java.lang.NoClassDefFoundError 读取数据库配置文件时报的错
用类加载器(ClassLoader)获取src路径下的配置文件，从而使JDBC获取数据连接对象(Connection)
需要注意的问题：①文件的路径需要时英文(不能含有中文)；②配置文件要放在src的根目录下；
如果文件路径是中文，会导致下图报错(java.io.FileNotFoundException):
如果数据库配置文件不在src的根目录下，会导致下图报错(java.lang.NoClassDefFoundError)：
数据库配置文件
url=jdbc:mysql:///db1
user=root
password=root
driver=com.mysql.jdbc.Driver
JDBC工具包类:
package JDBCUtils;
import java.io.FileReader;
import java.io.IOException;
import java.net.URL;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Properties;
/*
* JDBC工具类
*
*/
public class JDBCUtils {
private static String url;
private static String user;
private static String password;
private static String driver;
/*
* 文件的读取,只需要读取一次即可拿到这些值,使用静态代码块
*/
static {
//读取资源文件,获取值.
//1.创建Properties集合类
Properties pro = new Properties();
//获取src路径下的文件的方式--->ClassLoader(类加载器),可以加载字节码文件进内存,可以获取src下的资源文件的路径
//要获取ClassLoader,你得获取对应的字节码文件对象,随便一个字节码文件都可以,比如JDBCUtils字节码文件
ClassLoader classLoader = JDBCUtils.class.getClassLoader();
URL res = classLoader.getResource("jdbc.properties");
//传一个文件名,就可以获取它的Resource资源
//这个地方直接以src相对的根路径,只需要传jdbc.properties,就可以返回一个URL对象;
//URL表示统一资源定位符,可以定位一个文件的绝路路径,要获取字符串路径,有getPath()方法可以得到;
String path = res.getPath();
System.out.println(path);
try {
//2.加载文件
pro.load(new FileReader(path)); //最笨的方法是写上绝对路径，但写绝对路径不合适
//3.获取数据,赋值
url = pro.getProperty("url");
user = pro.getProperty("user");
password = pro.getProperty("password");
driver = pro.getProperty("driver");
//4.注册驱动
Class.forName(driver);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
/**
* 获取连接
* @return 连接对象
* @throws SQLException
*/
public static Connection getConnection() throws SQLException {
// return DriverManager.getConnection("jdbc:mysql:///db3", "root", "root");
return DriverManager.getConnection(url, user, password);
}
/**
* 释放资源
* @param stmt
* @param conn
*/
public static void close(Statement stmt,Connection conn) {
if( stmt != null) {
try {
stmt.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if( conn != null) {
try {
conn.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
/**
* 释放资源
* @param stmt
* @param conn
*/
public static void close(ResultSet rs,Statement stmt,Connection conn) {
if( rs != null) {
try {
stmt.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if( stmt != null) {
try {
stmt.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if( conn != null) {
try {
conn.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
数据库的记录封装成对象的类
package domain;
import java.util.Date;
/*
* 封装Emp表数据的JavaBean
*/
public class Emp {
private int id;
private String ename;
private int job_id;
private int mgr;
private Date joindate;
private double salary;
private double bonus;
private int dept_id;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getEname() {
return ename;
}
public void setEname(String ename) {
this.ename = ename;
}
public int getJob_id() {
return job_id;
}
public void setJob_id(int job_id) {
this.job_id = job_id;
}
public int getMgr() {
return mgr;
}
public void setMgr(int mgr) {
this.mgr = mgr;
}
public Date getJoindate() {
return joindate;
}
public void setJoindate(Date joindate) {
this.joindate = joindate;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public double getBonus() {
return bonus;
}
public void setBonus(double bounds) {
this.bonus = bounds;
}
public int getDept_id() {
return dept_id;
}
public void setDept_id(int dept_id) {
this.dept_id = dept_id;
}
@Override
public String toString() {
return "Emp [id=" + id + ", ename=" + ename + ", job_id=" + job_id + ", mgr=" + mgr + ", joindate=" + joindate
+ ", salary=" + salary + ", bounds=" + bonus + ", dept_id=" + dept_id + "]";
}
}
测试类
package JDBCUtils;
import java.sql.Connection;
import java.sql.Date;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import domain.Emp;
public class JDBCDemo8 {
/*
* 演示JDBC工具类
*/
public static void main(String[] args) {
List<Emp> list = new JDBCDemo8().FindAll2();
System.out.println(list);
}
public List<Emp> FindAll2(){
//变量抽取,扩展局部变量作用域
ResultSet rs=null;
Statement stmt=null;
Connection conn=null;
List<Emp> list = null;
try {
conn = JDBCUtils.getConnection();
//3.定义sql
String sql = "select * from emp";
//4.获取执行sql的对象
stmt = conn.createStatement();
//5.执行sql
rs = stmt.executeQuery(sql);
//6.遍历结果集,封装对象,装载集合
Emp emp = null; //让这个引用复用,减少栈内存多次创建这个引用
list = new ArrayList<Emp>();
while(rs.next()) {
//获取数据
int id = rs.getInt("id");
String ename = rs.getString("ename");
int job_id = rs.getInt("job_id");
int mgr = rs.getInt("mgr");
Date joindate = rs.getDate("joindate");
double salary = rs.getDouble("salary");
double bonus = rs.getDouble("bonus");
int dept_id = rs.getInt("dept_id");
System.out.println(id+"---"+ename+"---"+job_id+"---"+mgr+"---"+joindate+"---"
+salary+"---"+bonus+"---"+dept_id);
//创建对象,并赋值
emp = new Emp();
emp.setId(id);
emp.setEname(ename);
emp.setJob_id(job_id);
emp.setMgr(mgr);
emp.setJoindate(joindate);
emp.setSalary(salary);
emp.setBonus(bonus);
emp.setDept_id(dept_id);
//装载集合
list.add(emp);
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally {
//释放资源
JDBCUtils.close(rs,stmt,conn);
}
return list;
}
}
