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<Object> targetInThread =
             new NamedThreadLocal<>("Thread-local instance of bean '" + getTargetBeanName() + "'");

       /**
        * Set of managed targets, enabling us to keep track of the targets we've created.
        */
       private final Set<Object> targetSet = new HashSet<>();

     //省略无关代码......
       /**
        * Implementation of abstract getTarget() method.
        * We look for a target held in a ThreadLocal. If we don't find one,
        * we create one and bind it to the thread. No synchronization is required.
        */
       @Override
       public Object getTarget() throws BeansException {
          ++this.invocationCount;
          Object target = this.targetInThread.get();
          if (target == null) {
             if (logger.isDebugEnabled()) {
                logger.debug("No target for prototype '" + getTargetBeanName() + "' bound to thread: " +
                      "creating one and binding it to thread '" + Thread.currentThread().getName() + "'");
             }
             // Associate target with ThreadLocal.
             target = newPrototypeInstance();
             this.targetInThread.set(target);
             synchronized (this.targetSet) {
                this.targetSet.add(target);
             }
          }
          else {
             ++this.hitCount;
          }
          return target;
       }

       /**
        * Dispose of targets if necessary; clear ThreadLocal.
        * @see #destroyPrototypeInstance
        */
       @Override
       public void destroy() {
          logger.debug("Destroying ThreadLocalTargetSource bindings");
          synchronized (this.targetSet) {
             for (Object target : this.targetSet) {
                destroyPrototypeInstance(target);
             }
             this.targetSet.clear();
          }
          // Clear ThreadLocal, just in case.
          this.targetInThread.remove();
       }
    //省略无关代码......
    }

ThreadLocalTargetSource也就是和线程绑定的TargetSource，可以理解，其底层实现必然使用的是ThreadLocal。既然使用了ThreadLocal，也就是说我们需要注意两个问题：

• 目标对象必须声明为prototype类型，因为每个线程都会持有一个不一样的对象；
• 目标对象必须是无状态的，因为目标对象是和当前线程绑定的，而Spring是使用的线程池处理的请求，因而每个线程可能处理不同的请求，因而为了避免造成问题，目标对象必须是无状态的。

实现自定义的TargetSource

    package com.github.dqqzj.springboot.target;

    import org.springframework.aop.TargetSource;
    import org.springframework.util.Assert;

    import java.lang.reflect.Array;
    import java.util.concurrent.ThreadLocalRandom;
    import java.util.concurrent.atomic.AtomicInteger;

    /**
     * @author qinzhongjian
     * @date created in 2019-08-25 12:43
     * @description: TODO
     * @since JDK 1.8.0_212-b10z
     */
    public class DqqzjTargetSource implements TargetSource {
        private final AtomicInteger idx = new AtomicInteger();
        private final Object[] target;;
        public DqqzjTargetSource(Object[]  target) {
            Assert.notNull(target, "Target object must not be null");
            this.target = target;
        }
        @Override
        public Class<?> getTargetClass() {
            return target.getClass();
        }

        @Override
        public boolean isStatic() {
            return false;
        }

        @Override
        public Object getTarget() throws Exception {
            return this.target[this.idx.getAndIncrement() & this.target.length - 1];
        }

        @Override
        public void releaseTarget(Object target) throws Exception {

        }
    }

实现自定义TargetSource主要有两个点要注意，一个是getTarget()方法，该方法中需要实现获取目标对象的逻辑，另一个是isStatic()方法，这个方法告知Spring是否需要缓存目标对象，在非单例的情况下一般是返回false。

小结

   本文主要首先讲解了Spring是如果在源码层面支持TargetSource的，然后讲解了TargetSource的使用原理，接着对Spring提供的常见`TargetSource`进行了讲解，最后使用一个自定义的TargetSource讲解了其使用方式。