Prevent NPE during shutdown when the hook is already released

ibauersachs · ibauersachs · commit 34ba40e14de9 · 2026-01-18T16:12:06.000+01:00
diff --git a/src/main/java/org/xbill/DNS/NioClient.java b/src/main/java/org/xbill/DNS/NioClient.java
@@ -101,10 +101,14 @@ private static void close(boolean fromHook) {
     }
 
     if (!fromHook) {
-      try {
-        Runtime.getRuntime().removeShutdownHook(closeThread);
-      } catch (Exception ex) {
-        log.warn("Failed to remove shutdown hook, ignoring and continuing close");
+      synchronized (NIO_CLIENT_LOCK) {
+        if (closeThread != null) {
+          try {
+            Runtime.getRuntime().removeShutdownHook(closeThread);
+          } catch (Exception ex) {
+            log.warn("Failed to remove shutdown hook, ignoring and continuing close", ex);
+          }
+        }
       }
     }
 

Original file line number	Diff line number	Diff line change
`@@ -101,10 +101,14 @@ private static void close(boolean fromHook) {`
`101`	`101`	`}`
`102`	`102`
`103`	`103`	`if (!fromHook) {`
`104`		`- try {`
`105`		`- Runtime.getRuntime().removeShutdownHook(closeThread);`
`106`		`- } catch (Exception ex) {`
`107`		`- log.warn("Failed to remove shutdown hook, ignoring and continuing close");`
	`104`	`+ synchronized (NIO_CLIENT_LOCK) {`
	`105`	`+ if (closeThread != null) {`
	`106`	`+ try {`
	`107`	`+ Runtime.getRuntime().removeShutdownHook(closeThread);`
	`108`	`+ } catch (Exception ex) {`
	`109`	`+ log.warn("Failed to remove shutdown hook, ignoring and continuing close", ex);`
	`110`	`+ }`
	`111`	`+ }`
`108`	`112`	`}`
`109`	`113`	`}`
`110`	`114`