java – JDBC：连接返回NULL,该怎么办？

这是下面给出的程序的输出：

Connection made!
Schema Name:null

Successfully connected to null
Releasing all open resources ...

在establishConnection()内,conn被初始化为null.然后try块中的第一个语句应该与数据库建立连接,然后第三个语句打印conn的当前模式的名称.

根据API,getSchema()返回当前模式名称,如果没有则返回null.

这意味着没有与conn关联的模式(我认为模式名称与数据库名称相同)？任何人都可以建议我的预期是否正确,并且还建议为什么没有架构或与conn相关联的null？

public class ConnectDB {

    private Connection establishConnection() throws sqlException {
        Connection conn = null;
        try {
            conn = DriverManager.getConnection(
                    "jdbc:MysqL://localhost:3306/test_final1", "root", "password");
            System.out.println("Connection made!");
            System.out.println("Schema Name:"+conn.getSchema()+"\n");
        } catch (sqlException sqle) {
            System.err.println("sql Exception thrown while making connection");
            printsqlException(sqle);
        }
        return conn;
    }

    public static void main(String[] args) {
        ConnectDB cdb= new ConnectDB();
        Connection myconn=null;
        try{
            myconn=cdb.establishConnection();
            if(myconn!=null) System.out.println("Successfully connected to " + myconn.getSchema());
        }catch (sqlException e) {
              ConnectDB.printsqlException(e);
        } catch (Exception e) {
          e.printstacktrace(System.err);
        } finally {
          ConnectDB.closeConnection(myconn);
        }

    }

解决方法:

MysqL不支持模式的概念.对于MysqL,模式实际上是数据库.从MySQL Glossary开始：

schema

Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on. (…)

基于this answer from MySQL forums,您无法通过Connection#getSchema方法(自Java 7与JDBC 4一起添加)获取当前数据库(或数据库),但使用Connection#getCatalog除非您使用最新的JDBC jar驱动程序：

The JDBC driver (because of legacy, MysqL didn’t call them “schemas” until 5.0, and JDBC didn’t have a way to select schemas until JDBC4), calls databases “catalogs”, so thus you have to call getCatalogs() to get the list of databases

我做了一个脏的快速证明上面的斜体句子(除非你使用最新的JDBC jar驱动程序).使用getCatalog()使用Java 6：

public class DirtyQuickTest {
    private static final String url = "jdbc:MysqL://localhost:7841/test";
    private static final String user = "lmendozaj";
    private static final String password = "s3cr3t"; //I won't show you my password
    public static void main(String[] args) throws Exception {
        Connection con = null;
        try {
            Class.forName("com.MysqL.jdbc.Driver");
            con = DriverManager.getConnection(url, user, password);
            //commented since doesn't exists in Java 6
            //System.out.println(con.getSchema());
            System.out.println(con.getCatalog());
        } finally {
            con.close();
        }
    }
}

输出：

test

然后我在Java 8中执行相同的代码,并使用mysql-connector-java-5.1.31-bin.jar(当前最新的MysqL的Java连接器驱动程序)取消注释包含getSchema的语句.这是输出：

null
test

这意味着他们仍然不支持getSchema方法.

有关：

> Difference Between Schema / Database in MySQL