Springboot源码 TargetSource解析

本文讲解"Springboot源码 TargetSource解析"，用于解决相关问题。

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......@Overridepublic 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()*/@Overridepublic Object getTarget() throws BeansException {return newPrototypeInstance();}/*** Destroy the given independent instance.* @see #destroyPrototypeInstance*/@Overridepublic 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 AbstractPrototypeBasedTargetSourceimplements 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.*/@Overridepublic 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*/@Overridepublic 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;}@Overridepublic Class<?> getTargetClass() {return target.getClass();}@Overridepublic boolean isStatic() {return false;}@Overridepublic Object getTarget() throws Exception {return this.target[this.idx.getAndIncrement() & this.target.length - 1];}@Overridepublic 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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持亿速云。

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