The NIO cleaner hack wasn't working as expected, because the clear method was…

The NIO cleaner hack wasn't working as expected, because the clear method was called instead of the clean method. This has been fixed. Please note this hack is not enabled by default, to try it you need to set the system property "h2.nioCleanerHack" to true.

h2/src/main/org/h2/store/fs/FileObjectDiskMapped.java

@@ -64,8 +64,10 @@ public class FileObjectDiskMapped implements FileObject {
                 Method cleanerMethod = mapped.getClass().getMethod("cleaner");
                 cleanerMethod.setAccessible(true);
                 Object cleaner = cleanerMethod.invoke(mapped);
-                Method clearMethod = cleaner.getClass().getMethod("clear");
-                clearMethod.invoke(cleaner);
+                if (cleaner != null) {
+                    Method clearMethod = cleaner.getClass().getMethod("clean");
+                    clearMethod.invoke(cleaner);
+                }
                 useSystemGc = false;
             } catch (Throwable e) {
                 // useSystemGc is already true