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」：
@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：
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;
}再来完善字段相关信息：
@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
}把他们拼起来，成为完整的字段描述：
@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：
@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：
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;
}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：
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。
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