Springboot源码分析之TargetSource
- 2019 年 10 月 3 日
- 笔记
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。