fix: let Plugin implement AutoCloseable

CHANGELOG.md

@@ -7,6 +7,7 @@ Currently the versioning policy of this project follows [Semantic Versioning v2.
 ## Unreleased - 2022-??-??
 ### Changed
 - Updated documentation by adding parenthesis `()` to the negative odd check message ([#1995](https://github.com/spotbugs/spotbugs/issues/1995))
+- Let the Plugin class implement AutoCloseable so we can release the .jar file ([#2024](https://github.com/spotbugs/spotbugs/issues/2024))
 
 ### Fixed
 - Bumped Saxon-HE from 10.6 to 11.3 ([#1955](https://github.com/spotbugs/spotbugs/pull/1955), [#1999](https://github.com/spotbugs/spotbugs/pull/1999))

spotbugs/src/main/java/edu/umd/cs/findbugs/Plugin.java

@@ -20,10 +20,12 @@
 package edu.umd.cs.findbugs;
 
 import java.io.File;
+import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.URL;
+import java.net.URLClassLoader;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -53,7 +55,7 @@
  * @see PluginLoader
  * @author David Hovemeyer
  */
-public class Plugin {
+public class Plugin implements AutoCloseable {
 
 
     private static final String USE_FINDBUGS_VERSION = "USE_FINDBUGS_VERSION";
@@ -696,4 +698,12 @@ public static synchronized void removeCustomPlugin(Plugin plugin) {
         DetectorFactoryCollection.instance().unLoadPlugin(plugin);
     }
 
+    /**
+     * Closes the underlying {@link PluginLoader}, in turn this closes the {@link URLClassLoader}.
+     * When loading a custom plugin from a .jar file this method needs to be called to release the reference on that file.
+     */
+    @Override
+    public void close() throws IOException {
+        pluginLoader.close();
+    }
 }

spotbugs/src/main/java/edu/umd/cs/findbugs/PluginLoader.java

@@ -102,7 +102,7 @@
  * @see Plugin
  * @see PluginException
  */
-public class PluginLoader {
+public class PluginLoader implements AutoCloseable {
 
     private static final String XPATH_PLUGIN_SHORT_DESCRIPTION = "/MessageCollection/Plugin/ShortDescription";
     private static final String XPATH_PLUGIN_WEBSITE = "/FindbugsPlugin/@website";
@@ -140,6 +140,9 @@ public class PluginLoader {
 
     private final URI loadedFromUri;
 
+    // The classloaders we have created, so we can close then if we want to close the plugin
+    private List<URLClassLoader> createdClassLoaders = new ArrayList<>();
+
     /** plugin Id for parent plugin */
     String parentId;
 
@@ -196,7 +199,7 @@ public boolean hasParent() {
      */
     private PluginLoader(@Nonnull URL url, URI uri, ClassLoader parent, boolean isInitial, boolean optional) throws PluginException {
         URL[] loaderURLs = createClassloaderUrls(url);
-        classLoaderForResources = new URLClassLoader(loaderURLs);
+        classLoaderForResources = buildURLClassLoader(loaderURLs);
         loadedFrom = url;
         loadedFromUri = uri;
         jarName = getJarName(url);
@@ -205,15 +208,15 @@ private PluginLoader(@Nonnull URL url, URI uri, ClassLoader parent, boolean isIn
         optionalPlugin = optional;
         plugin = init();
         if (!hasParent()) {
-            classLoader = new URLClassLoader(loaderURLs, parent);
+            classLoader = buildURLClassLoader(loaderURLs, parent);
         } else {
             if (parent != PluginLoader.class.getClassLoader()) {
                 throw new IllegalArgumentException("Can't specify parentid " + parentId + " and provide a separate class loader");
             }
             Plugin parentPlugin = Plugin.getByPluginId(parentId);
             if (parentPlugin != null) {
                 parent = parentPlugin.getClassLoader();
-                classLoader = new URLClassLoader(loaderURLs, parent);
+                classLoader = buildURLClassLoader(loaderURLs, parent);
             }
         }
         if (classLoader == null) {
@@ -246,7 +249,7 @@ private static void finishLazyInitialization() {
                     i.remove();
                     try {
                         URL[] loaderURLs = PluginLoader.createClassloaderUrls(pluginLoader.loadedFrom);
-                        pluginLoader.classLoader = new URLClassLoader(loaderURLs, parent.getClassLoader());
+                        pluginLoader.classLoader = pluginLoader.buildURLClassLoader(loaderURLs, parent.getClassLoader());
                         pluginLoader.loadPluginComponents();
                         Plugin.putPlugin(pluginLoader.loadedFromUri, pluginLoader.plugin);
                     } catch (PluginException e) {
@@ -275,6 +278,50 @@ private static void finishLazyInitialization() {
         lazyInitialization = false;
     }
 
+    /**
+     * Creates a new {@link URLClassLoader} and adds it to the list of classloaders
+     * we need to close if we close the corresponding plugin
+     * 
+     * @param 
+     *            urls the URLs from which to load classes and resources
+     * @return a new {@link URLClassLoader}
+     */
+    private URLClassLoader buildURLClassLoader(URL[] urls) {
+        URLClassLoader urlClassLoader = new URLClassLoader(urls);
+        createdClassLoaders.add(urlClassLoader);
+
+        return urlClassLoader;
+    }
+
+    /**
+     * Creates a new {@link URLClassLoader} and adds it to the list of classloaders
+     * we need to close if we close the corresponding plugin
+     * 
+     * @param 
+     *            urls the URLs from which to load classes and resources
+     * @param 
+     *            parent the parent class loader for delegation
+     * @return a new {@link URLClassLoader}
+     */
+    private URLClassLoader buildURLClassLoader(URL[] urls, ClassLoader parent) {
+        URLClassLoader urlClassLoader = new URLClassLoader(urls, parent);
+        createdClassLoaders.add(urlClassLoader);
+
+        return urlClassLoader;
+    }
+
+    /**
+     * Closes the class loaders created in this {@link PluginLoader}
+     * @throws IOException if a class loader fails to close, in that case the other classloaders won't be closed
+     */
+    @Override
+    public void close() throws IOException {
+        for (URLClassLoader urlClassLoader : createdClassLoaders) {
+            urlClassLoader.close();
+        }
+    }
+
+
     /**
      * Patch for issue 3429143: allow plugins load classes/resources from 3rd
      * party jars