背景

參考
https://blog.csdn.net/m0_71263309/article/details/127353275?spm=1001.2101.3001.6650.1&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7EAD_ESQUERY%7Eyljh-1-127353275-blog-112536673.pc_relevant_landingrelevant&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7EAD_ESQUERY%7Eyljh-1-127353275-blog-112536673.pc_relevant_landingrelevant&utm_relevant_index=2

從 Android 2.3 開始熙揍，Google 把 Handler 的阻塞/喚醒方案從 Object#wait() / notify()阴孟，改成了用 Linux epoll 來(lái)實(shí)現(xiàn)

流程

1漆枚、epoll涉及到3個(gè)重要方法

int epoll_create(int size);
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);

2、在java層Handler的MessageQueue類有3個(gè)本地方法

class MessageQueue {
    private native static long nativeInit();
    private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
    private native static void nativeWake(long ptr);

}

3寺鸥、在Handler的native層也有對(duì)應(yīng)的MessageQueue、Looper、Handler秘血，在native層Looper的構(gòu)造方法里
- 調(diào)用epoll_create()方法創(chuàng)建了mEpollFd(epoll 對(duì)象)
- 調(diào)用epoll_ctl()方法mWakeEventFd（喚醒隊(duì)列作用）注冊(cè)到mEpollFd(epoll池)

/system/core/libutils/Looper.cpp
class looper {

    Looper::Looper() {
        int mWakeEventFd = eventfd();
        rebuildEpollLocked();
    }

    void rebuildEpollLocked(){
        創(chuàng)建了 epoll 對(duì)象
        int mEpollFd = epoll_create();
        mWakeEventFd（喚醒隊(duì)列作用）注冊(cè)到mEpollFd(epoll池)
        epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
    }
}

4、在java層的MessageQueue類調(diào)用本地方法 nativePollOnce()的時(shí)候评甜，最終調(diào)用是native層Looper的pollOnce()方法輪詢
- 如果沒(méi)有消息 epoll_wait() 使線程進(jìn)入到阻塞態(tài)灰粮，讓出 CPU 調(diào)度

/frameworks/base/core/jni/android_os_MessageQueue.cpp
class android_os_MessageQueue {

    //jni方法，轉(zhuǎn)到 NativeMessageQueue#pollOnce()
    void android_os_MessageQueue_nativePollOnce(){
        nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
    }
    class NativeMessageQueue : MessageQueue {

         Looper#pollOnce() 方法
        void pollOnce(){
            mLooper->pollOnce(timeoutMillis);
        }
    }
}

//system/core/libutils/Looper.cpp
class looper {

    int pollOnce(int timeoutMillis){
        int result = 0;
        for (;;) {
            if (result != 0) {
                return result;
            }
            result = pollInner(timeoutMillis);//超時(shí)
        }
    }

    int pollInner(int timeoutMillis){
        int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
    }
}

5忍坷、在java層如果有新的消息插入粘舟，會(huì)調(diào)用的native層的wake()方法

/system/core/libutils/Looper.cpp
class looper {

    void Looper::wake() {
        int inc = 1;
        write(mWakeEventFd, &inc);
    }
}

這個(gè)時(shí)候mWakeEventFd寫入了1，狀態(tài)由不可讀變成可讀佩研，內(nèi)核監(jiān)聽(tīng)到fd讀寫狀態(tài)的改變柑肴，會(huì)觸發(fā) epoll_wait() 方法調(diào)用，epoll_wait() 就會(huì)解除阻塞狀態(tài)旬薯，繼續(xù)執(zhí)行