【Practical API Design學習筆記】組件定位和交互

中心思想是：模塊間去耦合。

舉例說明：

  隨機顯示一個通過擾亂處理的不規則單詞，而用戶則要將擾亂後的單詞糾正爲正確的單詞。其中，一個類用來生成原始單詞，一個業務邏輯層用來擾亂單詞，還有一個用戶界面模塊負責將這個單詞展現給用戶看。定義接口以下：

public interface Scrambler {
  public String scramble(String word);
}

public interface WordLibrary {
  public String[] getWords();
}

public interface UI {
  public void display();
}

實現：

public abstract class Anagrams extends javax.swing.JFrame implements UI {
  protected abstract WordLibrary getWordLibrary();
  protected abstract Scrambler getScrambler();
  
  public void display() {
    initWord();
    setVisible(true);
  }
}

public final class Launcher {
  private static Class<? extends WordLibrary> wordLibrary;
  private static Class<? extends Scrambler> scrambler;
  private static Class<? extends UI> ui;
  private Launcher() {
  }
  public static void registerWordLibrary(Class<? extends WordLibrary> libraryClass) {
    wordLibrary = libraryClass;
  }
  public static void registerScrambler(Class<? extends Scrambler> scramblerClass) {
    scrambler = scramblerClass;
  }
  public static void registerUI(Class<? extends UI> uiClass) {
    ui = uiClass;
  }
  public static UI launch() throws Exception {
    WordLibrary w = wordLibrary.newInstance();
    Scrambler s = scrambler.newInstance();
    return ui.getConstructor(WordLibrary.class, Scrambler.class).newInstance(w, s);
  }
}

public class AnagramsWithConstructor extends Anagrams {
  private final WordLibrary library;
  private final Scrambler scrambler;
  public AnagramsWithConstructor(WordLibrary library, Scrambler scrambler) {
    this.library = library;
    this.scrambler = scrambler;
  }
  @Override
  protected WordLibrary getWordLibrary() {
    return library;
  }
  @Override
  protected Scrambler getScrambler() {
    return scrambler;
  }
}

  以上代碼註冊方式效率很是慢，特別是在要註冊的模塊很是多的狀況，都須要在系統啓動時就註冊，註冊多個模塊，就會進行不少模塊的編譯鏈接，效率很是慢。

註冊的通用作法：

不少註冊代碼都是相同的，須要將其提取出來：

private static Map<Class<?>,Object[]> instances = new LinkedHashMap<Class<?>,Object[]>();
public static void registerClass(Class<?> impl) {
  instances.put(impl, new Object[1]);
}
public static <T> T find(Class<T> whatType) {
  for (Map.Entry<Class<?>, Object[]> entry : instances.entrySet()) {
    if (whatType.isAssignableFrom(entry.getKey())) {
      if (entry.getValue()[0] == null) {
        try {
          entry.getValue()[0] = entry.getKey().newInstance();
        } catch (Exception ex) {
          throw new IllegalStateException(ex);
        }
      }
      return whatType.cast(entry.getValue()[0]);
    }
  }
  return null;
}

@Override
protected WordLibrary getWordLibrary() {
  try {
    if (wordLibrary == null) {
      String implName=System.getProperty("org.apidesign.anagram.api.WordLibrary");
      assert implName != null;
      Class<?> impl = Class.forName(implName);
      wordLibrary = (WordLibrary)impl.newInstance();
    }
    return wordLibrary;
  } catch (Exception ex) {
    throw new IllegalStateException(ex);
  }
}

@Override
protected Scrambler getScrambler() {
  try {
    if (scrambler == null) {
      String implName = System.getProperty("org.apidesign.anagram.api.Scrambler");
      assert implName != null;
      Class<?> impl = Class.forName(implName);
      scrambler = (Scrambler)impl.newInstance();
    }
    return scrambler;
  } catch (Exception ex) {
    throw new IllegalStateException(ex);
  }
}

配置文件有用XML的，好比Spring的配置文件：

<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-2.5.xsd"
  >
  <bean
    id="wordLibrary"
    class="org.apidesign.anagram.wordstatic.StaticWordLibrary"
  />
  <bean
    id="scrambler"
    class="org.apidesign.anagram.scramblersimple.SimpleScrambler"
  />
  <bean
    id="ui"
    class="org.apidesign.anagram.gui.AnagramsWithConstructor"
    autowire="autodetect"
  />
</beans>