Spring-实例化bean的3种方式

推荐先阅读：Spring全家桶

1.构造方法

提供可访问的构造方法

public class BookDaoImpl implements BookDao {
    // 默认空参构造，可不写
    public BookDaoImpl() {
        System.out.println("book dao constructor is running ....");
    }
    public void save() {
        System.out.println("book dao save ...");
    }
}

配置

<bean id="bookDao" class="com.zx.dao.impl.BookDaoImpl"/>

注意：无参构造方法如果不存在，将抛出异常BeanCreationException

2.静态工厂（了解）

//静态工厂创建对象
public class OrderDaoFactory {
    public static OrderDao getOrderDao(){
        return new OrderDaoImpl();
    }
}

配置

<bean id="orderDao" class="com.zx.factory.OrderDaoFactory" factory-method="getOrderDao"/>

3.实例工厂

public class UserDaoFactory {
    public UserDao getUserDao(){
        return new UserDaoImpl();
    }
}

配置

<bean id = "userDaoFactory" class = "com.zx.factory.UserDaoFactory" />
<bean id = "userDao" factory-method = "getUserDao" factory-bean = "userDaoFactory" />

注意：第一个bean配合使用的，实际无意义。工厂方法名不固定，每次需要配置。

实例工厂改良（实现FactoryBean）

public class UserDaoFactoryBean implements FactoryBean<UserDao> {
    //代替原始实例工厂中创建对象的方法
    public UserDao getObject() throws Exception {
        return new UserDaoImpl();
    }
    //返回所创建类的Class对象
    public Class<?> getObjectType() {
        return UserDao.class;
    }
    // 默认为true，单例，可以不重写
    default boolean isSingleton() {
        return true;
    }
}

配置

<bean id="userDao" class="com.zx.factory.UserDaoFactoryBean"/>