Add apollo-log4j2 module to support log4j2.xml integration

apollo-plugin/apollo-log4j2/README.md

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+# Apollo Log4j2
+
+This module could let you integrate log4j2 with apollo easily. You could create an `log42.xml` namespace.
+
+## How to use it?
+
+There are several steps that need to do:
+
+1. Add log4j2 dependency to your classpath
+   ```xml
+   <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+   </dependency>
+   <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-core</artifactId>
+   </dependency>
+   ```
+2. Add `apollo-log4j2` dependency to your classpath
+   ```xml
+   <dependency>
+       <groupId>com.ctrip.framework.apollo</groupId>
+       <artifactId>apollo-log4j2</artifactId>
+   </dependency>
+   ```
+3. Create a new namespace `log4j2.xml` (namespace format must be XML) in your apollo application
+4. Add system properties `apollo.log4j2.enabled` when you run java application
+   ```bash
+   -Dapollo.log4j2.enabled=true
+   ```
+6. Now run the java application, then it could load log4j2 content from Apollo

apollo-plugin/apollo-log4j2/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<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">
+    <parent>
+        <artifactId>apollo-plugin</artifactId>
+        <groupId>com.ctrip.framework.apollo</groupId>
+        <version>${revision}</version>
+        <relativePath>../pom.xml</relativePath>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>apollo-log4j2</artifactId>
+    <name>Apollo Log4j2</name>
+    <packaging>jar</packaging>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.ctrip.framework.apollo</groupId>
+            <artifactId>apollo-client</artifactId>
+            <optional>true</optional>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.logging.log4j</groupId>
+            <artifactId>log4j-core</artifactId>
+            <optional>true</optional>
+        </dependency>
+    </dependencies>
+</project>

apollo-plugin/apollo-log4j2/src/main/java/com/ctrip/framework/apollo/log4j2/ApolloLog4j2ConfigurationFactory.java

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+package com.ctrip.framework.apollo.log4j2;
+
+import com.ctrip.framework.apollo.ConfigFile;
+import com.ctrip.framework.apollo.ConfigService;
+import com.ctrip.framework.apollo.core.enums.ConfigFileFormat;
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.Configuration;
+import org.apache.logging.log4j.core.config.ConfigurationException;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
+import org.apache.logging.log4j.core.config.ConfigurationSource;
+import org.apache.logging.log4j.core.config.Order;
+import org.apache.logging.log4j.core.config.plugins.Plugin;
+import org.apache.logging.log4j.core.config.xml.XmlConfiguration;
+import org.apache.logging.log4j.util.Strings;
+
+/**
+ * @author nisiyong
+ */
+@Plugin(name = "ApolloLog4j2ConfigurationFactory", category = ConfigurationFactory.CATEGORY)
+@Order(50)
+public class ApolloLog4j2ConfigurationFactory extends ConfigurationFactory {
+
+  private final boolean isActive;
+
+  public ApolloLog4j2ConfigurationFactory() {
+    String enabled = System.getProperty("apollo.log4j2.enabled");
+    isActive = Boolean.parseBoolean(enabled);
+  }
+
+  @Override
+  protected boolean isActive() {
+    return this.isActive;
+  }
+
+  @Override
+  protected String[] getSupportedTypes() {
+    return new String[]{"*"};
+  }
+
+  @Override
+  public Configuration getConfiguration(LoggerContext loggerContext, String name, URI configLocation) {
+    return getConfiguration(loggerContext, null);
+  }
+
+  @Override
+  public Configuration getConfiguration(LoggerContext loggerContext, ConfigurationSource configurationSource) {
+    if (!isActive) {
+      return null;
+    }
+
+    ConfigFile configFile = ConfigService.getConfigFile("log4j2", ConfigFileFormat.XML);
+
+    if (configFile == null || Strings.isBlank(configFile.getContent())) {
+      return null;
+    }
+
+    byte[] bytes = configFile.getContent().getBytes(StandardCharsets.UTF_8);
+    try {
+      configurationSource = new ConfigurationSource(new ByteArrayInputStream(bytes));
+    } catch (IOException e) {
+      throw new ConfigurationException("Unable to initialize ConfigurationSource from Apollo", e);
+    }
+
+    // TODO add ConfigFileChangeListener, dynamic load log4j2.xml in runtime
+    LOGGER.debug("Initializing configuration ApolloLog4j2Configuration[namespace=log4j2.xml]\n{}", configFile.getContent());
+    return new XmlConfiguration(loggerContext, configurationSource);
+  }
+}

apollo-plugin/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<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">
+    <parent>
+        <artifactId>apollo-java</artifactId>
+        <groupId>com.ctrip.framework.apollo</groupId>
+        <version>${revision}</version>
+        <relativePath>../pom.xml</relativePath>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>apollo-plugin</artifactId>
+    <name>Apollo Plugin</name>
+    <packaging>pom</packaging>
+
+    <modules>
+        <module>apollo-log4j2</module>
+    </modules>
+
+</project>

pom.xml

@@ -82,6 +82,7 @@
     <module>apollo-client-config-data</module>
     <module>apollo-mockserver</module>
     <module>apollo-openapi</module>
+    <module>apollo-plugin</module>
   </modules>
 
   <dependencyManagement>