Answer by Ferrybig for How to unit test netty handler

In Netty, there are different ways to test your networking stack.

Testing ChannelHandlers

You can use Netty's EmbeddedChannel to mock a netty connection for testing, an example of this would be:

@Test
public void nettyTest() {
    EmbeddedChannel channel = new EmbeddedChannel(new StringDecoder(StandardCharsets.UTF_8));
    channel.writeInbound(Unpooled.wrappedBuffer(new byte[]{(byte)0xE2,(byte)0x98,(byte)0xA2}));
    String myObject = channel.readInbound();
    // Perform checks on your object
    assertEquals("☢", myObject);
}

This test above tests for StringDecoder ability to decode unicode correct (example from this bug posted by me)

You can also test the encoder direction using EmbeddedChannel, for this you should use writeOutBound and readInbound.

More Examples:

DelimiterBasedFrameDecoderTest.java:

@Test
public void testIncompleteLinesStrippedDelimiters() {
    EmbeddedChannel ch = new EmbeddedChannel(new DelimiterBasedFrameDecoder(8192, true,
            Delimiters.lineDelimiter()));
    ch.writeInbound(Unpooled.copiedBuffer("Test", Charset.defaultCharset()));
    assertNull(ch.readInbound());
    ch.writeInbound(Unpooled.copiedBuffer("Line\r\ng\r\n", Charset.defaultCharset()));
    assertEquals("TestLine", releaseLater((ByteBuf) ch.readInbound()).toString(Charset.defaultCharset()));
    assertEquals("g", releaseLater((ByteBuf) ch.readInbound()).toString(Charset.defaultCharset()));
    assertNull(ch.readInbound());
    ch.finish();
}

More examples on github.

ByteBuf

To test if you use your bytebufs, you can set a JVM parameter that checks for leaked ByteBuf, for this, you should add -Dio.netty.leakDetectionLevel=PARANOID to the startup parameters, or call the method ResourceLeakDetector.setLevel(PARANOID).