Adds a Configuration Converter API (#142)

Adds an API to perform automatic conversions between different logging configuration file formats and a basic implementation.
The implementation supports the following configuration file formats:

- Log4j Core 2 XML,
- Log4j Core 2 JSON,
- Log4j Core 2 YAML,
- Log4j Core 2 Properties (read-only),
- Log4j Core 3 Properties.

The API is extensible through `ServiceLoader` and allows integrators to provide support for additional configuration file formats.

Read-only support for the Log4j 1 Properties and Log4j 1 XML configuration formats will be provided in a separate PR.

**Note**: Currently the API is only accessible from Java code.
Its main purpose is to provide a "Migrate Log4j 1.x to Log4j Core 2.x", a "Migrate Logback to Log4j Core 2.x" and a "Migrate JUL to Log4j Core 2.x" [OpenRewrite recipe](https://docs.openrewrite.org/recipes/java/logging/log4j).
A `picocli`-based `log4j-transform-cli` tool to access all the goodies in this repository, will be provided later.

Closes apache/logging-log4j2#2080

log4j-codegen/pom.xml

@@ -43,24 +43,6 @@
 
   <dependencies>
 
-    <dependency>
-      <groupId>org.osgi</groupId>
-      <artifactId>org.osgi.annotation.bundle</artifactId>
-      <scope>provided</scope>
-    </dependency>
-
-    <dependency>
-      <groupId>org.osgi</groupId>
-      <artifactId>org.osgi.annotation.versioning</artifactId>
-      <scope>provided</scope>
-    </dependency>
-
-    <dependency>
-      <groupId>com.github.spotbugs</groupId>
-      <artifactId>spotbugs-annotations</artifactId>
-      <scope>provided</scope>
-    </dependency>
-
     <!-- Compile dependencies: the artifact is shaded, so limit these. -->
     <dependency>
       <groupId>info.picocli</groupId>

log4j-converter-config/pom.xml

@@ -0,0 +1,113 @@
+<?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-transform-parent</artifactId>
+    <version>${revision}</version>
+    <relativePath>../log4j-transform-parent</relativePath>
+  </parent>
+
+  <artifactId>log4j-converter-config</artifactId>
+  <name>Apache Log4j Configuration Converter</name>
+  <description>Converts various logging configuration formats to the Log4j Core 2.x format.</description>
+
+  <properties>
+    <!-- This artifact is an API: we need to generate its Javadoc -->
+    <maven.javadoc.skip>false</maven.javadoc.skip>
+
+    <!-- Remove after first release -->
+    <bnd.baseline.fail.on.missing>false</bnd.baseline.fail.on.missing>
+
+    <!-- Dependencies -->
+    <jackson.version>2.18.1</jackson.version>
+    <txw2.version>4.0.5</txw2.version>
+  </properties>
+
+  <dependencyManagement>
+    <dependencies>
+
+      <dependency>
+        <groupId>com.fasterxml.jackson</groupId>
+        <artifactId>jackson-bom</artifactId>
+        <version>${jackson.version}</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+
+    </dependencies>
+  </dependencyManagement>
+
+  <dependencies>
+
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+    </dependency>
+
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+    </dependency>
+
+    <!-- Used in the JSON configuration format -->
+    <dependency>
+      <groupId>com.fasterxml.jackson.core</groupId>
+      <artifactId>jackson-databind</artifactId>
+    </dependency>
+
+    <!-- Used in the v3 Properties configuration format -->
+    <dependency>
+      <groupId>com.fasterxml.jackson.dataformat</groupId>
+      <artifactId>jackson-dataformat-properties</artifactId>
+    </dependency>
+
+    <!-- Used in the YAML configuration format -->
+    <dependency>
+      <groupId>com.fasterxml.jackson.dataformat</groupId>
+      <artifactId>jackson-dataformat-yaml</artifactId>
+    </dependency>
+
+    <!-- Used in the XML configuration format -->
+    <dependency>
+      <groupId>org.glassfish.jaxb</groupId>
+      <artifactId>txw2</artifactId>
+      <version>${txw2.version}</version>
+    </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>
+
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+
+  </dependencies>
+</project>

log4j-converter-config/src/main/java/org/apache/logging/converter/config/ConfigurationConverter.java

@@ -0,0 +1,58 @@
+/*
+ * 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.converter.config;
+
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.Set;
+import org.apache.logging.converter.config.internal.DefaultConfigurationConverter;
+
+/**
+ * Service class to convert between different logging configuration formats.
+ */
+public interface ConfigurationConverter {
+
+    /**
+     * A default implementation of {@link ConfigurationConverter} that uses {@link java.util.ServiceLoader} to load additional formats.
+     * @see org.apache.logging.converter.config.spi.ConfigurationMapper
+     */
+    static ConfigurationConverter getInstance() {
+        return DefaultConfigurationConverter.INSTANCE;
+    }
+
+    /**
+     * Converts a logging configuration file from one format to another.
+     *
+     * @param inputStream The input configuration file, never {@code null}.
+     * @param inputFormat The input format. Must be one of the formats returned by {@link #getSupportedInputFormats()}.
+     * @param outputStream The output configuration file, never {@code null}.
+     * @param outputFormat The output format. Must be one of the formats returned by {@link #getSupportedOutputFormats()}.
+     * @throws ConfigurationConverterException If any kind of error occurs during the conversion process.
+     */
+    void convert(InputStream inputStream, String inputFormat, OutputStream outputStream, String outputFormat)
+            throws ConfigurationConverterException;
+
+    /**
+     * Returns the list of supported input formats.
+     */
+    Set<String> getSupportedInputFormats();
+
+    /**
+     * Returns the list of supported output formats.
+     */
+    Set<String> getSupportedOutputFormats();
+}

log4j-converter-config/src/main/java/org/apache/logging/converter/config/ConfigurationConverterException.java

@@ -0,0 +1,33 @@
+/*
+ * 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.converter.config;
+
+/**
+ * Exception thrown by {@link ConfigurationConverter}, when a problem occurs during the conversion.
+ */
+public class ConfigurationConverterException extends RuntimeException {
+
+    private static final long serialVersionUID = 1L;
+
+    public ConfigurationConverterException(final String message) {
+        super(message);
+    }
+
+    public ConfigurationConverterException(final String message, final Throwable cause) {
+        super(message, cause);
+    }
+}

log4j-converter-config/src/main/java/org/apache/logging/converter/config/internal/ComponentUtils.java

@@ -0,0 +1,156 @@
+/*
+ * 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.converter.config.internal;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+import java.util.function.Supplier;
+import org.apache.logging.converter.config.ConfigurationConverterException;
+import org.apache.logging.converter.config.spi.ConfigurationNode;
+import org.jspecify.annotations.Nullable;
+
+public final class ComponentUtils {
+
+    public static ConfigurationNodeBuilder newNodeBuilder() {
+        return new ConfigurationNodeBuilder();
+    }
+
+    public static ConfigurationNode createThresholdFilter(String level) {
+        return newNodeBuilder()
+                .setPluginName("ThresholdFilter")
+                .addAttribute("level", level)
+                .build();
+    }
+
+    public static ConfigurationNode createCompositeFilter(Iterable<? extends ConfigurationNode> filters) {
+        ConfigurationNodeBuilder builder = newNodeBuilder().setPluginName("Filters");
+        filters.forEach(builder::addChild);
+        return builder.build();
+    }
+
+    private ComponentUtils() {}
+
+    public static class ConfigurationNodeBuilder implements Supplier<ConfigurationNode> {
+
+        private @Nullable String pluginName;
+        private final Map<String, String> attributes = new TreeMap<>();
+        private final List<ConfigurationNode> children = new ArrayList<>();
+
+        protected ConfigurationNodeBuilder() {}
+
+        public ConfigurationNodeBuilder setPluginName(String pluginName) {
+            this.pluginName = pluginName;
+            return this;
+        }
+
+        public ConfigurationNodeBuilder addAttribute(String key, @Nullable String value) {
+            if (value != null) {
+                attributes.put(key, value);
+            }
+            return this;
+        }
+
+        public ConfigurationNodeBuilder addAttribute(String key, boolean value) {
+            attributes.put(key, String.valueOf(value));
+            return this;
+        }
+
+        public ConfigurationNodeBuilder addChild(ConfigurationNode child) {
+            children.add(child);
+            return this;
+        }
+
+        public ConfigurationNode build() {
+            if (pluginName == null) {
+                throw new ConfigurationConverterException("No plugin name specified");
+            }
+            return new ConfigurationNodeImpl(pluginName, attributes, children);
+        }
+
+        @Override
+        public ConfigurationNode get() {
+            return build();
+        }
+    }
+
+    private static final class ConfigurationNodeImpl implements ConfigurationNode {
+
+        private final String pluginName;
+        private final Map<String, String> attributes;
+        private final List<ConfigurationNode> children;
+
+        private ConfigurationNodeImpl(
+                final String pluginName,
+                final Map<String, String> attributes,
+                final Collection<ConfigurationNode> children) {
+            this.pluginName = pluginName;
+            this.attributes = Collections.unmodifiableMap(new TreeMap<>(attributes));
+            this.children = Collections.unmodifiableList(new ArrayList<>(children));
+        }
+
+        @Override
+        public String getPluginName() {
+            return pluginName;
+        }
+
+        @Override
+        public Map<String, String> getAttributes() {
+            return attributes;
+        }
+
+        @Override
+        public List<? extends ConfigurationNode> getChildren() {
+            return children;
+        }
+
+        private static void formatTo(ConfigurationNode node, StringBuilder builder, int indent) {
+            String indentation = getIndentation(indent);
+            builder.append(indentation).append("<").append(node.getPluginName());
+            for (final Map.Entry<String, String> entry : node.getAttributes().entrySet()) {
+                builder.append(" ")
+                        .append(entry.getKey())
+                        .append("=\"")
+                        .append(entry.getValue())
+                        .append("\"");
+            }
+            builder.append(">\n");
+            for (ConfigurationNode child : node.getChildren()) {
+                formatTo(child, builder, indent + 1);
+                builder.append('\n');
+            }
+            builder.append(indentation)
+                    .append("</")
+                    .append(node.getPluginName())
+                    .append(">");
+        }
+
+        private static String getIndentation(int indent) {
+            return String.join("", Collections.nCopies(indent, "  "));
+        }
+
+        @Override
+        public String toString() {
+            StringBuilder builder = new StringBuilder();
+            formatTo(this, builder, 0);
+            return builder.toString();
+        }
+    }
+}