Question

我正在尝试使用JDBC和Java，但我遇到了一些问题......

我在我的pom.xml文件中添加了这个依赖项：

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>6.0.3</version>
</dependency>

这是我的java代码：

public static void initDriver(){
    try {
        Class.forName("com.mysql.cj.jdbc.Driver");
    } catch ( ClassNotFoundException e ) {
        e.printStackTrace();
    } 
}

public void connectBDD(){
    initDriver();

    String url = new String("jdbc:mysql://localhost:"+BDDPort+"/"+BDDName);
    String utilisateur = BDDUser;
    String motDePasse = BDDPassword;
    Connection connexion = null;
    try {
        connexion = DriverManager.getConnection( url, utilisateur, motDePasse );    

        Statement statement = connexion.createStatement();

        ResultSet resultat = statement.executeQuery( "SHOW TABLES;" );

        System.out.println(resultat.toString());

        resultat.close();           

    } catch ( SQLException e ) {
        e.printStackTrace();
    } finally {stack trace I am getting : 


        if ( connexion != null ){
            try {
                connexion.close();
            } catch ( SQLException ignore ) {
            }
        }
    } 
}

执行此代码时，我可以从println请求中看到：com.mysql.cj.jdbc.result.ResultSetImpl@2e3fc542

我认为这是一个隐藏在它旁边的空指针异常，但我不知道如何解决它......

如果有人能帮助我......谢谢！

更新

这是我尝试的另一个代码及其stacktrace：

    ResultSet resultat = statement.executeQuery( "SELECT id FROM COURRIER;" );

    while ( resultat.next() ) {
        Integer password = resultat.getInt("id");

        System.out.println(password);
    }
    resultat.close();

堆栈追踪：

java.sql.SQLSyntaxErrorException: Table 'myDB.COURRIER' doesn't exist
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:686)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:663)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:653)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:115)
    at com.mysql.cj.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2041)
    at com.mysql.cj.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:1994)
    at com.mysql.cj.jdbc.StatementImpl.executeQuery(StatementImpl.java:1429)
    at utils.BDDInitializer.connectBDD(BDDInitializer.java:51)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:367)
    at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:274)
    at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:238)
    at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:161)
    at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:290)
    at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:242)
    at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:121)

Answer 1

这里的问题是您正在打印ResultSet本身。

下面显示了如何使用ResultSet：

    Connection con= DriverManager.getConnection( url, utilisateur, motDePasse );    

    Statement statement = con.createStatement();

    ResultSet rs = statement.executeQuery( "SELECT EMP_NAME from Employees" );

    while(rs.next()){
         String str = rs.getString("EMP_NAME");//here rs will be having multiple methods for multiple data types, EMP_NAME is the column name
         System.out.println(str);
    }

    rs.close();

这将有助于

Answer 2

您的代码实际上正在运行并执行查询而没有任何问题。您打印com.mysql.cj.jdbc.result.ResultSetImpl@2e3fc542之类的事实是因为您在查询返回的toString()上调用了ResultSet，并且ResultSet没有被覆盖toString()方法。

要以一种很好的方式打印ResultSet中的数据，您需要对其进行迭代并打印每行中的字段：

while (resultat.next()) {
    System.out.println(resultat.getString(1));
}

而不是现在拥有的东西：

System.out.println(resultat.toString());

为什么在Java中使用JDBC时出错？

2 个答案: