Springboot中TargetSource的作用是什么

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摘要：

其实proxy代理的不是target,而是TargetSource,这点非常重要，一定要分清楚！！！

通常情况下,一个代理对象只能代理一个target,每次方法调用的目标也是唯一固定的target。但是,如果让proxy代理TargetSource,可以使得每次方法调用的target实例都不同(当然也可以相同,这取决于TargetSource实现)。这种机制使得方法调用变得灵活,可以扩展出很多高级功能,如:单利，原型，本地线程，目标对象池、运行时目标对象热替换目标源等等。

Spring内置的TargetSource

SingletonTargetSource

    public class SingletonTargetSource implements TargetSource, Serializable {                /** Target cached and invoked using reflection. */            private final Object target;            //省略无关代码......            @Override            public Object getTarget() {                    return this.target;            }            //省略无关代码......    }

从这个目标源取得的目标对象是单例的，成员变量target缓存了目标对象，每次getTarget()都是返回这个对象。

PrototypeTargetSource

    public class PrototypeTargetSource extends AbstractPrototypeBasedTargetSource {           /**        * Obtain a new prototype instance for every call.        * @see #newPrototypeInstance()        */       @Override       public Object getTarget() throws BeansException {          return newPrototypeInstance();       }           /**        * Destroy the given independent instance.        * @see #destroyPrototypeInstance        */       @Override       public void releaseTarget(Object target) {          destroyPrototypeInstance(target);       }      //省略无关代码......    }

每次getTarget()将生成prototype类型的bean，即其生成的bean并不是单例的，因而使用这个类型的TargetSource时需要注意，封装的目标bean必须是prototype类型的。PrototypeTargetSource继承了AbstractBeanFactoryBasedTargetSource拥有了创建bean的能力。

    public abstract class AbstractPrototypeBasedTargetSource extends AbstractBeanFactoryBasedTargetSource {           //省略无关代码......       /**        * Subclasses should call this method to create a new prototype instance.        * @throws BeansException if bean creation failed        */       protected Object newPrototypeInstance() throws BeansException {          if (logger.isDebugEnabled()) {             logger.debug("Creating new instance of bean '" + getTargetBeanName() + "'");          }          return getBeanFactory().getBean(getTargetBeanName());       }           /**        * Subclasses should call this method to destroy an obsolete prototype instance.        * @param target the bean instance to destroy        */       protected void destroyPrototypeInstance(Object target) {          if (logger.isDebugEnabled()) {             logger.debug("Destroying instance of bean '" + getTargetBeanName() + "'");          }          if (getBeanFactory() instanceof ConfigurableBeanFactory) {             ((ConfigurableBeanFactory) getBeanFactory()).destroyBean(getTargetBeanName(), target);          }          else if (target instanceof DisposableBean) {             try {                ((DisposableBean) target).destroy();             }             catch (Throwable ex) {                logger.warn("Destroy method on bean with name '" + getTargetBeanName() + "' threw an exception", ex);             }          }       }          //省略无关代码......        }

可以看到，PrototypeTargetSource的生成prototype类型bean的方式主要是委托给BeanFactory进行的，因为BeanFactory自有一套生成prototype类型的bean的逻辑，因而PrototypeTargetSource也就具有生成prototype类型bean的能力，这也就是我们要生成的目标bean必须声明为prototype类型的原因。

ThreadLocalTargetSource

    public class ThreadLocalTargetSource extends AbstractPrototypeBasedTargetSource          implements ThreadLocalTargetSourceStats, DisposableBean {           /**        * ThreadLocal holding the target associated with the current        * thread. Unlike most ThreadLocals, which are static, this variable        * is meant to be per thread per instance of the ThreadLocalTargetSource class.        */       private final ThreadLocal