Support error log handler for Http server (#3700)

Fixes #3257

Signed-off-by: raccoonback <[email protected]>

Author: raccoonback

docs/modules/ROOT/pages/http-server.adoc

@@ -412,10 +412,13 @@ The following example uses a domain name containing a wildcard:
 include::{examples-dir}/sni/Application.java[lines=18..47]
 ----
 
-[[http-access-log]]
-== HTTP Access Log
+[[http-log]]
+== HTTP Log
 
-You can enable the `HTTP` access log either programmatically or by configuration. By default, it is disabled.
+You can enable the `HTTP` access or error log either programmatically or by configuration. By default, it is disabled.
+
+[[access-log]]
+=== Access Log
 
 You can use `-Dreactor.netty.http.server.accessLogEnabled=true` to enable the `HTTP` access log by configuration.
 
@@ -472,6 +475,63 @@ include::{examples-dir}/accessLog/CustomFormatAndFilterAccessLogApplication.java
 <1> Specifies the filter predicate to use
 <2> Specifies the custom format to apply
 
+[[error-log]]
+=== Error Log
+
+You can use `-Dreactor.netty.http.server.errorLogEnabled=true` to enable the `HTTP` error log by configuration.
+
+You can use the following configuration (for Logback or similar logging frameworks) to have a separate
+`HTTP` error log file:
+
+[source,xml]
+----
+<appender name="errorLog" class="ch.qos.logback.core.FileAppender">
+    <file>error_log.log</file>
+    <encoder>
+        <pattern>%msg%n</pattern>
+    </encoder>
+</appender>
+<appender name="async" class="ch.qos.logback.classic.AsyncAppender">
+    <appender-ref ref="errorLog" />
+</appender>
+
+<logger name="reactor.netty.http.server.ErrorLog" level="ERROR" additivity="false">
+    <appender-ref ref="async"/>
+</logger>
+----
+
+The following example enables it programmatically:
+
+{examples-link}/errorLog/Application.java
+----
+include::{examples-dir}/errorLog/Application.java[lines=21..32]
+----
+
+Calling this method takes precedence over the system property configuration.
+
+By default, the logging format is `[{datetime}] [error] [client {remote address}] {exception message}`, but you can
+specify a custom one as a parameter, as in the following example:
+
+{examples-link}/errorLog/CustomLogErrorFormatApplication.java
+----
+include::{examples-dir}/errorLog/CustomLogErrorFormatApplication.java[lines=22..36]
+----
+
+You can also filter `HTTP` error logs by using the `ErrorLogFactory#createFilter` method, as in the following example:
+
+{examples-link}/errorLog/FilterLogErrorApplication.java
+----
+include::{examples-dir}/errorLog/FilterLogErrorApplication.java[lines=22..36]
+----
+
+Note that this method can take a custom format parameter too, as in this example:
+
+{examples-link}/errorLog/CustomFormatAndFilterErrorLogApplication.java.java
+----
+include::{examples-dir}/errorLog/CustomFormatAndFilterErrorLogApplication.java[lines=23..40]
+----
+<1> Specifies the filter predicate to use
+<2> Specifies the custom format to apply
 
 [[HTTP2]]
 == HTTP/2

reactor-netty-core/src/main/java/reactor/netty/NettyPipeline.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2024 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2011-2025 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -89,6 +89,7 @@ public interface NettyPipeline {
 	String RIGHT                 = "reactor.right.";
 
 	String AccessLogHandler      = LEFT + "accessLogHandler";
+	String ErrorLogHandler       = LEFT + "errorLogHandler";
 	String ChannelMetricsHandler = LEFT + "channelMetricsHandler";
 	String ChunkedWriter         = LEFT + "chunkedWriter";
 	String CompressionHandler    = LEFT + "compressionHandler";

reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2024 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2011-2025 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -194,6 +194,12 @@ public final class ReactorNetty {
 	 */
 	public static final String ACCESS_LOG_ENABLED = "reactor.netty.http.server.accessLogEnabled";
 
+	/**
+	 * Specifies whether the Http Server error log will be enabled.
+	 * By default, it is disabled.
+	 */
+	public static final String ERROR_LOG_ENABLED = "reactor.netty.http.server.errorLogEnabled";
+
 	/**
 	 *  Specifies the zone id used by the access log.
 	 */

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/errorLog/Application.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2025 VMware, Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reactor.netty.examples.documentation.http.server.errorLog;
+
+import reactor.netty.DisposableServer;
+import reactor.netty.http.server.HttpServer;
+
+public class Application {
+
+	public static void main(String[] args) {
+		DisposableServer server =
+				HttpServer.create()
+						.errorLog(true)
+						.bindNow();
+
+		server.onDispose()
+				.block();
+	}
+}

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/errorLog/CustomFormatAndFilterErrorLogApplication.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025 VMware, Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reactor.netty.examples.documentation.http.server.errorLog;
+
+import reactor.netty.DisposableServer;
+import reactor.netty.http.server.HttpServer;
+import reactor.netty.http.server.logging.error.DefaultErrorLog;
+import reactor.netty.http.server.logging.error.ErrorLogFactory;
+
+public class CustomFormatAndFilterErrorLogApplication {
+
+	public static void main(String[] args) {
+		DisposableServer server =
+				HttpServer.create()
+						.errorLog(
+								true,
+								ErrorLogFactory.createFilter(
+										p -> p.cause() instanceof RuntimeException, //<1>
+										x -> DefaultErrorLog.create("method={}, uri={}", x.httpServerInfos().method(), x.httpServerInfos().uri()) //<2>
+								)
+						)
+						.bindNow();
+
+		server.onDispose()
+				.block();
+	}
+}

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/errorLog/CustomLogErrorFormatApplication.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2025 VMware, Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reactor.netty.examples.documentation.http.server.errorLog;
+
+import reactor.netty.DisposableServer;
+import reactor.netty.http.server.HttpServer;
+import reactor.netty.http.server.logging.error.DefaultErrorLog;
+
+public class CustomLogErrorFormatApplication {
+
+	public static void main(String[] args) {
+		DisposableServer server =
+				HttpServer.create()
+						.errorLog(
+								true,
+								x -> DefaultErrorLog.create("method={}, uri={}", x.httpServerInfos().method(), x.httpServerInfos().uri())
+						)
+						.bindNow();
+
+		server.onDispose()
+				.block();
+	}
+}

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/errorLog/FilterLogErrorApplication.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021-2025 VMware, Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reactor.netty.examples.documentation.http.server.errorLog;
+
+import reactor.netty.DisposableServer;
+import reactor.netty.http.server.HttpServer;
+import reactor.netty.http.server.logging.error.ErrorLogFactory;
+
+public class FilterLogErrorApplication {
+
+	public static void main(String[] args) {
+		DisposableServer server =
+				HttpServer.create()
+						.errorLog(
+								true,
+								ErrorLogFactory.createFilter(p -> p.cause() instanceof RuntimeException)
+						)
+						.bindNow();
+
+		server.onDispose()
+				.block();
+	}
+}

reactor-netty-http/src/main/java/reactor/netty/http/server/Http3Codec.java

@@ -35,6 +35,9 @@
 import reactor.netty.http.server.logging.AccessLog;
 import reactor.netty.http.server.logging.AccessLogArgProvider;
 import reactor.netty.http.server.logging.AccessLogHandlerFactory;
+import reactor.netty.http.server.logging.error.DefaultErrorLogHandler;
+import reactor.netty.http.server.logging.error.ErrorLog;
+import reactor.netty.http.server.logging.error.ErrorLogArgProvider;
 import reactor.util.Logger;
 import reactor.util.Loggers;
 import reactor.util.annotation.Nullable;
@@ -52,6 +55,8 @@ final class Http3Codec extends ChannelInitializer<QuicStreamChannel> {
 
 	final boolean                                                 accessLogEnabled;
 	final Function<AccessLogArgProvider, AccessLog>               accessLog;
+	final boolean                                                 errorLogEnabled;
+	final Function<ErrorLogArgProvider, ErrorLog>                 errorLog;
 	final HttpCompressionOptionsSpec                              compressionOptions;
 	final BiPredicate<HttpServerRequest, HttpServerResponse>      compressPredicate;
 	final ServerCookieDecoder                                     cookieDecoder;
@@ -74,6 +79,8 @@ final class Http3Codec extends ChannelInitializer<QuicStreamChannel> {
 	Http3Codec(
 			boolean accessLogEnabled,
 			@Nullable Function<AccessLogArgProvider, AccessLog> accessLog,
+			boolean errorLogEnabled,
+			@Nullable Function<ErrorLogArgProvider, ErrorLog> errorLog,
 			@Nullable HttpCompressionOptionsSpec compressionOptions,
 			@Nullable BiPredicate<HttpServerRequest, HttpServerResponse> compressPredicate,
 			ServerCookieDecoder decoder,
@@ -93,6 +100,8 @@ final class Http3Codec extends ChannelInitializer<QuicStreamChannel> {
 			boolean validate) {
 		this.accessLogEnabled = accessLogEnabled;
 		this.accessLog = accessLog;
+		this.errorLogEnabled = errorLogEnabled;
+		this.errorLog = errorLog;
 		this.compressionOptions = compressionOptions;
 		this.compressPredicate = compressPredicate;
 		this.cookieDecoder = decoder;
@@ -149,6 +158,10 @@ else if (metricsRecorder instanceof ContextAwareHttpServerMetricsRecorder) {
 			}
 		}
 
+		if (errorLogEnabled) {
+			p.addBefore(NettyPipeline.ReactiveBridge, NettyPipeline.ErrorLogHandler, new DefaultErrorLogHandler(errorLog));
+		}
+
 		channel.pipeline().remove(this);
 
 		if (log.isDebugEnabled()) {
@@ -159,6 +172,8 @@ else if (metricsRecorder instanceof ContextAwareHttpServerMetricsRecorder) {
 	static ChannelHandler newHttp3ServerConnectionHandler(
 			boolean accessLogEnabled,
 			@Nullable Function<AccessLogArgProvider, AccessLog> accessLog,
+			boolean errorLogEnabled,
+			@Nullable Function<ErrorLogArgProvider, ErrorLog> errorLog,
 			@Nullable HttpCompressionOptionsSpec compressionOptions,
 			@Nullable BiPredicate<HttpServerRequest, HttpServerResponse> compressPredicate,
 			ServerCookieDecoder decoder,
@@ -177,8 +192,8 @@ static ChannelHandler newHttp3ServerConnectionHandler(
 			@Nullable Function<String, String> uriTagValue,
 			boolean validate) {
 		return new Http3ServerConnectionHandler(
-				new Http3Codec(accessLogEnabled, accessLog, compressionOptions, compressPredicate, decoder, encoder, formDecoderProvider, forwardedHeaderHandler,
-						httpMessageLogFactory, listener, mapHandle, methodTagValue, metricsRecorder, minCompressionSize,
-						opsFactory, readTimeout, requestTimeout, uriTagValue, validate));
+				new Http3Codec(accessLogEnabled, accessLog, errorLogEnabled, errorLog, compressionOptions, compressPredicate, decoder, encoder,
+						formDecoderProvider, forwardedHeaderHandler, httpMessageLogFactory, listener, mapHandle, methodTagValue, metricsRecorder,
+						minCompressionSize, opsFactory, readTimeout, requestTimeout, uriTagValue, validate));
 	}
 }