add testcase & bug fixed

src/test/java/com/alibaba/druid/bvt/pool/DruidConnectionHolderTest.java

@@ -0,0 +1,40 @@
+package com.alibaba.druid.bvt.pool;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+
+import junit.framework.TestCase;
+
+import com.alibaba.druid.filter.FilterAdapter;
+import com.alibaba.druid.filter.FilterChain;
+import com.alibaba.druid.pool.DruidDataSource;
+import com.alibaba.druid.proxy.jdbc.ConnectionProxy;
+
+public class DruidConnectionHolderTest extends TestCase {
+
+    private DruidDataSource dataSource;
+
+    protected void setUp() throws Exception {
+        dataSource = new DruidDataSource();
+        dataSource.setUrl("jdbc:mock:xxx");
+        dataSource.setTestOnBorrow(false);
+        dataSource.setPoolPreparedStatements(true);
+        dataSource.getProxyFilters().add(new FilterAdapter() {
+
+            public int connection_getTransactionIsolation(FilterChain chain, ConnectionProxy connection)
+                                                                                                        throws SQLException {
+                throw new com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException();
+            }
+        });
+
+    }
+
+    protected void tearDown() throws Exception {
+        dataSource.close();
+    }
+
+    public void test_mysqlSyntaxError() throws Exception {
+        Connection conn = dataSource.getConnection();
+        conn.close();
+    }
+}