Add SLF4J to Log4j API bridge

NOTICE.txt

@@ -1,4 +1,4 @@
-Apache Logging JDK Bridges
+Apache Logging SLF4J Bridges
 Copyright 2024 The Apache Software Foundation
 
 This product includes software developed at

pom.xml

@@ -60,6 +60,7 @@
 
     <!-- Modules here must have a corresponding entry in `dependencyManagement > dependencies` block below! -->
     <module>log4j-api-to-slf4j</module>
+    <module>slf4j-to-log4j-api</module>
 
   </modules>
 
@@ -106,6 +107,12 @@
         <version>${project.version}</version>
       </dependency>
 
+      <dependency>
+        <groupId>org.apache.logging.log4j</groupId>
+        <artifactId>slf4j-to-log4j-api</artifactId>
+        <version>${project.version}</version>
+      </dependency>
+
     </dependencies>
   </dependencyManagement>
 

slf4j-to-log4j-api/pom.xml

@@ -0,0 +1,143 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one or more
+  ~ contributor license agreements.  See the NOTICE file distributed with
+  ~ this work for additional information regarding copyright ownership.
+  ~ The ASF licenses this file to you 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
+  ~
+  ~      http://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.
+  -->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>org.apache.logging.log4j</groupId>
+    <artifactId>log4j-slf4j-parent</artifactId>
+    <version>${revision}</version>
+    <relativePath>../parent</relativePath>
+  </parent>
+
+  <artifactId>slf4j-to-log4j-api</artifactId>
+
+  <name>SLF4J to Log4j API logging bridge</name>
+
+  <description>SLF4J 2 provider (binding) for the Apache Log4j API.
+    It forwards SLF4J 2 calls to the Log4j API.
+    This effectively allows using Log4j as an implementation of SLF4J 2.
+    (Refer to the `log4j-to-slf4j` artifact for forwarding the Log4j API to SLF4J.)</description>
+
+  <properties>
+    <!-- Dependency versions -->
+    <osgi.framework.version>1.8.0</osgi.framework.version>
+  </properties>
+
+  <dependencies>
+
+    <dependency>
+      <groupId>org.osgi</groupId>
+      <artifactId>org.osgi.framework</artifactId>
+      <version>${osgi.framework.version}</version>
+      <scope>provided</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+    </dependency>
+
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-api</artifactId>
+    </dependency>
+
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-core-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-api</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-params</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+  </dependencies>
+  <build>
+    <plugins>
+
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <executions>
+          <!-- Separate test execution to verify that the presence of both:
+            ~  * `log4j-slf4j2-impl` (bridge from SLF4J to Log4j API)
+            ~  * `log4j-to-slf4j` (bridge from Log4j API to SLF4J)
+            ~  does not cause a stack overflow.
+            -->
+          <execution>
+            <id>loop-test</id>
+            <goals>
+              <goal>test</goal>
+            </goals>
+            <configuration>
+              <additionalClasspathDependencies>
+                <dependency>
+                  <groupId>org.apache.logging.log4j</groupId>
+                  <artifactId>log4j-to-slf4j</artifactId>
+                  <version>${log4j.version}</version>
+                </dependency>
+              </additionalClasspathDependencies>
+              <includes>
+                <include>**/OverflowTest.java</include>
+              </includes>
+            </configuration>
+          </execution>
+          <execution>
+            <id>default-test</id>
+            <configuration>
+              <includes>
+                <include>**/*Test.java</include>
+              </includes>
+              <excludes>
+                <exclude>**/OverflowTest.java</exclude>
+              </excludes>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+
+    </plugins>
+  </build>
+</project>

slf4j-to-log4j-api/src/main/java/org/apache/logging/slf4j/Log4jEventBuilder.java

@@ -0,0 +1,163 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you 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
+ *
+ *      http://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 org.apache.logging.slf4j;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Supplier;
+import org.apache.logging.log4j.BridgeAware;
+import org.apache.logging.log4j.CloseableThreadContext;
+import org.apache.logging.log4j.CloseableThreadContext.Instance;
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogBuilder;
+import org.apache.logging.log4j.Logger;
+import org.slf4j.Marker;
+import org.slf4j.spi.CallerBoundaryAware;
+import org.slf4j.spi.LoggingEventBuilder;
+
+public class Log4jEventBuilder implements LoggingEventBuilder, CallerBoundaryAware {
+
+    private static final String FQCN = Log4jEventBuilder.class.getName();
+
+    private final Log4jMarkerFactory markerFactory;
+    private final Logger logger;
+    private final List<Object> arguments = new ArrayList<>();
+    private String message = null;
+    private org.apache.logging.log4j.Marker marker = null;
+    private Throwable throwable = null;
+    private Map<String, String> keyValuePairs = null;
+    private final Level level;
+    private String fqcn = FQCN;
+
+    public Log4jEventBuilder(final Log4jMarkerFactory markerFactory, final Logger logger, final Level level) {
+        this.markerFactory = markerFactory;
+        this.logger = logger;
+        this.level = level;
+    }
+
+    @Override
+    public LoggingEventBuilder setCause(final Throwable cause) {
+        this.throwable = cause;
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder addMarker(final Marker marker) {
+        this.marker = markerFactory.getLog4jMarker(marker);
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder addArgument(final Object p) {
+        arguments.add(p);
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder addArgument(final Supplier<?> objectSupplier) {
+        arguments.add(objectSupplier.get());
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder addKeyValue(final String key, final Object value) {
+        if (keyValuePairs == null) {
+            keyValuePairs = new HashMap<>();
+        }
+        keyValuePairs.put(key, String.valueOf(value));
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder addKeyValue(final String key, final Supplier<Object> valueSupplier) {
+        if (keyValuePairs == null) {
+            keyValuePairs = new HashMap<>();
+        }
+        keyValuePairs.put(key, String.valueOf(valueSupplier.get()));
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder setMessage(final String message) {
+        this.message = message;
+        return this;
+    }
+
+    @Override
+    public LoggingEventBuilder setMessage(final Supplier<String> messageSupplier) {
+        this.message = messageSupplier.get();
+        return this;
+    }
+
+    @Override
+    public void log() {
+        final LogBuilder logBuilder = logger.atLevel(level).withMarker(marker).withThrowable(throwable);
+        if (logBuilder instanceof BridgeAware) {
+            ((BridgeAware) logBuilder).setEntryPoint(fqcn);
+        }
+        if (keyValuePairs == null || keyValuePairs.isEmpty()) {
+            logBuilder.log(message, arguments.toArray());
+        } else {
+            try (final Instance c = CloseableThreadContext.putAll(keyValuePairs)) {
+                logBuilder.log(message, arguments.toArray());
+            }
+        }
+    }
+
+    @Override
+    public void log(final String message) {
+        setMessage(message);
+        log();
+    }
+
+    @Override
+    public void log(final String message, final Object arg) {
+        setMessage(message);
+        addArgument(arg);
+        log();
+    }
+
+    @Override
+    public void log(final String message, final Object arg0, final Object arg1) {
+        setMessage(message);
+        addArgument(arg0);
+        addArgument(arg1);
+        log();
+    }
+
+    @Override
+    public void log(final String message, final Object... args) {
+        setMessage(message);
+        for (final Object arg : args) {
+            addArgument(arg);
+        }
+        log();
+    }
+
+    @Override
+    public void log(final Supplier<String> messageSupplier) {
+        setMessage(messageSupplier);
+        log();
+    }
+
+    @Override
+    public void setCallerBoundary(String fqcn) {
+        this.fqcn = fqcn;
+    }
+}