未找到类加载 JDBC org.postgresql.Driver

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我正在做一个网络项目，我最近安装了 postgres 9.1.1

I'm working on a web project and I recently installed postgres 9.1.1

postgresql 服务器已启动并正在运行.我可以像往常一样通过 psql 连接，一切都从我从 8.5 制作的数据库的转储中加载并正确保存.

The postgresql server is up and running. I can connect via psql as usual and everything is loaded and properly saved from a dump of the db I made from 8.5.

所以我也在这里下载了 9.1 postgres 版本的 JDBC4 驱动程序:jdbc.postgresql/download/postgresql-jdbc-9.1-901.src.tar.gz

So I also downloaded the JDBC4 driver for 9.1 postgres version here: jdbc.postgresql/download/postgresql-jdbc-9.1-901.src.tar.gz

我通过 eclipse 使用项目属性将其添加到 java 构建路径中.

I added it to the java build path using the project properties via eclipse.

这是我用来提供到其他类的数据库连接的代码(即它是一个单例，只有当现有对象关闭或为空时，我才能获得一个新连接，一次只从一个对象)

This is the code I use to provide db connection to other classes (i.e. it's a singleton, I get a new connection only if the existing is either closed or null, from one object at a time only)

public abstract class DBConnection { private static Connection connection = null; public static void connect() { try { if (connection == null) { String host = "127.0.0.1"; String database = "xxxxx"; String username = "xxxxx"; String password = "xxxxx"; String url = "jdbc:postgresql://" + host + "/" + database; String driverJDBC = "org.postgresql.Driver"; Class.forName(driverJDBC); connection = DriverManager.getConnection(url, username, password); //line firing the class not found exception } else if (connection.isClosed()) { connection = null; connect(); } } catch (SQLException e) { e.printStackTrace(System.err); } catch (Exception e) { e.printStackTrace(System.err); } } public static void disconnect() { if (connection != null) { try { connection.close(); } catch (SQLException e) { Logger.getLogger(DBConnection.class.getName()).log( Level.SEVERE, null, e); } } } public static Connection getConnection() { try { if (connection != null && !connection.isClosed()) { return connection; } else { connect(); return connection; } } catch (SQLException e) { Logger.getLogger(DBConnection.class.getName()).log(Level.SEVERE, null, e); return null; } } @Override public void finalize() { if (connection != null) { try { connection.close(); } catch (SQLException e) { Logger.getLogger(DBConnection.class.getName()).log( Level.SEVERE, null, e); } } } }

正如我在运行项目时在标题中所写的那样，一个类要求连接到这个类，我总是得到一个 Class Not Found Exception，因为它显然无法加载 org.postgresql.Driver.class 驱动程序位于项目 ~/lib/org.postgresql-9.1-901.jdbc4.jar 的子文件夹中，正如我所说，通过 eclipse 项目属性添加到构建路径中.

As I wrote in the title when I run the project and a class asks for a connection to this class I always get a Class Not Found Exception, Since it apparently can't load the org.postgresql.Driver.class The driver is located in a subfolder of the project ~/lib/org.postgresql-9.1-901.jdbc4.jar and as I said added to the build path via eclipse project properties.

我还提供了一个示例查询，以便查看我的类访问 DBConnection 的通常行为:

I'm also providing a sample query to let see the usual behavior of my classes to access the DBConnection:

public static final User validateUserCredentials(String id, String pswd) { Connection connection = DBConnection.getConnection(); Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE, (connection!=null)?"connection not null":"connection null"); Statement stmt = null; Logger.getLogger(Home.class.getName()).log(Level.SEVERE, "validating credentials for user: username : " + id + " password : " + pswd); String sql = "Select * from fuser where id = '" + id + "'"; ResultSet resultset = null; try { stmt = connection.createStatement(); resultset = stmt.executeQuery(sql); Logger.getLogger(Credentials.class.getName()) .log(Level.SEVERE, sql); resultset.next(); String password = resultset.getString("pswd"); if (pswd.equals(password)) return new User(id, pswd); } catch (SQLException ex) { Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE, null, ex); } finally { if (stmt != null) stmt = null; if (resultset != null) resultset = null; if (connection != null) { try { connection.close(); } catch (SQLException e) { } connection = null; } } return null; }

推荐答案

我正在从事一个网络项目，我最近安装了 postgres 9.1.1

I'm working on a web project and I recently installed postgres 9.1.1

...

我通过eclipse使用项目属性将它添加到java构建路径中.

I added it to the java build path using the project properties via eclipse.

那是错误的方式.该 JAR 必须直接放在 Web 项目的 /WEB-INF/lib 文件夹中，而无需在项目属性中修改 Build Path.该文件夹是 webapp 运行时类路径的标准部分.

That's the wrong way. That JAR has to be dropped straight in /WEB-INF/lib folder of the web project without fiddling with the Build Path in the project's properties. That folder is standard part of webapp's runtime classpath.

与具体问题无关:您的 DBConnection 类存在重大设计缺陷.您已将 Connection 声明为 static，这实质上使您的连接不是线程安全的.使用连接池并且永远不要将 Connection(也不是 Statement 或 ResultSet)分配为类/实例变量.它们应该在与您执行查询的位置完全相同的 try-finally 块中创建和关闭.此外，您还有一个 SQL 注入漏洞.使用 PreparedStatement 而不是在 SQL 字符串中连接用户控制的变量.

Unrelated to the concrete problem: you've a major design flaw in your DBConnection class. You've declared Connection as static which essentially makes your connection not threadsafe. Use a connection pool and never assign the Connection (nor Statement nor ResultSet) as a class/instance variable. They should be created and closed in the very same try-finally block as where you're executing the query. Further you've there also a SQL injection hole. Use PreparedStatement instead of concatenating user-controlled variables in the SQL string.