深入理解SpringAOP之代理对象

　　本篇文章主要带大家简单分析一下AOP的代理对象，至于AOP是什么，如何配置等基础性知识，不在这里讨论。阅读前请先参考：代理模式，在这之前我们需要了解springframework的三个核心接口与getBean方法

一、FactoryBean&BeanFactory&ObjectFactory

　　这三个接口都为Springframework的核心接口，虽然这三个名字很像，但是意义却千差万别。面试的时候也常问它们之间的区别。BeanFactory本身就是一个bean的工厂，同时也是我们的IOC容器，而FactoryBean是一个特殊的Bean,我们可以来看看这个接口：

/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
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package org.springframework.beans.factory;

import org.springframework.lang.Nullable;

/**
 * Interface to be implemented by objects used within a {@link BeanFactory} which
 * are themselves factories for individual objects. If a bean implements this
 * interface, it is used as a factory for an object to expose, not directly as a
 * bean instance that will be exposed itself.
 *
 * 
NB: A bean that implements this interface cannot be used as a normal bean.
 * A FactoryBean is defined in a bean style, but the object exposed for bean
 * references ({@link #getObject()}) is always the object that it creates.
 *
 * 
FactoryBeans can support singletons and prototypes, and can either create
 * objects lazily on demand or eagerly on startup. The {@link SmartFactoryBean}
 * interface allows for exposing more fine-grained behavioral metadata.
 *
 * 
This interface is heavily used within the framework itself, for example for
 * the AOP {@link org.springframework.aop.framework.ProxyFactoryBean} or the
 * {@link org.springframework.jndi.JndiObjectFactoryBean}. It can be used for
 * custom components as well; however, this is only common for infrastructure code.
 *
 * 
{@code FactoryBean} is a programmatic contract. Implementations are not
 * supposed to rely on annotation-driven injection or other reflective facilities.
 * {@link #getObjectType()} {@link #getObject()} invocations may arrive early in
 * the bootstrap process, even ahead of any post-processor setup. If you need access
 * other beans, implement {@link BeanFactoryAware} and obtain them programmatically.
 *
 * 
Finally, FactoryBean objects participate in the containing BeanFactory's
 * synchronization of bean creation. There is usually no need for internal
 * synchronization other than for purposes of lazy initialization within the
 * FactoryBean itself (or the like).
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 08.03.2003
 * @see org.springframework.beans.factory.BeanFactory
 * @see org.springframework.aop.framework.ProxyFactoryBean
 * @see org.springframework.jndi.JndiObjectFactoryBean
 */
public interface FactoryBean {

    /**
     * Return an instance (possibly shared or independent) of the object
     * managed by this factory.
     * 
As with a {@link BeanFactory}, this allows support for both the
     * Singleton and Prototype design pattern.
     * 
If this FactoryBean is not fully initialized yet at the time of
     * the call (for example because it is involved in a circular reference),
     * throw a corresponding {@link FactoryBeanNotInitializedException}.
     * 
As of Spring 2.0, FactoryBeans are allowed to return {@code null}
     * objects. The factory will consider this as normal value to be used; it
     * will not throw a FactoryBeanNotInitializedException in this case anymore.
     * FactoryBean implementations are encouraged to throw
     * FactoryBeanNotInitializedException themselves now, as appropriate.
     * @return an instance of the bean (can be {@code null})
     * @throws Exception in case of creation errors
     * @see FactoryBeanNotInitializedException
     */
    @Nullable
    T getObject() throws Exception;

    /**
     * Return the type of object that this FactoryBean creates,
     * or {@code null} if not known in advance.
     * 
This allows one to check for specific types of beans without
     * instantiating objects, for example on autowiring.
     * 
In the case of implementations that are creating a singleton object,
     * this method should try to avoid singleton creation as far as possible;
     * it should rather estimate the type in advance.
     * For prototypes, returning a meaningful type here is advisable too.
     * 
This method can be called before this FactoryBean has
     * been fully initialized. It must not rely on state created during
     * initialization; of course, it can still use such state if available.
     * 
NOTE: Autowiring will simply ignore FactoryBeans that return
     * {@code null} here. Therefore it is highly recommended to implement
     * this method properly, using the current state of the FactoryBean.
     * @return the type of object that this FactoryBean creates,
     * or {@code null} if not known at the time of the call
     * @see ListableBeanFactory#getBeansOfType
     */
    @Nullable
    Class getObjectType();

    /**
     * Is the object managed by this factory a singleton? That is,
     * will {@link #getObject()} always return the same object
     * (a reference that can be cached)?
     * 
NOTE: If a FactoryBean indicates to hold a singleton object,
     * the object returned from {@code getObject()} might get cached
     * by the owning BeanFactory. Hence, do not return {@code true}
     * unless the FactoryBean always exposes the same reference.
     * 
The singleton status of the FactoryBean itself will generally
     * be provided by the owning BeanFactory; usually, it has to be
     * defined as singleton there.
     * 
NOTE: This method returning {@code false} does not
     * necessarily indicate that returned objects are independent instances.
     * An implementation of the extended {@link SmartFactoryBean} interface
     * may explicitly indicate independent instances through its
     * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean}
     * implementations which do not implement this extended interface are
     * simply assumed to always return independent instances if the
     * {@code isSingleton()} implementation returns {@code false}.
     * 
The default implementation returns {@code true}, since a
     * {@code FactoryBean} typically manages a singleton instance.
     * @return whether the exposed object is a singleton
     * @see #getObject()
     * @see SmartFactoryBean#isPrototype()
     */
    default boolean isSingleton() {
        return true;
    }

}

View Code

　　这里面有三个方法，分别为：getObject，getObjectType，isSingleton。根据文档解释，它只是一个生产对象的工厂，被Spring管理 。这个工厂负责提供我们需要的对象。当需要特殊的方式创建Bean时，则考虑实现该接口。我举个例子来说明：

package org.hzgj.spring.study;

import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Component;

@Component
public class WaterFactoryBean implements FactoryBean {

    @Override
    public Water getObject() throws Exception {
        Water water=new Water();
        water.setCapacity(20);
        return water;
    }

    @Override
    public Class getObjectType() {
        return Water.class;
    }

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

//.....

package org.hzgj.spring.study;

@Deprecated
public class Water {

    private int capacity;

    public int getCapacity() {
        return capacity;
    }

    public void setCapacity(int capacity) {
        this.capacity = capacity;
    }

    public void test() {
        System.out.println("test");
    }

    @Deprecated
    public void test1() {
        System.out.println("test1");
    }
}
//.....

  ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-config.xml");
        Water water = applicationContext.getBean(Water.class);
        water.test1();
// ....能够成功获取到对象

　　在上面例子里，我们本身是要获得Water对象，那么此时Water对象实际上是通过FactoryBean创建的，因此我们在获取对象时可以添加我们自己的逻辑。

　　下面我们根据源代码来追溯一下getBean与BeanFactory关联，具体可以参考一下AbstractBeanFactory的doGetBean方法，那么在这里简单的说明一下执行过程：

　　1) 如果是单例对象的Bean会去缓存中获取

　　　　我们先看一下getSinglelone方法：

/** Cache of singleton objects: bean name --> bean instance */
    private final Map singletonObjects = new ConcurrentHashMap(256);


/** Cache of singleton factories: bean name --> ObjectFactory */
    private final Map> singletonFactories = new HashMap>(16);

/** Cache of early singleton objects: bean name --> bean instance */
    private final Map earlySingletonObjects = new HashMap(16);


/**
     * Return the (raw) singleton object registered under the given name.
     * 
Checks already instantiated singletons and also allows for an early
     * reference to a currently created singleton (resolving a circular reference).
     * @param beanName the name of the bean to look for
     * @param allowEarlyReference whether early references should be created or not
     * @return the registered singleton object, or {@code null} if none found
     */
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }

　　　　在这里我们获取单例对象时一定和ObjectFactory有关系

　　2）从它的parentBeanFactory中获取

// Check if bean definition exists in this factory.
            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);
                }
            }

 

　　3）处理bean的dependsOn

final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                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);
                        getBean(dep);
                    }
                }

 

　　4）根据bean的scope类型来获取对应的bean

    // Create bean instance.
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory