Springboot源码 TargetSource解析

Springboot源码 TargetSource解析

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

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

//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 12:4XxZNswPx3

* @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。

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