RxJava的消息发送和线程切换实现原理
rxjava是一个在java虚拟机上的响应式扩展,通过使用可观察的序列将异步和基于事件的程序组合起来的一个库。
它扩展了观察者模式来支持数据/事件序列,并且添加了操作符,这些操作符允许你声明性地组合序列,同时抽象出要关注的问题:比如低级线程、同步、线程安全和并发数据结构等。
rxjava相信大家都非常了解吧,今天分享一下rxjava的消息发送和线程源码的分析。最后并分享一个相关demo,让大家更加熟悉我们天天都在用的框架。
消息订阅发送
首先让我们看看消息订阅发送最基本的代码组成:
observable observable = observable.create(new observableonsubscribe<string>() { @override public void subscribe(observableemitter<string> emitter) throws exception { emitter.onnext("jack1"); emitter.onnext("jack2"); emitter.onnext("jack3"); emitter.oncomplete(); } }); observer<string> observer = new observer<string>() { @override public void onsubscribe(disposable d) { log.d(tag, "onsubscribe"); } @override public void onnext(string s) { log.d(tag, "onnext : " + s); } @override public void onerror(throwable e) { log.d(tag, "onerror : " + e.tostring()); } @override public void oncomplete() { log.d(tag, "oncomplete"); } }; observable.subscribe(observer);
代码很简单,observable为被观察者,observer为观察者,然后通过observable.subscribe(observer),把观察者和被观察者关联起来。被观察者发送消息(emitter.onnext("内容")),观察者就可以在onnext()方法里回调出来。
我们先来看observable,创建是用observable.create()方法进行创建,源码如下:
public static <t> observable<t> create(observableonsubscribe<t> source) { objecthelper.requirenonnull(source, "source is null"); return rxjavaplugins.onassembly(new observablecreate<t>(source)); } public static <t> t requirenonnull(t object, string message) { if (object == null) { throw new nullpointerexception(message); } return object; } public static <t> observable<t> onassembly(@nonnull observable<t> source) { function<? super observable, ? extends observable> f = onobservableassembly; if (f != null) { return apply(f, source); } return source; }
可以看出,create()方法里最主要的还是创建用observableonsubscribe传入创建了一个observablecreate对象并且保存而已。
public final class observablecreate<t> extends observable<t> { final observableonsubscribe<t> source; public observablecreate(observableonsubscribe<t> source) { this.source = source; } }
接着是创建observer,这比较简单只是单纯创建一个接口对象而已
public interface observer<t> { void onsubscribe(@nonnull disposable d); void onnext(@nonnull t t); void onerror(@nonnull throwable e); void oncomplete(); }
订阅发送消息
observable.subscribe(observer)的subscribe方法如下:
public final void subscribe(observer<? super t> observer) { objecthelper.requirenonnull(observer, "observer is null"); try { observer = rxjavaplugins.onsubscribe(this, observer); objecthelper.requirenonnull(observer, "plugin returned null observer"); subscribeactual(observer); } catch (nullpointerexception e) { // nopmd throw e; } catch (throwable e) { exceptions.throwiffatal(e); rxjavaplugins.onerror(e); nullpointerexception npe = new nullpointerexception("actually not, but can't throw other exceptions due to rs"); npe.initcause(e); throw npe; } } //objecthelper.requirenonnull()方法 public static <t> t requirenonnull(t object, string message) { if (object == null) { throw new nullpointerexception(message); } return object; } //rxjavaplugins.onsubscribe()方法 public static <t> observer<? super t> onsubscribe(@nonnull observable<t> source, @nonnull observer<? super t> observer) { bifunction<? super observable, ? super observer, ? extends observer> f = onobservablesubscribe; if (f != null) { return apply(f, source, observer); } return observer; }
从上面源码可以看出requirenonnull()只是做非空判断而已,而rxjavaplugins.onsubscribe()也只是返回最终的观察者而已。所以关键代码是抽象方法subscribeactual(observer);那么subscribeactual对应哪个代码段呢?
还记得observable.create()创建的observablecreate类吗,这就是subscribeactual()具体实现类,源码如下:
protected void subscribeactual(observer<? super t> observer) { createemitter<t> parent = new createemitter<t>(observer); observer.onsubscribe(parent); try { source.subscribe(parent); } catch (throwable ex) { exceptions.throwiffatal(ex); parent.onerror(ex); } }
从上面的代码可以看出,首先创建了一个createemitter对象并传入observer,然后回到observer的onsubscribe()方法,而source就是我们之前创建observablecreate传入的observableonsubscribe对象。
class createemitter<t> extends atomicreference<disposable> implements observableemitter<t>, disposable { }
而createemitter又继承observableemitter接口,又回调observableonsubscribe的subscribe方法,对应着我们的:
observable observable = observable.create(new observableonsubscribe<string>() { @override public void subscribe(observableemitter<string> emitter) throws exception { emitter.onnext("jack1"); emitter.onnext("jack2"); emitter.onnext("jack3"); emitter.oncomplete(); } });
当它发送消息既调用emitter.onnext()方法时,既调用了createemitter的onnext()方法:
public void onnext(t t) { if (t == null) { onerror(new nullpointerexception("onnext called with null. null values are generally not allowed in 2.x operators and sources.")); return; } if (!isdisposed()) { observer.onnext(t); } }
可以看到最终又回调了观察者的onnext()方法,把被观察者的数据传输给了观察者。有人会问
isdisposed()是什么意思,是判断要不要终止传递的,我们看emitter.oncomplete()源码:
public void oncomplete() { if (!isdisposed()) { try { observer.oncomplete(); } finally { dispose(); } } } public static boolean dispose(atomicreference<disposable> field) { disposable current = field.get(); disposable d = disposed; if (current != d) { current = field.getandset(d); if (current != d) { if (current != null) { current.dispose(); } return true; } } return false; } public static boolean isdisposed(disposable d) { return d == disposed; }
dispose()方法是终止消息传递,也就付了个disposed常量,而isdisposed()方法就是判断这个常量而已。这就是整个消息订阅发送的过程,用的是观察者模式。
线程切换
在上面模板代码的基础上,线程切换只是改变了如下代码:
observable.subscribeon(schedulers.io()) .observeon(androidschedulers.mainthread()) .subscribe(observer);
下面我们对线程切换的源码进行一下分析,分为两部分:subscribeon()和observeon()
subscribeon()
首先是subscribeon()源码如下:
public final observable<t> subscribeon(scheduler scheduler) { objecthelper.requirenonnull(scheduler, "scheduler is null"); return rxjavaplugins.onassembly(new observablesubscribeon<t>(this, scheduler)); }
我们传进去了一个scheduler类,scheduler是一个调度类,能够延时或周期性地去执行一个任务。
scheduler有如下类型:
类型 | 使用方式 | 含义 | 使用场景 |
---|---|---|---|
ioscheduler | schedulers.io() | io操作线程 | 读写sd卡文件,查询数据库,访问网络等io密集型操作 |
newthreadscheduler | schedulers.newthread() | 创建新线程 | 耗时操作等 |
singlescheduler | schedulers.single() | 单例线程 | 只需一个单例线程时 |
computationscheduler | schedulers.computation() | cpu计算操作线程 | 图片压缩取样、xml,json解析等cpu密集型计算 |
trampolinescheduler | schedulers.trampoline() | 当前线程 | 需要在当前线程立即执行任务时 |
handlerscheduler | androidschedulers.mainthread() | android主线程 | 更新ui等 |
接着就没什么了,只是返回一个observablesubscribeon对象而已。
observeon()
首先看源码如下:
public final observable<t> observeon(scheduler scheduler) { return observeon(scheduler, false, buffersize()); } public final observable<t> observeon(scheduler scheduler, boolean delayerror, int buffersize) { objecthelper.requirenonnull(scheduler, "scheduler is null"); objecthelper.verifypositive(buffersize, "buffersize"); return rxjavaplugins.onassembly(new observableobserveon<t>(this, scheduler, delayerror, buffersize)); }
这里也是没什么,只是最终返回一个observableobserveon对象而已。
接着还是像原来那样调用subscribe()方法进行订阅,看起来好像整体变化不大,就是封装了一些对象而已,不过着恰恰是rxjava源码的精华,当他再次调用subscribeactual()方法时,已经不是之前的observablecreate()里subscribeactual方法了,而是最先调用observableobserveon的subscribeactual()方法,对应源码如下:
protected void subscribeactual(observer<? super t> observer) { if (scheduler instanceof trampolinescheduler) { source.subscribe(observer); } else { scheduler.worker w = scheduler.createworker(); source.subscribe(new observeonobserver<t>(observer, w, delayerror, buffersize)); } }
在这里有两点要讲,一点是observeonobserver是执行观察者的线程,后面还会详解,然后就是source.subscribe,这个source.subscribe调的是observablesubscribeon的subscribe方法,而subscribe方法因为继承的也是observable,是observable里的方法,所以和上面的observablecreate一样的方法,所以会调用observablesubscribeon里的subscribeactual()方法,对应的代码如下:
public void subscribeactual(final observer<? super t> s) { final subscribeonobserver<t> parent = new subscribeonobserver<t>(s); s.onsubscribe(parent); parent.setdisposable(scheduler.scheduledirect(new subscribetask(parent))); }
上面代码中,首先把observeonobserver返回给来的用subscribeonobserver“包装”起来,然后在回调observer的onsubscribe(),就是对应模板代码的onsubscribe()方法。
接着看subscribetask类的源码:
final class subscribetask implements runnable { private final subscribeonobserver<t> parent; subscribetask(subscribeonobserver<t> parent) { this.parent = parent; } @override public void run() { source.subscribe(parent); } }
其中的source.subscribe(parent),就是我们执行子线程的回调方法,对应我们模板代码里的被观察者的subscribe()方法。它放在run()方法里,并且继承runnable,说明这个类主要是线程运行。接着看scheduler.scheduledirect()方法对应的源码如下:
public disposable scheduledirect(@nonnull runnable run) { return scheduledirect(run, 0l, timeunit.nanoseconds); } public disposable scheduledirect(@nonnull runnable run, long delay, @nonnull timeunit unit) { final worker w = createworker(); final runnable decoratedrun = rxjavaplugins.onschedule(run); disposetask task = new disposetask(decoratedrun, w); w.schedule(task, delay, unit); return task; }
在这里,createworker()也是一个抽象方法,调用的是我们的调度类对应的schedulers类里面的方法,这里是ioscheduler类,
public final class ioscheduler extends scheduler{ final atomicreference<cachedworkerpool> pool; //省略.... public worker createworker() { return new eventloopworker(pool.get()); } static final class eventloopworker extends scheduler.worker { private final compositedisposable tasks; private final cachedworkerpool pool; private final threadworker threadworker; final atomicboolean once = new atomicboolean(); eventloopworker(cachedworkerpool pool) { this.pool = pool; this.tasks = new compositedisposable(); this.threadworker = pool.get(); } //省略.... @nonnull @override public disposable schedule(@nonnull runnable action, long delaytime, @nonnull timeunit unit) { if (tasks.isdisposed()) { // don't schedule, we are unsubscribed return emptydisposable.instance; } return threadworker.scheduleactual(action, delaytime, unit, tasks); } } } static final class cachedworkerpool implements runnable { //省略.... threadworker get() { if (allworkers.isdisposed()) { return shutdown_thread_worker; } while (!expiringworkerqueue.isempty()) { threadworker threadworker = expiringworkerqueue.poll(); if (threadworker != null) { return threadworker; } } threadworker w = new threadworker(threadfactory); allworkers.add(w); return w; } //省略.... }
这就是ioscheduler的createworker()的方法,其实最主要的意思就是获取线程池,以便于生成子线程,让subscribetask()可以运行。然后直接调用 w.schedule(task, delay, unit)方法让它在线程池里执行。上面中那threadworker的源码如下:
static final class threadworker extends newthreadworker { private long expirationtime; threadworker(threadfactory threadfactory) { super(threadfactory); this.expirationtime = 0l; } //省略代码.... } public class newthreadworker extends scheduler.worker implements disposable { private final scheduledexecutorservice executor; public newthreadworker(threadfactory threadfactory) { executor = schedulerpoolfactory.create(threadfactory); } public scheduledrunnable scheduleactual(final runnable run, long delaytime, @nonnull timeunit unit, @nullable disposablecontainer parent) { runnable decoratedrun = rxjavaplugins.onschedule(run); scheduledrunnable sr = new scheduledrunnable(decoratedrun, parent); if (parent != null) { if (!parent.add(sr)) { return sr; } } future<?> f; try { if (delaytime <= 0) { f = executor.submit((callable<object>)sr); } else { f = executor.schedule((callable<object>)sr, delaytime, unit); } sr.setfuture(f); } catch (rejectedexecutionexception ex) { if (parent != null) { parent.remove(sr); } rxjavaplugins.onerror(ex); } return sr; } }
可以看到,这就调了原始的javaapi来进行线程池操作。
然后最后一环在子线程调用source.subscribe(parent)方法,然后回调刚开始创建的observablecreate的subscribeactual(),既:
protected void subscribeactual(observer<? super t> observer) { createemitter<t> parent = new createemitter<t>(observer); observer.onsubscribe(parent); try { source.subscribe(parent); } catch (throwable ex) { exceptions.throwiffatal(ex); parent.onerror(ex); } }
进行消息的订阅绑定。
当我们在调用 emitter.onnext(内容)时,是在io线程里的,那回调的onnext()又是什么时候切换的?那就是前面为了整个流程流畅性没讲的在observeon()里的observeonobserver是执行观察者的线程的过程。
class observeonobserver<t> extends basicintqueuedisposable<t> implements observer<t>, runnable { //省略代码.... observeonobserver(observer<? super t> actual, scheduler.worker worker, boolean delayerror, int buffersize) { this.actual = actual; this.worker = worker; this.delayerror = delayerror; this.buffersize = buffersize; } @override public void onsubscribe(disposable s) { if (disposablehelper.validate(this.s, s)) { this.s = s; if (s instanceof queuedisposable) { @suppresswarnings("unchecked") queuedisposable<t> qd = (queuedisposable<t>) s; int m = qd.requestfusion(queuedisposable.any | queuedisposable.boundary); if (m == queuedisposable.sync) { sourcemode = m; queue = qd; done = true; actual.onsubscribe(this); schedule(); return; } if (m == queuedisposable.async) { sourcemode = m; queue = qd; actual.onsubscribe(this); return; } } queue = new spsclinkedarrayqueue<t>(buffersize); actual.onsubscribe(this); } } @override public void onnext(t t) { if (done) { return; } if (sourcemode != queuedisposable.async) { queue.offer(t); } schedule(); } void schedule() { if (getandincrement() == 0) { worker.schedule(this); } } //省略代码.... }
当调用emitter.onnext(内容)方法,会调用上面的onnext()方法,然后在这个方法里会把数据压入一个队列,然后执行worker.schedule(this)方法,work是什么呢,还记得androidschedulers.mainthread()吗,这个对应这个handlerscheduler这个类,所以createworker()对应着:
private static final class mainholder { static final scheduler default = new handlerscheduler(new handler(looper.getmainlooper())); } public worker createworker() { return new handlerworker(handler); } private static final class handlerworker extends worker { private final handler handler; private volatile boolean disposed; handlerworker(handler handler) { this.handler = handler; } @override public disposable schedule(runnable run, long delay, timeunit unit) { if (run == null) throw new nullpointerexception("run == null"); if (unit == null) throw new nullpointerexception("unit == null"); if (disposed) { return disposables.disposed(); } run = rxjavaplugins.onschedule(run); scheduledrunnable scheduled = new scheduledrunnable(handler, run); message message = message.obtain(handler, scheduled); message.obj = this; // used as token for batch disposal of this worker's runnables. handler.sendmessagedelayed(message, unit.tomillis(delay)); if (disposed) { handler.removecallbacks(scheduled); return disposables.disposed(); } return scheduled; } }
在next()方法里,运用android自带的handler消息机制,通过把方法包裹在message里,同通过handler.sendmessagedelayed()发送消息,就会在ui线程里回调next()方法,从而实现从子线程切换到android主线程的操作。我们在主线程拿到数据就可以进行各种在主线程的操作了。
总结一下:
observablecreate 一> observablesubscribeon 一> observableobserveon为初始化顺序
当调用observable.subscribe(observer)时的执行顺序
observableobserveon 一> observablesubscribeon 一> observablecreate
当发送消息的执行顺序
observablecreate 一> observablesubscribeon 一> observableobserveon
以上就是消息订阅和线程切换的源码的所有讲解了。
为了让你们理解更清楚,我仿照rxjava写了大概的消息订阅和线程切换的最基本代码和基本功能,以帮助你们理解
https://github.com/jack921/rxjava2demo
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。
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