Springboot源碼分析之TargetSource
- 2019 年 10 月 3 日
- 筆記
摘要：
其實我第一次看見這個東西的時候也是不解，代理目標源不就是一個class嘛還需要封裝幹嘛。。。
其實proxy代理的不是target,而是TargetSource,這點非常重要，一定要分清楚！！！
通常情況下,一個代理對象只能代理一個target,每次方法調用的目標也是唯一固定的target。但是,如果讓proxy代理TargetSource,可以使得每次方法調用的target實例都不同(當然也可以相同,這取決於TargetSource實現)。這種機制使得方法調用變得靈活,可以擴展出很多高級功能,如:單利，原型，本地執行緒，目標對象池、運行時目標對象熱替換目標源等等。
Spring內置的TargetSource
SingletonTargetSource
public class SingletonTargetSource implements TargetSource, Serializable { /** Target cached and invoked using reflection. */ private final Object target; //省略無關程式碼...... @Override public Object getTarget() { return this.target; } //省略無關程式碼...... }從這個目標源取得的目標對象是單例的，成員變數target快取了目標對象，每次getTarget()都是返回這個對象。
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); } //省略無關程式碼...... }每次getTarget()將生成prototype類型的bean，即其生成的bean並不是單例的，因而使用這個類型的TargetSource時需要注意，封裝的目標bean必須是prototype類型的。PrototypeTargetSource繼承了AbstractBeanFactoryBasedTargetSource擁有了創建bean的能力。
public abstract class AbstractPrototypeBasedTargetSource extends AbstractBeanFactoryBasedTargetSource { //省略無關程式碼...... /** * 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); } } } //省略無關程式碼...... }可以看到，PrototypeTargetSource的生成prototype類型bean的方式主要是委託給BeanFactory進行的，因為BeanFactory自有一套生成prototype類型的bean的邏輯，因而PrototypeTargetSource也就具有生成prototype類型bean的能力，這也就是我們要生成的目標bean必須聲明為prototype類型的原因。
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<>(); //省略無關程式碼...... /** * 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(); } //省略無關程式碼...... }ThreadLocalTargetSource也就是和執行緒綁定的TargetSource，可以理解，其底層實現必然使用的是ThreadLocal。既然使用了ThreadLocal，也就是說我們需要注意兩個問題：
- 目標對象必須聲明為prototype類型，因為每個執行緒都會持有一個不一樣的對象；
- 目標對象必須是無狀態的，因為目標對象是和當前執行緒綁定的，而Spring是使用的執行緒池處理的請求，因而每個執行緒可能處理不同的請求，因而為了避免造成問題，目標對象必須是無狀態的。
實現自定義的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 { } }實現自定義TargetSource主要有兩個點要注意，一個是getTarget()方法，該方法中需要實現獲取目標對象的邏輯，另一個是isStatic()方法，這個方法告知Spring是否需要快取目標對象，在非單例的情況下一般是返回false。
小結
本文主要首先講解了Spring是如果在源碼層面支援TargetSource的，然後講解了TargetSource的使用原理，接著對Spring提供的常見`TargetSource`進行了講解，最後使用一個自定義的TargetSource講解了其使用方式。