lock free driver

This PR ensure that after a driver is closed, no new session could be started. While if there are sessions that has already started before driver.close, the sessions will probably crash becasue connection get closed in driver.close.

Author: Zhen

driver/src/main/java/org/neo4j/driver/internal/BaseDriver.java

@@ -18,7 +18,7 @@
  */
 package org.neo4j.driver.internal;
 
-import java.util.concurrent.locks.ReentrantReadWriteLock;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.neo4j.driver.internal.security.SecurityPlan;
 import org.neo4j.driver.v1.AccessMode;
@@ -34,8 +34,7 @@ abstract class BaseDriver implements Driver
     private final SecurityPlan securityPlan;
     protected final Logger log;
 
-    private final ReentrantReadWriteLock closedLock = new ReentrantReadWriteLock();
-    private boolean closed;
+    private AtomicBoolean closed = new AtomicBoolean( false );
 
     BaseDriver( SecurityPlan securityPlan, Logging logging )
     {
@@ -46,16 +45,8 @@ abstract class BaseDriver implements Driver
     @Override
     public final boolean isEncrypted()
     {
-        closedLock.readLock().lock();
-        try
-        {
-            assertOpen();
-            return securityPlan.requiresEncryption();
-        }
-        finally
-        {
-            closedLock.readLock().unlock();
-        }
+        assertOpen();
+        return securityPlan.requiresEncryption();
     }
 
     @Override
@@ -67,33 +58,16 @@ public final Session session()
     @Override
     public final Session session( AccessMode mode )
     {
-        closedLock.readLock().lock();
-        try
-        {
-            assertOpen();
-            return newSessionWithMode( mode );
-        }
-        finally
-        {
-            closedLock.readLock().unlock();
-        }
+        assertOpen();
+        return newSessionWithMode( mode );
     }
 
     @Override
     public final void close()
     {
-        closedLock.writeLock().lock();
-        try
-        {
-            if ( !closed )
-            {
-                closeResources();
-            }
-        }
-        finally
+        if ( closed.compareAndSet(false, true) )
         {
-            closed = true;
-            closedLock.writeLock().unlock();
+            closeResources();
         }
     }
 
@@ -103,7 +77,7 @@ public final void close()
 
     private void assertOpen()
     {
-        if ( closed )
+        if ( closed.get() )
         {
             throw new IllegalStateException( "This driver instance has already been closed" );
         }