泛型dao

感冒了，也好几天没更，说好了写一下泛型DAO的，

简单说一下自己的理解，下面会附上代码

DAO是指操作数据库和实体类连接用的，而泛型指的就是能满足项目里所有的增删改查之类的所有功能，

具体实现的方法当然是定义泛型类，然后在实体类里实现泛型接口里的所有方法，还运用了一些类反射的知识，其实很简单，不过这是框架里的思维，可以在开发项目的时候节省很多时间，提高效率！

public interface BaseDao<T> {


void add(T t);
void del(T t);
void update(T t);
T selectAll(T t);
}

//////////////////////////////////////////////////////////////

public class BaseDaoImp<T> implements BaseDao<T> {


public static final String SQL_INSERT = "insert";
public static final String SQL_DELETE = "delete";
public static final String SQL_UPDATE = "update";
public static final String SQL_SELECT = "select";


private Class<T> EntityClass;
private Connection conn;
private PreparedStatement ps;
private String sql;
private Object argType[];
private ResultSet rs;


@SuppressWarnings("unchecked")
public BaseDaoImp() {


// getGenericSuperclass()方法可以获取当前对象的超类的type
ParameterizedType type = (ParameterizedType) getClass().getGenericSuperclass();
// 从一个泛型类型中获得第一个泛型类型的实体类
EntityClass = (Class<T>) type.getActualTypeArguments()[0];
}


// sql拼接函数 形如 : insert into Student(Sid,Sname,Sage,Ssex) values(?,?,?,?)
private String getSql(String sqlType) {
StringBuffer sql = new StringBuffer();
// 通过反射获取实体类中的所有变量
Field fields[] = EntityClass.getDeclaredFields();


if (sqlType.equals(SQL_INSERT)) {
sql.append("insert into " + EntityClass.getSimpleName());
sql.append("(");
for (int i = 0; fields != null && i < fields.length; i++) {
fields[i].setAccessible(true);// 因为类中的属性是private的，所以这句话必须加
String column = fields[i].getName();
sql.append(column).append(",");
}
sql = sql.deleteCharAt(sql.length() - 1);// 删除最后一个字符
sql.append(") values (");
for (int i = 0; fields != null && i < fields.length; i++) {
sql.append("?,");
}
sql = sql.deleteCharAt(sql.length() - 1);
sql.append(")");// 注意结尾分号
} else if (sqlType.equals(SQL_DELETE)) {
sql.append("delete from " + EntityClass.getSimpleName() + " where id=?");
} else if (sqlType.equals(SQL_UPDATE)) {
sql.append("update " + EntityClass.getSimpleName() + " set ");
for (int i = 0; fields != null && i < fields.length; i++) {
fields[i].setAccessible(true);
String column = fields[i].getName();
if (column.equals("id")) {
continue;
}
sql.append(column).append("=").append("?,");
}
sql.deleteCharAt(sql.length() - 1);
sql.append(" where id=?");
} else if (sqlType.equals(SQL_SELECT)) {
sql.append("select * from " + EntityClass.getSimpleName() + " where id=?");
}
return sql.toString();
}


// 获取参数
private Object[] setArgs(T entity, String sqlType) throws IllegalAccessException, IllegalArgumentException {
Field fields[] = EntityClass.getDeclaredFields();


if (sqlType.equals(SQL_INSERT)) {
Object[] obj = new Object[fields.length];
for (int i = 0; obj != null && i < fields.length; i++) {
fields[i].setAccessible(true);
obj[i] = fields[i].get(entity);// 拿到entity中的值
}
return obj;
} else if (sqlType.equals(SQL_DELETE)) {
Object[] obj = new Object[1];
fields[0].setAccessible(true);
obj[0] = fields[0].get(entity);
return obj;
} else if (sqlType.equals(SQL_UPDATE)) {
Object[] tempobj = new Object[fields.length];
for (int i = 0; tempobj != null && i < fields.length; i++) {
fields[i].setAccessible(true);
tempobj[i] = fields[i].get(entity);
}
Object[] obj = new Object[fields.length];
System.arraycopy(tempobj, 1, obj, 0, tempobj.length - 1);
obj[obj.length - 1] = tempobj[0];
return obj;
} else if (sqlType.equals(SQL_SELECT)) {
Object obj[] = new Object[1];
fields[0].setAccessible(true);
obj[0] = fields[0].get(entity);
return obj;
}
return null;
}


@Override
public void add(T t) {
sql = this.getSql(SQL_INSERT);//获取sql
//赋值：
try {
argType = setArgs(t , SQL_INSERT);
conn = ConnectionManager.getConnection();
ps = conn.prepareStatement(sql);
ps = ConnectionManager.setPreparedStatementParam(ps, argType);
ps.executeUpdate();
} catch (IllegalAccessException | IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
ConnectionManager.releaseDB(rs, ps, conn);
}

}


@Override
public void del(T t) {
sql = this.getSql(SQL_DELETE);
try {
argType = this.setArgs(t , SQL_DELETE);
conn = ConnectionManager.getConnection();
ps = conn.prepareStatement(sql);
ps = ConnectionManager.setPreparedStatementParam(ps, argType);
ps.executeUpdate();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
ConnectionManager.releaseDB(rs, ps, conn);
}

}


@Override
public void update(T t) {
sql = this.getSql(SQL_UPDATE);
try {
argType = this.setArgs(t , SQL_UPDATE);
conn = ConnectionManager.getConnection();
ps = conn.prepareStatement(sql);
ps = ConnectionManager.setPreparedStatementParam(ps, argType);
ps.executeUpdate();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
ConnectionManager.releaseDB(rs, ps, conn);
}
}


@Override
public T selectAll(T t) {
sql = this.getSql(SQL_SELECT);
T obj = null;
try {
            argType = setArgs(t, SQL_SELECT);
            conn = ConnectionManager.getConnection();
            ps = conn.prepareStatement(sql);
            ps = ConnectionManager.setPreparedStatementParam(ps, argType);
            rs = ps.executeQuery();
            Field fields[] = EntityClass.getDeclaredFields();
            while (rs.next()) {
                obj = EntityClass.newInstance();
                for (int i = 0; i < fields.length; i++) {
                    fields[i].setAccessible(true);
                    fields[i].set(obj, rs.getObject(fields[i].getName()));
                }
            }
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return obj;
}


}