rxjava2源码解析（三）线程池原理分析

引言

先贴上这个系列的连接。
rxjava2源码解析（一）基本流程分析
rxjava2源码解析（二）线程切换分析
上一篇说了rxjava2的线程切换，可是没有深刻说其中的线程池。这篇咱们来深扒一下。

observeOn

仍是先说observeOn,直接看源码：

public ObservableObserveOn(ObservableSource<T> source, Scheduler scheduler, boolean delayError, int bufferSize) {
        super(source);
        this.scheduler = scheduler;
        this.delayError = delayError;
        this.bufferSize = bufferSize;
    }

    @Override
    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));
        }
    }
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这段代码咱们上篇看到过，这里再重复一下。obsererOn是切换下游观察者线程，咱们看ObserveOnObserver中的onNext方法是如何切换线程的。

static final class ObserveOnObserver<T> extends BasicIntQueueDisposable<T>
    implements Observer<T>, Runnable {
        ObserveOnObserver(Observer<? super T> actual, Scheduler.Worker worker, boolean delayError, int bufferSize) {
            this.downstream = actual;
            this.worker = worker;
            this.delayError = delayError;
            this.bufferSize = bufferSize;
        }
        
        @Override
        public void onNext(T t) {
            if (done) {
                return;
            }

            if (sourceMode != QueueDisposable.ASYNC) {
                queue.offer(t);
            }
            schedule();
        }
        
        void schedule() {
            if (getAndIncrement() == 0) {
                //ObserveOnObserver继承了runnable接口，意味着能够当作是线程任务来执行。这里表明着在新线程中执行run方法。
                worker.schedule(this);
            }
        }
        
        //ObserveOnObserver继承了runnable接口
        @Override
        public void run() {
            if (outputFused) {
                drainFused();
            } else {
                drainNormal();
            }
        }
        
        void drainNormal() {
            int missed = 1;
            final SimpleQueue<T> q = queue;
            final Observer<? super T> a = downstream;
            for (;;) {
                if (checkTerminated(done, q.isEmpty(), a)) {
                    return;
                }
                for (;;) {
                    ····//省略一些判断的代码
                    v = q.poll();
                    //这里就能够看到，将下游的onNext方法，切换到新线程执行。
                    a.onNext(v);
                }
                ···
            }
        }
        
    }
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这是上游的处理器执行onNext,传到这里，使用以前设置的线程执行下游的onNext方法。

Worker

这个worker究竟是什么？咱们先看scheduler的createWorker方法：

public abstract Worker createWorker();
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在Scheduler类中，createWorker只是一个接口，子类会重写这个方法，咱们就以Schedulers.newThread()这个方法建立的Scheduler为例，来看看这里面的原理。

//Schedulers类中的newThread静态方法，这里的hock咱们暂且不理，直接返回NEW_THREAD
    public static Scheduler newThread() {
        return RxJavaPlugins.onNewThreadScheduler(NEW_THREAD);
    }
    
    //Schedulers类中定义了NEW_THREAD和其余THREAD
        static {
        SINGLE = RxJavaPlugins.initSingleScheduler(new SingleTask());

        COMPUTATION = RxJavaPlugins.initComputationScheduler(new ComputationTask());

        IO = RxJavaPlugins.initIoScheduler(new IOTask());

        TRAMPOLINE = TrampolineScheduler.instance();

        NEW_THREAD = RxJavaPlugins.initNewThreadScheduler(new NewThreadTask());
    }
    
    //NewThreadTask是Schedulers的静态内部类，继承自Callable接口，其中call方法返回一个Scheduler
    static final class NewThreadTask implements Callable<Scheduler> {
        @Override
        public Scheduler call() throws Exception {
            return NewThreadHolder.DEFAULT;
        }
    }
    
    //NewThreadHolder一样是一个静态内部类，里面只有一个静态参数DEFAULT，这里咱们就找到了newThread方法返回的本尊NewThreadScheduler
    static final class NewThreadHolder {
        static final Scheduler DEFAULT = new NewThreadScheduler();
    }
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如上面代码和注释所示，咱们直接看NewThreadScheduler的源码：

/**
 * Schedules work on a new thread.
 */
public final class NewThreadScheduler extends Scheduler {

    final ThreadFactory threadFactory;

    private static final String THREAD_NAME_PREFIX = "RxNewThreadScheduler";
    private static final RxThreadFactory THREAD_FACTORY;

    /** The name of the system property for setting the thread priority for this Scheduler. */
    private static final String KEY_NEWTHREAD_PRIORITY = "rx2.newthread-priority";

    static {
        //这里Thread.MIN_PRIORITY为1，Thread.MAX_PRIORITY为10.Thread.NORM_PRIORITY为5.若是咱们不作任何更改，这里的priority的值就为5.
        int priority = Math.max(Thread.MIN_PRIORITY, Math.min(Thread.MAX_PRIORITY,
                Integer.getInteger(KEY_NEWTHREAD_PRIORITY, Thread.NORM_PRIORITY)));

        THREAD_FACTORY = new RxThreadFactory(THREAD_NAME_PREFIX, priority);
    }

    public NewThreadScheduler() {
        this(THREAD_FACTORY);
    }

    public NewThreadScheduler(ThreadFactory threadFactory) {
        this.threadFactory = threadFactory;
    }

    @NonNull
    @Override
    public Worker createWorker() {
        return new NewThreadWorker(threadFactory);
    }
}
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这里createWorker方法返回的是一个NewThreadWorker对象。咱们总算找到了worker的来源，须要注意这里的构造参数是threadFactory。来看看NewThreadWorker的源码。

public class NewThreadWorker extends Scheduler.Worker implements Disposable {
    private final ScheduledExecutorService executor;

    volatile boolean disposed;

    public NewThreadWorker(ThreadFactory threadFactory) {
        executor = SchedulerPoolFactory.create(threadFactory);
    }

    @NonNull
    @Override
    public Disposable schedule(@NonNull final Runnable run) {
        return schedule(run, 0, null);
    }

    @NonNull
    @Override
    public Disposable schedule(@NonNull final Runnable action, long delayTime, @NonNull TimeUnit unit) {
        if (disposed) {
            return EmptyDisposable.INSTANCE;
        }
        return scheduleActual(action, delayTime, unit, null);
    }
    
    @NonNull
    public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, @NonNull TimeUnit unit, @Nullable DisposableContainer parent) {
        //hock机制
        Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
        //用一个ScheduledRunnable把传入的runnable包装一下，本质上没区别。
        ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, parent);

        ....//省略判断性代码
        Future<?> f;
        try {
            if (delayTime <= 0) {
                //executor由构造方法中建立
                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;
    }
    ....
}
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这里咱们就能够看到，前面调用worker.schedule(this),最终走到了executor.submit(sr)。这里的sr只是前面ObserveOnObserver的包装。executor在构造方法中建立。来看看executor是什么：

//SchedulerPoolFactory类中的静态方法
    public static ScheduledExecutorService create(ThreadFactory factory) {
        final ScheduledExecutorService exec = Executors.newScheduledThreadPool(1, factory);
        tryPutIntoPool(PURGE_ENABLED, exec);
        return exec;
    }
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//Executors类的静态方法
    public static ScheduledExecutorService newScheduledThreadPool(
            int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }
    
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OK，executor是一个ScheduledThreadPoolExecutor，标准的工做线程池。核心线程数为1，threadFactory是前面NewThreadWorker构造参数中的RxThreadFactory。他会给thread按照命名格式进行命名。

public final class RxThreadFactory extends AtomicLong implements ThreadFactory {

    public RxThreadFactory(String prefix) {
        this(prefix, Thread.NORM_PRIORITY, false);
    }

    public RxThreadFactory(String prefix, int priority) {
        this(prefix, priority, false);
    }

    public RxThreadFactory(String prefix, int priority, boolean nonBlocking) {
        this.prefix = prefix;
        this.priority = priority;
        this.nonBlocking = nonBlocking;
    }

    @Override
    public Thread newThread(Runnable r) {
        StringBuilder nameBuilder = new StringBuilder(prefix).append('-').append(incrementAndGet());
        String name = nameBuilder.toString();
        Thread t = nonBlocking ? new RxCustomThread(r, name) : new Thread(r, name);
        t.setPriority(priority);
        t.setDaemon(true);
        return t;
    }
    
    ...
}
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总结一下：

• observeOn在subscribe方法中，新建一个worker对象。这个worker对象是根据设置的scheduler建立的。而后在新建一个ObserveOnObserver对象，将上游与之订阅。
• 在ObserveOnObserver的onNext方法中，会调用worker.schedule(this)，将自己做为runnable传入到worker中。
• 以newThreadScheduler为例，他建立的worker是一个NewThreadWorker实例。在实例构造方法中，会根据传入的threadFactory新建一个ScheduledThreadPool线程池。
• NewThreadWorker的shedule方法，就是将ObserveOnObserver做为一个runnable放在一个新的线程池中执行。
• ObserveOnObserver的run方法，就是用来执行下游的onNext，将数据传输下去。从而达到了，切换下游onNext线程的目的。

subscribeOn

subscribeOn是用来切换上游发射器线程。切换原理上一篇有说过，其中线程池相关跟上面observeOn差很少，这里就不赘述了。

总结

上面就是rxjava2线程切换原理分析了，后面再有人面试问你rxjava2里面的线程池是哪种，你就能够自信的说出：ScheduledThreadPool。