什么是“静态工厂”方法?


当前回答

我们避免提供对数据库连接的直接访问,因为它们是资源密集型的。因此,我们使用一个静态工厂方法getDbConnection,它在低于限制时创建一个连接。否则,它将尝试提供一个“备用”连接,如果没有则会出现异常而失败。

public class DbConnection{
   private static final int MAX_CONNS = 100;
   private static int totalConnections = 0;

   private static Set<DbConnection> availableConnections = new HashSet<DbConnection>();

   private DbConnection(){
     // ...
     totalConnections++;
   }

   public static DbConnection getDbConnection(){

     if(totalConnections < MAX_CONNS){
       return new DbConnection();

     }else if(availableConnections.size() > 0){
         DbConnection dbc = availableConnections.iterator().next();
         availableConnections.remove(dbc);
         return dbc;

     }else {
         throw new NoDbConnections();
     }
   }

   public static void returnDbConnection(DbConnection dbc){
     availableConnections.add(dbc);
     //...
   }
}

其他回答

与构造函数不同,具有名称,可以阐明代码。 不需要在每次调用时创建一个新对象-对象 如果需要,可以缓存和重用。 可以返回其返回类型的子类型-特别是,Can 返回调用方不知道其实现类的对象。 这是许多框架中非常有价值和广泛使用的特性 它们使用接口作为静态工厂方法的返回类型。

标准:/ / www.javapractices.com/topic/TopicAction.do ? Id = 21

当您希望确保只有一个实例将返回要使用的具体类时,静态工厂方法很好。

例如,在一个数据库连接类中,您可能希望只有一个类创建数据库连接,因此如果您决定从Mysql切换到Oracle,您只需更改一个类中的逻辑,而应用程序的其余部分将使用新的连接。

如果您希望实现数据库池,那么也可以在不影响应用程序其余部分的情况下完成。

它保护应用程序的其余部分不受您可能对工厂所做的更改的影响,这就是目的。

它是静态的原因是,如果你想跟踪一些有限的资源(套接字连接或文件句柄的数量),那么这个类可以跟踪已经传递和返回了多少,这样你就不会耗尽有限的资源。

静态工厂的优点之一是API可以返回对象,而不需要将它们的类设为public。这导致了非常紧凑的API。在java中,这是通过Collections类实现的,它隐藏了大约32个类,这使得它的集合API非常紧凑。

可读性可以通过静态工厂方法来提高:

比较

public class Foo{
  public Foo(boolean withBar){
    //...
  }
}

//...

// What exactly does this mean?
Foo foo = new Foo(true);
// You have to lookup the documentation to be sure.
// Even if you remember that the boolean has something to do with a Bar
// you might not remember whether it specified withBar or withoutBar.

to

public class Foo{
  public static Foo createWithBar(){
    //...
  }

  public static Foo createWithoutBar(){
    //...
  }
}

// ...

// This is much easier to read!
Foo foo = Foo.createWithBar();

One of the advantages of the static factory methods with private constructor(object creation must have been restricted for external classes to ensure instances are not created externally) is that you can create instance-controlled classes. And instance-controlled classes guarantee that no two equal distinct instances exist(a.equals(b) if and only if a==b) during your program is running that means you can check equality of objects with == operator instead of equals method, according to Effective java.

The ability of static factory methods to return the same object from repeated invocations allows classes to maintain strict control over what instances exist at any time. Classes that do this are said to be instance-controlled. There are several reasons to write instance-controlled classes. Instance control allows a class to guarantee that it is a singleton (Item 3) or noninstantiable (Item 4). Also, it allows an immutable class (Item 15) to make the guarantee that no two equal instances exist: a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use the == operator instead of the equals(Object) method, which may result in improved performance. Enum types (Item 30) provide this guarantee.

摘自Effective Java, Joshua Bloch(第1项,第6页)