Merge pull request #4917 from Monits:fix-4913

[cli] Properly wait for cpd-gui until close #4917

docs/pages/release_notes.md

@@ -25,6 +25,7 @@ This is a {{ site.pmd.release_type }} release.
   * [#494](https://github.com/pmd/pmd/issues/494): \[core] Adopt JApiCmp to enforce control over API changes
 * cli
   * [#4791](https://github.com/pmd/pmd/issues/4791): \[cli] Could not find or load main class
+  * [#4913](https://github.com/pmd/pmd/issues/4913): \[cli] cpd-gui closes immediately
 * apex-errorprone
   * [#3953](https://github.com/pmd/pmd/issues/3953): \[apex] EmptyCatchBlock false positive with formal (doc) comments
 * java-bestpractices

pmd-cli/src/main/java/net/sourceforge/pmd/cli/commands/internal/CpdGuiCommand.java

@@ -15,6 +15,13 @@ public class CpdGuiCommand implements Runnable {
     @Override
     public void run() {
         new GUI();
+
+        // wait for the process to be killed by the GUI
+        try {
+            Thread.currentThread().join();
+        } catch (InterruptedException ignored) {
+            // noop
+        }
     }
 
 }