有關Ehcache的內容的引用和Java的deep copy

項目使用Ehcache來做爲程序和數據庫之間的緩衝， 使用過程當中會對cache對象作修改， 如plan.setLangSymbol()，發現後面使用cache的地方，取到的數據都是修改後的，因此猜想是cache的淺引用形成的。

實際上，stackoverflow也有人提到此問題《Cache.get() returns deep copy of element?》

至於如何作deep copy，stackoverflow也提到《Copying cached Map<String , List> object into temporary Map<String , List> object》，通過實驗，確認Guava的Maps.newHashMap(Map)工具類方法也是shallow copy。

一種方法就是循環對象內容，將每一個屬性賦值給從新構建的新對象。

另外有個更簡單的方法，《A Java deep clone (deep copy) example》使用對象的序列化特性完成，看以下代碼：

  /**
   * This method makes a "deep clone" of any object it is given.
   */
  public static Object deepClone(Object object) {
    try {
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      ObjectOutputStream oos = new ObjectOutputStream(baos);
      oos.writeObject(object);
      ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
      ObjectInputStream ois = new ObjectInputStream(bais);
      return ois.readObject();
    }
    catch (Exception e) {
      e.printStackTrace();
      return null;
    }
  }
}
 
/**
 * These classes implement Serializable so we can write them out and
 * read them back in as a stream of bytes.
 */
class Person implements Serializable {
  String firstName, lastName;
  Address address;
  public Person(String firstName, String lastName, Address address) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.address = address;
  }
  public String toString() {
    StringBuilder sb = new StringBuilder();
    sb.append("First Name: " + firstName + "\n");
    sb.append("Last Name: " + lastName + "\n");
    sb.append("Street: " + address.street + "\n");
    return sb.toString();
  }
}

很簡單！