未找到类加载JDBC org.postgresql.Driver [英] Class not found loading JDBC org.postgresql.Driver
问题描述
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.org/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;
}
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：
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: http://jdbc.postgresql.org/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;
}
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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