我们知道，在设计一个JAVA bean的时候，要把这些BEAN 的数据存放在数据库中的表结构，然而这些数据库中的表直接又有些特殊的关系，例如员工与部门直接有一对多的关系，学生与老师直接又多对多的关系，那么这些表的关系如何表示呢？
首先在建立数据库的时候就应该建立这样的对应关系。
一对多 ，只要建立两个表就能建立这样的关系，因为你可以把多方的那个表设置一个Foreign Key 属性 ，下面是一个部门和员工的表结构关系
在mysql 数据库上应该这样建立表结构：
create table department(
id int primary key,
name varchar(100)
);
create table employee(
id int primary key,
name varchar(100),
salary float(8,2),
dept_id int,
constraint dept_id_fk foreign key (dept_id) references department(id)//这个其实是约束条件，不是表格的属性值。
);
在java 程序的javabean中应该如何做呢
public class Department {
private Integer id;
private String name;
private Set<Employee> emps = new HashSet<Employee>();//查看部门，就能查看得到部门下面有哪些员工，所以存放在一个Set集合中。
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set<Employee> getEmps() {
return emps;
}
public void setEmps(Set<Employee> emps) {
this.emps = emps;
}
@Override
public String toString() {
return "Department [emps=" + emps + ", id=" + id + ", name=" + name
+ "]";
}
}
public class Employee {
private Integer id;
private String name;
private Float salary;
//private Department dept = new Department();
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Float getSalary() {
return salary;
}
public void setSalary(Float salary) {
this.salary = salary;
}
@Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", salary=" + salary
+ "]";
}
}
在DAO层 如何实现增加 查询数据呢？增加一个部门和查询一个部门的时候要不要显示员工呢?
public class DeparmentDao {
private QueryRunner qr = new QueryRunner(JdbcUtil.getDataSource());
public void addDepartment(Department dept){
try{
//先保存部门的基本信息
String sql = "insert into department values(?,?)";
Object params[] = {dept.getId(),dept.getName()};
qr.update(sql, params);
//得到员工，如果有员工信息，保存员工信息
Set<Employee> emps = dept.getEmps();
if(emps!=null&&emps.size()>0){
for(Employee e:emps){
sql = "insert into employee values(?,?,?,?)";
params = new Object[]{e.getId(),e.getName(),e.getSalary(),dept.getId()};
qr.update(sql, params);
}
}
}catch(Exception e){
throw new RuntimeException(e);
}
}
//每个部门中的员工要不要查出来？看需求
public List<Department> findDepts(boolean lazy){
try{
//部门的基本信息
String sql = "select * from department";
List<Department> depts = qr.query(sql, new BeanListHandler<Department>(Department.class));
if(depts!=null&&depts.size()>0){
for(Department dept:depts){
if(lazy){
//懒的
sql = "select id from employee where dept_id=?";
}else{
//饿的
sql = "select * from employee where dept_id=?";
}
List<Employee> emps = qr.query(sql, new BeanListHandler<Employee>(Employee.class), dept.getId());
for(Employee e:emps){
dept.getEmps().add(e);
}
}
}
return depts;
}catch(Exception e){
throw new RuntimeException(e);
}
}
//每个部门中的员工要不要查出来？看需求.默认情况下，不查
public List<Department> findDepts(){
return findDepts(true);
}
}
---------------------------------------------------------------------------------------------------------------------------
多对多的关系
---------------------------------------------------------------------------------
下面以老师和学生的关系来说明这个结构
数据库中：
create table teacher(
id int primary key,
name varchar(100),
salary float(8,2)
);
create table student(
id int primary key,
name varchar(100),
grade varchar(100)
);
create table teacher_student(
t_id int,
s_id int,
primary key(t_id,s_id),
constraint t_id_fk foreign key(t_id) references teacher(id),
constraint s_id_fk foreign key(s_id) references student(id)
);
如何写javabean 和 dao呢 ？
public class Teacher {
private Integer id;
private String name;
private Float salary;
private Set<Student> stus = new HashSet<Student>();
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Float getSalary() {
return salary;
}
public void setSalary(Float salary) {
this.salary = salary;
}
public Set<Student> getStus() {
return stus;
}
public void setStus(Set<Student> stus) {
this.stus = stus;
}
}
public class Student {
private Integer id;
private String name;
private String grade;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getGrade() {
return grade;
}
public void setGrade(String grade) {
this.grade = grade;
}
@Override
public String toString() {
return "Student [grade=" + grade + ", id=" + id + ", name=" + name+ "]";
}
}
public class TeacherDao {
private QueryRunner qr = new QueryRunner(JdbcUtil.getDataSource());
public void addTeacher(Teacher t) throws SQLException{
//保存教师基本信息
String sql = "insert into teacher values(?,?,?)";
Object params[] = {t.getId(),t.getName(),t.getSalary()};
qr.update(sql, params);
//保存学生基本信息
//第3方表
Set<Student> stus = t.getStus();
if(stus!=null&&stus.size()>0){
for(Student s:stus){
sql = "insert into student values(?,?,?)";
params = new Object[]{s.getId(),s.getName(),s.getGrade()};
qr.update(sql, params);
sql = "insert into teacher_student values(?,?)";
params = new Object[]{t.getId(),s.getId()};;
qr.update(sql, params);
}
}
}
public List<Teacher> findTeacher(boolean lazy) throws SQLException{
String sql = "select * from teacher";
List<Teacher> ts = qr.query(sql, new BeanListHandler<Teacher>(Teacher.class));
if(ts!=null&&ts.size()>0){
for(Teacher t:ts){
if(lazy){
sql = "select id from student where id in (select s_id from teacher_student where t_id=?)";
}else{
sql = "select * from student where id in (select s_id from teacher_student where t_id=?)";
}
List<Student> stus = qr.query(sql, new BeanListHandler<Student>(Student.class), t.getId());
for(Student s:stus){
t.getStus().add(s);
}
}
}
return ts;
}
}
工具表工具
public class JdbcUtil {
private static DataSource ds;
private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>();
static{
try{
InputStream in = JdbcUtil.class.getClassLoader().getResourceAsStream("dbcpconfig.properties");
Properties props = new Properties();
props.load(in);
BasicDataSourceFactory factory = new BasicDataSourceFactory();
ds = factory.createDataSource(props);
}catch(Exception e){
throw new ExceptionInInitializerError(e);
}
}
public static DataSource getDataSource(){
return ds;
}
public static Connection getConnection() throws SQLException{
Connection conn = tl.get();
if(conn==null){
conn = ds.getConnection();
tl.set(conn);
}
return conn;
}
public static void startTransaction() throws SQLException{
Connection conn = tl.get();
if(conn==null){
conn = ds.getConnection();
tl.set(conn);
}
conn.setAutoCommit(false);
}
public static void rollback()throws SQLException{
Connection conn = tl.get();
if(conn==null){
conn = ds.getConnection();
tl.set(conn);
}
conn.rollback();
}
public static void commit()throws SQLException{
Connection conn = tl.get();
if(conn==null){
conn = ds.getConnection();
tl.set(conn);
}
conn.commit();
tl.remove();
}
public static void release(ResultSet rs,Statement stmt,Connection conn){
if(rs!=null){
try{
rs.close();
}catch(Exception e){
e.printStackTrace();
}
rs = null;
}
if(stmt!=null){
try{
stmt.close();
}catch(Exception e){
e.printStackTrace();
}
stmt = null;
}
if(conn!=null){
try{
conn.close();
}catch(Exception e){
e.printStackTrace();
}
conn = null;
}
}
}
dbcpconfig.properties的文件 中内容
#连接设置
driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/day15 #这个是你的数据库地址
username=root #这个是你的用户名
password=sorry # 这个是你 密码
#<!-- 初始化连接 -->
initialSize=10
#最大连接数量
maxActive=20
#<!-- 最大空闲连接 -->
maxIdle=6
#<!-- 最小空闲连接 -->
minIdle=3
#<!-- 超时等待时间以毫秒为单位 6000毫秒/1000等于60秒 -->
maxWait=60000
#JDBC驱动建立连接时附带的连接属性属性的格式必须为这样：[属性名=property;]
#注意："user" 与 "password" 两个属性会被明确地传递，因此这里不需要包含他们。
connectionProperties=useUnicode=true;characterEncoding=utf8
#指定由连接池所创建的连接的自动提交（auto-commit）状态。
defaultAutoCommit=true
#driver default 指定由连接池所创建的连接的只读（read-only）状态。
#如果没有设置该值，则“setReadOnly”方法将不被调用。（某些驱动并不支持只读模式，如：Informix）
defaultReadOnly=
#driver default 指定由连接池所创建的连接的事务级别（TransactionIsolation）。
#可用值为下列之一：（详情可见javadoc。）NONE,READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE
defaultTransactionIsolation=REPEATABLE_READ