[008]Cursor引发的一系列思考

王小二

发布于 2020-06-08 10:45:32

1.2K0

发布于 2020-06-08 10:45:32

前言

在[007]一次Binder通信最大可以传输多大的数据？这个文章，我得到了一个结论，就是正常情况下一次Binder通信最大可以传输的数据的大小是1MB-8KB。突然想到我们在通过ContentResolver对象调用ContentProvider的调用query返回Cursor的时候，本质上这是一次Binder通信，那这个Cursor对象大小有没有限制呢？是不是也要小于1MB-8KB？

Cursor的实现原理

rameworks/base/libs/androidfw/CursorWindow.cpp

status_t CursorWindow::create(const String8& name, size_t size, CursorWindow** outCursorWindow) {
    String8 ashmemName("CursorWindow: ");
    ashmemName.append(name);

    status_t result;
    //调用ashmem_create_region创建一个共享内存
    int ashmemFd = ashmem_create_region(ashmemName.string(), size);
    if (ashmemFd < 0) {
        result = -errno;
    } else {
        //设置匿名共享内存可读可写
        result = ashmem_set_prot_region(ashmemFd, PROT_READ | PROT_WRITE);
        if (result >= 0) {
            //调用mmap进行内存映射
            void* data = ::mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, ashmemFd, 0);
            if (data == MAP_FAILED) {
                result = -errno;
            } else {
                result = ashmem_set_prot_region(ashmemFd, PROT_READ);
                if (result >= 0) {
                    CursorWindow* window = new CursorWindow(name, ashmemFd,
                            data, size, false /*readOnly*/);
                  //省略部分代码
    return result;
}

//序列化只传递两个数据1.mName 2. mAshmemFd
status_t CursorWindow::writeToParcel(Parcel* parcel) {
    status_t status = parcel->writeString8(mName);//一个名字
    if (!status) {
        status = parcel->writeDupFileDescriptor(mAshmemFd);//匿名共享内存的文件描述FD
    }
    return status;
}

从代码来看Cursor的真实实现应该是CursorWindow.cpp，CursorWindow的Data数据真实实现是匿名共享内存，在序列化到Parcel的时候，只需要传递String和匿名共享内存的FD就好了。

这样子看来Cursor的大小是不受限制的，不懂匿名共享内存的可以先看一下[006]匿名共享内存（Ashmem）的使用

其实在Android Framework中对此有一定的限制，请注意在CursorWindow::create会传递一个size的参数。

frameworks/base/core/java/android/database/CursorWindow.java

    public CursorWindow(String name) {
        this(name, getCursorWindowSize());
    }

    public CursorWindow(String name, @BytesLong long windowSizeBytes) {
        mStartPos = 0;
        mName = name != null && name.length() != 0 ? name : "<unnamed>";
        //windowSizeBytes默认是getCursorWindowSize的返回值就是2048*1024
        mWindowPtr = nativeCreate(mName, (int) windowSizeBytes);
        if (mWindowPtr == 0) {
            throw new CursorWindowAllocationException("Cursor window allocation of " +
                    windowSizeBytes + " bytes failed. " + printStats());
        }
        mCloseGuard.open("close");
        recordNewWindow(Binder.getCallingPid(), mWindowPtr);
    }

    private static int getCursorWindowSize() {
        if (sCursorWindowSize < 0) {
            //<integer name="config_cursorWindowSize">2048</integer>
            sCursorWindowSize = Resources.getSystem().getInteger(
                    com.android.internal.R.integer.config_cursorWindowSize) * 1024;
        }
        return sCursorWindowSize;
    }

总结：

Cursor的Data区域是基于匿名共享内存实现的，所以Binder进程传递的Cursor对象，本质上就是一个String和FD（根本不用担心超出Binder的1MB-8KB的限制导致异常），但是这个匿名共享内存的大小是有限制的，安卓系统中Cursor的data匿名共享内存的大小限制是2MB。当然可以通过调用public CursorWindow(String name, @BytesLong long windowSizeBytes)来设置Cursor的Data区域大小

意外发现Parcel.cpp中的writeBlob方法

frameworks/native/libs/binder/Parcel.cpp

// Maximum size of a blob to transfer in-place.
static const size_t BLOB_INPLACE_LIMIT = 16 * 1024;

status_t Parcel::writeBlob(size_t len, bool mutableCopy, WritableBlob* outBlob)
{
    if (len > INT32_MAX) {
        // don't accept size_t values which may have come from an
        // inadvertent conversion from a negative int.
        return BAD_VALUE;
    }

    status_t status;
    //如果mAllowFds为false或者len小于等于16kb，使用普通的方式保存数据
    if (!mAllowFds || len <= BLOB_INPLACE_LIMIT) {
        ALOGV("writeBlob: write in place");
        status = writeInt32(BLOB_INPLACE);
        if (status) return status;

        void* ptr = writeInplace(len);
        if (!ptr) return NO_MEMORY;

        outBlob->init(-1, ptr, len, false);
        return NO_ERROR;
    }
    //使用匿名共享内存的方式保存数据
    ALOGV("writeBlob: write to ashmem");
    int fd = ashmem_create_region("Parcel Blob", len);
    if (fd < 0) return NO_MEMORY;
    //创建可读可写的匿名共享内存fd
    int result = ashmem_set_prot_region(fd, PROT_READ | PROT_WRITE);
    if (result < 0) {
        status = result;
    } else {
        //申请物理内存
        void* ptr = ::mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
        if (ptr == MAP_FAILED) {
            status = -errno;
        } else {
            if (!mutableCopy) {
                result = ashmem_set_prot_region(fd, PROT_READ);
            }
            if (result < 0) {
                status = result;
            } else {
                //把数据写到申请的匿名共享内存上
                status = writeInt32(mutableCopy ? BLOB_ASHMEM_MUTABLE : BLOB_ASHMEM_IMMUTABLE);
                if (!status) {
                    status = writeFileDescriptor(fd, true /*takeOwnership*/);
                    if (!status) {
                        outBlob->init(fd, ptr, len, mutableCopy);
                        return NO_ERROR;
                    }
                }
            }
        }
        ::munmap(ptr, len);
    }
    ::close(fd);
    return status;
}

frameworks/base/core/java/android/os/Parcel.java

    /** @hide */
    public final void restoreAllowFds(boolean lastValue) {
        nativeRestoreAllowFds(mNativePtr, lastValue);
    }

    /**
     * Write a blob of data into the parcel at the current {@link #dataPosition},
     * growing {@link #dataCapacity} if needed.
     * @param b Bytes to place into the parcel.
     * {@hide}
     * {@SystemApi}
     */
    public final void writeBlob(byte[] b) {
        writeBlob(b, 0, (b != null) ? b.length : 0);
    }

    /**
     * Write a blob of data into the parcel at the current {@link #dataPosition},
     * growing {@link #dataCapacity} if needed.
     * @param b Bytes to place into the parcel.
     * @param offset Index of first byte to be written.
     * @param len Number of bytes to write.
     * {@hide}
     * {@SystemApi}
     */
    public final void writeBlob(byte[] b, int offset, int len) {
        if (b == null) {
            writeInt(-1);
            return;
        }
        Arrays.checkOffsetAndCount(b.length, offset, len);
        nativeWriteBlob(mNativePtr, b, offset, len);
    }

发现其实Parcel有隐藏接口，可以通过restoreAllowFds来让writeBlob的接口内部以匿名共享内存的方式存储数据。

猜想：在Intent跳转另一个页面的时候，能否通过调用隐藏接口来进行大数据的传输？

常规的操作

    private void test1() {
        Intent intent = new Intent(this, Main2Activity.class);
        byte[] bytes = new byte[1024*1024];//创建一个1M的数据
        intent.putExtra("kobe", bytes);
        this.startActivity(intent);
    }

出现异常

E AndroidRuntime: Caused by: android.os.TransactionTooLargeException: data parcel size 1048944 bytes
E AndroidRuntime:   at android.os.BinderProxy.transactNative(Native Method)
E AndroidRuntime:   at android.os.BinderProxy.transact(Binder.java:1127)
E AndroidRuntime:   at android.app.IActivityManager$Stub$Proxy.startActivity(IActivityManager.java:3754)
E AndroidRuntime:   at android.app.Instrumentation.execStartActivity(Instrumentation.java:1669)

因为Binder调用的限制是1M-8K，我们传递一个1M的数据，肯定报错了。

特殊操作：调用Parcel的隐藏接口

    private void test2() {
        Intent intent = new Intent(this, Main2Activity.class);
        Data data = new Data();
        data.bytes = new byte[1024*1024];//创建一个1M的数据，并保存在Data中的bytes
        intent.putExtra("kobe", data);//把一个Parcelable对象传进去
        this.startActivity(intent);
    }

//Data实现了Parcelable接口
public class Data implements Parcelable {

    public byte[] bytes;

    @Override
    public int describeContents() {
        return 0;
    }
    //序列化
    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.restoreAllowFds(true);//设置Parcel的AllowFds为true
        dest.writeBlob(bytes);//把数据传递写到Parcel里面，存储方式是匿名共享内存
    }

    public Data() {
    }
    //反序列化
    protected Data(Parcel in) {
        this.bytes = in.readBlob();//把数据从Parcel中读出来
    }

    public static final Parcelable.Creator<Data> CREATOR = new Parcelable.Creator<Data>() {
        @Override
        public Data createFromParcel(Parcel source) {
            return new Data(source);
        }

        @Override
        public Data[] newArray(int size) {
            return new Data[size];
        }
    };
}

写完代码的我感觉非常好，程序跑起来，正准备宣布我的新发现，结果发现还是我太年轻了

E AndroidRuntime: Caused by: java.lang.IllegalArgumentException: File descriptors passed in Intent
E AndroidRuntime:   at android.os.Parcel.readException(Parcel.java:2009)
E AndroidRuntime:   at android.os.Parcel.readException(Parcel.java:1951)
E AndroidRuntime:   at android.app.IActivityManager$Stub$Proxy.startActivity(IActivityManager.java:4425)
E AndroidRuntime:   at android.app.Instrumentation.execStartActivity(Instrumentation.java:1613)
E AndroidRuntime:   at android.app.Activity.startActivityForResult(Activity.java:4501)
E AndroidRuntime:   at android.app.Activity.startActivityForResult(Activity.java:4459)
E AndroidRuntime:   at android.app.Activity.startActivity(Activity.java:4820)
E AndroidRuntime:   at android.app.Activity.startActivity(Activity.java:4788)
E AndroidRuntime:   at com.tct.blobtest.MainActivity.test2(MainActivity.java:28)
E AndroidRuntime:   at com.tct.blobtest.MainActivity.onCreate(MainActivity.java:14)
E AndroidRuntime:   at android.app.Activity.performCreate(Activity.java:7023)
E AndroidRuntime:   at android.app.Activity.performCreate(Activity.java:7014)
E AndroidRuntime:   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1215)
E AndroidRuntime:   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2861)