我正在开发一个Web项目，我最近安装了postgres 9.1.1
postgresql服务器启动并运行。我可以像往常一样通过psql进行连接，并且所有内容都已加载并从我从8.5生成的db转储中正确保存。
所以我还下载了用于9.1 postgres的JDBC4驱动程序版本： http：// jdbc。 postgresql/download/postgresql-jdbc-9.1-901.src.tar.gz
我使用项目属性将其添加到java构建路径通过eclipse。
这是我用来提供与其他类的数据库连接的代码（即它是一个单例，只有当现有关闭或者null，仅从一个对象开始）
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）; //行触发类未找到异常 }否则if（connection.isClosed（））{ connection = null; connect（）; } } catch（SQLException e）{ e.printStackTrace（System.err）; } catch（例外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（）; 返回连接; } } catch（SQLException e）{ Logger.getLogger（DBConnection.class.getName（））。log（Level.SEVERE， null，e）; 返回null; } } @Override public void finalize（）{ if（connection！= null）{ try { connection.close（）; } catch（SQLException e）{ Logger.getLogger（DBConnection.class.getName（））。log（ Level.SEVERE，null，e）; } } } }正如我在标题中写的那样，当我运行项目并且一个类要求连接到这个类时，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。
<我还提供了一个示例查询，让我们看看我的类通常的行为来访问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）; 语句stmt = null; Logger.getLogger（Home.class.getName（））。log（Level.SEVERE，验证用户凭据：用户名：+ id +密码：+ pswd）; String sql =选择*来自fuser，其中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））返回新用户（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; } } 返回null; }解决方案
我正在开发网络项目，我最近安装了postgres 9.1.1
...
我使用项目属性通过eclipse将其添加到java构建路径。
这是错误的方式。必须直接在Web项目的 / WEB-INF / lib 文件夹中删除JAR，而不必在项目属性中摆弄 Build Path 。该文件夹是webapp运行时类路径的标准部分。
无关具体问题：你你的 DBConnection 类中存在一个主要的设计缺陷。您已将 Connection 声明为 static ，这实际上使您的连接不是线程安全。使用连接池，永远不要分配连接（也不要语句，也不要 ResultSet ）作为类/实例变量。它们应该在您执行查询的同一块 try-finally 块中创建和关闭。此外，你还有一个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。
另请参阅：- JDBC MySql连接池实践以避免耗尽连接池
- 从连接池获取数据库连接
- 我使用JDBC连接池吗？
I'm working on a web project and I recently installed postgres 9.1.1
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.
So I also downloaded the JDBC4 driver for 9.1 postgres version here: jdbc.postgresql/download/postgresql-jdbc-9.1-901.src.tar.gz
I added it to the java build path using the project properties via eclipse.
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); } } } }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.
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; }解决方案
I'm working on a web project and I recently installed postgres 9.1.1
...
I added it to the java build path using the project properties via eclipse.
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.
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.
See also:- JDBC MySql connection pooling practices to avoid exhausted connection pool
- Get database connection from a connection pool
- Am I Using JDBC Connection Pooling?
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