Convert BufferedImage into byte[] without I/O

Question

Hi I have a BufferedImage instance in memory and want to convert it into byte[] to encode as base64 string without I/O operation for performance consideration. I was using the following API:

ByteArrayOutputStream baos = new ByteArrayOutputStream ();
ImageIO.write(image,"png",baos);
return baos.toByteArray();

However, this API still implicitly writes the image to the OS temp directory, which will lead to failure in case that the underlying OS temp directory is full and the temp file cannot be created. Stack Trace:

Caused by: java.io.IOException: No space left on device
    at java.io.RandomAccessFile.write(RandomAccessFile.java:493)
    at javax.imageio.stream.FileCacheImageOutputStream.write(FileCacheImageOutputStream.java:134)
    at javax.imageio.stream.ImageOutputStreamImpl.write(ImageOutputStreamImpl.java:66)
    at com.sun.imageio.plugins.png.PNGImageWriter.write_magic(PNGImageWriter.java:376)
    at com.sun.imageio.plugins.png.PNGImageWriter.write(PNGImageWriter.java:1115)
    at javax.imageio.ImageWriter.write(ImageWriter.java:628)
    at javax.imageio.ImageIO.write(ImageIO.java:1480)
    at javax.imageio.ImageIO.write(ImageIO.java:1554)

Is there an efficient (like in-memory conversion or efficient I/O) way to do the conversion without I/O? Please advise.

Answer 1

Disable the ImageIO cache through the ImageIO.setUseCache() method:

ImageIO.setUseCache(false);

It is on by default according to the javadoc:

Sets a flag indicating whether a disk-based cache file should be used when creating ImageInputStreams and ImageOutputStreams.

When reading from a standard InputStream>, it may be necessary to save previously read information in a cache since the underlying stream does not allow data to be re-read. Similarly, when writing to a standard OutputStream, a cache may be used to allow a previously written value to be changed before flushing it to the final destination.

The cache may reside in main memory or on disk. Setting this flag to false disallows the use of disk for future streams, which may be advantageous when working with small images, as the overhead of creating and destroying files is removed.

On startup, the value is set to true.

Answer 2

Both mentions of ImageIO.setUseCache(false) is correct. However, if you don't like to disable disk caching for ImageIO globally, an alternative is to explicitly wrap the stream in a MemoryCacheImageOutputStream (which does in-memory caching instead of disk caching):

ByteArrayOutputStream baos = new ByteArrayOutputStream ();
ImageOutputStream stream = new MemoryCacheImageOutputStream(baos);
ImageIO.write(image, "png", stream);
stream.close();
return baos.toByteArray();

Answer 3

ImageIO by default writes it's cache to disk even when you only use streams. Try disabling the cache with:

ImageIO.setUseCache(false);