我目前正在处理一个需要加载 mysql 驱动程序运行时并使用 java 连接到数据库的需求。
我正在使用 URLClassLoader
加载 jar 文件
File f = new File("D:/Pallavi/workspace/WarInstallation/mysql-connector-java-5.0.4-bin.jar"); //Jar path
URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURL()},System.class.getClassLoader());
Class sqldriver = urlCl.loadClass("com.mysql.jdbc.Driver"); // Runtime loading
Driver ds = (Driver) sqldriver.newInstance(); //Compilation failing as "sqldriver" class of type Driver is not found
//I am using now java.sql.Driver to remove the compilation error
sqldriver = Class.forName("com.mysql.jdbc.Driver", true, sqldriver.getClassLoader()).newInstance(); //Runtime fail.. "Driver" Class not Found Exception.
尽管类加载正常,但无论我尝试使用哪个驱动程序,我都无法建立数据库连接(没有找到合适的驱动程序...)。
请建议一种加载 jdbc "com.mysql.jdbc.Driver
"类运行时的方法。
如果您需要任何进一步的信息,请告诉我,因为这很紧急。
提前致谢。
最佳答案
在回答您的问题之前我有三个问题:
声明 1:
ya, I have set the classpath of mysql jar in the environment variables, do we need to set it through system properties?
问题 1:为什么要依赖自定义类加载器,当系统类加载器可以从类路径轻松获取类时?
您不需要mysql***.jar
的显式类路径使用自定义类加载器。声明 2:
Class sqldriver = urlCl.loadClass("com.mysql.jdbc.Driver"); // Runtime loading<br/> //Compilation failing as "sqldriver" class of type Driver is not found<br/> Driver ds = (Driver) sqldriver.newInstance();
Q2:声明
Compilation failing ...
非常矛盾。您的编译器是否正在寻找这样的类来生成您的类!?
我确定不是。可能是错误在运行时出现java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
.我还怀疑评论应该与下面的声明 3 一起使用。
如果是CNFE
,你的文件路径到mysql***.jar
是错的。先修复它。声明 3:
//I am using now java.sql.Driver to remove the compilation error<br/> //Runtime fail.. "Driver" Class not Found Exception. sqldriver = Class.forName("com.mysql.jdbc.Driver", true, sqldriver.getClassLoader()).newInstance();
Q3:声明
... "Driver" Class not Found Exception
值得怀疑。因为,这条语句不会被编译。那怎么会是运行时失败呢。 ..!?
我还怀疑该评论应该与上面的声明 2 一致。
在这里,您需要调用newInstance()
然后转换为java.sql.Driver
在分配给sqlDriver
之前多变的。因为Class.forName( ...
只返回Class
object associated with the class or interface with the given string name .
如果上述声明 2 的问题得到解决,您可以应用此修复程序进行进一步测试。
希望您能澄清这些陈述。
我在下面提供了一个有效的示例代码,并为您显示了经过测试的输出。
import java.io.File; // and others as required
public class MySQLDriveClassLoader {
public static void main( String [] args ) throws Exception {
//File f = new File( "/home/ravinder/soft-dump/mysql-connector-java-5.1.18-bin.jar" );
File f = new File( "E:\\Soft_Dump\\mysql-connector-java-5.0.4\\mysql-connector-java-5.0.4-bin.jar" );
URLClassLoader urlCl = new URLClassLoader( new URL[] { f.toURI().toURL() }, System.class.getClassLoader() );
Class mySqlDriver = urlCl.loadClass( "com.mysql.jdbc.Driver" );
//*** Start: DEBUG *************************
//mySqlDriver.con // On pressing CTRL+SPACEBAR, after .con, IDE shows "No default proposals"
// meaning it still is not an instance of Driver, and hence can't call a method from Driver class.
//Incompatible conditional operand types Class and Driver
//System.out.println( mySqlDriver instanceof java.sql.Driver ) );
System.out.println( "mySqlDriver: " + mySqlDriver );
System.out.println( "Is this interface? = " + mySqlDriver.isInterface() );
Class interfaces[] = mySqlDriver.getInterfaces();
int i = 1;
for( Class _interface : interfaces ) {
System.out.println( "Implemented Interface Name " + ( i++ ) + " = " + _interface.getName() );
} // for(...)
Constructor constructors[] = mySqlDriver.getConstructors();
for( Constructor constructor : constructors ) {
System.out.println( "Constructor Name = " + constructor.getName() );
System.out.println( "Is Constructor Accessible? = " + constructor.isAccessible() );
} // for(...)
//*** End : DEBUG *************************
Driver sqlDriverInstance = ( Driver ) mySqlDriver.newInstance();
System.out.println( "sqlDriverInstance: " + sqlDriverInstance );
Connection con = null;
try {
/******************************************************************
// You may fail to register the above driver
// hence don't depend on DriverManager to get Connected
//DriverManager.registerDriver( sqlDriverInstance );
//Driver driver = DriverManager.getDriver( "com.mysql.jdbc.Driver" ); // ( "jdbc:mysql" );
Enumeration<Driver> enumDrivers = DriverManager.getDrivers();
while ( enumDrivers.hasMoreElements() ) {
Driver driver = enumDrivers.nextElement();
System.out.println( "driver: " + driver );
} // while drivers
//******************************************************************/
String dbUrl = "jdbc:mysql://:3306/test";
Properties userDbCredentials = new Properties();
userDbCredentials.put( "user", "root" );
userDbCredentials.put( "password", "password" );
// No suitable driver found for ...
//con = DriverManager.getConnection( dbUrl, "root", "password" );
// safely use driver to connect
con = sqlDriverInstance.connect( dbUrl, userDbCredentials );
System.out.println( "con: " + con );
Statement stmt = con.createStatement();
String sql = "select now()";
ResultSet rs = stmt.executeQuery( sql );
if ( rs.next() ) {
System.out.println( rs.getString( 1 ) );
} // if rs
} catch( Exception e ) {
e.printStackTrace(); // only for quick debug
} finally {
try { if ( con != null ) con.close(); } catch ( Exception ignoreThis ) {}
}
} // psvm(...)
} // class MySQLDriveClassLoader
编译运行成功,输出如下:
mySqlDriver: class com.mysql.jdbc.Driver
Is this interface? = false
Implemented Interface Name 1 = java.sql.Driver
Constructor Name = com.mysql.jdbc.Driver
Is Constructor Accessible? = false
sqlDriverInstance: com.mysql.jdbc.Driver@1270b73
con: com.mysql.jdbc.Connection@32fb4f
2012-05-29 03:52:12.0
关于java - 加载并连接到 mysql jdbc 驱动程序运行时,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/10780919/