We should be able to use the ByteBuffer cleaner on java8 (and earlier… (#8234)

* We should be able to use the ByteBuffer cleaner on java8 (and earlier versions) even if sun.misc.Unsafe is not present.

Motivation:

At the moment we have a hard dependency on sun.misc.Unsafe to use the Cleaner on Java8 and earlier. This is not really needed as we can still use pure reflection if sun.misc.Unsafe is not present.

Modifications:

Refactor Cleaner6 to fallback to pure reflection if sun.misc.Unsafe is not present on system.

Result:

More timely releasing of direct memory on Java8 and earlier when sun.misc.Unsafe is not present.

common/src/main/java/io/netty/util/internal/CleanerJava6.java

@@ -21,6 +21,8 @@ import io.netty.util.internal.logging.InternalLoggerFactory;
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.nio.ByteBuffer;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
 
 
 /**
@@ -32,19 +34,50 @@ import java.nio.ByteBuffer;
 final class CleanerJava6 implements Cleaner {
     private static final long CLEANER_FIELD_OFFSET;
     private static final Method CLEAN_METHOD;
+    private static final Field CLEANER_FIELD;
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(CleanerJava6.class);
 
     static {
-        long fieldOffset = -1;
-        Method clean = null;
+        long fieldOffset;
+        Method clean;
+        Field cleanerField;
         Throwable error = null;
-        if (PlatformDependent0.hasUnsafe()) {
-            ByteBuffer direct = ByteBuffer.allocateDirect(1);
+        final ByteBuffer direct = ByteBuffer.allocateDirect(1);
+        try {
+            Object mayBeCleanerField = AccessController.doPrivileged(new PrivilegedAction<Object>() {
+                @Override
+                public Object run() {
                     try {
                         Field cleanerField =  direct.getClass().getDeclaredField("cleaner");
+                        if (!PlatformDependent.hasUnsafe()) {
+                            // We need to make it accessible if we do not use Unsafe as we will access it via
+                            // reflection.
+                            cleanerField.setAccessible(true);
+                        }
+                        return cleanerField;
+                    } catch (Throwable cause) {
+                        return cause;
+                    }
+                }
+            });
+            if (mayBeCleanerField instanceof Throwable) {
+                throw (Throwable) mayBeCleanerField;
+            }
+
+            cleanerField = (Field) mayBeCleanerField;
+
+            final Object cleaner;
+
+            // If we have sun.misc.Unsafe we will use it as its faster then using reflection,
+            // otherwise let us try reflection as last resort.
+            if (PlatformDependent.hasUnsafe()) {
                 fieldOffset = PlatformDependent0.objectFieldOffset(cleanerField);
-                Object cleaner = PlatformDependent0.getObject(direct, fieldOffset);
+                cleaner = PlatformDependent0.getObject(direct, fieldOffset);
+            } else {
+                fieldOffset = -1;
+                cleaner = cleanerField.get(direct);
+            }
             clean = cleaner.getClass().getDeclaredMethod("clean");
             clean.invoke(cleaner);
         } catch (Throwable t) {
@@ -52,21 +85,21 @@ final class CleanerJava6 implements Cleaner {
             fieldOffset = -1;
             clean = null;
             error = t;
+            cleanerField = null;
         }
-        } else {
-            error = new UnsupportedOperationException("sun.misc.Unsafe unavailable");
-        }
+
         if (error == null) {
             logger.debug("java.nio.ByteBuffer.cleaner(): available");
         } else {
             logger.debug("java.nio.ByteBuffer.cleaner(): unavailable", error);
         }
+        CLEANER_FIELD = cleanerField;
         CLEANER_FIELD_OFFSET = fieldOffset;
         CLEAN_METHOD = clean;
     }
 
     static boolean isSupported() {
-        return CLEANER_FIELD_OFFSET != -1;
+        return CLEANER_FIELD_OFFSET != -1 || CLEANER_FIELD != null;
     }
 
     @Override
@@ -74,13 +107,45 @@ final class CleanerJava6 implements Cleaner {
         if (!buffer.isDirect()) {
             return;
         }
+        if (System.getSecurityManager() == null) {
             try {
-            Object cleaner = PlatformDependent0.getObject(buffer, CLEANER_FIELD_OFFSET);
-            if (cleaner != null) {
-                CLEAN_METHOD.invoke(cleaner);
-            }
+                freeDirectBuffer0(buffer);
             } catch (Throwable cause) {
                 PlatformDependent0.throwException(cause);
             }
+        } else {
+            freeDirectBufferPrivileged(buffer);
+        }
+    }
+
+    private static void freeDirectBufferPrivileged(final ByteBuffer buffer) {
+        Throwable cause = AccessController.doPrivileged(new PrivilegedAction<Throwable>() {
+            @Override
+            public Throwable run() {
+                try {
+                    freeDirectBuffer0(buffer);
+                    return null;
+                } catch (Throwable cause) {
+                    return cause;
+                }
+            }
+        });
+        if (cause != null) {
+            PlatformDependent0.throwException(cause);
+        }
+    }
+
+    private static void freeDirectBuffer0(ByteBuffer buffer) throws Exception {
+        final Object cleaner;
+        // If CLEANER_FIELD_OFFSET == -1 we need to use reflection to access the cleaner, otherwise we can use
+        // sun.misc.Unsafe.
+        if (CLEANER_FIELD_OFFSET == -1) {
+            cleaner = CLEANER_FIELD.get(buffer);
+        } else {
+            cleaner = PlatformDependent0.getObject(buffer, CLEANER_FIELD_OFFSET);
+        }
+        if (cleaner != null) {
+            CLEAN_METHOD.invoke(cleaner);
+        }
     }
 }

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -164,7 +164,7 @@ public final class PlatformDependent {
 
         MAYBE_SUPER_USER = maybeSuperUser0();
 
-        if (!isAndroid() && hasUnsafe()) {
+        if (!isAndroid()) {
             // only direct to method if we are not running on android.
             // See https://github.com/netty/netty/issues/2604
             if (javaVersion() >= 9) {