委托模式，简单工厂方法的演进

回顾简单工厂方法

将factory类作为animal接口及其子类的容器（ioc思想），如果需要获取Animal，只需要从factory中获取即可。

具体的代码实现

接口和实现

public interface Animal {
	void run();
}		

public class Dog implements Animal{
	@Override
	public void run() {
		System.out.println("狗跑步");
	}
}

public class Person implements Animal{
	@Override
	public void run() {
		System.out.println("人走路");
	}
}

public class Pig implements Animal{
	@Override
	public void run() {
		System.out.println("猪睡觉");
	}
}

简单工厂方法的封装

public class AnimalFactory {
	public static Animal getAnimalByType(String type) {
		 
		if ("Person".equals(type)) {
			return new Person();
		}
		if ("Pig".equals(type)) {
			return new Pig();
		}
		if ("Dog".equals(type)) {
			return new Dog();
		}
		
		throw new UnsupportedOperationException();
	}
}

封装client测试类

public class Client {
	public static void main(String[] args) {
		// 通过简单工厂方法获取Animal
		Animal animal= AnimalFactory.getAnimalByType("Pig");
		animal.run();
	}
}

测试结果

猪睡觉

弊端，factory是独立的一个类，需要单独维护，并且没有抽象

改进，通过委托方式

AnimalDelegete类属于委托代理类

public class AnimalDelegete implements Animal {
	
	private final Animal delegate;
	
	public AnimalDelegete(String type) {
		if ("Person".equals(type)) {
			delegate = new Person();
		} else if ("Pig".equals(type)) {
			delegate = new Pig();
		} else if ("Dog".equals(type)) {
			delegate = new Dog();
		} else {
			throw new UnsupportedOperationException();
		}
	}

	@Override
	public void run() {
		delegate.run();
	}
}

测试类Client2

public class Client2 {
	public static void main(String[] args) {
		// 通过委托类获取Animal
		Animal animal= new AnimalDelegete("Pig");
		animal.run();
	}
}

优点

通过一个委托类，也是接口的子类来维护子类的统一实现，简直6的飞起

mybaties中的委托类例子

RoutingStatementHandler

public class RoutingStatementHandler implements StatementHandler {

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    return delegate.prepare(connection, transactionTimeout);
  }

  @Override
  public void parameterize(Statement statement) throws SQLException {
    delegate.parameterize(statement);
  }

  @Override
  public void batch(Statement statement) throws SQLException {
    delegate.batch(statement);
  }

  @Override
  public int update(Statement statement) throws SQLException {
    return delegate.update(statement);
  }

  @Override
  public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
    return delegate.query(statement, resultHandler);
  }

  @Override
  public  Cursor queryCursor(Statement statement) throws SQLException {
    return delegate.queryCursor(statement);
  }

  @Override
  public BoundSql getBoundSql() {
    return delegate.getBoundSql();
  }

  @Override
  public ParameterHandler getParameterHandler() {
    return delegate.getParameterHandler();
  }
}