Java 数据库连接类一直将连接返回为 Null

Question

我已经为我的数据库连接创建了一个公共(public)类，因为我在整个应用程序中运行各种数据库查询

每次我调用类 con 总是返回 Null但是，如果我在类之外使用它，并直接在 Activity 中使用它，那么它会连接并运行。我不知道为什么，非常感谢任何正确方向的指示。

这是我的 ConnectionManager 类:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;


public class ConnectionManager {
    static String serverName ="192.168.1.5";
    static String serverPort ="1433";
    static String databaseName ="DBNAME";
    static String db = String.format("jdbc:jtds:sqlserver://%s:%s/%s", serverName, serverPort, databaseName);
    static String un = "sa";
    static String pass = "";
    static Connection con;


    public static Connection getConnection() {
        try {
            try {
                Class.forName("net.sourceforge.jtds.jdbc.Driver");
                con = DriverManager.getConnection(db, un, pass);
            } catch (SQLException ex) {
                // log an exception. fro example:
                System.out.println("Failed to create the database connection.");
            }
        } catch (ClassNotFoundException ex) {
            // log an exception. for example:
            System.out.println("Driver not found.");
        }
        return con;
    }
}

在我的 Activity 中，我这样调用它:

  try {

            con = ConnectionManager.getConnection();
            if (con == null) {
                z = "Check Your Internet Access!";
                Toast.makeText(StockEnquiry.this, z, Toast.LENGTH_LONG).show();
            } else {
                stmt = con.createStatement();
                String query = "select * from Details where ID= '1111'  ";
                rs = stmt.executeQuery(query);
                if (rs.next()) {
                    String sCode = rs.getString("Short_Code");
                    String sDesc = rs.getString("Short_Desc");
                    Toast.makeText(StockEnquiry.this, (sCode + " " + sDesc), Toast.LENGTH_LONG).show();
                    con.close();
                } else {
                    Toast.makeText(StockEnquiry.this,"Invalid Code!", Toast.LENGTH_LONG).show();
                }

                Toast.makeText(StockEnquiry.this, "Message1", Toast.LENGTH_LONG).show();
            }
        } catch (Exception ex) {
            isSuccess = false;
            z = ex.getMessage();
            Toast.makeText(StockEnquiry.this, "Appears to have failed in catch", Toast.LENGTH_LONG).show();
        }
    }
}

Answer 1

所以我得到的消息是:android.os.NetworkOnMainThreadException'经过一番挖掘后，我相信我正面临这个问题，因为我没有将它作为异步任务运行。

可以取消这个限制

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

StrictMode.setThreadPolicy(policy); 

因此连接类如下所示。

import android.os.StrictMode;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;


public class ConnectionManager {
    static StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
    static String serverName ="192.168.1.1";
    static String serverPort ="1433";
    static String databaseName ="DatabaseName";
    static String db = String.format("jdbc:jtds:sqlserver://%s:%s/%s", serverName, serverPort, databaseName);
    static String un = "Username";
    static String pass = "Password";
    static Connection con;


    public static Connection getConnection() {
        try {
            try {
                StrictMode.setThreadPolicy(policy);
                Class.forName("net.sourceforge.jtds.jdbc.Driver");
                con = DriverManager.getConnection(db, un, pass);
            } catch (SQLException ex) {
                // log an exception. fro example:
                System.out.println("Failed to create the database connection.");
            }
        } catch (ClassNotFoundException ex) {
            // log an exception. for example:
            System.out.println("Driver not found.");
        }
        return con;
    }
}

从 Activity 中调用代码与原始帖子保持不变。

作为由此产生的链式 react ，您可能会遇到应用程序崩溃/锁定或因网络连接/覆盖不佳而运行缓慢。所以它有点牺牲，但可以通过其他方式处理。