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<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。
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。