mysql JDBC 连接 NoClassDefFoundError

Question

I am trying to test mySQL JDBC connection on a windows 2008 server. I have downloaded the JDBC driver and it created a jar file at "C:\\Program Files\\MySQL\\MySQL Connector J\\mysql-connector-java-5.1.34-bin.jar". When I am running a small program to test my jdbc connection , I am getting "NoClassDefFound" error. What am i missing here?

I did set up jdbc jar in classpath

C:\>echo %CLASSPATH%
C:\Program Files\MySQL\MySQL Connector J\mysql-connector-java-5.1.34-bin.jar;

I have placed the jar in same location where I have my DBDemo.java (C:\\test)

DemoDB.java

import java.sql.*;
import java.util.Properties;
public class DBDemo
{
// The JDBC Connector Class.
private static final String dbClassName = "com.mysql.jdbc.Driver";
// Connection string. emotherearth is the database the program
// is connecting to. You can include user and password after this
// by adding (say) ?user=paulr&password=paulr. Not recommended!
private static final String CONNECTION =
                      "jdbc:mysql://127.0.0.1/emotherearth";
public static void main(String[] args) throws
                         ClassNotFoundException,SQLException
{
    System.out.println(dbClassName);
    // Class.forName(xxx) loads the jdbc classes and
    // creates a drivermanager class factory
    Class.forName(dbClassName);
    // Properties for user and password. Here the user and password are both 'paulr'
    Properties p = new Properties();
    p.put("user","paulr");
    p.put("password","paulr");
    // Now try to connect
    Connection c = DriverManager.getConnection(CONNECTION,p);
    System.out.println("It works !");
    c.close();
}
}

*Error *

C:\Program Files\Java\jdk1.6.0_45\bin>java c:\test\DBDemo
Exception in thread "main" java.lang.NoClassDefFoundError: c:\test\DBDemo
Caused by: java.lang.ClassNotFoundException: c:\test\DBDemo
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
 Could not find the main class: c:\test\DBDemo.  Program will exit.

Answer 1

C:\Program Files\MySQL\MySQL Connector J\mysql-connector-java-5.1.34-bin.jar;

Your CLASSPATH seems to be not having . that is the current working directory where the DBDemo would be there. So add . also to your CLASSPATH . After that your CLASSPATH will be like

.;C:\Program Files\MySQL\MySQL Connector J\mysql-connector-java-5.1.34-bin.jar;

EDIT

Also try extracting the contents of mysql-connector-java-5.1.34-bin.jar into a folder and change the CLASSPATH according to that.

Answer 2

Exception in thread "main" java.lang.NoClassDefFoundError

One of the places java tries to find your .class file is your current directory. So if your .class file is in c:\\test\\ , you should change your current directory to that.

To change your directory, type the following command at the prompt and press Enter:

cd c:\test\

Also

set CLASSPATH = .;C:\Program Files\MySQL\MySQL Connector J\mysql-connector-java-5.1.34-bin.jar;C:\Program Files\Java\jdk1.6.0_45\lib\tools.jar;

set PATH = C:\Program Files\Java\jdk1.6.0_45\bin

Executing your program using this command should correct the problem:

c:\test>java DemoDB

Answer 3

这个问题可能有多种原因，但最常见的一个是类或依赖项无法相互看到，您必须验证每个类或依赖项的导入，如果您重命名了包、类或依赖项，则JVM找不到寻求阶级或依赖。