android分析之Parcel

将数据打包，跨进程传输（通过Binder）。看看这货究竟是啥玩意：

Parcel.java ：

public final class Parcel {
    private static final boolean DEBUG_RECYCLE = false;
    private static final String TAG = "Parcel";

    @SuppressWarnings({"UnusedDeclaration"})
    private int mNativePtr; // used by native code，非static 

    /**
     * Flag indicating if {@link #mNativePtr} was allocated by this object,
     * indicating that we're responsible for its lifecycle.
     */
    private boolean mOwnsNativeParcelObject;//非static，从上面解释可以看到，它标识"mNativePtr”是否可用，如果是从NativeCode分配的，则要负责它的生命周期

    private RuntimeException mStack;

    private static final int POOL_SIZE = 6;
    private static final Parcel[] sOwnedPool = new Parcel[POOL_SIZE];//static，类拥有，下同。作为Parcel的缓冲池使用。
    private static final Parcel[] sHolderPool = new Parcel[POOL_SIZE];

　　下面看调用Parcel的obtain（）时的过程：

    static protected final Parcel obtain() {
        final Parcel[] pool = sHolderPool;//尝试从sHolderPool这个缓冲池取
        synchronized (pool) {
            Parcel p;
            for (int i=0; i<POOL_SIZE; i++) {//POOL_SIZE为6
                p = pool[i];
                if (p != null) {
                    pool[i] = null;
                    if (DEBUG_RECYCLE) {
                        p.mStack = new RuntimeException();
                    }
                    p.init(obj);//找到一个可用的（非null），初始化这个Parcel
                    return p;
                }
            }
        }
        return new Parcel(0);//如果6个都不可用（即缓冲池空了），则新new一个出来
    }

    private Parcel(int nativePtr) {
        if (DEBUG_RECYCLE) {
            mStack = new RuntimeException();
        }
        //Log.i(TAG, "Initializing obj=0x" + Integer.toHexString(obj), mStack);
        init(nativePtr);//简单调用
    }

    private void init(int nativePtr) {
        if (nativePtr != 0) {
            mNativePtr = nativePtr;
            mOwnsNativeParcelObject = false;
        } else {
            mNativePtr = nativeCreate();//调用Native CODE
            mOwnsNativeParcelObject = true;//表明，需要负责本Parcel对象的生命周期。后面有几个方法会根据该boolean值决定是否释放Native CODE生成的对象。
        }
    }

　　小结：Parcel(.java)逻辑很简单，从sHolderPool或者sOwnedPool中找不等于null的，取出来重新使用。否则，调用Native Code重新生成一个Parcel。在这里，有mNativePtr和mOwnsNativeParcelObject两个对象的成员变量，用来标识所生成的Native层的Parcel是否需要释放/销毁。

Parcel.h(.cpp)分析：

在Parcel.h中，存在许多字段。实际上，可以将Parcel看作管理一块内存的一个管理者。

    status_t            mError;
    uint8_t*            mData;//指针，从字面上看应该是指向数据的指针
    size_t              mDataSize;//表明数据大小，已经存储的数据大小
    size_t              mDataCapacity;//应该是内存空间的容量
    mutable size_t      mDataPos;//mutable修饰，说明这个变量要及时反映出最新值，类比数组中position下标

    size_t*             mObjects;//指针，可以看作数组。其存储的是每个保存在Parcel对象所申请的内存的大小
    size_t              mObjectsSize;//与上面数组配合使用
    size_t              mObjectsCapacity;
    mutable size_t      mNextObjectHint;

    mutable bool        mFdsKnown;
    mutable bool        mHasFds;
    bool                mAllowFds;

    release_func        mOwner;
    void*               mOwnerCookie;

　　从上面各个字段来看，还是很经典的内存管理方式，这样一般有：内存起始地址（对应上面mData）、内存总容量（对应mDataCapacity）、内存已用容量（对应mDataSize）、当前可用的内存位置（mDataPos）。在Parcel里还更加细分了，每个存储在内存中的对象大小。粒度更细。

Parcel::Parcel()//构造函数
{
    initState();
}
...
void Parcel::initState()//简单给各个成员变量赋初值
{
    mError = NO_ERROR;
    mData = 0;
    mDataSize = 0;
    mDataCapacity = 0;
    mDataPos = 0;
    ALOGV("initState Setting data size of %p to %d\n", this, mDataSize);
    ALOGV("initState Setting data pos of %p to %d\n", this, mDataPos);
    mObjects = NULL;
    mObjectsSize = 0;
    mObjectsCapacity = 0;
    mNextObjectHint = 0;
    mHasFds = false;
    mFdsKnown = true;
    mAllowFds = true;
    mOwner = NULL;
}

　　在setDataCapacity、setDataSize等函数中，调用到continueWrite，这个函数是真正的申请内存函数：

status_t Parcel::continueWrite(size_t desired)
{
    // If shrinking, first adjust for any objects that appear
    // after the new data size.
    size_t objectsSize = mObjectsSize;
    if (desired < mDataSize) {//表明需要缩小申请的内存容量
        if (desired == 0) {
            objectsSize = 0;
        } else {
            while (objectsSize > 0) {
                if (mObjects[objectsSize-1] < desired)//找到一个对象的内存大小小于所要申请的内存大小
                    break;
                objectsSize--;
            }
        }
    }

    if (mOwner) {//mOwner是一个回调函数指针
        // If the size is going to zero, just release the owner's data.
        if (desired == 0) {
            freeData();//释放数据
            return NO_ERROR;
        }

        // If there is a different owner, we need to take
        // posession.
        uint8_t* data = (uint8_t*)malloc(desired);//分配内存
        if (!data) {
            mError = NO_MEMORY;
            return NO_MEMORY;
        }
        size_t* objects = NULL;

        if (objectsSize) {
            objects = (size_t*)malloc(objectsSize*sizeof(size_t));//分配objectSize*sizeof(size_t)大小的内存
            if (!objects) {
                mError = NO_MEMORY;
                return NO_MEMORY;
            }

            // Little hack to only acquire references on objects
            // we will be keeping.
            size_t oldObjectsSize = mObjectsSize;
            mObjectsSize = objectsSize;
            acquireObjects();//给各个对象增加强、弱引用计数——加入需要的话
            mObjectsSize = oldObjectsSize;
        }

        if (mData) {
            memcpy(data, mData, mDataSize < desired ? mDataSize : desired);//拷贝到data（新申请的）
        }
        if (objects && mObjects) {
            memcpy(objects, mObjects, objectsSize*sizeof(size_t));
        }
        //ALOGI("Freeing data ref of %p (pid=%d)\n", this, getpid());
        mOwner(this, mData, mDataSize, mObjects, mObjectsSize, mOwnerCookie);
        mOwner = NULL;

        mData = data;
        mObjects = objects;
        mDataSize = (mDataSize < desired) ? mDataSize : desired;
        ALOGV("continueWrite Setting data size of %p to %d\n", this, mDataSize);
        mDataCapacity = desired;
        mObjectsSize = mObjectsCapacity = objectsSize;
        mNextObjectHint = 0;

    } else if (mData) {
        if (objectsSize < mObjectsSize) {
            // Need to release refs on any objects we are dropping.
            const sp<ProcessState> proc(ProcessState::self());
            for (size_t i=objectsSize; i<mObjectsSize; i++) {
                const flat_binder_object* flat
                    = reinterpret_cast<flat_binder_object*>(mData+mObjects[i]);
                if (flat->type == BINDER_TYPE_FD) {
                    // will need to rescan because we may have lopped off the only FDs
                    mFdsKnown = false;
                }
                release_object(proc, *flat, this);//尝试释放对象
            }
            size_t* objects =
                (size_t*)realloc(mObjects, objectsSize*sizeof(size_t));
            if (objects) {
                mObjects = objects;
            }
            mObjectsSize = objectsSize;
            mNextObjectHint = 0;
        }

        // We own the data, so we can just do a realloc().
        if (desired > mDataCapacity) {
            uint8_t* data = (uint8_t*)realloc(mData, desired);//在原mData位置上重新分配内存
            if (data) {
                mData = data;
                mDataCapacity = desired;
            } else if (desired > mDataCapacity) {
                mError = NO_MEMORY;
                return NO_MEMORY;
            }
        } else {
            if (mDataSize > desired) {
                mDataSize = desired;
                ALOGV("continueWrite Setting data size of %p to %d\n", this, mDataSize);
            }
            if (mDataPos > desired) {
                mDataPos = desired;
                ALOGV("continueWrite Setting data pos of %p to %d\n", this, mDataPos);
            }
        }

    } else {
        // This is the first data.  Easy!
        uint8_t* data = (uint8_t*)malloc(desired);//直接申请所需大小
        if (!data) {
            mError = NO_MEMORY;
            return NO_MEMORY;
        }

        if(!(mDataCapacity == 0 && mObjects == NULL
             && mObjectsCapacity == 0)) {
            ALOGE("continueWrite: %d/%p/%d/%d", mDataCapacity, mObjects, mObjectsCapacity, desired);
        }

        mData = data;
        mDataSize = mDataPos = 0;
        ALOGV("continueWrite Setting data size of %p to %d\n", this, mDataSize);
        ALOGV("continueWrite Setting data pos of %p to %d\n", this, mDataPos);
        mDataCapacity = desired;
    }

    return NO_ERROR;
}

　　小结：上面内存分配管理比较细致，总的来说就是“要么新申请一块内存”、“要么复用一块内存”，“释放内存”，外加对象的生命周期的控制。