搞过Java的码农都知道，在J2EE开发中一个（确切地说，应该是一类）很重要的框架，那就是ORM（Object Relational Mapping，对象关系映射）。它把Java中的类和数据库中的表关联起来，可以像操作对象那样操作数据表，十分方便。给码农们节约了大量的时间去摸鱼。其实它的本质一点都不复杂，而最核心的就是怎么实现对象和表之间的转换。之前对反射和注解有了一点了解，所以就试着来实现咱们自己的缝合怪。

首先，需要建立一个「表格」：

/**
 * 类注解，将类注解成数据库表
 *
 * @author xiangwang
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface DBTable {
   String name() default "";
}

然后，定义需要的数据库数据类型：

/**
 * 字段类型枚举
 *
 * @author xiangwang
 */
public enum Type {
   CHAR,
   STRING,
   BOOLEAN,
   INTEGER,
   LONG,
   FLOAT,
   DOUBLE,
   DATETIME
}

 

/**
 * 数据库字段类型
 *
 * @author xiangwang
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ColumnType {
   Type value() default Type.INTEGER;
}

再来完善字段相关信息：

/**
 * 字段信息
 *
 * @author xiangwang
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ExtraInfo {
   String name() default "";
   int length() default 0;
}

 

/**
 * 明确字段约束
 *
 * @author xiangwang
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Constraints {
    boolean primaryKey() default false;
    boolean allowNull() default true;
    boolean unique() default false;
    // 还可以增加默认值
}

把他们拼起来，成为完整的字段描述：

/**
 * 拼装注解，形成完整的字段嵌套注解
 *
 * @author xiangwang
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TableColumn {
   ColumnType columntype() default @ColumnType;
   ExtraInfo extrainfo() default @ExtraInfo;
   Constraints constraints() default @Constraints;
}

最后，创建实体类，应用刚才写好的这些注解：

/**
 * 用户实体类
 *
 * @author xiangwang
 */
@DBTable(name = "User")
public class User {
   @TableColumn(
         columntype = @ColumnType(Type.INTEGER),
         extrainfo = @ExtraInfo(name = "id", length = 4),
         constraints = @Constraints(primaryKey = true))
   private String id;

   @TableColumn(
         columntype = @ColumnType(Type.STRING),
         extrainfo = @ExtraInfo(name = "name", length = 32),
         constraints = @Constraints(primaryKey = false, allowNull = false, unique = true))
   private String name;

   @TableColumn(
         columntype = @ColumnType(Type.INTEGER),
         extrainfo = @ExtraInfo(name = "age", length = 4),
         constraints = @Constraints(primaryKey = false))
   private Integer age;

   public String getId() { return id; }
   public void setId(String id) { this.id = id; }

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

   public Integer getAge() { return age; }
   public void setAge(Integer age) { this.age = age; }

   @Override
   public String toString() {
      return "User [id=" + id + ", name=" + name + ", age=" + age + "]";
   }
}

来看看ORM是怎么工作的吧：

/**
 * 解析类型注解
 */
private static String getColumnType(ColumnType columntype) {
   String type = "";
   switch (columntype.value()) {
      case CHAR:
         type += "CHAR";
         break;
      case STRING:
         type += "VARCHAR";
         break;
      case BOOLEAN:
         type += "BIT";
         break;
      case INTEGER:
         type += "INT";
         break;
      case LONG:
         type += "BIGINT";
         break;
      case FLOAT:
         type += "FLOAT";
         break;
      case DOUBLE:
         type += "DOUBLE";
         break;
      case DATETIME:
         type += "DATETIME";
         break;
      default:
         type += "VARCHAR";
         break;
   }
   return type;
}

 

/**
 * 解析信息注解
 */
private static String getExtraInfo(ExtraInfo extrainfo) {
   String info = "";
   if (null != extrainfo.name()) {
      info = extrainfo.name();
   } else {
      return null;
   }
   if (0 < extrainfo.length()) {
      info += " (" + extrainfo.length() + ")";
   } else {
      return null;
   }
   return info;
}

 

/**
 * 解析约束注解
 */
private static String getConstraints(Constraints con) {
   String constraints = "";
   if (con.primaryKey()) {
      constraints += " PRIMARY KEY";
   }
   if (!con.allowNull()) {
      constraints += " NOT NULL";
   }
   if (con.unique()) {
      constraints += " UNIQUE";
   }

   return constraints;
}

做了那么多的铺垫，终于到了临门一脚了，实现一个缝合怪了：

/**
 * 临门一脚：实现一个缝合怪
 */
private static void createTable(List<String> list) {
   for (String className : list) {
      Class<?> clazz;
      try {
         clazz = Class.forName(className);
         DBTable dbTable = clazz.getAnnotation(DBTable.class);
         if (dbTable == null) {// 无DBTable注解
            continue;
         }
         // 转大写
         String tableName = clazz.getSimpleName().toUpperCase();
         StringBuilder sql = new StringBuilder("CREATE TABLE " + tableName + "(");
         for (Field field : clazz.getDeclaredFields()) {
            // 反射得到注解
            Annotation[] anns = field.getDeclaredAnnotations();
            if (anns.length < 1) {
               continue;
            }
            String columnInfo = "";
            // 类型判断
            if (anns[0] instanceof TableColumn) {
               TableColumn column = (TableColumn) anns[0];
               String type = getColumnType(column.columntype());
               columnInfo = getExtraInfo(column.extrainfo());
               // 代替(
               columnInfo = columnInfo.replace("(", type + "(");
               columnInfo += getConstraints(column.constraints());
            }
            sql.append("\n " + columnInfo + ",");
         }
         // 删除尾部的逗号
         String tableCreate = sql.substring(0, sql.length() - 1) + "\n);";
         System.out.println(tableCreate);
      } catch (ClassNotFoundException e) {
         e.printStackTrace();
      }
   }
}

验证效果的时候到了：

public static void main(String[] args) {
   Class<?> clazz = User.class;
   List<String> list = new ArrayList<>();
   list.add(clazz.getName());

   createTable(list);
}

当然，实际的运营于生产环境中的ORM框架可要比这个小玩意复杂多了。但千变万变，原理不变，ORM的核心——反射 + 注解——就是这么玩的。