[spring源码学习]六、IOC源码-BeanFactory和factory-bean

一、代码实例

　　在我们分析spring的IOC源码的时候，发现除了配置标准的bean，并且通过getBean(beanName)的方法获取到一个bean的实例外，似乎还有这不少其他获取的方法，例如在第四节，我们发现得到bean实例后，在第26节，会判断是否继承了FactoryBean，然后调用它的方法获取真实的bean，在配置文件中我们发现一个factory-bean方法，这些都说明，我们应该可以使用一个beanFactory获取一个bean，此节重点讨论这部分的实现。

　　代码如下：

　　1、car类

package com.zjl.factorybean;

public class Car {
    public Car(String name) {
        this.name=name;
    }
    String name;
    public void run(){
        System.out.println(this.name+" is running");
    }
}

　　2、person类

package com.zjl.factorybean;

public class Person {
    public Person(String name) {
        this.name=name;
    }
    public String name;
    public int age;
    
    public Car car;
    
    public void sayHello(){
        System.out.println(this.name+" say hello");
    }
    public Car createCar(){
        return new Car("奥迪");
    }
}

　　3、用来获取person的FactoryBean，必须继承FactoryBean接口　

package com.zjl.factorybean;
import org.springframework.beans.factory.FactoryBean;

public class PersonFactory implements FactoryBean<Person> {
    String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public Person getObject() throws Exception {
        return new Person(name);
    }

    @Override
    public Class<Person> getObjectType() {
        return Person.class;
    }

    @Override
    public boolean isSingleton() {
        return true;
    }

}

　　4、配置文件

 <bean id="personFactory" class="com.zjl.factorybean.PersonFactory">
        <property name="name" value="zhangsan"></property>
    </bean>  
    
    <bean id="car" factory-method="createCar" factory-bean="personFactory"></bean>

　　5、测试类

package com.zjl.factorybean;

import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.beans.factory.xml.XmlBeanDefinitionReader;
import org.springframework.core.io.ClassPathResource;


public class Test {
    public static void main(String[] args) throws Exception {
        DefaultListableBeanFactory beanFacory=new DefaultListableBeanFactory();
        XmlBeanDefinitionReader reader=new XmlBeanDefinitionReader(beanFacory);
        reader.loadBeanDefinitions(new ClassPathResource("factorybean.xml"));
        
        Person person=(Person)beanFacory.getBean("personFactory");
        person.sayHello();
        Person person2=(Person)beanFacory.getBean("personFactory");
        System.out.println("person==person2 is "+(person==person2));
        
        Car car=(Car)beanFacory.getBean("car");
        car.run();
        Car car2=(Car)beanFacory.getBean("car");
        System.out.println("car==car2 is "+(car==car2));
    }
}

　　6、测试结果

zhangsan say hello
person==person2 is true
奥迪 is running
car==car2 is true

　　7、结论

　　我们可以看到：

　　a)通过getBean(beanName)方法获取到的直接就是Person的实例，而不是BeanFactory或者PersonFactory的实例。

　　b)每次获取到的Person实例都是同一个，根据接口中的方法isSingleton方法，猜测于此有关

　　c)bean的id为car，并没有配置我们常见的class配置，他应该是执行了Person的createCar方法

　　d)car也遵循单例模式

二、FactoryBean代码解析

　　1、通过我们对bean的加载过程，发现所有的配置无论是spring默认bean的 配置，还是客户自定义的配置均无差别的被解析后存放在beanDefinitionMap中，所以解析配置文件过程不再重复。

　　2、Person person=(Person)beanFacory.getBean("personFactory");生成bean过程与第四部分重复的跳过

　　3、来到bean生成实例后的地方

bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd)

　　4、进入方法，判断如果是bean不是FactoryBean的实例且beanName是&开头，抛出错误。是FactoryBean的实例，且以&开头，则直接返回实例。

　　　　将实例转化为FactoryBean的实例，并且调用getObjectFromFactoryBean(factory, beanName, !synthetic)方法

　　注：也就是我们要获得定义的通过personFactory返回PersonFactory的实例，可以使用beanFacory.getBean("&personFactory")进行获取，然后调用getObject也可以返回Person的实例，不过这个需要自己控制单例模式

    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());
        }//bean不是FactoryBean的实例且beanName是&开头,报错

        // 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.
　　　　 //bean是FactoryBean的实例，且beanName以&开头，返回实例

        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;
    }

　　5、调用factory实例的isSingleton方法和containsSingleton(beanName)，判断是否是单例模式，单例模式的话，从factoryBeanObjectCache中尝试读取，否则直接生成。

　　注：我们可以看到，通过FactoryBean的对象是否是单例模式取决于bean定义的范围和方法isSingleton同时为单例才可以

    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) {
　　　　　　　　　　　　//入口，调用getObject方法
                    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) {
                            try {
                                object = postProcessObjectFromFactoryBean(object, beanName);
                            }
                            catch (Throwable ex) {
                                throw new BeanCreationException(beanName,
                                        "Post-processing of FactoryBean's singleton object failed", ex);
                            }
                        }
                        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;
        }
    }

　　6、doGetObjectFromFactoryBean中调用getObject方法，返回实例

　　7、调用bean后处理postProcessObjectFromFactoryBean并放入缓存factoryBeanObjectCache

　　到此，我们已经获取到了真正的bean，并且也知道了怎么获取原来定义的FactoryBean的实例，但是，似乎少了一个方法，那就说FactoryBean中的getObjectType，我们需要回头去找哪里漏掉了

　　我翻遍了源代码，并没有找到此方法调用的地方，事实上通过改动

    @Override
    public Class<Car> getObjectType() {
        return Car.class;
    }

或者 

    @Override
    public Class<Person> getObjectType() {
        return null;
    }

都不会影响代码执行结果的正确性，那么我们猜想，是否仅仅是一个预留，并无实际用处，或者客户自定义使用方法。

三、FactoryBean实例

　　在上一步查找源代码过程中，我们有了新的发现，spring自定义了抽象类AbstractFactoryBean和大量他的子类，包括我们常见的list，map，set，object等，我们也来研究下

1、首先是他的继承关系

public abstract class AbstractFactoryBean<T>
        implements FactoryBean<T>, BeanClassLoaderAware, BeanFactoryAware, InitializingBean, DisposableBean {

我们看到继承了几个熟悉的接口，包括FactoryBean和InitializingBean，还有几个BeanClassLoaderAware, BeanFactoryAware, DisposableBean，这些接口分别是：

FactoryBean：通过getObjectBean生成bean实例

InitializingBean：在实例化后执行afterPropertiesSet方法

以上我们比较熟悉，其余三个接口简单了解下

BeanClassLoaderAware：注入classLoad

BeanFactoryAware:注入一个BeanFactory

DisposableBean：销毁bean默认调用destroy方法

我们这里重点关注FactoryBean的三个接口实现：

2、是否单例，通过外部注入

    public boolean isSingleton() {
        return this.singleton;
    }

3、getObject方法，如果是单例且已经创建，返回单例模式，未创建调用getEarlySingletonInstance方法，不是单例模式，调用createInstance方法

    @Override
    public final T getObject() throws Exception {
        if (isSingleton()) {
            return (this.initialized ? this.singletonInstance : getEarlySingletonInstance());
        }
        else {
            return createInstance();
        }
    }

4、getEarlySingletonInstance方法，通过第5步判断，是否为接口，如果是通过动态代理，创建对象

    private T getEarlySingletonInstance() throws Exception {
        Class<?>[] ifcs = getEarlySingletonInterfaces();
        if (ifcs == null) {
            throw new FactoryBeanNotInitializedException(
                    getClass().getName() + " does not support circular references");
        }
        if (this.earlySingletonInstance == null) {
            this.earlySingletonInstance = (T) Proxy.newProxyInstance(
                    this.beanClassLoader, ifcs, new EarlySingletonInvocationHandler());
        }
        return this.earlySingletonInstance;
    }

5、此处调用getObjectType，判断是否为空或者是否为接口，如果是，返回，否则为空

    protected Class<?>[] getEarlySingletonInterfaces() {
        Class<?> type = getObjectType();
        return (type != null && type.isInterface() ? new Class<?>[] {type} : null);
    }

6、动态代理的InvocationHandler 类，似乎什么都没干，存疑

private class EarlySingletonInvocationHandler implements InvocationHandler {

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            if (ReflectionUtils.isEqualsMethod(method)) {
                // Only consider equal when proxies are identical.
                return (proxy == args[0]);
            }
            else if (ReflectionUtils.isHashCodeMethod(method)) {
                // Use hashCode of reference proxy.
                return System.identityHashCode(proxy);
            }
            else if (!initialized && ReflectionUtils.isToStringMethod(method)) {
                return "Early singleton proxy for interfaces " +
                        ObjectUtils.nullSafeToString(getEarlySingletonInterfaces());
            }
            try {
                return method.invoke(getSingletonInstance(), args);
            }
            catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
        }
    }

分析到这里，似乎这个类什么都没做，我们具体看个实例类ListFactoryBean，此类注入了以下参数：

a、sourceList是一个list

b、targetListClass是一个Class

c、重写了createInstance()，将sourceList修改后注入

public class ListFactoryBean extends AbstractFactoryBean<List<Object>> {

    private List<?> sourceList;

    @SuppressWarnings("rawtypes")
    private Class<? extends List> targetListClass;


    /**
     * Set the source List, typically populated via XML "list" elements.
     */
    public void setSourceList(List<?> sourceList) {
        this.sourceList = sourceList;
    }

    /**
     * Set the class to use for the target List. Can be populated with a fully
     * qualified class name when defined in a Spring application context.
     * <p>Default is a {@code java.util.ArrayList}.
     * @see java.util.ArrayList
     */
    @SuppressWarnings("rawtypes")
    public void setTargetListClass(Class<? extends List> targetListClass) {
        if (targetListClass == null) {
            throw new IllegalArgumentException("'targetListClass' must not be null");
        }
        if (!List.class.isAssignableFrom(targetListClass)) {
            throw new IllegalArgumentException("'targetListClass' must implement [java.util.List]");
        }
        this.targetListClass = targetListClass;
    }


    @Override
    @SuppressWarnings("rawtypes")
    public Class<List> getObjectType() {
        return List.class;
    }

    @Override
    @SuppressWarnings("unchecked")
    protected List<Object> createInstance() {
        if (this.sourceList == null) {
            throw new IllegalArgumentException("'sourceList' is required");
        }
        List<Object> result = null;
        if (this.targetListClass != null) {
            result = BeanUtils.instantiateClass(this.targetListClass);
        }
        else {
            result = new ArrayList<Object>(this.sourceList.size());
        }
        Class<?> valueType = null;
        if (this.targetListClass != null) {
            valueType = GenericCollectionTypeResolver.getCollectionType(this.targetListClass);
        }
        if (valueType != null) {
            TypeConverter converter = getBeanTypeConverter();
            for (Object elem : this.sourceList) {
                result.add(converter.convertIfNecessary(elem, valueType));
            }
        }
        else {
            result.addAll(this.sourceList);
        }
        return result;
    }

}

ListFactoryBean