Add constants for manifest's attributes names

pf4j/src/main/java/org/pf4j/ManifestPluginDescriptorFinder.java

 
     private static final Logger log = LoggerFactory.getLogger(ManifestPluginDescriptorFinder.class);
 
+    public static final String PLUGIN_ID = "Plugin-Id";
+    public static final String PLUGIN_DESCRIPTION = "Plugin-Description";
+    public static final String PLUGIN_CLASS = "Plugin-Class";
+    public static final String PLUGIN_VERSION = "Plugin-Version";
+    public static final String PLUGIN_PROVIDER = "Plugin-Provider";
+    public static final String PLUGIN_DEPENDENCIES = "Plugin-Dependencies";
+    public static final String PLUGIN_REQUIRES = "Plugin-Requires";
+    public static final String PLUGIN_LICENSE = "Plugin-License";
+
     @Override
     public boolean isApplicable(Path pluginPath) {
         return Files.exists(pluginPath) && (Files.isDirectory(pluginPath) || FileUtils.isJarFile(pluginPath));
 
         // TODO validate !!!
         Attributes attributes = manifest.getMainAttributes();
-        String id = attributes.getValue("Plugin-Id");
+        String id = attributes.getValue(PLUGIN_ID);
         pluginDescriptor.setPluginId(id);
 
-        String description = attributes.getValue("Plugin-Description");
+        String description = attributes.getValue(PLUGIN_DESCRIPTION);
         if (StringUtils.isNullOrEmpty(description)) {
             pluginDescriptor.setPluginDescription("");
         } else {
             pluginDescriptor.setPluginDescription(description);
         }
 
-        String clazz = attributes.getValue("Plugin-Class");
+        String clazz = attributes.getValue(PLUGIN_CLASS);
         if (StringUtils.isNotNullOrEmpty(clazz)) {
             pluginDescriptor.setPluginClass(clazz);
         }
 
-        String version = attributes.getValue("Plugin-Version");
+        String version = attributes.getValue(PLUGIN_VERSION);
         if (StringUtils.isNotNullOrEmpty(version)) {
             pluginDescriptor.setPluginVersion(version);
         }
 
-        String provider = attributes.getValue("Plugin-Provider");
+        String provider = attributes.getValue(PLUGIN_PROVIDER);
         pluginDescriptor.setProvider(provider);
-        String dependencies = attributes.getValue("Plugin-Dependencies");
+        String dependencies = attributes.getValue(PLUGIN_DEPENDENCIES);
         pluginDescriptor.setDependencies(dependencies);
 
-        String requires = attributes.getValue("Plugin-Requires");
+        String requires = attributes.getValue(PLUGIN_REQUIRES);
         if (StringUtils.isNotNullOrEmpty(requires)) {
             pluginDescriptor.setRequires(requires);
         }
 
-        pluginDescriptor.setLicense(attributes.getValue("Plugin-License"));
+        pluginDescriptor.setLicense(attributes.getValue(PLUGIN_LICENSE));
 
         return pluginDescriptor;
     }