CodeQL documentation

Inefficient output stream

ID: java/inefficient-output-stream
Kind: problem
Severity: warning
Precision: very-high
Tags:
   - efficiency
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

The classes java.io.OutputStream and java.io.FilterOutputStream only require subclasses to implement the method write(byte b). Typically, uses of OutputStreams will not write single bytes, but an array via the write(byte[] b, int off, int len) method. The default implementation of this method, which you are not required to override, calls write(byte b) for each byte in the array. If this method involves I/O, such as accessing the network or disk, this is likely to incur significant overhead.

Recommendation

Always provide an implementation of the write(byte[] b, int off, int len) method.

Example

The following example shows a subclass of OutputStream that simply wraps a DigestOutputStream to confirm that the data it writes to a file has the expected MD5 hash. Without an implementation of write(byte[] b, int off, int len) this will be very slow, because it makes a call to DigestOutputStream.write(byte b) and FileOutputStream.write(byte b) for each byte written.

public class DigestCheckingFileOutputStream extends OutputStream {
	private DigestOutputStream digest;
	private byte[] expectedMD5;

	public DigestCheckingFileOutputStream(File file, byte[] expectedMD5)
		throws IOException, NoSuchAlgorithmException {
			this.expectedMD5 = expectedMD5;
			digest = new DigestOutputStream(new FileOutputStream(file),
											MessageDigest.getInstance("MD5"));
		}

	@Override
	public void write(int b) throws IOException {
		digest.write(b);
	}

	@Override
	public void close() throws IOException {
		super.close();

		digest.close();
		byte[] md5 = digest.getMessageDigest().digest();
		if (expectedMD5 != null && !Arrays.equals(expectedMD5, md5)) {
			throw new InternalError();
		}
	}
}

The example can be updated to use a more efficient method. In this case, calls to write(byte[] b, int off, int len) are simply forwarded to DigestOutputStream.write(byte[] b, int off, int len).

public class DigestCheckingFileOutputStream extends OutputStream {
	private DigestOutputStream digest;
	private byte[] expectedMD5;

	public DigestCheckingFileOutputStream(File file, byte[] expectedMD5)
		throws IOException, NoSuchAlgorithmException {
			this.expectedMD5 = expectedMD5;
			digest = new DigestOutputStream(new FileOutputStream(file),
											MessageDigest.getInstance("MD5"));
		}

	@Override
	public void write(int b) throws IOException {
		digest.write(b);
	}

	@Override
	public void write(byte[] b, int off, int len) throws IOException {
		digest.write(b, off, len);
	}

	@Override
	public void close() throws IOException {
		super.close();

		digest.close();
		byte[] md5 = digest.getMessageDigest().digest();
		if (expectedMD5 != null && !Arrays.equals(expectedMD5, md5)) {
			throw new InternalError();
		}
	}
}

References