Android一个APP里面最少有几个线程

参考

https://www.jianshu.com/p/92bff8d6282f
https://www.jianshu.com/p/8a820d93c6aa

线程查看

Android一个进程里面最少包含5个线程，分别为：

main线程(主线程）
FinalizerDaemon线程
终结者守护线程。对于重写了成员函数finalize的对象，它们被GC决定回收时，并没有马上被回收，而是被放入到一个队列中，等待FinalizerDaemon守护线程去调用它们的成员函数finalize，然后再被回收。
FinalizerWatchdogDaemon线程
监控终结者守护线程。用来监控FinalizerDaemon线程的执行。一旦检测那些重定了成员函数finalize的对象在执行成员函数finalize时超出一定的时候，那么就会退出VM。
HeapTaskDaemon线程
堆栈守护线程。用来执行堆栈的操作，也就是用来将那些空闲的堆内存归还给系统。
ReferenceQueueDaemon线程。
引用队列守护线程。我们知道，在创建引用对象的时候，可以关联一个队列。当被引用对象引用的对象被GC回收的时候，被引用对象就会被加入到其创建时关联的队列去。这个加入队列的操作就是由ReferenceQueueDaemon守护线程来完成的。这样应用程序就可以知道哪些被引用对象引用的对象已经被回收了。

下图是创建的一个仅有hello World！页面的工程，线程包含以下的这些。

在这里插入图片描述

刚开始我比较疑惑的是FileObserver 这个线程是否也是每个进程所必须包含的线程。后来我查看了一下Daemons创建的过程，能确定的是Android启动一个APP最少包含ReferenceQueueDaemon线程、FinalizerDaemon线程、FinalizerWatchdogDaemon线程、HeapTaskDaemon线程，以及在ActivityThread中开启的主线程。如下：

public final class Daemons {
    private static final int NANOS_PER_MILLI = 1000 * 1000;
    private static final int NANOS_PER_SECOND = NANOS_PER_MILLI * 1000;
    private static final long MAX_FINALIZE_NANOS = 10L * NANOS_PER_SECOND;

    public static void start() {
        ReferenceQueueDaemon.INSTANCE.start();//开启ReferenceQueueDaemon线程
        FinalizerDaemon.INSTANCE.start();//开启FinalizerDaemon线程
        FinalizerWatchdogDaemon.INSTANCE.start();//开启FinalizerWatchdogDaemon线程
        HeapTaskDaemon.INSTANCE.start();//开启HeapTaskDaemon线程
    }

    public static void startPostZygoteFork() {
        ReferenceQueueDaemon.INSTANCE.startPostZygoteFork();
        FinalizerDaemon.INSTANCE.startPostZygoteFork();
        FinalizerWatchdogDaemon.INSTANCE.startPostZygoteFork();
        HeapTaskDaemon.INSTANCE.startPostZygoteFork();
    }

    public static void stop() {
        HeapTaskDaemon.INSTANCE.stop();
        ReferenceQueueDaemon.INSTANCE.stop();
        FinalizerDaemon.INSTANCE.stop();
        FinalizerWatchdogDaemon.INSTANCE.stop();
    }
    ...
}

1.main线程

2. ReferenceQueueDaemon线程。

代码块

3. FinalizerDaemon线程

    private static class FinalizerDaemon extends Daemon {
        private static final FinalizerDaemon INSTANCE = new FinalizerDaemon();
        private final ReferenceQueue<Object> queue = FinalizerReference.queue;
        private final AtomicInteger progressCounter = new AtomicInteger(0);
        // Object (not reference!) being finalized. Accesses may race!
        private Object finalizingObject = null;

        FinalizerDaemon() {
            super("FinalizerDaemon");
        }

        @Override public void runInternal() {
            // This loop may be performance critical, since we need to keep up with mutator
            // generation of finalizable objects.
            // We minimize the amount of work we do per finalizable object. For example, we avoid
            // reading the current time here, since that involves a kernel call per object.  We
            // limit fast path communication with FinalizerWatchDogDaemon to what's unavoidable: A
            // non-volatile store to communicate the current finalizable object, e.g. for
            // reporting, and a release store (lazySet) to a counter.
            // We do stop the  FinalizerWatchDogDaemon if we have nothing to do for a
            // potentially extended period.  This prevents the device from waking up regularly
            // during idle times.

            // Local copy of progressCounter; saves a fence per increment on ARM and MIPS.
            int localProgressCounter = progressCounter.get();

            while (isRunning()) {
                try {
                    // Use non-blocking poll to avoid FinalizerWatchdogDaemon communication
                    // when busy.
                    FinalizerReference<?> finalizingReference = (FinalizerReference<?>)queue.poll();
                    if (finalizingReference != null) {
                        finalizingObject = finalizingReference.get();
                        progressCounter.lazySet(++localProgressCounter);
                    } else {
                        finalizingObject = null;
                        progressCounter.lazySet(++localProgressCounter);
                        // Slow path; block.
                        FinalizerWatchdogDaemon.INSTANCE.goToSleep();
                        finalizingReference = (FinalizerReference<?>)queue.remove();
                        finalizingObject = finalizingReference.get();
                        progressCounter.set(++localProgressCounter);
                        FinalizerWatchdogDaemon.INSTANCE.wakeUp();
                    }
                    doFinalize(finalizingReference);
                } catch (InterruptedException ignored) {
                } catch (OutOfMemoryError ignored) {
                }
            }
        }

        @FindBugsSuppressWarnings("FI_EXPLICIT_INVOCATION")
        private void doFinalize(FinalizerReference<?> reference) {
            FinalizerReference.remove(reference);
            Object object = reference.get();
            reference.clear();
            try {
                object.finalize();
            } catch (Throwable ex) {
                // The RI silently swallows these, but Android has always logged.
                System.logE("Uncaught exception thrown by finalizer", ex);
            } finally {
                // Done finalizing, stop holding the object as live.
                finalizingObject = null;
            }
        }
    }

4. FinalizerWatchdogDaemon线程

代码块

5. HeapTaskDaemon线程

private static class HeapTaskDaemon extends Daemon {
        private static final HeapTaskDaemon INSTANCE = new HeapTaskDaemon();

        HeapTaskDaemon() {
            super("HeapTaskDaemon");
        }

        // Overrides the Daemon.interupt method which is called from Daemons.stop.
        public synchronized void interrupt(Thread thread) {
            VMRuntime.getRuntime().stopHeapTaskProcessor();
        }

        @Override public void runInternal() {
            synchronized (this) {
                if (isRunning()) {
                  // Needs to be synchronized or else we there is a race condition where we start
                  // the thread, call stopHeapTaskProcessor before we start the heap task
                  // processor, resulting in a deadlock since startHeapTaskProcessor restarts it
                  // while the other thread is waiting in Daemons.stop().
                  VMRuntime.getRuntime().startHeapTaskProcessor();
                }
            }
            // This runs tasks until we are stopped and there is no more pending task.
            VMRuntime.getRuntime().runHeapTasks();
        }
    }

查看VMRuntime的源码发现 startHeapTaskProcessor()、runHeapTasks()均是native方法。

    public native void requestConcurrentGC();
    public native void concurrentGC();
    public native void requestHeapTrim();
    public native void trimHeap();
    public native void startHeapTaskProcessor();
    public native void stopHeapTaskProcessor();
    public native void runHeapTasks();