bug fixed for multi-thread use one connection.

src/main/java/com/alibaba/druid/pool/DruidConnectionHolder.java

@@ -24,6 +24,7 @@ import java.util.Date;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.locks.ReentrantLock;
 
 import javax.sql.ConnectionEventListener;
 import javax.sql.StatementEventListener;
@@ -68,6 +69,7 @@ public final class DruidConnectionHolder {
     protected boolean                             discard                  = false;
     protected final Map<String, Object>           variables;
     protected final Map<String, Object>           globleVariables;
+    final ReentrantLock                           lock                     = new ReentrantLock();
 
     public DruidConnectionHolder(DruidAbstractDataSource dataSource, PhysicalConnectionInfo pyConnectInfo)
                                                                                                           throws SQLException{
@@ -191,11 +193,21 @@ public final class DruidConnectionHolder {
     }
 
     public void addTrace(DruidPooledStatement stmt) {
-        statementTrace.add(stmt);
+        lock.lock();
+        try {
+            statementTrace.add(stmt);
+        } finally {
+            lock.unlock();
+        }
     }
 
     public void removeTrace(DruidPooledStatement stmt) {
-        statementTrace.remove(stmt);
+        lock.lock();
+        try {
+            statementTrace.remove(stmt);
+        } finally {
+            lock.unlock();
+        }
     }
 
     public List<ConnectionEventListener> getConnectionEventListeners() {
@@ -285,11 +297,17 @@ public final class DruidConnectionHolder {
         connectionEventListeners.clear();
         statementEventListeners.clear();
 
-        for (Object item : statementTrace.toArray()) {
-            Statement stmt = (Statement) item;
-            JdbcUtils.close(stmt);
+        lock.lock();
+        try {
+            for (Object item : statementTrace.toArray()) {
+                Statement stmt = (Statement) item;
+                JdbcUtils.close(stmt);
+            }
+            
+            statementTrace.clear();
+        } finally {
+            lock.unlock();
         }
-        statementTrace.clear();
 
         conn.clearWarnings();
     }

src/main/java/com/alibaba/druid/pool/DruidDataSource.java

@@ -1679,8 +1679,13 @@ public class DruidDataSource extends DruidAbstractDataSource implements DruidDat
 
         if (requireDiscard) {
             if (holder.statementTrace != null) {
-                for (Statement stmt : holder.statementTrace) {
-                    JdbcUtils.close(stmt);
+                holder.lock.lock();
+                try {
+                    for (Statement stmt : holder.statementTrace) {
+                        JdbcUtils.close(stmt);
+                    }
+                } finally {
+                    holder.lock.unlock();
                 }
             }
 

src/main/java/com/alibaba/druid/pool/DruidPooledConnection.java

@@ -69,13 +69,14 @@ public class DruidPooledConnection extends PoolableWrapper implements javax.sql.
     private volatile     boolean               abandoned            = false;
     protected            StackTraceElement[]   connectStackTrace;
     protected            Throwable             disableError         = null;
-    public               ReentrantLock         lock                 = new ReentrantLock();
+    final                ReentrantLock         lock;
 
     public DruidPooledConnection(DruidConnectionHolder holder){
         super(holder.getConnection());
 
         this.conn = holder.getConnection();
         this.holder = holder;
+        this.lock = holder.lock;
         dupCloseLogEnable = holder.getDataSource().isDupCloseLogEnable();
         ownerThread = Thread.currentThread();
         connectedTimeMillis = System.currentTimeMillis();