却让子类来决定具体实例化哪一个类.当一个类无法预料要创建哪种类的对象或是一个类需要由子类来指定创建的对象时我们就需要用到Factory Method 模式了.简单说来,Factory Method可以根据不同的条件产生不同的实例,当然这些不同的实例通常是属于相同的类型,具有共同的父类.Factory Method把创建这些实例的具体过程封装起来了,简化了客户端的应用,也改善了程序的扩展性,使得将来可以做最小的改动就可以加入新的待创建的类. 通常我们将Factory Method作为一种标准的创建对象的方法,当发现需要更多的灵活性的时候,就开始考虑向其它创建型模式转化

/**
 * 
 * The Factory Method is a creational design pattern which uses factory methods to deal with the
 * problem of creating objects without specifying the exact class of object that will be created.
 * This is done by creating objects via calling a factory method either specified in an interface
 * and implemented by child classes, or implemented in a base class and optionally overridden by
 * derived classes—rather than by calling a constructor.
 * <p>
 * In this Factory Method example we have an interface ({@link Blacksmith}) with a method for
 * creating objects ({@link Blacksmith#manufactureWeapon}). The concrete subclasses (
 * {@link OrcBlacksmith}, {@link ElfBlacksmith}) then override the method to produce objects of
 * their liking.
 * 
 */

模式结构

diagram1.png

源码分析

创建对象接口类

/**
 * 
 * The interface containing method for producing objects.
 * 
 */
public interface Blacksmith {

  Weapon manufactureWeapon(WeaponType weaponType);

}

创建对象的实现类

/**
 * 
 * Concrete subclass for creating new objects.
 * 
 */
public class ElfBlacksmith implements Blacksmith {

  public Weapon manufactureWeapon(WeaponType weaponType) {
    return new ElfWeapon(weaponType);
  }

}

测试

public class App {

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

  private final Blacksmith blacksmith;
  
  /**
   * Creates an instance of <code>App</code> which will use <code>blacksmith</code> to manufacture 
   * the weapons for war.
   * <code>App</code> is unaware which concrete implementation of {@link Blacksmith} it is using.
   * The decision of which blacksmith implementation to use may depend on configuration, or
   * the type of rival in war.
   * @param blacksmith a non-null implementation of blacksmith
   */
  public App(Blacksmith blacksmith) {
    this.blacksmith = blacksmith;
  }
  
  /**
   * Program entry point
   * 
   * @param args command line args
   */
  public static void main(String[] args) {
    // Lets go to war with Orc weapons
    App app = new App(new OrcBlacksmith());
    app.manufactureWeapons();
    
    // Lets go to war with Elf weapons
    app = new App(new ElfBlacksmith());
    app.manufactureWeapons();
  }
  
  private void manufactureWeapons() {
    Weapon weapon;
    weapon = blacksmith.manufactureWeapon(WeaponType.SPEAR);
    LOGGER.info(weapon.toString());
    weapon = blacksmith.manufactureWeapon(WeaponType.AXE);
    LOGGER.info(weapon.toString());
  }