1、加载数据库驱动。（JDBC本nq不能直接访问数据库Q它需要依赖于JDBC驱动E序。）
2、与数据库徏立连接。（格式QDriverManager.getConnection("jdbc:mysql://localhostQ或?27.0.0.1Q?端口?数据库名","用户?, "q入数据库的密码");Q?/p>
3、发送查询或更新语句到数据库?/p>
4、执行查询ƈq回查询l果?/p>
看一下下面的例子?/p>
/**
* DriverManager 驱动E序理?nbsp; 在数据库和相应驱动程序之间徏立连?br> * Connection 对象代表与数据库的连接，也就是在已经加蝲的Driver和数据库之间建立q接
* Statement 提供在基层连接上q行SQL语句Qƈ且访问结?br> * ResultSet 在Statement执行SQL语句Ӟ有时?x)返回ResultSetl果集，包含的是查询的结果集
*/
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class Tester {
public static void main(String[] args) {
query();
}
public static void query(){
Connection conn = null;
try {
//1.加蝲数据库驱?br> Class.forName("com.mysql.jdbc.Driver");
//DriverManager 驱动E序理?nbsp; 在数据库和相应驱动程序之间徏立连?br> //2.获得数据库连?br> conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/jdbc_db", "root", "1234");
//3.创徏语句
String sql = "select * from UserTbl";
//q回一个执行SQL的句?br> Statement stmt = conn.createStatement();
//q回查询?br> //4.执行语句
ResultSet rs = stmt.executeQuery(sql);
//5.遍历l果?br> while(rs.next()){
int id = rs.getInt(1);
String username = rs.getString(2);
String password = rs.getString(3);
int age = rs.getInt(4);
System.out.println(id+":"+username+":"+password+":"+age);
}
} catch (Exception e) {
e.printStackTrace();
}finally{
if(conn!=null){
try { //关闭数据库连?br> conn.close();
} catch (SQLException e) {
conn = null;
e.printStackTrace();
}
}
}
}
}
q行l果Q?
要注意的是在获得数据库连接时Ҏ(gu)你所用的数据库种c选择加蝲的connector。加载方法先连接数据库的jar包放q你的Project的一个Folder内，然后叛_选择build path-----add to build path可以了。第三步创徏的SQL应根据你具体的需要编写?
获得数据库连接还可以通过下面两种Ҏ(gu)Q?
// W一U方?br> public Connection getConnection(String driver, String url, String user,
String password) {
Connection conn = null;
try {
// Class.forName加蝲驱动
Class.forName(driver);
// DriverManager获得q接
conn = DriverManager.getConnection(url, user, password);
return conn;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
// W二U方?br> public Connection openConnection() {
String driver = "";
String url = "";
String user = "";
String password = "";
Properties prop = new Properties();
Connection conn = null;
try {
// 加蝲属性文?br> prop.load(this.getClass().getClassLoader().getResourceAsStream(
"DBConfig.properties"));
driver = prop.getProperty("driver");
url = prop.getProperty("url");
user = prop.getProperty("user");
password = prop.getProperty("password");
// Class.forName加蝲驱动
Class.forName(driver);
// DriverManager获得q接
conn = DriverManager.getConnection(url, user, password);
return conn;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}
W二U方法比较常用，因ؓ(f)做修改的时候不需要对E序q行大的修改Q只要在配置文g中进行增、删、改可以了?/p>
写一下关于数据库表的操作SQLQ内容不重要Q重要的是理解结构和含义哈?/p>
//有自增的语句
CREATE TABLE person(
id INT PRIMARY KEY NOT NULL AUTO_INCREMENT ,
name VARCHAR(20) NOT NULL ,
age INT
) ;
/*创徏学生?/
create table Student(
Sno char(9) primary key,/*列玩整性约束条ӞSno是主?/
Sname char(20) unique, /*Sname 取唯一?/
Ssex char(2),
Sage smallint,
Sdept char (20),
);
/*创徏评?/
create table Course(
Cno char(4) primary key, /*列表完整性约束条ӞCno是主?/
Cname char(40),
Cpno char(4), /*Cpno的含义是先修?/
Ccredit smallint,
foreign key (Cpno) references Course(Cno)
/*表完整性的U束条gQCpno是外码，被参照表是CourseQ被参照列式Cno*/
);
/*创徏学生评?/
create table SC(
Sno char (9),
Cno char(4),
Grade smallint,
primary key(Sno,Cno),/*ȝ׃个属性构成，必须作ؓ(f)表完整性进行定?/
foreign key (Sno) references Student(Sno),/*表完整性约束条ӞSno是外码，被参照表是Student*/
foreign key (Cno) references course(Cno),/*表完整性约束条ӞSno是外码，被参照表是Student*/
);
/*向Student表增加“入学时间”列 */
alter table Student add S_entrance date;
/*修改Student中Sage的属?/
alter table Student alter column Sage int;
/*修改Course表中Cname为唯一?/
alter table Course add unique (Cname);
/*删除表Student*/
Drop Table Student CascadeQ（cascade 删除表是该表没有限制条gQ在删除基本表的同时Q相关的依赖对象,例如 视图{，都被删除
restrict删除的表不能被其他约束所引用Q不能有视图Q触发器。存储过E或函数{?Q?
/*** 表的索引 ***/
/*在Student表的Sname列上建立一个聚族烦?/
create cluster intdex Stusname on Student(Sname);
/*在三个表中徏立唯一索引*/
create unique index Stuno on Student(Sno);
create unique index Coucno on Course(Cno);
create unique index SCno on SC(Sno ASC, Cno DESC);
/*删除Student表中stusname索引*/
drop index Student;
/*** 数据查询 ***/
一、普通查?
//查询全体学生的详l记?br>select *
from Student;
//查询l过计算的?br>select Sname Q?004-Sage
from Student;
//查询全体学生的姓名、出生年份和所在的院系Q要求用写的字母表C所有系?br>1、select Sname,'year of birth:' birth, 2004-Sage birthday,LOWER(Sdept) departement
from Student;
2、select Sname,'year of birth:', 2004-Sage ,LOWER(Sdept)
from Student;
//查询选修评的学生学?br>select QallQSno
from SC;
//查询选修评的学生学P消除取值重复的行）
select distinct Sno
from SC;
二、条件查?
//查询全体计算机系的所有学?nbsp;
select Sname
from Student
where Sdept='CS';
//查询所有年龄在20岁以下的学生姓名和年?br>select Sname Sage
from Student
where Sage<20;
//成W有不?qing)格学生的学?br>select distinct Sno
from SC
where Grade<60;
//q里不在20?0岁之间的学生姓名、系别、和q龄
select Sname,Sdept,Sage
from Student
where Sage not between 20 and 23;
//查询在CS、MA、IS学生的姓名和性别
select Sname,Ssex
from Student
where Sdept in('CS','MA','IS');
//查询不在CS、MA、IS学生的姓名和性别
select Sname,Ssex
from Student
where Sdept not in('CS','MA','IS');
//查询学号200215121的学生详l情?br>select *
from Student
where Sno like '200215121'; Qlike 可用=替换Q?
//查询所有姓刘的学生姓名和性别
select *
from Student
where Sname like '?'; Q? {符L(fng)含义Q?br>//删除q龄大于25岁的
delete from Student where age>25;
]]>
U程本n的数据通常只有寄存器数据和E序执行时的堆栈D，所以线E的切换比进E的负担要小。线E不能自动运行，必须跻n在某一q程中，pE触发。一个进E可以有多个U程且进E间不共用线E?/p>
实现多线E有两种Ҏ(gu)Q一是承Threadc，二是实现Runnable接口?/p>
l承Threadcd建线E时Q首先要定义一个Threadcȝ子类Qƈ在该子类中重写runQ）Ҏ(gu)。runQ）Ҏ(gu)是线E体Q说明该U程的具体操作。需要创建线E时Q只需创徏该子cȝ对象在调用其startQ）Ҏ(gu)卛_?/p>
实现Runnable接口创徏U程必须重写该接口的runQ）Ҏ(gu)QRunnable接口只有一个runQ）Ҏ(gu)Q?/p>
׃Java是单l承语言Q不直接支持多承，如果一个类已经是其他类的子c，׃能在l承ThreadҎ(gu)使该cL为线E类Q这时就要采用实现Runnable接口的方式?/p>
两种Ҏ(gu)创徏的线E中h相同目标对象的线E可以共享内存单元，但是实现Runnable接口的线E去创徏目标对象的类可以是某个特定类的子c，因此实现Runnable接口创徏U程比承Threadcd建线E更灉|?/p>
丑և个小例子来看一下哈
l承Threadcd现的多线E：(x)
class MyThread1 extends Thread{
private String name;
public MyThread1(String name){
this.name= name;
}
public void run(){
for(int i=0;i<15;i++){
System.out.println(this.name+"---->Is Running");
}
}
}
public class ThreadDemo01{
public static void main(String[] args){
Runnable r1 = new MyThread1("U程A");
Runnable r2 = new MyThread1("U程B");
Runnable r3 = new MyThread1("U程C");
Runnable r4 = new MyThread1("U程D");
Thread t1 = new Thread (r1);
Thread t2 = new Thread (r2);
Thread t3 = new Thread (r3);
Thread t4 = new Thread (r4);
t1.start();
t2.start();
t3.start();
t4.start();
//也可以这样写
/* MyThread mt1 = new MyThread("U程A");
MyThread mt2 = new MyThread("U程B");
MyThread mt3 = new MyThread("U程C");
MyThread mt4 = new MyThread("U程D");
mt1.start();
mt2.start();
mt3.start();
mt4.start();*/
}
}
0.0Q！q行l果好长Q就不截图了哈，可以自己试试?/p>
实现Runnable接口的多U程Q实C׃n资源Q：(x)
class MyThread2 implements Runnable{
//定义十张?/p>
private int ticket=10;
public void run(){
for(int i=0;i<500;i++){
if(this.ticket>0){
System.out.println("卖票-------"+(this.ticket--));
}
}
}
}
public class ThreadDemo02 {
public static void main(String[] args){
MyThread2 mt =new MyThread2();
//׃n同一资源
Thread mt1 = new Thread(mt);
Thread mt2 = new Thread(mt);
Thread mt3 = new Thread(mt);
mt1.start();
mt2.start();
mt3.start();
}
}
q行l果Q?
public class Error1 {
public static void main(String[] args){
String str = null;
System.out.print(str.substring(1));
}
}
昄错误Q?font style="font-size: 10pt; color: #ff0000" size="2">Exception in thread "main" java.lang.NullPointerException?br /> 正确代码为：(x)
public class Error1 {
public static void main(String[] args){
String str = "null";
System.out.print(str.substring(2));
}
}
正确的运行结果ؓ(f)Q?font style="font-size: 8pt" size="2">llQnull的后两位llQ?/font>
2.数组界异常Q定义的数组arr长度?Q正常的下标为（0,1,2,3,4,5,Q，所以arr[6]是非法的?br /> public class Error2 {
public static void main(String[] args){
int j = 0;
int arr[] = {1,2,3,4,5,6};
for(int i=0;i<arr.length;i++){
j +=arr[i];
arr[6] = j;
System.out.print(j);
}
}
}
昄错误QException in thread "main" java.lang.ArrayIndexOutOfBoundsException: 6?br /> 正确代码Q?br /> public class Error2 {
public static void main(String[] args){
int j = 0;
int arr[] = {1,2,3,4,5,6};
for(int i=0;i<arr.length;i++){
j +=arr[i];
arr[5] = j;
System.out.print(j);
}
}
}
正确的运行结果：(x)1 3 6 10 15 30?br />
]]>