android thread使用測試

本文主要是想測試 threadLoop 線程是不是迴圈執行。

1. ThreadTest.cpp

#include <binder/IPCThreadState.h>#include <binder/ProcessState.h>#include <utils/threads.h>namespace android {    class ThreadTest : public Thread {        virtual bool threadLoop();    };    bool ThreadTest::threadLoop() {        printf("threadLoop\n");        sleep(1);        return true;    }}using namespace android;int main() {        sp<ThreadTest> thr = new ThreadTest();    thr->run();    ProcessState::self()->startThreadPool();    IPCThreadState::self()->joinThreadPool();    return 0;}

2. Android.mk

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE_TAGS:= optionalLOCAL_SRC_FILES := \    ThreadTest.cppLOCAL_SHARED_LIBRARIES := \    libutils \    libbinderLOCAL_MODULE := ThreadTestinclude $(BUILD_EXECUTABLE)

3. 執行

可以看到每隔1秒，列印一次threadLoop，所以threadLoop是迴圈執行。

若用指標代替android的sp，即StrongPointer，則只列印一次threadLoop。如下所示：

//sp<ThreadTest> thr = new ThreadTest();
ThreadTest *thr = new ThreadTest();

或者在threadLoop函數中return false，也只列印一次threadLoop。

原因是Thread類的實現中，

先由pthreads或win32_threads實現建立thread，並將線程入口設為_threadLoop。

_threadLoop再調用thread類實現者(ThreadTest)的threadLoop，並將這個threadLoop

放在do-while迴圈中，只有當threadLoop返回false或線程對象指標不是sp實現時，會退出迴圈。

_threadLoop示意代碼：

int Thread::_threadLoop(void* user){    Thread* const self = static_cast<Thread*>(user);    sp<Thread> strong(self->mHoldSelf);    wp<Thread> weak(strong);    self->mHoldSelf.clear();#ifdef HAVE_ANDROID_OS    // this is very useful for debugging with gdb    self->mTid = gettid();#endif    bool first = true;    do {        bool result;        if (first) {            first = false;            self->mStatus = self->readyToRun();            result = (self->mStatus == NO_ERROR);            if (result && !self->exitPending()) {                // Binder threads (and maybe others) rely on threadLoop                // running at least once after a successful ::readyToRun()                // (unless, of course, the thread has already been asked to exit                // at that point).                // This is because threads are essentially used like this:                //   (new ThreadSubclass())->run();                // The caller therefore does not retain a strong reference to                // the thread and the thread would simply disappear after the                // successful ::readyToRun() call instead of entering the                // threadLoop at least once.                result = self->threadLoop();            }        } else {            result = self->threadLoop();        }        // establish a scope for mLock        {        Mutex::Autolock _l(self->mLock);        if (result == false || self->mExitPending) {            self->mExitPending = true;            self->mRunning = false;            // clear thread ID so that requestExitAndWait() does not exit if            // called by a new thread using the same thread ID as this one.            self->mThread = thread_id_t(-1);            // note that interested observers blocked in requestExitAndWait are            // awoken by broadcast, but blocked on mLock until break exits scope            self->mThreadExitedCondition.broadcast();            break;        }        }                // Release our strong reference, to let a chance to the thread        // to die a peaceful death.        strong.clear();        // And immediately, re-acquire a strong reference for the next loop        strong = weak.promote();    } while(strong != 0);        return 0;}

關於sp,wp，可參考：

深入理解android常用類