support dynamic preparedstatement.

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

@@ -288,7 +288,9 @@ public class DruidPooledStatement extends PoolableWrapper implements Statement {
     public void close() throws SQLException {
         if (!this.closed) {
             clearResultSet();
-            stmt.close();
+            if (stmt != null) {
+                stmt.close();
+            }
             this.closed = true;
 
             if (conn.getConnectionHolder() != null) {

src/main/java/com/alibaba/druid/proxy/jdbc/PreparedStatementProxyImpl.java

@@ -47,7 +47,7 @@ import com.alibaba.druid.proxy.jdbc.JdbcParameter.TYPE;
  */
 public class PreparedStatementProxyImpl extends StatementProxyImpl implements PreparedStatementProxy {
 
-    protected final PreparedStatement statement;
+    private PreparedStatement statement;
     protected final String            sql;
     private JdbcParameter[]           parameters     = new JdbcParameter[16];
     private int                       parametersSize = 0;
@@ -70,6 +70,11 @@ public class PreparedStatementProxyImpl extends StatementProxyImpl implements Pr
         return paramMap;
     }
 
+    protected void setStatement(PreparedStatement statement) {
+        super.statement = statement;
+        this.statement = statement;
+    }
+
     void setParameter(int jdbcIndex, JdbcParameter parameter) {
         int index = jdbcIndex - 1;
 

src/main/java/com/alibaba/druid/proxy/jdbc/StatementProxyImpl.java

@@ -35,7 +35,7 @@ import com.alibaba.druid.stat.JdbcSqlStat;
 public class StatementProxyImpl extends WrapperProxyImpl implements StatementProxy {
 
     private final ConnectionProxy  connection;
-    private final Statement        statement;
+    protected Statement        statement;
 
     protected String               lastExecuteSql;
     protected long                 lastExecuteStartNano;
@@ -94,6 +94,10 @@ public class StatementProxyImpl extends WrapperProxyImpl implements StatementPro
 
     @Override
     public void cancel() throws SQLException {
+        if (this.statement == null) {
+            return;
+        }
+
         FilterChainImpl chain = createChain();
         chain.statement_cancel(this);
         recycleFilterChain(chain);
@@ -101,6 +105,10 @@ public class StatementProxyImpl extends WrapperProxyImpl implements StatementPro
 
     @Override
     public void clearBatch() throws SQLException {
+        if (this.statement == null) {
+            return;
+        }
+
         if (batchSqlList == null) {
             batchSqlList = new ArrayList<String>();
         }
@@ -113,6 +121,10 @@ public class StatementProxyImpl extends WrapperProxyImpl implements StatementPro
 
     @Override
     public void clearWarnings() throws SQLException {
+        if (this.statement == null) {
+            return;
+        }
+
         FilterChainImpl chain = createChain();
         chain.statement_clearWarnings(this);
         recycleFilterChain(chain);
@@ -120,6 +132,10 @@ public class StatementProxyImpl extends WrapperProxyImpl implements StatementPro
 
     @Override
     public void close() throws SQLException {
+        if (this.statement == null) {
+            return;
+        }
+
         FilterChainImpl chain = createChain();
         chain.statement_close(this);
         recycleFilterChain(chain);