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)
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