improve MSSQLValidConnectionChecker & add testcase

src/test/java/com/alibaba/druid/bvt/pool/vendor/MSSQLValidConnectionCheckerTest.java

@@ -0,0 +1,40 @@
+package com.alibaba.druid.bvt.pool.vendor;
+
+import java.sql.SQLException;
+
+import junit.framework.TestCase;
+
+import org.junit.Assert;
+
+import com.alibaba.druid.mock.MockConnection;
+import com.alibaba.druid.pool.vendor.MSSQLValidConnectionChecker;
+
+public class MSSQLValidConnectionCheckerTest extends TestCase {
+
+    public void test_0() throws Exception {
+        MSSQLValidConnectionChecker checker = new MSSQLValidConnectionChecker();
+
+        MockConnection conn = new MockConnection();
+
+        Assert.assertTrue(checker.isValidConnection(conn, "select 1", 10));
+
+    }
+
+    public void test_closed() throws Exception {
+        MSSQLValidConnectionChecker checker = new MSSQLValidConnectionChecker();
+
+        MockConnection conn = new MockConnection();
+
+        conn.close();
+        Assert.assertFalse(checker.isValidConnection(conn, "select 1", 10));
+    }
+
+    public void test_error() throws Exception {
+        MSSQLValidConnectionChecker checker = new MSSQLValidConnectionChecker();
+
+        MockConnection conn = new MockConnection();
+        conn.setError(new SQLException());
+
+        Assert.assertFalse(checker.isValidConnection(conn, "select 1", 10));
+    }
+}