add DruidConnectionHolder testcase & bug fixed

src/test/java/com/alibaba/druid/bvt/pool/DruidConnectionHolderTest4.java

@@ -0,0 +1,59 @@
+package com.alibaba.druid.bvt.pool;
+
+import java.lang.reflect.Field;
+import java.sql.PreparedStatement;
+
+import junit.framework.TestCase;
+
+import org.junit.Assert;
+
+import com.alibaba.druid.pool.DruidConnectionHolder;
+import com.alibaba.druid.pool.DruidDataSource;
+import com.alibaba.druid.pool.DruidPooledConnection;
+
+public class DruidConnectionHolderTest4 extends TestCase {
+
+    private DruidDataSource dataSource;
+
+    protected void setUp() throws Exception {
+        dataSource = new DruidDataSource();
+        dataSource.setUrl("jdbc:mock:xxx");
+        dataSource.setTestOnBorrow(false);
+        dataSource.setPoolPreparedStatements(true);
+
+    }
+
+    protected void tearDown() throws Exception {
+        dataSource.close();
+    }
+
+    public void test_toString() throws Exception {
+        DruidPooledConnection conn = dataSource.getConnection();
+
+        DruidConnectionHolder holder = conn.getConnectionHolder();
+
+        Field field = DruidConnectionHolder.class.getDeclaredField("statementPool");
+        field.setAccessible(true);
+        Assert.assertNull(field.get(holder));
+
+        holder.toString();
+
+        Assert.assertNull(field.get(holder));
+
+        holder.getStatementPool();
+
+        Assert.assertNotNull(field.get(holder));
+
+        holder.toString();
+
+        PreparedStatement stmt = conn.prepareStatement("select 1");
+        stmt.execute();
+        stmt.close();
+
+        conn.close();
+        
+        Assert.assertEquals(1, holder.getStatementPool().size());
+        
+        holder.toString();
+    }
+}