java 静态工厂，

java 静态工厂，

public abstract class Foo {

//继承了Foo的类的集合

private static Map implementations = null;

    private static synchronized void initMapIfNecessary() { if (null == implementations) { implementations = new HashMap(); }     }                  public static Foo getInstance(String key) { initMaoIfNecessary(); Class c = (Class) implementations.get(key); if(c == null) return new DefaultFoo(); try { return (Foo) c.newInstance(); } catch(Exception e) { return new DefaultFoo(); }     } }