源码分析FactoryBean 接口的核心功能

￡神魔★判官ぃ 2023-10-02 20:22 19阅读 0赞

源码分析FactoryBean 接口的核心功能是今天的主要内容，下面我们通用一个测试类为入口来详细其核心源码：

1、测试类：

import com.enjoy.jack.bean.Student;
    import org.springframework.beans.factory.FactoryBean;
    import org.springframework.stereotype.Component;
    
    /**
     * 灵活定义需要我们自己创建的实例的时候，我们可以实现factoryBean接口
     * 在getObject方法里面定义实例化过程
     */
    @Component
    public class FactoryBeanDemo implements FactoryBean {
        @Override
        public Object getObject() throws Exception {
            return new Student();
        }
    
        @Override
        public Class<?> getObjectType() {
            return Student.class;
        }
    }

实体：

@Data
    public class Student {
        private String username = "nandao";
    }

2、核心配置：spring.xml

3、运行测试：

@Test
        public void test20() {
            ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
            Student student = (Student)applicationContext.getBean("factoryBeanDemo");
            System.out.println(student);
    
            FactoryBeanDemo factoryBeanDemo = (FactoryBeanDemo)applicationContext.getBean("&factoryBeanDemo");
            System.out.println(factoryBeanDemo);
        }

4、结果：

![a89147dc02074d1abb69e1b81839e013.png][]

发现没有“&”号为什么根据Id获取到的是getObject方法的返回对象，下面看看源码

5、源码入口：AbstractBeanFactory 类的doGetBean 接口

//从缓存中拿实例
    		// Eagerly check singleton cache for manually registered singletons.
    		Object sharedInstance = getSingleton(beanName);
    		//如果缓存里面能拿到实例
    		if (sharedInstance != null && args == null) {
    			if (logger.isTraceEnabled()) {
    				if (isSingletonCurrentlyInCreation(beanName)) {
    					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
    							"' that is not fully initialized yet - a consequence of a circular reference");
    				}
    				else {
    					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
    				}
    			}
    			//该方法是FactoryBean接口的调用入口
    			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    		}

点击 getObjectForBeanInstance(sharedInstance, name, beanName, null);接口

protected Object getObjectForBeanInstance(
    			Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
    
    		// Don't let calling code try to dereference the factory if the bean isn't a factory.
            //判断对象前面是否家里“&” 符号，如果加了，可能直接返回当前实例的对象；如果没加继续往下走，最终返回getObject里面的对象。
    		if (BeanFactoryUtils.isFactoryDereference(name)) {
    			if (beanInstance instanceof NullBean) {
    				return beanInstance;
    			}
    			if (!(beanInstance instanceof FactoryBean)) {
    				throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
    			}
    			if (mbd != null) {
    				mbd.isFactoryBean = true;
    			}
    			return beanInstance;
    		}
    
    		// Now we have the bean instance, which may be a normal bean or a FactoryBean.
    		// If it's a FactoryBean, we use it to create a bean instance, unless the
    		// caller actually wants a reference to the factory.
    		//如果实例不是FactoryBean类型的
    		if (!(beanInstance instanceof FactoryBean)) {
    			return beanInstance;
    		}
    
    		//如果代码能走下来，则说明 beanName不是以&开头，并且beanInstance是FactoryBean类型的
    		Object object = null;
    		if (mbd != null) {
    			mbd.isFactoryBean = true;
    		}
    		else {
    			//从缓存里面拿FactoryBean类型的实例
    			object = getCachedObjectForFactoryBean(beanName);
    		}
    		if (object == null) {
    			// Return bean instance from factory.
    			FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
    			// Caches object obtained from FactoryBean if it is a singleton.
    			if (mbd == null && containsBeanDefinition(beanName)) {
    				mbd = getMergedLocalBeanDefinition(beanName);
    			}
    			boolean synthetic = (mbd != null && mbd.isSynthetic());
    			//重点看,里面是调用getObject方法，获取对象信息
    			object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    		}
    		return object;
    	}

6、点击 BeanFactoryUtils.isFactoryDereference(name）

public static boolean isFactoryDereference(@Nullable String name) {
            //判断name里面首个字符串是不是“&”,如果是直接返回true
            //	String FACTORY_BEAN_PREFIX = "&";
    		return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
    	}

这里就判断了id中有没有存在“&”符号，如果有则返回true，返回true就直接return beanInstance;把实例本身返回了，而实例本身就是例子中的FactoryBeanDemo

7、点击 getObjectFromFactoryBean(factory, beanName, !synthetic);进入父类FactoryBeanRegistrySupport

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    		if (factory.isSingleton() && containsSingleton(beanName)) {
    			synchronized (getSingletonMutex()) {
    				Object object = this.factoryBeanObjectCache.get(beanName);
    				if (object == null) {
    					//调用getObject方法，点击进入
    					object = doGetObjectFromFactoryBean(factory, beanName);
    					// Only post-process and store if not put there already during getObject() call above
    					// (e.g. because of circular reference processing triggered by custom getBean calls)
    					Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
    					if (alreadyThere != null) {
    						object = alreadyThere;
    					}
    					else {
    						if (shouldPostProcess) {
    							if (isSingletonCurrentlyInCreation(beanName)) {
    								// Temporarily return non-post-processed object, not storing it yet..
    								return object;
    							}
    							beforeSingletonCreation(beanName);
    							try {
    								object = postProcessObjectFromFactoryBean(object, beanName);
    							}
    							catch (Throwable ex) {
    								throw new BeanCreationException(beanName,
    										"Post-processing of FactoryBean's singleton object failed", ex);
    							}
    							finally {
    								afterSingletonCreation(beanName);
    							}
    						}
    						if (containsSingleton(beanName)) {
    							//把实例缓存到factoryBeanObjectCache map中，这个是单独缓存FactoryBean类型实例的map
    							this.factoryBeanObjectCache.put(beanName, object);
    						}
    					}
    				}
    				return object;
    			}
    		}
    		else {
    			Object object = doGetObjectFromFactoryBean(factory, beanName);
    			if (shouldPostProcess) {
    				try {
    					object = postProcessObjectFromFactoryBean(object, beanName);
    				}
    				catch (Throwable ex) {
    					throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
    				}
    			}
    			return object;
    		}
    	}

点击 doGetObjectFromFactoryBean(factory, beanName);方法

private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
    		Object object;
    		try {
    			if (System.getSecurityManager() != null) {
    				AccessControlContext acc = getAccessControlContext();
    				try {
    					object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
    				}
    				catch (PrivilegedActionException pae) {
    					throw pae.getException();
    				}
    			}
    			else {
    				//调用getObject方法
    				object = factory.getObject();
    			}
    		}
    		catch (FactoryBeanNotInitializedException ex) {
    			throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    		}
    		catch (Throwable ex) {
    			throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    		}
    
    		// Do not accept a null value for a FactoryBean that's not fully
    		// initialized yet: Many FactoryBeans just return null then.
    		if (object == null) {
    			if (isSingletonCurrentlyInCreation(beanName)) {
    				throw new BeanCurrentlyInCreationException(
    						beanName, "FactoryBean which is currently in creation returned null from getObject");
    			}
    			object = new NullBean();
    		}
    		return object;
    	}

8、此处 factory.getObject(); 就是调到了业务方法,如下：

![188ab9f7c0fa42bf83c3c30da342c6c3.png][]

这里就调到了getObject方法，所以拿到的对象就是getObject方法返回的对象。

总之，从分析源码我们可以看到，就是一个“&”符号控制了代码的流转，有“&”拿到的是类本身实例，没有“&”拿到的getObject返回实例，到此FactoryBean分析完毕。

[a89147dc02074d1abb69e1b81839e013.png]: https://img-blog.csdnimg.cn/a89147dc02074d1abb69e1b81839e013.png
[188ab9f7c0fa42bf83c3c30da342c6c3.png]: https://img-blog.csdnimg.cn/188ab9f7c0fa42bf83c3c30da342c6c3.png