[JENKINS-32765] Perhaps it makes more sense to make this a property of the...

[JENKINS-32765] Perhaps it makes more sense to make this a property of the PluginManager rather than the classic strategy

- Also expose via servlet context parameter of same name to allow multiple instances of Jenkins in the same servlet container to have different directories
- As we are no longer a constant, change the system property to camelCase

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -88,12 +88,6 @@ public class ClassicPluginStrategy implements PluginStrategy {
         }
     };
 
-    /**
-     * If set this should be the absolute file path to the the base directory for all exploded .hpi/.jpi plugins.
-     * If {@literal null} then plugins will be exploded into {@literal JENKINS_HOME/plugins}.
-     */
-    private static final String WORK_DIR = System.getProperty(ClassicPluginStrategy.class.getName() + ".WORK_DIR");
-
     private PluginManager pluginManager;
 
     /**
@@ -168,10 +162,8 @@ public class ClassicPluginStrategy implements PluginStrategy {
             if (archive.isDirectory()) {// already expanded
                 expandDir = archive;
             } else {
-                expandDir = WORK_DIR == null
-                        ? new File(archive.getParentFile(), getBaseName(archive.getName()))
-                        : new File(WORK_DIR, getBaseName(archive.getName()));
-                ;
+                File f = pluginManager.getWorkDir();
+                expandDir =  new File(f == null ? archive.getParentFile() : f, getBaseName(archive.getName()));
                 explode(archive, expandDir);
             }
 

core/src/main/java/hudson/PluginManager.java

@@ -149,6 +149,12 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
      */
     public final File rootDir;
 
+    /**
+     * If non-null, the base directory for all exploded .hpi/.jpi plugins. Controlled by the system property / servlet
+     * context parameter {@literal hudson.PluginManager.workDir}.
+     */
+    private final File workDir;
+
     /**
      * @deprecated as of 1.355
      *      {@link PluginManager} can now live longer than {@link jenkins.model.Jenkins} instance, so
@@ -199,6 +205,11 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
         this.rootDir = rootDir;
         if(!rootDir.exists())
             rootDir.mkdirs();
+        String workDir = System.getProperty(PluginManager.class.getName()+".workDir");
+        if (workDir == null) {
+            workDir = context.getInitParameter(PluginManager.class.getName() + ".workDir");
+        }
+        this.workDir = workDir == null ? null : new File(workDir);
 
         strategy = createPluginStrategy();
 
@@ -218,6 +229,14 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
         return new Api(this);
     }
 
+    /**
+     * If non-null, the base directory for all exploded .hpi/.jpi plugins.
+     * @return the base directory for all exploded .hpi/.jpi plugins or {@code null} to leave this up to the strategy.
+     */
+    public File getWorkDir() {
+        return workDir;
+    }
+
     /**
      * Find all registered overrides (intended to allow overriding/adding views)
      * @return List of extensions