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。
image.png
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();
如何查看当前项目包含几个线程
在Android studio中点击Profile 图标，点击 CPU，显示如下图，点击 Record，然后再点击 Stop，即可生成。
image.png
