arturobernalg · Oct 6, 2026
diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ByteSseEntityConsumer.java
@@ -240,10 +240,15 @@ private void handleLine(final byte[] buf, final int len) {
     }
 
     private void dispatch() {
+        if (id != null) {
+            cb.onLastEventId(id);
+        }
+
         if (data.length() == 0) {
             type = null;
             return;
         }
+
         final int n = data.length();
         if (n > 0 && data.charAt(n - 1) == '\n') {
             data.setLength(n - 1);

diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/DefaultEventSource.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/DefaultEventSource.java
@@ -323,7 +323,7 @@ private void doConnect() {
         final SimpleRequestBuilder rb = SimpleRequestBuilder.get(uri);
         rb.setHeader(HttpHeaders.ACCEPT, TEXT_EVENT_STREAM.getMimeType());
         rb.setHeader(HttpHeaders.CACHE_CONTROL, "no-cache");
-        if (lastEventId != null) {
+        if (lastEventId != null && !lastEventId.isEmpty()) {
             rb.setHeader("Last-Event-ID", lastEventId);
         }
         for (final Map.Entry<String, String> e : headers.entrySet()) {
@@ -393,11 +393,13 @@ public void onOpen() {
                 dispatch(listener::onOpen);
             }
 
+            @Override
+            public void onLastEventId(final String id) {
+                lastEventId = id;
+            }
+
             @Override
             public void onEvent(final String id, final String type, final String data) {
-                if (id != null) {
-                    lastEventId = id;
-                }
                 dispatch(() -> listener.onEvent(id, type, data));
             }
 

diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ServerSentEventReader.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/ServerSentEventReader.java
@@ -48,6 +48,9 @@ public interface Callback {
         void onComment(String comment);
 
         void onRetryChange(long retryMs);
+
+        default void onLastEventId(final String id) {
+        }
     }
 
     private final Callback cb;
@@ -159,11 +162,16 @@ public void line(final String line) {
     }
 
     private void dispatch() {
+        if (id != null) {
+            cb.onLastEventId(id);
+        }
+
         if (data.length() == 0) {
             // spec: a blank line with no "data:" accumulates nothing -> just clear type
             type = null;
             return;
         }
+
         final int n = data.length();
         if (n > 0 && data.charAt(n - 1) == '\n') {
             data.setLength(n - 1);

diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseCallbacks.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseCallbacks.java
@@ -64,4 +64,12 @@ public interface SseCallbacks {
      * @param retryMs new retry delay in milliseconds (non-negative)
      */
     void onRetry(long retryMs);
+
+    /**
+     * Notifies that the last event ID has been committed by the SSE dispatch step.
+     *
+     * @param id the last event ID, possibly empty to reset it
+     */
+    default void onLastEventId(final String id) {
+    }
 }
diff --git a/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java b/httpclient5-sse/src/main/java/org/apache/hc/client5/http/sse/impl/SseEntityConsumer.java
@@ -130,6 +130,11 @@ public void releaseResources() {
         reader = null;
     }
 
+    @Override
+    public void onLastEventId(final String id) {
+        cb.onLastEventId(id);
+    }
+
     // ServerSentEventReader.Callback
 
     @Override

diff --git a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ByteSseEntityConsumerTest.java
@@ -27,6 +27,7 @@
 package org.apache.hc.client5.http.sse;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.nio.ByteBuffer;
@@ -43,6 +44,7 @@ static final class Cb implements SseCallbacks {
         boolean opened;
         String id, type, data;
         Long retry;
+        String lastEventId;
 
         @Override
         public void onOpen() {
@@ -60,6 +62,11 @@ public void onEvent(final String id, final String type, final String data) {
         public void onRetry(final long retryMs) {
             retry = retryMs;
         }
+
+        @Override
+        public void onLastEventId(final String id) {
+            lastEventId = id;
+        }
     }
 
 
@@ -96,4 +103,32 @@ void emitsRetry() throws Exception {
 
         assertEquals(Long.valueOf(2500L), cb.retry);
     }
+
+    @Test
+    void updatesLastEventIdWithoutDispatchingEvent() throws Exception {
+        final Cb cb = new Cb();
+        final ByteSseEntityConsumer c = new ByteSseEntityConsumer(cb);
+
+        c.streamStart(ContentType.parse("text/event-stream"));
+
+        final byte[] p = "id: 42\n\n".getBytes(StandardCharsets.UTF_8);
+        c.consume(ByteBuffer.wrap(p));
+
+        assertEquals("42", cb.lastEventId);
+        assertNull(cb.data);
+    }
+
+    @Test
+    void resetsLastEventIdWithoutDispatchingEvent() throws Exception {
+        final Cb cb = new Cb();
+        final ByteSseEntityConsumer c = new ByteSseEntityConsumer(cb);
+
+        c.streamStart(ContentType.parse("text/event-stream"));
+
+        final byte[] p = "id: 42\n\nid:\n\n".getBytes(StandardCharsets.UTF_8);
+        c.consume(ByteBuffer.wrap(p));
+
+        assertEquals("", cb.lastEventId);
+        assertNull(cb.data);
+    }
 }
diff --git a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ServerSentEventReaderTest.java b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/ServerSentEventReaderTest.java
@@ -36,6 +36,7 @@ class ServerSentEventReaderTest {
 
     static final class Capt implements ServerSentEventReader.Callback {
         String id, type, data, comment;
+        String lastEventId;
         Long retry;
 
         @Override
@@ -45,6 +46,11 @@ public void onEvent(final String id, final String type, final String data) {
             this.data = data;
         }
 
+        @Override
+        public void onLastEventId(final String id) {
+            this.lastEventId = id;
+        }
+
         @Override
         public void onComment(final String comment) {
             this.comment = comment;
@@ -101,4 +107,36 @@ void ignoresIdWithNul() {
         assertNull(c.id);
         assertEquals("d", c.data);
     }
+
+    @Test
+    void updatesLastEventIdOnBlankLineWithoutDispatchingEvent() {
+        final Capt c = new Capt();
+        final ServerSentEventReader r = new ServerSentEventReader(c);
+
+        r.line("id: 42");
+
+        assertNull(c.lastEventId);
+
+        r.line("");
+
+        assertEquals("42", c.lastEventId);
+        assertNull(c.data);
+    }
+
+    @Test
+    void resetsLastEventIdOnBlankLineWithoutDispatchingEvent() {
+        final Capt c = new Capt();
+        final ServerSentEventReader r = new ServerSentEventReader(c);
+
+        r.line("id: 42");
+        r.line("");
+
+        assertEquals("42", c.lastEventId);
+
+        r.line("id:");
+        r.line("");
+
+        assertEquals("", c.lastEventId);
+        assertNull(c.data);
+    }
 }
diff --git a/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java b/httpclient5-sse/src/test/java/org/apache/hc/client5/http/sse/SseEntityConsumerTest.java
@@ -27,6 +27,7 @@
 package org.apache.hc.client5.http.sse;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 
@@ -43,6 +44,7 @@ static final class Cb implements SseCallbacks {
         boolean opened;
         String id, type, data;
         Long retry;
+        String lastEventId;
 
         @Override
         public void onOpen() {
@@ -60,6 +62,11 @@ public void onEvent(final String id, final String type, final String data) {
         public void onRetry(final long retryMs) {
             retry = retryMs;
         }
+
+        @Override
+        public void onLastEventId(final String id) {
+            lastEventId = id;
+        }
     }
 
     @Test
@@ -86,4 +93,28 @@ void rejectsWrongContentType() {
             fail("Should have thrown");
         } catch (final Exception expected) { /* ok */ }
     }
+
+    @Test
+    void updatesLastEventIdWithoutDispatchingEvent() throws Exception {
+        final Cb cb = new Cb();
+        final SseEntityConsumer c = new SseEntityConsumer(cb);
+
+        c.streamStart(ContentType.parse("text/event-stream"));
+        c.data(CharBuffer.wrap("id: 42\n\n"), false);
+
+        assertEquals("42", cb.lastEventId);
+        assertNull(cb.data);
+    }
+
+    @Test
+    void resetsLastEventIdWithoutDispatchingEvent() throws Exception {
+        final Cb cb = new Cb();
+        final SseEntityConsumer c = new SseEntityConsumer(cb);
+
+        c.streamStart(ContentType.parse("text/event-stream"));
+        c.data(CharBuffer.wrap("id: 42\n\nid:\n\n"), false);
+
+        assertEquals("", cb.lastEventId);
+        assertNull(cb.data);
+    }
 }
diff --git a/...client5-sse/src/test/java/org/apache/hc/client5/http/sse/impl/DefaultEventSourceTest.java b/...client5-sse/src/test/java/org/apache/hc/client5/http/sse/impl/DefaultEventSourceTest.java
@@ -26,6 +26,7 @@
  */
 package org.apache.hc.client5.http.sse.impl;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -49,6 +50,7 @@
 import org.apache.hc.core5.concurrent.FutureCallback;
 import org.apache.hc.core5.function.Supplier;
 import org.apache.hc.core5.http.HttpHost;
+import org.apache.hc.core5.http.HttpRequest;
 import org.apache.hc.core5.http.nio.AsyncPushConsumer;
 import org.apache.hc.core5.http.nio.AsyncRequestProducer;
 import org.apache.hc.core5.http.nio.AsyncResponseConsumer;
@@ -127,6 +129,7 @@ void callerSchedulerIsNotShutdownOnCancel() {
 
     static final class CapturingClient extends CloseableHttpAsyncClient {
         volatile FutureCallback<Void> lastCallback;
+        volatile HttpRequest lastRequest;
 
         @Override
         public void start() { }
@@ -156,8 +159,19 @@ protected <T> Future<T> doExecute(
                 final HandlerFactory<AsyncPushConsumer> pushHandlerFactory,
                 final HttpContext context,
                 final FutureCallback<T> callback) {
-            @SuppressWarnings("unchecked") final FutureCallback<Void> cb = (FutureCallback<Void>) callback;
+
+            @SuppressWarnings("unchecked")
+            final FutureCallback<Void> cb = (FutureCallback<Void>) callback;
             this.lastCallback = cb;
+
+            try {
+                requestProducer.sendRequest(
+                        (request, entityDetails, requestContext) -> this.lastRequest = request,
+                        context);
+            } catch (final Exception ex) {
+                throw new IllegalStateException(ex);
+            }
+
             return new CompletableFuture<>();
         }
 
@@ -265,4 +279,48 @@ public V get(final long timeout, final TimeUnit unit) {
             return null;
         }
     }
+
+    @Test
+    void doesNotSendEmptyLastEventId() {
+        final RecordingScheduler scheduler = new RecordingScheduler();
+        final CapturingClient client = new CapturingClient();
+
+        final DefaultEventSource es = new DefaultEventSource(
+                client,
+                URI.create("http://example.org/sse"),
+                Collections.emptyMap(),
+                (id, type, data) -> { },
+                scheduler,
+                null,
+                null,
+                SseParser.CHAR);
+
+        es.setLastEventId("");
+        es.start();
+
+        assertFalse(client.lastRequest.containsHeader("Last-Event-ID"));
+    }
+
+    @Test
+    void sendsNonEmptyLastEventId() {
+        final RecordingScheduler scheduler = new RecordingScheduler();
+        final CapturingClient client = new CapturingClient();
+
+        final DefaultEventSource es = new DefaultEventSource(
+                client,
+                URI.create("http://example.org/sse"),
+                Collections.emptyMap(),
+                (id, type, data) -> { },
+                scheduler,
+                null,
+                null,
+                SseParser.CHAR);
+
+        es.setLastEventId("42");
+        es.start();
+
+        assertEquals(
+                "42",
+                client.lastRequest.getFirstHeader("Last-Event-ID").getValue());
+    }
 }