【Android 异步操作】Handler 机制 ( MessageQueue 消息队列的阻塞机制 | Java 层机制 | native 层阻塞机制 | native 层解除阻塞机制 )

韩曙亮

发布于 2023-03-28 18:49:28

1.1K0

发布于 2023-03-28 18:49:28

文章目录

一、MessageQueue 的 Java 层机制

之前在【Android 异步操作】手写 Handler ( 消息队列 MessageQueue | 消息保存到链表 | 从链表中获取消息 ) 中 , 模仿 Android 的 MessageQueue 手写的 MessageQueue , 使用了如下同步机制 ,

从消息队列 MessageQueue 中取出消息 Message ,

如果当前链表为空 , 此时会调用 wait 方法阻塞 , 直到消息入队时 , 链表中有了元素 , 会调用 notify 解除该阻塞 ;

在实际的 Android 中的消息队列 MessageQueue 的同步机制是在 native 层实现的 ;

在创建消息队列 MessageQueue 时 , 调用了 nativeInit() 方法 , 销毁 MessageQueue 时调用 nativeDestroy 方法 ;

如果调用 next 获取下一个消息时 , 如果当前消息队列 MessageQueue 中没有消息 , 此时需要阻塞 , 调用 nativePollOnce 即可实现在 native 阻塞线程 ;

    // 初始化 MessageQueue 时调用的方法 
    private native static long nativeInit();
    // 销毁 MessageQueue 时调用的方法 
    private native static void nativeDestroy(long ptr);
    // 线程阻塞方法
    @UnsupportedAppUsage
    private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
    // 线程唤醒方法 
    private native static void nativeWake(long ptr);
    private native static boolean nativeIsPolling(long ptr);
    private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);

	// 此处初始化 MessageQueue , 调用了 nativeInit 方法 
    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
    }
    
	// 获取消息队列中的下一个消息 
    @UnsupportedAppUsage
    Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
			// 此处阻塞线程 
            nativePollOnce(ptr, nextPollTimeoutMillis);
        }
    }

二、MessageQueue 的 native 层阻塞机制

线程阻塞方法 private native void nativePollOnce(long ptr, int timeoutMillis) , 是 native 方法 , 该方法在 frameworks/base/core/jni/android_os_MessageQueue.cpp 中实现 ,

从 Java 层传入 long 类型 , 然后转为 NativeMessageQueue* 类型指针 ,

该 Java 层传入的 long 类型是初始化消息队列时 , 由 nativeInit 方法返回 , 是消息队列在 Native 层的指针 ,

之后 NativeMessageQueue 指针调用了其本身的 pollOnce 函数 , 该函数中 , 主要调用了 Looper 的 pollOnce 函数 , mLooper->pollOnce(timeoutMillis) ;

frameworks/base/core/jni/android_os_MessageQueue.cpp 中阻塞相关源码 :

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    mPollEnv = env;
    mPollObj = pollObj;
    // 这是主要操作 
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

    if (mExceptionObj) {
        env->Throw(mExceptionObj);
        env->DeleteLocalRef(mExceptionObj);
        mExceptionObj = NULL;
    }
}

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jlong ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

参考 : frameworks/base/core/jni/android_os_MessageQueue.cpp

继续查看 Native 层 Looper.cpp 的 pollOnce 方法 ,

Looper.cpp 源码路径是 system/core/libutils/Looper.cpp ,

在该方法中 , 最终调用 Looper.cpp 的 pollInner 方法 ,

在 pollInner 方法中 , 调用了 epoll_wait 方法 , 该方法就是等待方法 , 在该方法中会监听 mEpollFd 文件句柄 ,

       #include <sys/epoll.h>

       int epoll_wait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout);
       int epoll_pwait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout,
                      const sigset_t *sigmask);

参考 : epoll_wait

system/core/libutils/Looper.cpp 中阻塞相关源码 :

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
        while (mResponseIndex < mResponses.size()) {
            const Response& response = mResponses.itemAt(mResponseIndex++);
            int ident = response.request.ident;
            if (ident >= 0) {
                int fd = response.request.fd;
                int events = response.events;
                void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE
                ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
                        "fd=%d, events=0x%x, data=%p",
                        this, ident, fd, events, data);
#endif
                if (outFd != NULL) *outFd = fd;
                if (outEvents != NULL) *outEvents = events;
                if (outData != NULL) *outData = data;
                return ident;
            }
        }

        if (result != 0) {
#if DEBUG_POLL_AND_WAKE
            ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
            if (outFd != NULL) *outFd = 0;
            if (outEvents != NULL) *outEvents = 0;
            if (outData != NULL) *outData = NULL;
            return result;
        }

        result = pollInner(timeoutMillis);
    }
}

int Looper::pollInner(int timeoutMillis) {
	// ... 
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
}

参考 : system/core/libutils/Looper.cpp

三、MessageQueue 的 native 层解除阻塞机制

在 MessageQueue 消息队列的 Java 层 , 将 Message 消息插入到链表表头后 , 调用了 nativeWake 方法 , 唤醒了线程 , 即解除了阻塞 ;

public final class MessageQueue {
    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }
}

在 native 层的 frameworks/base/core/jni/android_os_MessageQueue.cpp 实现了上述

Java 层定义的 private native static void nativeWake(long ptr) 方法 ,

static const JNINativeMethod gMessageQueueMethods[] = {
    /* name, signature, funcPtr */
    { "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
    { "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
    { "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
    { "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
    { "nativeIsPolling", "(J)Z", (void*)android_os_MessageQueue_nativeIsPolling },
    { "nativeSetFileDescriptorEvents", "(JII)V",
            (void*)android_os_MessageQueue_nativeSetFileDescriptorEvents },
};

参考 : frameworks/base/core/jni/android_os_MessageQueue.cpp

下面是 frameworks/base/core/jni/android_os_MessageQueue.cpp 中的相关方法实现 ,

在 android_os_MessageQueue_nativeWake 方法中调用了本身的 wake 方法 ,

在 wake 方法中调用了 system/core/libutils/Looper.cpp 中的 wake 方法 ;

void NativeMessageQueue::wake() {
	// 此处调用了 Looper 的 wake 函数 
    mLooper->wake();
}

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->wake();
}

参考 : frameworks/base/core/jni/android_os_MessageQueue.cpp

查看 system/core/libutils/Looper.cpp 中的 wake 方法 , 在该方法中 ,

ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t))) 代码说明 ,

向 mWakeEventFd 文件句柄写入了数据 ;

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ wake", this);
#endif

    uint64_t inc = 1;
    ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
    if (nWrite != sizeof(uint64_t)) {
        if (errno != EAGAIN) {
            LOG_ALWAYS_FATAL("Could not write wake signal to fd %d: %s",
                    mWakeEventFd, strerror(errno));
        }
    }
}

参考 : system/core/libutils/Looper.cpp

阻塞的时候使用的是 mEpollFd 文件句柄 ,

唤醒的时候使用的是 mWakeEventFd 文件句柄 ,

下面分析这两个文件句柄之间的联系 ;

Looper 构造函数 , 调用了 rebuildEpollLocked() 方法 ,

在 rebuildEpollLocked 方法中调用 mEpollFd = epoll_create(EPOLL_SIZE_HINT) , 创建了一个句柄 ,

调用 int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem) 注册事件监听 ,

监听的事件是 eventItem.events = EPOLLIN 事件 ,

该事件代表 , 向 mWakeEventFd 文件句柄写入数据 , 此时就对应解除 epoll_wait 阻塞 ;

system/core/libutils/Looper.cpp 中 Looper 构造函数 , rebuildEpollLocked 方法 :

Looper::Looper(bool allowNonCallbacks) :
        mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
        mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
        mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
    mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
    LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
                        strerror(errno));

    AutoMutex _l(mLock);
    rebuildEpollLocked();
}


void Looper::rebuildEpollLocked() {
    // Close old epoll instance if we have one.
    if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
        ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
        close(mEpollFd);
    }

    // Allocate the new epoll instance and register the wake pipe.
    // 创建句柄 
    mEpollFd = epoll_create(EPOLL_SIZE_HINT);
    LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(errno));

    struct epoll_event eventItem;
    memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
    // 注册监听的事件
    eventItem.events = EPOLLIN;
    eventItem.data.fd = mWakeEventFd;
    // 注册事件监听 
    int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
                        strerror(errno));

    for (size_t i = 0; i < mRequests.size(); i++) {
        const Request& request = mRequests.valueAt(i);
        struct epoll_event eventItem;
        request.initEventItem(&eventItem);

        int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
        if (epollResult < 0) {
            ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",
                  request.fd, strerror(errno));
        }
    }
}

MessageQueue 消息队列是通过 Linux 的 epoll 机制实现的阻塞 ;

三、MessageQueue 的 native 层 JNI 方法动态注册

JNI 动态注册 , 消息队列 MessageQueue 中的注册方法 , 使用的是动态注册 ,

static const JNINativeMethod gMessageQueueMethods[] = {
    /* name, signature, funcPtr */
    { "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
    { "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
    { "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
    { "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
    { "nativeIsPolling", "(J)Z", (void*)android_os_MessageQueue_nativeIsPolling },
    { "nativeSetFileDescriptorEvents", "(JII)V",
            (void*)android_os_MessageQueue_nativeSetFileDescriptorEvents },
};

// 动态注册 JNI 函数 
int register_android_os_MessageQueue(JNIEnv* env) {
    int res = RegisterMethodsOrDie(env, "android/os/MessageQueue", gMessageQueueMethods,
                                   NELEM(gMessageQueueMethods));

    jclass clazz = FindClassOrDie(env, "android/os/MessageQueue");
    gMessageQueueClassInfo.mPtr = GetFieldIDOrDie(env, clazz, "mPtr", "J");
    gMessageQueueClassInfo.dispatchEvents = GetMethodIDOrDie(env, clazz,
            "dispatchEvents", "(II)I");

    return res;
}

详情参考 : 【Android NDK 开发】JNI 动态注册 ( 动态注册流程 | JNI_OnLoad 方法 | JNINativeMethod 结构体 | GetEnv | RegisterNatives )

三、MessageQueue 的 native 层完整代码 android_os_MessageQueue.cpp

完整的 android_os_MessageQueue.cpp 代码 :

/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#define LOG_TAG "MessageQueue-JNI"

#include <nativehelper/JNIHelp.h>
#include <android_runtime/AndroidRuntime.h>

#include <utils/Looper.h>
#include <utils/Log.h>
#include "android_os_MessageQueue.h"

#include "core_jni_helpers.h"

namespace android {

static struct {
    jfieldID mPtr;   // native object attached to the DVM MessageQueue
    jmethodID dispatchEvents;
} gMessageQueueClassInfo;

// Must be kept in sync with the constants in Looper.FileDescriptorCallback
static const int CALLBACK_EVENT_INPUT = 1 << 0;
static const int CALLBACK_EVENT_OUTPUT = 1 << 1;
static const int CALLBACK_EVENT_ERROR = 1 << 2;


class NativeMessageQueue : public MessageQueue, public LooperCallback {
public:
    NativeMessageQueue();
    virtual ~NativeMessageQueue();

    virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj);

    void pollOnce(JNIEnv* env, jobject obj, int timeoutMillis);
    void wake();
    void setFileDescriptorEvents(int fd, int events);

    virtual int handleEvent(int fd, int events, void* data);

private:
    JNIEnv* mPollEnv;
    jobject mPollObj;
    jthrowable mExceptionObj;
};


MessageQueue::MessageQueue() {
}

MessageQueue::~MessageQueue() {
}

bool MessageQueue::raiseAndClearException(JNIEnv* env, const char* msg) {
    if (env->ExceptionCheck()) {
        jthrowable exceptionObj = env->ExceptionOccurred();
        env->ExceptionClear();
        raiseException(env, msg, exceptionObj);
        env->DeleteLocalRef(exceptionObj);
        return true;
    }
    return false;
}

NativeMessageQueue::NativeMessageQueue() :
        mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
    mLooper = Looper::getForThread();
    if (mLooper == NULL) {
        mLooper = new Looper(false);
        Looper::setForThread(mLooper);
    }
}

NativeMessageQueue::~NativeMessageQueue() {
}

void NativeMessageQueue::raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj) {
    if (exceptionObj) {
        if (mPollEnv == env) {
            if (mExceptionObj) {
                env->DeleteLocalRef(mExceptionObj);
            }
            mExceptionObj = jthrowable(env->NewLocalRef(exceptionObj));
            ALOGE("Exception in MessageQueue callback: %s", msg);
            jniLogException(env, ANDROID_LOG_ERROR, LOG_TAG, exceptionObj);
        } else {
            ALOGE("Exception: %s", msg);
            jniLogException(env, ANDROID_LOG_ERROR, LOG_TAG, exceptionObj);
            LOG_ALWAYS_FATAL("raiseException() was called when not in a callback, exiting.");
        }
    }
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    mPollEnv = env;
    mPollObj = pollObj;
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

    if (mExceptionObj) {
        env->Throw(mExceptionObj);
        env->DeleteLocalRef(mExceptionObj);
        mExceptionObj = NULL;
    }
}

void NativeMessageQueue::wake() {
    mLooper->wake();
}

void NativeMessageQueue::setFileDescriptorEvents(int fd, int events) {
    if (events) {
        int looperEvents = 0;
        if (events & CALLBACK_EVENT_INPUT) {
            looperEvents |= Looper::EVENT_INPUT;
        }
        if (events & CALLBACK_EVENT_OUTPUT) {
            looperEvents |= Looper::EVENT_OUTPUT;
        }
        mLooper->addFd(fd, Looper::POLL_CALLBACK, looperEvents, this,
                reinterpret_cast<void*>(events));
    } else {
        mLooper->removeFd(fd);
    }
}

int NativeMessageQueue::handleEvent(int fd, int looperEvents, void* data) {
    int events = 0;
    if (looperEvents & Looper::EVENT_INPUT) {
        events |= CALLBACK_EVENT_INPUT;
    }
    if (looperEvents & Looper::EVENT_OUTPUT) {
        events |= CALLBACK_EVENT_OUTPUT;
    }
    if (looperEvents & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP | Looper::EVENT_INVALID)) {
        events |= CALLBACK_EVENT_ERROR;
    }
    int oldWatchedEvents = reinterpret_cast<intptr_t>(data);
    int newWatchedEvents = mPollEnv->CallIntMethod(mPollObj,
            gMessageQueueClassInfo.dispatchEvents, fd, events);
    if (!newWatchedEvents) {
        return 0; // unregister the fd
    }
    if (newWatchedEvents != oldWatchedEvents) {
        setFileDescriptorEvents(fd, newWatchedEvents);
    }
    return 1;
}


// ----------------------------------------------------------------------------

sp<MessageQueue> android_os_MessageQueue_getMessageQueue(JNIEnv* env, jobject messageQueueObj) {
    jlong ptr = env->GetLongField(messageQueueObj, gMessageQueueClassInfo.mPtr);
    return reinterpret_cast<NativeMessageQueue*>(ptr);
}

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
    if (!nativeMessageQueue) {
        jniThrowRuntimeException(env, "Unable to allocate native queue");
        return 0;
    }

    nativeMessageQueue->incStrong(env);
    return reinterpret_cast<jlong>(nativeMessageQueue);
}

static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->decStrong(env);
}

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jlong ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->wake();
}

static jboolean android_os_MessageQueue_nativeIsPolling(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    return nativeMessageQueue->getLooper()->isPolling();
}

static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz,
        jlong ptr, jint fd, jint events) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->setFileDescriptorEvents(fd, events);
}

// ----------------------------------------------------------------------------

// 动态注册 JNI 函数的结构体 
// 每个结构体中的元素是 Java 方法名称 , 方法签名 , C++ 中的方法指针 
static const JNINativeMethod gMessageQueueMethods[] = {
    /* name, signature, funcPtr */
    { "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
    { "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
    { "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
    { "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
    { "nativeIsPolling", "(J)Z", (void*)android_os_MessageQueue_nativeIsPolling },
    { "nativeSetFileDescriptorEvents", "(JII)V",
            (void*)android_os_MessageQueue_nativeSetFileDescriptorEvents },
};

// 动态注册 JNI 函数 
int register_android_os_MessageQueue(JNIEnv* env) {
    int res = RegisterMethodsOrDie(env, "android/os/MessageQueue", gMessageQueueMethods,
                                   NELEM(gMessageQueueMethods));

    jclass clazz = FindClassOrDie(env, "android/os/MessageQueue");
    gMessageQueueClassInfo.mPtr = GetFieldIDOrDie(env, clazz, "mPtr", "J");
    gMessageQueueClassInfo.dispatchEvents = GetMethodIDOrDie(env, clazz,
            "dispatchEvents", "(II)I");

    return res;
}

} // namespace android

参考 : frameworks/base/core/jni/android_os_MessageQueue.cpp

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原始发表：2020-11-05，如有侵权请联系 cloudcommunity@tencent.com 删除

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