Adds DI and SPI examples

messaging/src/test/java/messagebroker/handlers/EchoMessageHandlerTest.java

@@ -0,0 +1,39 @@
+package messagebroker.handlers;
+
+import messagebroker.MessageHandlerContext;
+import messagebroker.UnknownMessageFormatException;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.io.OutputStream;
+
+import static org.mockito.Mockito.*;
+
+public class EchoMessageHandlerTest {
+
+    private final MessageHandlerContext context = mock(MessageHandlerContext.class);
+    private EchoMessageHandler messageHandler;
+
+    public EchoMessageHandlerTest() {
+        when(context.getOutputStream()).thenReturn(mock(OutputStream.class));
+    }
+
+    @Before()
+    public void setUp() {
+        messageHandler = new EchoMessageHandler(context);
+    }
+
+    @Test
+    public void shouldHandleEchoMessage() throws UnknownMessageFormatException, IOException {
+        var message = "ECHO Some message";
+        messageHandler.handleMessage(message);
+        verify(context.getOutputStream()).flush();
+    }
+
+    @Test(expected = UnknownMessageFormatException.class)
+    public void shouldThrowWhenMessageIsUnknown() throws UnknownMessageFormatException {
+        var message = "Bad message";
+        messageHandler.handleMessage(message);
+    }
+}