Use ByteBuffer.allocateDirect(). If you use ByteBuffer.allocate() to create a byte buffer, JNI call GetDirectBufferAddress(jobject:ByteBuffer) returns NULL.

src/main/java/org/xerial/snappy/Snappy.java

@@ -27,6 +27,11 @@ public class Snappy
      */
     public static int compress(ByteBuffer uncompressed, ByteBuffer compressed) {
 
+        if (!uncompressed.isDirect())
+            throw new IllegalArgumentException("input is not a direct buffer");
+        if (!compressed.isDirect())
+            throw new IllegalArgumentException("destination is not a direct buffer");
+
         // input: uncompressed[pos(), limit())
         // output: compressed
         int uPos = uncompressed.position();
@@ -54,6 +59,11 @@ public class Snappy
      */
     public static boolean decompress(ByteBuffer compressed, ByteBuffer decompressed) {
 
+        if (!compressed.isDirect())
+            throw new IllegalArgumentException("input is not a direct buffer");
+        if (!decompressed.isDirect())
+            throw new IllegalArgumentException("destination is not a direct buffer");
+
         int cPos = compressed.position();
         int cLen = compressed.remaining();
 
@@ -66,6 +76,9 @@ public class Snappy
     }
 
     public static int getUncompressedLength(ByteBuffer compressed) {
+        if (!compressed.isDirect())
+            throw new IllegalArgumentException("input is not a direct buffer");
+
         return SnappyNative.getUncompressedLength(compressed, compressed.position(), compressed.remaining());
     }
 

src/main/java/org/xerial/snappy/SnappyNative.cpp

@@ -16,8 +16,8 @@ JNIEXPORT jstring JNICALL Java_org_xerial_snappy_SnappyNative_nativeLibraryVersi
 JNIEXPORT jint JNICALL Java_org_xerial_snappy_SnappyNative_rawCompress
   (JNIEnv* env, jclass self, jobject uncompressed, jint upos, jint ulen, jobject compressed, jint cpos)
 {
-	char* uncompressedBuffer = (char*) env->GetDirectBufferAddress(uncompressed);
-	char* compressedBuffer = (char*) env->GetDirectBufferAddress(compressed);
+	char* uncompressedBuffer = (char*) env->GetDirectBufferAddress(uncompressed) + upos;
+	char* compressedBuffer = (char*) env->GetDirectBufferAddress(compressed) + cpos;
 	size_t compressedLength;
 
 	snappy::RawCompress(uncompressedBuffer, (size_t) ulen, compressedBuffer, &compressedLength);
@@ -63,11 +63,11 @@ JNIEXPORT jint JNICALL Java_org_xerial_snappy_SnappyNative_getUncompressedLength
   (JNIEnv * env, jclass self, jobject compressed, jint cpos, jint clen)
 {
 	char* compressedBuffer = (char*) env->GetDirectBufferAddress(compressed);
-	size_t result;
-	std::string s = "hello world";
-	//snappy::GetUncompressedLength(compressedBuffer, (size_t) clen, &result);
-	snappy::GetUncompressedLength(s.c_str(), s.length(), &result);
+	if(compressedBuffer == 0)
+		return (jint) -1;
+	compressedBuffer += cpos;
 
+	snappy::GetUncompressedLength(compressedBuffer, (size_t) clen, &result);
 	return (jint) result;
 }
 

src/test/java/org/xerial/snappy/SnappyTest.java

@@ -9,6 +9,8 @@
 //--------------------------------------
 package org.xerial.snappy;
 
+import static org.junit.Assert.*;
+
 import java.nio.ByteBuffer;
 
 import org.junit.Test;
@@ -24,17 +26,40 @@ public class SnappyTest
         _logger.info("version: " + version);
     }
 
+    @Test
+    public void directBufferCheck() throws Exception {
+
+        try {
+            ByteBuffer src = ByteBuffer.allocate(1024);
+            src.put("hello world".getBytes());
+            src.flip();
+            ByteBuffer dest = ByteBuffer.allocate(1024);
+            int maxCompressedLen = Snappy.compress(src, dest);
+        }
+        catch (IllegalArgumentException e) {
+            // detected non-direct buffer. OK
+            return;
+        }
+
+        fail("shouldn't reach here");
+
+    }
+
     @Test
     public void load() throws Exception {
 
-        ByteBuffer src = ByteBuffer.allocate(1024);
+        ByteBuffer src = ByteBuffer.allocateDirect(1024);
         src.put("hello world".getBytes());
-        ByteBuffer dest = ByteBuffer.allocate(1024);
 
         src.flip();
         int maxCompressedLen = Snappy.getMaxCompressedLength(src.remaining());
         _logger.info("max compressed length:" + maxCompressedLen);
-        //long uncompressedLen = Snappy.getUncompressedLength(dest);
-        //
+
+        int uncompressedLen = Snappy.getUncompressedLength(src);
+        _logger.info("uncompressed length: " + uncompressedLen);
+
+        ByteBuffer dest = ByteBuffer.allocateDirect(1024);
+        Snappy.compress(src, dest);
+
     }
 }