Fix and simplify freeing a direct buffer / Fix Android support

Motivation:

6e8ba291cf introduced a regression in Android because Android does not have sun.nio.ch.DirectBuffer (see #2330.)  I also found PlatformDependent0.freeDirectBuffer() and freeDirectBufferUnsafe() are pretty much same after the commit and the unsafe version should be removed.

Modifications:

- Do not use the pooled allocator in Android because it's too resource hungry for Androids.
- Merge PlatformDependent0.freeDirectBuffer() and freeDirectBufferUnsafe() into one method.
- Make the Unsafe unavailable when sun.nio.ch.DirectBuffer is unavailable.  We could keep the Unsafe available and handle the sun.nio.ch.DirectBuffer case separately, but I don't want to complicate our code just because of that.  All supported JDK versions have sun.nio.ch.DirectBuffer if the Unsafe is available.

Result:

Simpler code. Fixes Android support (#2330)

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -16,6 +16,7 @@
 package io.netty.buffer;
 
 import io.netty.util.CharsetUtil;
+import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.SystemPropertyUtil;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
@@ -48,7 +49,10 @@ public final class ByteBufUtil {
             HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i       & 0x0F];
         }
 
-        String allocType = SystemPropertyUtil.get("io.netty.allocator.type", "pooled").toLowerCase(Locale.US).trim();
+        String allocType = SystemPropertyUtil.get(
+                "io.netty.allocator.type", PlatformDependent.isAndroid() ? "unpooled" : "pooled");
+        allocType = allocType.toLowerCase(Locale.US).trim();
+
         ByteBufAllocator alloc;
         if ("unpooled".equals(allocType)) {
             alloc = UnpooledByteBufAllocator.DEFAULT;

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

@@ -251,12 +251,8 @@ public final class PlatformDependent {
      * the current platform does not support this operation or the specified buffer is not a direct buffer.
      */
     public static void freeDirectBuffer(ByteBuffer buffer) {
-        if (buffer.isDirect()) {
-            if (hasUnsafe()) {
-                PlatformDependent0.freeDirectBufferUnsafe(buffer);
-            } else {
-                PlatformDependent0.freeDirectBuffer(buffer);
-            }
+        if (hasUnsafe()) {
+            PlatformDependent0.freeDirectBuffer(buffer);
         }
     }
 

common/src/main/java/io/netty/util/internal/PlatformDependent0.java

@@ -48,39 +48,40 @@ final class PlatformDependent0 {
     private static final boolean UNALIGNED;
 
     static {
-        ByteBuffer direct = ByteBuffer.allocateDirect(1);
-        Field cleanerField;
+        boolean directBufferFreeable = false;
         try {
-            cleanerField = direct.getClass().getDeclaredField("cleaner");
-            cleanerField.setAccessible(true);
-            Cleaner cleaner = (Cleaner) cleanerField.get(direct);
-            cleaner.clean();
+            Class<?> cls = Class.forName("sun.nio.ch.DirectBuffer", false, PlatformDependent0.class.getClassLoader());
+            Method method = cls.getMethod("cleaner");
+            if ("sun.misc.Cleaner".equals(method.getReturnType().getName())) {
+                directBufferFreeable = true;
+            }
         } catch (Throwable t) {
-            cleanerField = null;
+            // We don't have sun.nio.ch.DirectBuffer.cleaner().
         }
-        logger.debug("java.nio.ByteBuffer.cleaner: {}", cleanerField != null? "available" : "unavailable");
+        logger.debug("sun.nio.ch.DirectBuffer.cleaner(): {}", directBufferFreeable? "available" : "unavailable");
 
         Field addressField;
         try {
             addressField = Buffer.class.getDeclaredField("address");
             addressField.setAccessible(true);
             if (addressField.getLong(ByteBuffer.allocate(1)) != 0) {
+                // A heap buffer must have 0 address.
                 addressField = null;
             } else {
-                direct = ByteBuffer.allocateDirect(1);
+                ByteBuffer direct = ByteBuffer.allocateDirect(1);
                 if (addressField.getLong(direct) == 0) {
+                    // A direct buffer must have non-zero address.
                     addressField = null;
                 }
-                Cleaner cleaner = (Cleaner) cleanerField.get(direct);
-                cleaner.clean();
             }
         } catch (Throwable t) {
+            // Failed to access the address field.
             addressField = null;
         }
         logger.debug("java.nio.Buffer.address: {}", addressField != null? "available" : "unavailable");
 
         Unsafe unsafe;
-        if (addressField != null && cleanerField != null) {
+        if (addressField != null && directBufferFreeable) {
             try {
                 Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
                 unsafeField.setAccessible(true);
@@ -104,6 +105,7 @@ final class PlatformDependent0 {
                     throw e;
                 }
             } catch (Throwable cause) {
+                // Unsafe.copyMemory(Object, long, Object, long, long) unavailable.
                 unsafe = null;
             }
         } else {
@@ -111,6 +113,7 @@ final class PlatformDependent0 {
             // Let's just pretend unsafe is unavailable for overall simplicity.
             unsafe = null;
         }
+
         UNSAFE = unsafe;
 
         if (unsafe == null) {
@@ -144,19 +147,6 @@ final class PlatformDependent0 {
         UNSAFE.throwException(t);
     }
 
-    static void freeDirectBufferUnsafe(ByteBuffer buffer) {
-        try {
-            Cleaner cleaner = ((DirectBuffer) buffer).cleaner();
-            if (cleaner == null) {
-                throw new IllegalArgumentException(
-                        "attempted to deallocate the buffer which was allocated via JNIEnv->NewDirectByteBuffer()");
-            }
-            cleaner.clean();
-        } catch (Throwable t) {
-            // Nothing we can do here.
-        }
-    }
-
     static void freeDirectBuffer(ByteBuffer buffer) {
         if (!(buffer instanceof DirectBuffer)) {
             return;