JDBC上关于数据库中多表操作一对多关系和多对多关系的实现方法

黑马程序员

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>(); //????????????????????????????Set????

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 {
//??????????
String sql = "insert into department values(?,?)";
Object[] params = { dept.getId(), dept.getName() };
qr.update(sql, params);

//???????????????????
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);
}
}

//??????????????????
public List<Department> findDepts(boolean lazy) {
try {
//???????
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) {
//??
sql = "select id from employee where dept_id=?";
} else {
//??
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);
}
}

//??????????????????.????????
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 {
//????????
String sql = "insert into teacher values(?,?,?)";
Object[] params = { t.getId(), t.getName(), t.getSalary() };
qr.update(sql, params);

//????????
//?3??
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