SharedMemory use, transfer byte data across processes

background

I encountered a requirement at work that needed to transfer pictures across processes. Android’s SharedMemory was used in the project. Here is a brief introduction to its use.

API to add this feature

API level 27

Class interface

See the official document
https://developer.android.google.cn/reference/android/os/SharedMemory?hl=en

Cross-process use (here to transfer picture bytes as an example)

1. The server creates and saves data:

    // 1、使用文件流把文件读入到内存
    FileInputStream inputStream = new FileInputStream(jpegFile);
    byte[] bytes = new byte[(int) jpegFile.length()];
    inputStream.read(bytes);

    // 2、创建sharedMemory跨进程传输
    SharedMemory sharedMemory = SharedMemory.create("test", bytes.length);
    // 3、mapReadWrite获取ByteBuffer
    ByteBuffer buffer = sharedMemory.mapReadWrite();
    // 4、put数据
    buffer.put(bytes);
    // 5、把sharedMemory权限设置为只读，create默认是读写权限都有，这里可以避免客户端更改数据
    sharedMemory.setProtect(OsConstants.PROT_READ);
    // 6、使用完需要unmap
    SharedMemory.unmap(buffer);
    return sharedMemory;

Two, the client obtains data display

    // 1、通过aidl拿到SharedMemory
    SharedMemory sharedMemory = iService.takeScreenshot();
    if (sharedMemory == null) {
    
    
        return;
    }
    // 2、mapReadOnly获取到存了数据的ByteBuffer
    ByteBuffer byteBuffer = sharedMemory.mapReadOnly();
    int len = byteBuffer.limit() - byteBuffer.position();
    byte[] bytes = new byte[len];
    // 3、借助byteBuffer获取到数据
    byteBuffer.get(bytes);
    // 4、把字节转换成bitmap并显示到imageView
    Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
    imageView.setVisibility(View.VISIBLE);
    imageView.setImageBitmap(bitmap);
    // 5、使用完unmap同时close sharedMemory释放内存
    SharedMemory.unmap(byteBuffer);
    sharedMemory.close();

Precautions

1. When the server service is destroyed, remember sharedMemory.close
2. When the server transmits sharedMemory, remember to setProtect(OsConstants.PROT_READ)
3. When the client finishes using sharedMemory, it is best to unmap to avoid memory occupation