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# Plugin Registry
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Plugin mapping information for groupId-level metadata, enabling Maven plugin resolution by prefix and maintaining plugin directory indices.
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## Capabilities
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### Plugin Class
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Plugin information for groupId directories, providing mappings between plugin prefixes, names, and artifactIds.
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```java { .api }
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/**
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 * Plugin mapping information for groupId directories
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 * Maps plugin prefixes to artifactIds for Maven plugin resolution
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 */
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public class Plugin implements Serializable, Cloneable {
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  // Constructors
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  public Plugin();
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  // Plugin identification
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  public String getName();
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  public void setName(String name);
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  public String getPrefix();
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  public void setPrefix(String prefix);
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  public String getArtifactId();
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  public void setArtifactId(String artifactId);
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  // Object operations
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  public Plugin clone();
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}
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```
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**Usage Examples:**
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```java
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import org.apache.maven.artifact.repository.metadata.Plugin;
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// Create plugin mapping for compiler plugin
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Plugin compilerPlugin = new Plugin();
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compilerPlugin.setName("Apache Maven Compiler Plugin");
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compilerPlugin.setPrefix("compiler");
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compilerPlugin.setArtifactId("maven-compiler-plugin");
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// Create plugin mapping for surefire plugin
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Plugin surefirePlugin = new Plugin();
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surefirePlugin.setName("Maven Surefire Plugin");
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surefirePlugin.setPrefix("surefire");
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surefirePlugin.setArtifactId("maven-surefire-plugin");
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// Plugin properties
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System.out.println("Plugin name: " + compilerPlugin.getName());
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System.out.println("Plugin prefix: " + compilerPlugin.getPrefix());
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System.out.println("Plugin artifactId: " + compilerPlugin.getArtifactId());
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```
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### Plugin Management in Metadata
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Plugin registration and management at the groupId level for enabling prefix-based plugin resolution.
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```java { .api }
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// Plugin management methods in Metadata class
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public List<Plugin> getPlugins();
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public void setPlugins(List<Plugin> plugins);
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public void addPlugin(Plugin plugin);
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public void removePlugin(Plugin plugin);
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```
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**Usage Examples:**
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```java
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import org.apache.maven.artifact.repository.metadata.Metadata;
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import org.apache.maven.artifact.repository.metadata.Plugin;
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import java.util.List;
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// Create groupId-level metadata
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Metadata groupMetadata = new Metadata();
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groupMetadata.setGroupId("org.apache.maven.plugins");
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// Add compiler plugin
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Plugin compilerPlugin = new Plugin();
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compilerPlugin.setName("Apache Maven Compiler Plugin");
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compilerPlugin.setPrefix("compiler");
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compilerPlugin.setArtifactId("maven-compiler-plugin");
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groupMetadata.addPlugin(compilerPlugin);
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// Add surefire plugin
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Plugin surefirePlugin = new Plugin();
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surefirePlugin.setName("Maven Surefire Plugin");
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surefirePlugin.setPrefix("surefire");
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surefirePlugin.setArtifactId("maven-surefire-plugin");
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groupMetadata.addPlugin(surefirePlugin);
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// Add failsafe plugin
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Plugin failsafePlugin = new Plugin();
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failsafePlugin.setName("Maven Failsafe Plugin");
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failsafePlugin.setPrefix("failsafe");
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failsafePlugin.setArtifactId("maven-failsafe-plugin");
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groupMetadata.addPlugin(failsafePlugin);
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// Access plugin list
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List<Plugin> plugins = groupMetadata.getPlugins();
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System.out.println("Number of plugins: " + plugins.size());
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// Find plugin by prefix
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for (Plugin plugin : plugins) {
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    if ("compiler".equals(plugin.getPrefix())) {
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        System.out.println("Found compiler plugin: " + plugin.getArtifactId());
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        break;
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    }
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}
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// Remove a plugin
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groupMetadata.removePlugin(failsafePlugin);
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```
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### Plugin Resolution Workflow
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How plugins are resolved using the registry information stored in groupId-level metadata.
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**Plugin Resolution Process:**
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1. **Prefix lookup**: Maven searches groupId-level metadata for plugin prefix
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2. **ArtifactId mapping**: Maps prefix to specific artifactId
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3. **Plugin loading**: Loads plugin using resolved groupId/artifactId combination
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**Example Resolution:**
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```java
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// When user runs: mvn compiler:compile
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// Maven resolves this to: org.apache.maven.plugins:maven-compiler-plugin
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// Step 1: Look in org.apache.maven.plugins group metadata
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Metadata groupMetadata = readGroupMetadata("org.apache.maven.plugins");
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// Step 2: Find plugin with prefix "compiler"
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Plugin resolvedPlugin = null;
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for (Plugin plugin : groupMetadata.getPlugins()) {
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    if ("compiler".equals(plugin.getPrefix())) {
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        resolvedPlugin = plugin;
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        break;
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    }
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}
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// Step 3: Use resolved artifactId
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if (resolvedPlugin != null) {
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    String artifactId = resolvedPlugin.getArtifactId(); // "maven-compiler-plugin"
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    // Maven can now load: org.apache.maven.plugins:maven-compiler-plugin
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}
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```
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### Plugin Metadata Merging
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Plugin merging logic during metadata consolidation from multiple sources.
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**Merging Behavior:**
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- **Deduplication by prefix**: Plugins with same prefix are not duplicated
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- **No overwrites**: Existing plugins are preserved during merge operations
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- **Additive merging**: New plugins from source metadata are added to target
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**Usage Examples:**
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```java
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// Local metadata with some plugins
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Metadata localMetadata = new Metadata();
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localMetadata.setGroupId("org.apache.maven.plugins");
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Plugin compilerPlugin = new Plugin();
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compilerPlugin.setPrefix("compiler");
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compilerPlugin.setArtifactId("maven-compiler-plugin");
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localMetadata.addPlugin(compilerPlugin);
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// Remote metadata with additional plugins
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Metadata remoteMetadata = new Metadata();
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remoteMetadata.setGroupId("org.apache.maven.plugins");
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Plugin surefirePlugin = new Plugin();
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surefirePlugin.setPrefix("surefire");
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surefirePlugin.setArtifactId("maven-surefire-plugin");
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remoteMetadata.addPlugin(surefirePlugin);
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// Merge remote plugins into local metadata
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boolean changed = localMetadata.merge(remoteMetadata);
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if (changed) {
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    // Local metadata now contains both compiler and surefire plugins
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    System.out.println("Plugins merged successfully");
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    System.out.println("Total plugins: " + localMetadata.getPlugins().size());
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}
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```
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## Common Plugin Prefixes
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Standard Maven plugin prefixes commonly found in repositories:
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| Prefix | ArtifactId | Purpose |
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|--------|------------|---------|
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| compiler | maven-compiler-plugin | Java compilation |
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| surefire | maven-surefire-plugin | Unit test execution |
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| failsafe | maven-failsafe-plugin | Integration test execution |
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| jar | maven-jar-plugin | JAR file creation |
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| war | maven-war-plugin | WAR file creation |
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| clean | maven-clean-plugin | Clean lifecycle |
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| install | maven-install-plugin | Install to local repository |
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| deploy | maven-deploy-plugin | Deploy to remote repository |
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| site | maven-site-plugin | Site generation |
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| assembly | maven-assembly-plugin | Assembly creation |
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## Error Handling
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Plugin registry includes robust error handling:
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- **Null safety**: Empty plugin lists instead of null collections
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- **Clone support**: Deep cloning preserves all plugin data
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- **Serialization**: Full serialization support for persistence
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- **Duplicate prevention**: Merge operations prevent duplicate prefixes