java - 如何使用 try-with-resources 检查 JDBC 连接是 Activity 的还是关闭的？

Question

一旦try-with-resources语句执行完成后如何检查JDBC连接是 Activity 的还是关闭的？

代码:

try (Connection conn = ConnectionString.getConnection();
     PreparedStatement psmt = conn.prepareStatement(SQL_Queries.GET_PUBLISHER_ID_QUERY);
     ResultSet rs = psmt.executeQuery();)
{
   psmt.setString(1, appId);
   while (rs.next()) {
     publisherId = rs.getString(Schema.PUBLISHER_PUBLISHID);
   }

   //System.out.println(conn != null ? "live" : "close");
}

Answer 1

这个想法是，使用 try-with-resource block ，您正在为连接创建一个内部范围。 try-with-resource 关闭连接并且 conn 对象范围结束，因此准备好进行垃圾回收。 Read more about scope here.

要检查这一点，请运行此代码

String url, username, pass;
url = "your-connection-url";
username = "db-username";
pass = "db-password";
Connection outerConnection = null;

try (Connection innerConnection = DriverManager.getConnection(url, username, pass);
     PreparedStatement psmt = innerConnection.prepareStatement("SELECT 1");
     ResultSet rs = psmt.executeQuery();)
{
   while (rs.next()) {
       System.out.println(rs.getString(1));
   }

   outerConnection = innerConnection;

   // after the try catch, innerConnection won't exist anymore
   // (the scope ends, the compiler compains if you access it outside the block!)
} catch (SQLException e) 
{
   e.printStackTrace();
}

if (outerConnection != null) {
    try {
       System.out.println("Is closed? " + outerConnection.isClosed());
    }
    catch (SQLException e) {
       e.printStackTrace();
    }
}
else {
    System.out.println("Is null");
}

在控制台中您将得到

1
Is closed? true