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Javabean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