FactoryBean使用方法

一般来说我们自己写的Bean在只继承自己的接口时创建Bean的过程是交给IOC容器来实现的。但是某些特殊情况如果单纯的交给Spring的IOC容器来实现会配置非常复杂有时甚至没办法实现。所以这种情况下Spring为我们提供了FactoryBean这个接口！这个接口有3个方法

public interface FactoryBean<T> {
    T getObject() throws Exception;
    Class<?> getObjectType();
    boolean isSingleton();
}

• getObject方法用来获得一个Object对象当是单例时会将Object对象放入IOC容器缓存起来保证只会生成一个对象
• isSingleton判断是否是单例模式
• getObjectType返回对象的class
  如果我们现在有个很复杂的对象需要实例，单纯的使用IOC容器帮我们创建比较复杂，所以需要自己写创建类的过程，这个时候就可以让你的类实现FactoryBean这个接口，然后自己实现创建对象的方法（getObject()）方法。下面我从网上摘抄的一个简单的案例：在创建Car对象比较复杂，需要使用properties文件来生成，如果单纯使用IOC容器可能无法创建，所以需要自己写个CarFactoryBean来创建Car对象
• 1.Car类的基本属性

public   class  Car  {    
       private   int maxSpeed ;    
       private  String brand ;    
       private   double price ;    
      //get/set方法

• 2.创建方式复杂所以需要自己创建Car对象，实现FactroyBean然后自己实现getObject方法

public   class  CarFactoryBean  implements  FactoryBean<Car>  {    
    private  String carInfo ;    
    public  Car getObject ()   throws  Exception  {    
        Car car =  new  Car () ;    
        String []  infos =  carInfo .split ( "," ) ;    
        car.setBrand ( infos [ 0 ]) ;    
        car.setMaxSpeed ( Integer. valueOf ( infos [ 1 ])) ;    
        car.setPrice ( Double. valueOf ( infos [ 2 ])) ;    
        return  car;    
    }    
    public  Class<Car> getObjectType ()   {    
        return  Car. class ;    
    }    
    public   boolean  isSingleton ()   {    
        return   false ;    
    }    
    public  String getCarInfo ()   {    
        return   this . carInfo ;    
    }    
    
    // 接受逗号分割符设置属性信息    
    public   void  setCarInfo ( String carInfo )   {    
        this . carInfocarInfo  = carInfo;    
    }    
}

• 3.配置一个Car的Bean

<bean id="car" class="com.test.factorybean.CarFactoryBean" carInfo="超级跑车,400,2000000"/> 

实现源码分析

前面基本使用方式已经知道了，再来看看Spring如何实现的。在使用getBean()的时候可以找到最终调用的是AbstractBeanFactory.doGetBean()

    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        //1.如果是FactoryBean这种Bean都是以&开头。所以需要去掉&
        //2.如果name的别名，则需要在SimpleAliasRegistry里面进行判断获取的真正的那么
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        //从缓存中获取对象，避免重复创建
        Object sharedInstance = getSingleton(beanName);
        System.out.println(sharedInstance);
        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 + "'");
                }
            }
            /*这里 的 getObjectForBeanInstance 完成 的 是 FactoryBean 的 相关 处理，
            以 取得 FactoryBean 的 生产 结果, BeanFactory 和 FactoryBean 的 区别 已经 在前面 讲过， 这个 过程 在后面 还会 详细 地 分析*/
            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.
            /*
            这里 对 IoC 容器 中的 BeanDefintion 是否 存在 进行检查，
            检查 是否 能在 当前 的 BeanFactory 中 取得 需要 的 Bean。
            如果 在 当前 的 工厂 中 取 不到， 则 到 双亲 BeanFactory 中 去取；
            如果 当前 的 双亲 工厂 取 不到， 那就 顺着 双亲 BeanFactory 链 一直 向上 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 {
                //通过beanName获得BeanDefinition对象  //这里 根据 Bean 的 名字 取得 BeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                //检查bean配置是否设置了depend-on属性。即实例A需要先实例B
                //获取 当前 Bean 的 所有 依赖 Bean， 这样 会 触发 getBean 的 递归 调用， 直到 取 到 一个 没有
                // 任何 依赖 的 Bean 为止
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dep : dependsOn) {
                        if (isDependent(beanName, dep)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                        }
                        registerDependentBean(dep, beanName);
                        try {
                            //Bean依赖别的bean，直接递归getBean即可
                            getBean(dep);
                        } catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }
                /*这里 通过 调用 createBean 方法 创建 Singleton bean 的 实例， 这里 有一个 回 调 函数 getObject，
                会在 getSingleton 中 调用 ObjectFactory 的 createBean*/

                // 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.
        // 这里 对 创建 的 Bean 进行 类型 检查， 如果 没有 问题， 就 返回 这个 新 创建 的 Bean， 这个 Bean 已经 //是 包含 了 依赖 关系 的 Bean
        if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
            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;
    }

通过上面的源码可以看到先通过IOC的 createBean(beanName, mbd, args);最后都会走bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);这个方法就是FactoryBean调用getObject()方法的地方。看一下源码

    protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }

        // 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.
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        if (mbd == null) {
            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());
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }

    

分析一下上面的源码。首先判断当前获得的Bean是否是FactoryBean如果不是直接返回，如果是FactoryBean那么从缓存中获得，如果缓存中没有则创建也就是最后object = getObjectFromFactoryBean(factory, beanName, !synthetic);

    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) {
                    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 (object != null && 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)) {
                            this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
                        }
                    }
                }
                return (object != NULL_OBJECT ? object : null);
            }
        }
        else {
            Object object = doGetObjectFromFactoryBean(factory, beanName);
            if (object != null && shouldPostProcess) {
                try {
                    object = postProcessObjectFromFactoryBean(object, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
                }
            }
            return object;
        }
    }

上面的源码会做一些前置后置处理。主要创建Object的方法是object = doGetObjectFromFactoryBean(factory, beanName);

    private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
            throws BeanCreationException {

        Object object;
        try {
            if (System.getSecurityManager() != null) {
                AccessControlContext acc = getAccessControlContext();
                try {
                    object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                        @Override
                        public Object run() throws Exception {
                                return factory.getObject();
                            }
                        }, acc);
                }
                catch (PrivilegedActionException pae) {
                    throw pae.getException();
                }
            }
            else {
                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 && isSingletonCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(
                    beanName, "FactoryBean which is currently in creation returned null from getObject");
        }
        return object;
    }

看上面的源码object = factory.getObject();最终都是通过你自己写的FactoryBean里面的getObject()来返回你需要的对象

框架中如何使用

在这节里面我们来使用一个框架来看看FactoryBean是如何在框架中使用的。Mybatis这个ORM框架现在在企业中使用已经很广泛了，所以我们来分析一下这个框架里面如何利用FactoryBean。在spring和Mybatis整合中有这样一个类SqlSessionFactoryBean作用是用来生成SqlSessionFactory

先看一下这个类的签名public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent>类的签名继承了FactoryBean<SqlSessionFactory>并且提供了泛型那么表示当时用getBean的使用会生成一个SqlSessionFactory而我们知道在Mybatis里面生成SqlSessionFactory是很复杂的，需要定位资源，然后解析资源最后build生成SqlSessionFactory这个过程是很复杂的单纯交给spring来处理比较麻烦，所以需要自己来实现这个创建的过程。所以Mybatis自己实现了getObject方法

    public SqlSessionFactory getObject() throws Exception {
        if (this.sqlSessionFactory == null) {
            afterPropertiesSet();
        }
        return this.sqlSessionFactory;
    }

当SqlSessionFactory是空时，调用afterPropertiesSet();这个方法是InitializingBean接口里面的方法。

    public void afterPropertiesSet() throws Exception {
        notNull(dataSource, "Property 'dataSource' is required");
        notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
        state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
                "Property 'configuration' and 'configLocation' can not specified with together");

        this.sqlSessionFactory = buildSqlSessionFactory();
    }

最后调用buildSqlSessionFactory();这个方法会收集各种配置数据，最终调用Mybatis的sqlSessionFactoryBuilder.build(configuration);来生成Object