加載並連接到mysql jdbc驅動運行時

Question

我目前正在需要加載mysql驅動運行時並使用java連接到數據庫的需求。

我使用URLClassLoader加載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. 

雖然類加載罰款，我不能建立數據庫連接（中找不到合適的驅動程序...）不管我的驅動程序。

請建議加載jdbc「com.mysql.jdbc.Driver」類運行時的方法。 讓我知道，如果您需要任何進一步的信息，因爲這是迫切的。

在此先感謝。

Answer 1

DriverManager忽略在運行時加載的類，它只對System加載器加載的類有效。

您可以創建一個虛擬驅動程序類來封裝您的實際數據庫驅動程序。源代碼可以找到here。

題外話：

File.toURL已被棄用，而不是從使用toURLFile上URI

URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURI().toURL()},System.class.getClassLoader()); 

Answer 2

獲取URL我有三個問題之前，我回答你的問題：

1. 聲明1：

   ya, I have set the classpath of mysql jar in the environment variables, do we need to set it through system properties?

   Q1：爲什麼是依靠自定義類加載器，當一個類是現成的，從類路徑系統類加載器？
   您不需要明確的類路徑mysql***.jar來使用自定義類加載器。

2. 聲明2：

   Class sqldriver = urlCl.loadClass("com.mysql.jdbc.Driver"); // Runtime loading
//Compilation failing as "sqldriver" class of type Driver is not found
Driver ds = (Driver) sqldriver.newInstance();

   Q2：自稱Compilation failing ...是非常矛盾的。你的編譯器是否在尋找這樣的類來生成你的類？
   我相信它不是。可能是錯誤是在運行時與java.lang.ClassNotFoundException: com.mysql.jdbc.Driver。我也懷疑這個評論應該在你的聲明3下面。
   如果它是一個CNFE，您的文件路徑mysql***.jar是錯誤的。先修復它。

3. 聲明3：

   //I am using now java.sql.Driver to remove the compilation error
//Runtime fail.. "Driver" Class not Found Exception. sqldriver = Class.forName("com.mysql.jdbc.Driver", true, sqldriver.getClassLoader()).newInstance();

   Q3：聲稱... "Driver" Class not Found Exception是suspectful。因爲，這個聲明不會被編譯。那麼它怎麼會是一個運行時失敗。 ..！？
   我也懷疑這個評論應該和你的聲明2一致。
   而在這裏，您需要先撥打newInstance()，然後在分配給sqlDriver變量之前投射到java.sql.Driver。因爲Class.forName(...只返回Class object associated with the class or interface with the given string name。
   如果在上發佈聲明2已修復，則可以應用此修復程序進一步測試。

讓我希望你能得到這些聲明澄清。

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我有一個工作示例代碼，下面顯示了一個經過測試的輸出。

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 

成功編譯和運行，導致下面的輸出：

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 

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