详解Java注解实现自己的ORM

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface DBTable {
   String name() default "";
}

public enum Type {
   CHAR,
   STRING,
   BOOLEAN,
   INTEGER,
   LONG,
   FLOAT,
   DOUBLE,
   DATETIME
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ColumnType {
   Type value() default Type.INTEGER;
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Extrainfo {
   String name() default "";
   int length() default 0;
}

@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
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TableColumn {
   ColumnType columntype() default @ColumnType;
   ExtraInfo extrainfo() default @ExtraInfo;
   Constraints constraints() default @Constraints;
}

@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 + "]";
   }
}

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;
}

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;
}

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;
}

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();
      }
   }
}

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

   createTable(list);
}