【Hibernate】模拟Hibernate

先定义一个实体类Student:

public class Student {

	private int id;
	private String name;
	
	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;
	}
}

模拟的Session类：

public class Session {

	String tableName = "t_student";  // 对应的数据库表
	Map<String, String> cfs = new HashMap<>();  // 模拟数据表中的列属性(column)对应实体类的属性(field)
	
	String[] methodNames;
	
	public Session(){
		cfs.put("_id", "id");  // _id是表中的属性，id是实体类的属性
		cfs.put("_name", "name");
		
		methodNames = new String[cfs.size()];
	}
	
	public void save(Student s) throws Exception{
		String sql = createSQL();
		
		Class.forName("com.mysql.jdbc.Driver");
		Connection conn = (Connection) DriverManager.getConnection("jdbc:mysql://localhost/db_hibernate3", "root", "root");
		PreparedStatement ps = (PreparedStatement) conn.prepareStatement(sql);
		
		//
		for (int i = 0; i < methodNames.length; i++) {
			Method m = s.getClass().getMethod(methodNames[i]);
			Class type = m.getReturnType();
			
			if (type.getName().equals("java.lang.String")) {
				String returnValue = (String) m.invoke(s);
				ps.setString(i+1, returnValue);
			}
			if (type.getName().equals("int")) {
				Integer returnValue = (Integer) m.invoke(s);
				ps.setInt(i+1, returnValue);
			}
		}
		
		ps.executeUpdate();
		ps.close();
		conn.close();
	}
	
	public String createSQL(){
		String str1 = "";
		int index = 0;
		for (String s : cfs.keySet()) {	
			String value = cfs.get(s);
			value = Character.toUpperCase(value.charAt(0)) + value.substring(1);
			methodNames[index] = "get" + value;  // 实体类里的getXXX方法
			
			str1 += s + ",";
			index++;
		}
		str1 = str1.substring(0, str1.length()-1);
		//System.out.println(str1);
		
		String str2 = "";
		for (int i = 0; i < cfs.size(); i++) {
			str2 += "?,";
		}
		str2 = str2.substring(0, str2.length()-1);
		//System.out.println(str2);
		
		String sql = "insert into " +tableName+ "(" +str1+ ")" + " values (" +str2+ ")";
		System.out.println(sql);
		return sql;
	}
}

测试：

public class StudentTest {

	public static void main(String[] args) throws Exception {

		Student s = new Student();
		s.setId(111);
		s.setName("Hibernate");
		
		Session session = new Session();
		session.save(s);
	}
}

解析：

当执行save()方法时，先根据实体类对应的表的字段拼接sql语句，再根据表的字段对应的类的属性得到各个getXXX字符串；
再根据要save(对象)，利用反射得到对象的getXXX方法；
再通过invoke()方法，得到方法执行结果，再作为PreparedStatement执行的参数