add isolated unit test

logbook-servlet/src/test/java/org/zalando/logbook/servlet/LogbookFilterTest.java

@@ -1,9 +1,22 @@
 package org.zalando.logbook.servlet;
 
+import io.undertow.servlet.util.EmptyEnumeration;
+import jakarta.servlet.DispatcherType;
+import jakarta.servlet.FilterChain;
 import jakarta.servlet.FilterConfig;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
 import org.junit.jupiter.api.Test;
+import org.zalando.logbook.Logbook;
 
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 final class LogbookFilterTest {
 
@@ -27,4 +40,28 @@ void shouldCallDestroy() {
         new LogbookFilter().destroy();
     }
 
+    @Test
+    void shouldHandleIOExceptionOnFlushBufferAndWriteResponse() throws Exception {
+        Logbook logbook = mock(Logbook.class);
+        Logbook.RequestWritingStage requestWritingStage = mock(Logbook.RequestWritingStage.class);
+        Logbook.ResponseWritingStage responseWritingStage = mock(Logbook.ResponseWritingStage    .class);
+        LogbookFilter filter = new LogbookFilter(logbook);
+        HttpServletRequest request = mock(HttpServletRequest.class);
+        HttpServletResponse response = mock(HttpServletResponse.class);
+        FilterChain chain = mock(FilterChain.class);
+
+        when(logbook.process(any())).thenReturn(requestWritingStage);
+        when(requestWritingStage.write()).thenReturn(requestWritingStage);
+        when(requestWritingStage.process(any())).thenReturn(responseWritingStage);
+        when(request.getHeaderNames()).thenReturn(EmptyEnumeration.instance());
+        when(request.getDispatcherType()).thenReturn(DispatcherType.REQUEST);
+        when(request.getAttribute(any())).thenReturn(new AtomicBoolean(false));
+
+        doThrow(new IOException("Simulated IOException")).when(response).flushBuffer();
+
+        filter.doFilter(request, response, chain);
+
+        verify(responseWritingStage).write();
+    }
+
 }