diff --git a/src/Makefile b/src/Makefile
index d8ee52a..6cf9f17 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3,7 +3,7 @@ JAR=jniadapter.jar
 
 all: $(JAR) $(LIBRARY)
 
-$(JAR): org/pEp/jniadapter/pEpException.java
+$(JAR): org/pEp/jniadapter/pEpException.java org/pEp/jniadapter/AbstractEngine.java org/pEp/jniadapter/Engine.java
 	javac org/pEp/jniadapter/*.java
 	jar cf $@ org/pEp/jniadapter/*.class
 
diff --git a/src/org/pEp/jniadapter/AbstractEngine.java b/src/org/pEp/jniadapter/AbstractEngine.java
index 90d52f9..f0a5612 100644
--- a/src/org/pEp/jniadapter/AbstractEngine.java
+++ b/src/org/pEp/jniadapter/AbstractEngine.java
@@ -5,17 +5,17 @@ abstract class AbstractEngine implements AutoCloseable {
         System.loadLibrary("pEpJNI");
     }
 
-    protected native void init() throws pEpException;
-    protected native void release();
+    private native void init() throws pEpException;
+    private native void release();
 
     private long handle;
-    protected long getHandle() { return handle; }
+    final protected long getHandle() { return handle; }
 
     public AbstractEngine() throws pEpException {
         init();
     }
 
-    public void close() {
+    final public void close() {
         release();
     }
 }