Springboot源码分析之TargetSource

摘要：

其实我第一次看见这个东西的时候也是不解，代理目标源不就是一个class嘛还需要封装干嘛。。。

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