Hibernate O/R Mapping模拟

作为SSH中的重要一环，有必要理解一下Hibernate对 O/R Mapping的实现。

主要利用java的反射机制来得到完整的SQL语句。

准备工作：

1. Object

Student实体类:

public class Student {
    private int id;
    private String name;
    private int age;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

}

2. Relationship

_student表：

-- database: hibernate
create table _student(_id int primary key auto_increment,_age int,_name varchar(20));

3. 模拟O/R Mapping：

针对Student实体类的SessionStu （完成Student对象的持久化）：

public class SessionStu {

    private String tableName;
    private Map<String, String> field2Column = new HashMap<String,String>();
    private String[] fields;
    
    public SessionStu() {
        // 以下内容应当通过解析xml的方式build出来
        // 该示例仅仅演示Hibernate的核心部分：O/R Mapping (利用反射得到完整sql)
        tableName = "_student";
        field2Column.put("id","_id");
        field2Column.put("name","_name");
        field2Column.put("age","_age");
        fields = new String[field2Column.size()];
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    
    public void save(Student student) throws Exception {
        // TODO Auto-generated method stub
        String sql = createSQL();
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/hibernate","root","");
        PreparedStatement ps = conn.prepareStatement(sql);
        ps = setParameters(ps,student);
        ps.executeUpdate();
        ps.close();
        conn.close();
    }
    
    //important
    private PreparedStatement setParameters(PreparedStatement ps, Student student) throws Exception{
        for (int i = 0; i < fields.length; i++) {
            String get_method = "get"+(char)(fields[i].charAt(0)-32)+fields[i].substring(1);
            Method m = student.getClass().getMethod(get_method);
            String type = m.getReturnType().getName();
            String te = type.substring(type.lastIndexOf(".")+1);
            switch (te) {
            case "int":
                ps.setInt(i+1, (int) m.invoke(student));
                break;
            case "String":
                ps.setString(i+1, (String) m.invoke(student));
            default:
                break;
            }
        }
        return ps;        
    }
    
    private String createSQL(){
        String columnsStr = "";
        int index = 0;
        for (String key : field2Column.keySet()) {
            fields[index] = key;
            columnsStr += (field2Column.get(key) + ",");
            index++;
        }
        columnsStr = columnsStr.substring(0, columnsStr.length()-1);
        String unknow = "";
        for (int i = 0; i < field2Column.size(); i++) {
            unknow += "?,";
        }
        unknow = unknow.substring(0,unknow.length()-1);
        String sql = "insert into " + this.tableName + " (" + columnsStr +") values (" + unknow +")";
        System.out.println(sql);
        return sql;
    }
    
}

4. Test：

public class TestSessionSave {

    public static void main(String[] args) throws Exception {
        Student student = new Student();
        student.setAge(20);
        student.setId(5);
        student.setName("hibernate");
        SessionStu ss = new SessionStu();
        ss.save(student);
    }
}

5. Done