java - Class.forName ("org.postgresql.Driver") 错误

Question

我有简单的类(class)。然后我尝试编译并运行它，我看到了 ClassNotFoundException。

import java.sql.*;

public class DBProcessor{

private static String serverAdres = "127.0.0.1:5432";

private static String DBname = "dota";
private static String clientName = "postgres";
private static String password = "master";
private static Connection connection;

public static void connect() {
    try {
        Class.forName("org.postgresql.Driver");
    } catch (ClassNotFoundException e) {
        System.err.println("Where is your PostgreSQL JDBC Driver? "
                + "Include in your library path!");
        e.printStackTrace();
    }

    try {
        connection = DriverManager.getConnection("jdbc:postgresql://" + serverAdres + "/" + DBname, clientName, password);

    } catch (SQLException e) {
        System.err.println("Connection Failed! Check output console");
        e.printStackTrace();
    }

    if (connection == null) {
        System.err.println("Failed to make connection!");
    }
}

public static void main(String[] args){
    DBProcessor db = new DBProcessor();
    db.connect();
}
}

我正在使用 Windows 命令:

C:\rmi2>javac -classpath postgresql.jar DBProcessor.java
C:\rmi2>java DBProcessor
Where is your PostgreSQL JDBC Driver? Include in your library path!
java.lang.ClassNotFoundException: org.postgresql.Driver
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at DBProcessor.connect(DBProcessor.java:12)
at DBProcessor.main(DBProcessor.java:34)
Connection Failed! Check output console
java.sql.SQLException: No suitable driver found for jdbc:postgresql://127.0.0.1:
5432/dota
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at DBProcessor.connect(DBProcessor.java:20)
at DBProcessor.main(DBProcessor.java:34)
Failed to make connection!

我做错了什么？所以，这是 postgrsql 库 https://www.dropbox.com/s/idx5l0kub5rn1b8/postgresql.jar?dl=0

Answer 1

在命令行中，这个

java DBProcessor

应该是

java -cp .;postgresql.jar DBProcessor

而且，因为它包含 java.sql.Driver，当您让它工作时，您可以删除

// try {
//    Class.forName("org.postgresql.Driver");
// } catch (ClassNotFoundException e) {
//    System.err.println("Where is your PostgreSQL JDBC Driver? "
//            + "Include in your library path!");
//    e.printStackTrace();
// }

根据 DriverManager Java文档，

The DriverManager methods getConnection and getDrivers have been enhanced to support the Java Standard Edition Service Provider mechanism. JDBC 4.0 Drivers must include the file META-INF/services/java.sql.Driver. This file contains the name of the JDBC drivers implementation of java.sql.Driver. For example, to load the my.sql.Driver class, the META-INF/services/java.sql.Driver file would contain the entry:

 my.sql.Driver

Applications no longer need to explictly load JDBC drivers using Class.forName(). Existing programs which currently load JDBC drivers using Class.forName() will continue to work without modification.