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摘要:
其實(shí)我第一次看見(jiàn)這個(gè)東西的時(shí)候也是不解,代理目標(biāo)源不就是一個(gè)class嘛還需要封裝干嘛。。。
其實(shí)proxy
代理的不是target
,而是TargetSource
,這點(diǎn)非常重要,一定要分清楚?。?!
通常情況下,一個(gè)代理對(duì)象只能代理一個(gè)target,每次方法調(diào)用的目標(biāo)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(diào)用的target實(shí)例都不同(當(dāng)然也可以相同,這取決于TargetSource實(shí)現(xiàn))。這種機(jī)制使得方法調(diào)用變得靈活,可以擴(kuò)展出很多高級(jí)功能,如:單利,原型,本地線(xiàn)程,目標(biāo)對(duì)象池、運(yùn)行時(shí)目標(biāo)對(duì)象熱替換目標(biāo)源等等。
Spring內(nèi)置的TargetSource
SingletonTargetSource
public class SingletonTargetSource implements TargetSource, Serializable { /** Target cached and invoked using reflection. */ private final Object target; //省略無(wú)關(guān)代碼...... @Override public Object getTarget() { return this.target; } //省略無(wú)關(guān)代碼...... }
從這個(gè)目標(biāo)源取得的目標(biāo)對(duì)象是單例的,成員變量target緩存了目標(biāo)對(duì)象,每次getTarget()
都是返回這個(gè)對(duì)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(wú)關(guān)代碼...... }
每次getTarget()
將生成prototype
類(lèi)型的bean,即其生成的bean并不是單例的,因而使用這個(gè)類(lèi)型的TargetSource
時(shí)需要注意,封裝的目標(biāo)bean必須是prototype類(lèi)型的。
PrototypeTargetSource
繼承了AbstractBeanFactoryBasedTargetSource
擁有了創(chuàng)建bean的能力。
public abstract class AbstractPrototypeBasedTargetSource extends AbstractBeanFactoryBasedTargetSource { //省略無(wú)關(guān)代碼...... /** * 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(wú)關(guān)代碼...... }
可以看到,PrototypeTargetSource
的生成prototype類(lèi)型bean的方式主要是委托給BeanFactory進(jìn)行的,因?yàn)?code>BeanFactory自有一套生成prototype類(lèi)型的bean的邏輯,因而PrototypeTargetSource
也就具有生成prototype類(lèi)型bean的能力,這也就是我們要生成的目標(biāo)bean必須聲明為prototype類(lèi)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(wú)關(guān)代碼...... /** * 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(wú)關(guān)代碼...... }
ThreadLocalTargetSource
也就是和線(xiàn)程綁定的TargetSource
,可以理解,其底層實(shí)現(xiàn)必然使用的是ThreadLocal。既然使用了ThreadLocal,也就是說(shuō)我們需要注意兩個(gè)問(wèn)題:
目標(biāo)對(duì)象必須聲明為prototype類(lèi)型,因?yàn)槊總€(gè)線(xiàn)程都會(huì)持有一個(gè)不一樣的對(duì)象;
目標(biāo)對(duì)象必須是無(wú)狀態(tài)的,因?yàn)槟繕?biāo)對(duì)象是和當(dāng)前線(xiàn)程綁定的,而Spring是使用的線(xiàn)程池處理的請(qǐng)求,因而每個(gè)線(xiàn)程可能處理不同的請(qǐng)求,因而為了避免造成問(wèn)題,目標(biāo)對(duì)象必須是無(wú)狀態(tài)的。
實(shí)現(xiàn)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(shí)現(xiàn)自定義TargetSource主要有兩個(gè)點(diǎn)要注意,一個(gè)是getTarget()
方法,該方法中需要實(shí)現(xiàn)獲取目標(biāo)對(duì)象的邏輯,另一個(gè)是isStatic()
方法,這個(gè)方法告知Spring是否需要緩存目標(biāo)對(duì)象,在非單例的情況下一般是返回false。
小結(jié)
本文主要首先講解了Spring是如果在源碼層面支持TargetSource的,然后講解了TargetSource的使用原理,接著對(duì)Spring提供的常見(jiàn)`TargetSource`進(jìn)行了講解,最后使用一個(gè)自定義的TargetSource講解了其使用方式。
以上就是本文的全部?jī)?nèi)容,希望對(duì)大家的學(xué)習(xí)有所幫助,也希望大家多多支持億速云。
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