在使用Camera2 API开发应用程序时，我们可能需要手动控制设备的白平衡。为此，我们可以通过使用CameraCaptureSession.CaptureRequest.Builder对象设置WHITE_BALANCE模式来实现手动白平衡。

以下是一些示例代码，演示了如何实现手动白平衡：

//获取CameraCaptureSession.CaptureRequest.Builder对象
CameraCaptureSession.CaptureRequest.Builder builder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);

//手动设置白平衡模式
builder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
builder.set(CaptureRequest.CONTROL_AWB_MODE, CaptureRequest.CONTROL_AWB_MODE_OFF);
builder.set(CaptureRequest.COLOR_CORRECTION_MODE, CaptureRequest.COLOR_CORRECTION_MODE_TRANSFORM_MATRIX);

//手动设置白平衡值
Rational rationalRed = new Rational(255, 255);
Rational rationalGreen = new Rational(255, 255);
Rational rationalBlue = new Rational(255, 255);
builder.set(CaptureRequest.COLOR_CORRECTION_GAINS, new RggbChannelVector(rationalRed.floatValue(), rationalGreen.floatValue(), rationalGreen.floatValue(), rationalBlue.floatValue()));

//创建CaptureSession
mCameraDevice.createCaptureSession(Arrays.asList(mSurface), new CameraCaptureSession.StateCallback() {
@Override
public void onConfigured(CameraCaptureSession cameraCaptureSession) {
//设置相机拍摄的自定义请求
cameraCaptureSession.setRepeatingRequest(builder.build(), null, mBackgroundHandler);
}

@Override    
public void onConfigureFailed(CameraCaptureSession cameraCaptureSession) {    
    Log.e(TAG, "Failed to configure camera");    
}    

}, mBackgroundHandler);

在上面的代码实现中，我们通过设置WHITE_BALANCE模式为CONTROL_AWB_MODE_OFF来手动控制白平衡。然后，我们在COLOR_CORRECTION_GAINS中设置了用户定义的RggbChannelVector对象，以手动设置白平衡值。

在创建CaptureSession时，