JDBC上關於數據庫中多表操作一對多關係和多對多關係的實現方法

我們知道，在設計一個JAVA bean的時候，要把這些BEAN 的數據存放在數據庫中的表結構，然而這些數據庫中的表直接又有些特殊的關係，例如員工與部門直接有一對多的關係，學生與老師直接又多對多的關係，那麼這些表的關係如何表示呢？
首先在建立數據庫的時候就應該建立這樣的對應關係。
一對多，只要建立兩個表就能建立這樣的關係，因爲你可以把多方的那個表設置一個Foreign Key 屬性，下面是一個部門和員工的表結構關係
在mysql 數據庫上應該這樣建立表結構：
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)//這個其實是約束條件，不是表格的屬性值。
);

在java 程序的javabean中應該如何做呢
public class Department {
private Integer id;
private String name;
private Set<Employee> emps = new HashSet<Employee>();//查看部門，就能查看得到部門下面有哪些員工，所以存放在一個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層如何實現增加查詢數據呢？增加一個部門和查詢一個部門的時候要不要顯示員工呢?
public class DeparmentDao {
private QueryRunner qr = new QueryRunner(JdbcUtil.getDataSource());
public void addDepartment(Department dept){
try{
//先保存部門的基本信息
String sql = "insert into department values(?,?)";
Object params[] = {dept.getId(),dept.getName()};
qr.update(sql, params);
//得到員工，如果有員工信息，保存員工信息
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);
}
}
//每個部門中的員工要不要查出來？看需求
public List<Department> findDepts(boolean lazy){
try{
//部門的基本信息
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){
//懶的
sql = "select id from employee where dept_id=?";
}else{
//餓的
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);
}
}
//每個部門中的員工要不要查出來？看需求.默認情況下，不查
public List<Department> findDepts(){
return findDepts(true);
}
}
---------------------------------------------------------------------------------------------------------------------------多對多的關係---------------------------------------------------------------------------------
下面以老師和學生的關係來說明這個結構
數據庫中：
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)
);
如何寫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{
//保存教師基本信息
String sql = "insert into teacher values(?,?,?)";
Object params[] = {t.getId(),t.getName(),t.getSalary()};
qr.update(sql, params);
//保存學生基本信息
//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的文件中內容
#連接設置
driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/day15 #這個是你的數據庫地址
username=root #這個是你的用戶名
password=sorry # 這個是你密碼

#
initialSize=10

#最大連接數量
maxActive=20

#
maxIdle=6

#
minIdle=3

#
maxWait=60000

#JDBC驅動建立連接時附帶的連接屬性屬性的格式必須爲這樣：[屬性名=property;]
#注意："user" 與 "password" 兩個屬性會被明確地傳遞，因此這裏不需要包含他們。
connectionProperties=useUnicode=true;characterEncoding=utf8

#指定由連接池所創建的連接的自動提交（auto-commit）狀態。
defaultAutoCommit=true

#driver default 指定由連接池所創建的連接的只讀（read-only）狀態。
#如果沒有設置該值，則“setReadOnly”方法將不被調用。（某些驅動並不支持只讀模式，如：Informix）
defaultReadOnly=

#driver default 指定由連接池所創建的連接的事務級別（TransactionIsolation）。
#可用值爲下列之一：（詳情可見javadoc。）NONE,READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE
defaultTransactionIsolation=REPEATABLE_READ