ioc到底做了些什么

约定不等于承诺〃 2022-03-01 04:50 320阅读 0赞

IOC 是inversion of control  
控制反转,但是反转了什么?  
一般来说:我们创建对象是这样的;

Object obj=new Object();

但是逐渐感觉,这样的劳动重复性太高,能否减少这些无意义的重复劳动 。  
ioc就是做这件事的。  
来用测试代码体验一下:  
大致逻辑:  
现在有**接口BookService.java**,和**实现了接口的BookServiceImpl.java**  
接口 BookService:

package top.forethought.framework.ioc;
    
    public interface BookService { 
        void addBook();
    }

实现类:

package top.forethought.framework.ioc;
    
    public class BookServiceImpl implements BookService { 
        @Override
        public void addBook() { 
            System.out.println(" addBook @ BookServiceImpl");
        }
        public BookServiceImpl(){ 
            System.out.println("constructor BookServiceImpl");
        }
    }

## 不使用IOC ##

测试类:

@Test
        public void noneIoc(){ 
            BookService bookService=new BookServiceImpl();
            bookService.addBook();
        }

输出:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70]

## 使用ioc: ##

思路:将创建对象的控制权交给spring,即控制反转  
实现方式:

1.  将需要创建的对象配置在spring配置文件中的 < bean >标签内
2.  通过读取配置文件,拿到ApplicationContext(可以理解为整个应用,恶心的被翻译成上下文环境)
3.  调用getBean() 方法获取到ioc中的bean

配置文件:

<table> 
 <thead> 
  <tr> 
   <th>属性名</th> 
   <th>含义</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>id</td> 
   <td>对象在ioc中标识符</td> 
  </tr> 
  <tr> 
   <td>class</td> 
   <td>该对象对应的实现类</td> 
  </tr> 
  <tr> 
   <td>scope</td> 
   <td>bean 的作用域,默认是singleton,还有一种是 prototype,每次getBean 都是得到新的对象</td> 
  </tr> 
  <tr> 
   <td>property</td> 
   <td>这是给bean中的 成员属性注入值时使用(基本的值,可以使用 ),如果需要引用其他bean,需要使用 &lt; property name=“xx” ref=“otherBeanId”&gt;(注意:对应属性需要在实现类中需要由setter方法)</td> 
  </tr> 
 </tbody> 
</table>

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:context="http://www.springframework.org/schema/context"
           xsi:schemaLocation=
                   "http://www.springframework.org/schema/context
         http://www.springframework.org/schema/context/spring-context.xsd
         http://www.springframework.org/schema/beans
         http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="bookServiceId" class="top.forethought.framework.ioc.BookServiceImpl" ></bean>
    </beans>

测试类;

@Test
        public void byIoc(){ 
            String xmlPath="top/forethought/framework/configs/ioc-config.xml";
            ApplicationContext context= new ClassPathXmlApplicationContext(xmlPath);
             BookService bookService= (BookService) context.getBean("bookServiceId");
            bookService.addBook();
            for(int i=0;i<10;i++){ 
                bookService= (BookService) context.getBean("bookServiceId");
                System.out.println(bookService.hashCode());
            }
        }

scope=“singleton” 时输出:每次获取都是同一个bean  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 1]  
scope=“prototype” 时输出:每次获取,**都是得到一个新的bean**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 2]

## ioc 底层是如何实现的? ##

暂时猜测是通过**工厂模式**创建对象:  
追踪源码看看,全部是beanFactory 结尾的,仅仅是从名字就可以看出用到的设计模式 **Factory+反射**

**BeanFactory**:BeanFactory，以Factory结尾，表示它是一个工厂类(**接口**)，用于管理Bean的一个工厂(**产物是任意bean**)。在Spring中，BeanFactory是IOC容器的核心接口，它的职责包括：实例化、定位、配置应用程序中的对象及建立这些对象间的依赖)  
**FactoryBean**: 以Bean结尾，表示它是一个特殊的Bean，它是实现了FactoryBean接口的Bean,具有工厂生产对象的能力,**只能生产特定的对象**，(**T 类型的对象**），比如ThreadPoolExecutorFactoryBean 是返回 ExecutorService 类型的对象

ThreadPoolExecutorFactoryBean extends ExecutorConfigurationSupport
    		implements FactoryBean<ExecutorService>, InitializingBean, DisposableBean

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 3]

--------------------

**以下为源码探索部分,不太确定是否合理,暂时每太理清逻辑,暂时先忽略**

--------------------

1.  AbstractBeanFactory.class:  
    getBean(String name) 方法:返回一个共享的(单例模式)或者是独立的对象(原型模式)  
    ![在这里插入图片描述][20190321210054851.png]  
    getBean 调用了doGetBean 方法

doGetBean 代码逻辑:

protected <T> T doGetBean(
    			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
    			throws BeansException { 
    
    		final String beanName = transformedBeanName(name);
    		Object bean;
    
    		// Eagerly check singleton cache for manually registered singletons.
    		Object sharedInstance = getSingleton(beanName);
    		if (sharedInstance != null && args == null) { 
    			if (logger.isDebugEnabled()) { 
    				if (isSingletonCurrentlyInCreation(beanName)) { 
    					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
    							"' that is not fully initialized yet - a consequence of a circular reference");
    				}
    				else { 
    					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
    				}
    			}
    			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    		}
    
    		else { 
    			// Fail if we're already creating this bean instance:
    			// We're assumably within a circular reference.
    			if (isPrototypeCurrentlyInCreation(beanName)) { 
    				throw new BeanCurrentlyInCreationException(beanName);
    			}
    
    			// Check if bean definition exists in this factory.
    			// 检查当前工厂是否有这个bean 的定义,如果没有,从父工厂查找
    			BeanFactory parentBeanFactory = getParentBeanFactory();
    			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { 
    				// Not found -> check parent.
    				String nameToLookup = originalBeanName(name);
    				if (args != null) { 
    					// Delegation to parent with explicit args.
    					return (T) parentBeanFactory.getBean(nameToLookup, args);
    				}
    				else { 
    					// No args -> delegate to standard getBean method.
    					return parentBeanFactory.getBean(nameToLookup, requiredType);
    				}
    			}
    
    			if (!typeCheckOnly) { 
    				markBeanAsCreated(beanName);
    			}
    
    			try { 
    				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
    				checkMergedBeanDefinition(mbd, beanName, args);
                    //确保被依赖项先初始化
    				// Guarantee initialization of beans that the current bean depends on.
    			  // dependOn 是 bean 的依赖项
    				String[] dependsOn = mbd.getDependsOn();
    				if (dependsOn != null) { 
    					for (String dep : dependsOn) { 
    					// 如果beanName 是dep的依赖项 ,这说明出现循环依赖,抛出异常
    						if (isDependent(beanName, dep)) { 
    							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
    									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
    						}
    						// 将 beanName 依赖的dep 注册
    						//Register a dependent bean for the given bean
    						registerDependentBean(dep, beanName);
    						getBean(dep);//这相当于执行了dep 类型的对象的创建
    					}
    				}
    
    // 创建当前需要创建的bean
    				// Create bean instance.
    				if (mbd.isSingleton()) { 
    					sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { 
    						@Override
    						public Object getObject() throws BeansException { 
    							try { 
    								return createBean(beanName, mbd, args);
    							}
    							catch (BeansException ex) { 
    								// Explicitly remove instance from singleton cache: It might have been put there
    								// eagerly by the creation process, to allow for circular reference resolution.
    								// Also remove any beans that received a temporary reference to the bean.
    								destroySingleton(beanName);
    								throw ex;
    							}
    						}
    					});
    					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    				}
    
    				else if (mbd.isPrototype()) { 
    					// It's a prototype -> create a new instance.
    					Object prototypeInstance = null;
    					try { 
    						beforePrototypeCreation(beanName);
    						prototypeInstance = createBean(beanName, mbd, args);
    					}
    					finally { 
    						afterPrototypeCreation(beanName);
    					}
    					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
    				}
    
    				else { 
    					String scopeName = mbd.getScope();
    					final Scope scope = this.scopes.get(scopeName);
    					if (scope == null) { 
    						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
    					}
    					try { 
    						Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() { 
    							@Override
    							public Object getObject() throws BeansException { 
    								beforePrototypeCreation(beanName);
    								try { 
    									return createBean(beanName, mbd, args);
    								}
    								finally { 
    									afterPrototypeCreation(beanName);
    								}
    							}
    						});
    						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
    					}
    					catch (IllegalStateException ex) { 
    						throw new BeanCreationException(beanName,
    								"Scope '" + scopeName + "' is not active for the current thread; consider " +
    								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
    								ex);
    					}
    				}
    			}
    			catch (BeansException ex) { 
    				cleanupAfterBeanCreationFailure(beanName);
    				throw ex;
    			}
    		}
    
    		// Check if required type matches the type of the actual bean instance.
    		if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) { 
    			try { 
    				return getTypeConverter().convertIfNecessary(bean, requiredType);
    			}
    			catch (TypeMismatchException ex) { 
    				if (logger.isDebugEnabled()) { 
    					logger.debug("Failed to convert bean '" + name + "' to required type '" +
    							ClassUtils.getQualifiedName(requiredType) + "'", ex);
    				}
    				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    			}
    		}
    		return (T) bean;
    	}
    
    	@Override
    	public boolean containsBean(String name) { 
    		String beanName = transformedBeanName(name);
    		if (containsSingleton(beanName) || containsBeanDefinition(beanName)) { 
    			return (!BeanFactoryUtils.isFactoryDereference(name) || isFactoryBean(name));
    		}
    		// Not found -> check parent.
    		BeanFactory parentBeanFactory = getParentBeanFactory();
    		return (parentBeanFactory != null && parentBeanFactory.containsBean(originalBeanName(name)));
    	}

1.  如果是单例bean,就从缓存取 // 单例bean 从缓存取  
    if (mbd.isSingleton()) \{  
    instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);  
    \}
2.  如果没有,就创建 //createBeanInstance(beanName, mbd, args)
3.  spring 底层使用了ConcurrentHashMap,ThreadLocal,这是为了线程安全着想

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70]: /images/20220301/ac8c9002afb64af1aebd0b397e41e4ce.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 1]: /images/20220301/ae26c5b19c1b4765a814d708b21c2a23.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 2]: /images/20220301/0ef35ae2df2642dc8450d9c26d994654.png
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[20190321210054851.png]: /images/20220301/34c364a7a2b64ce4989a2a160ee9b254.png