【原创】基于注解运行时动态ORM映射

因为当前项目采用了分表策略，故一个实体会对应多个相同结构的表。只是映射的表名不同而已~项目又使用憨包儿呢特（Hibernate），让我采用原生SQL总感受不是那么爽，咋办呢？！第一念头就是若是可以动态映射就好啦，也就是如今想查table1只需将实体对应的 table映射为table1便可。咋个实现动态映射呢？！也就是须要动态改变类的注解。又咋个可以动态改变类的注解呢？！本质上修改字节码，从新加载类便可，也就是所谓的字节码加强功能~  看都看不懂字节码咋个修改呢？！仍是须要站在巨人的肩上才行。ASM以及JDK自带的字节码加强都不用，立刻想到CGlib,javassist。。。仍是痛苦的折腾了良久，为了你们不走个人弯路~ 直接上代码先~

动态映射代码以下:

 1: public class ClassPoolUtils {

 2: 

 3: 

 4:     /**

 5:  * 动态ORM映射

 6:  * 

 7:  * @param entityClassName 待映射的实体全限定类名

 8:  * @param tableName 待映射的表名

 9:  * @return 映射后的类对象

 10:  */

 11:     public static Class<?> tableMapping(String entityClassName, String tableName){

 12:         Class<?> c = null;

 13:         try {

 14:             ClassPool classPool = ClassPool.getDefault();

 15:             classPool.appendClassPath(new ClassClassPath(ClassPoolUtils.class));

 16:             classPool.importPackage("javax.persistence");

 17:             CtClass clazz = classPool.get(entityClassName);

 18:             ClassFile classFile = clazz.getClassFile();

 19: 

 20:             System.out.println("加强前Entity01:" + clazz.getAnnotation(Entity.class));

 21:             System.out.println("加强前Table02:" + clazz.getAnnotation(Table.class));

 22: 

 23:             ConstPool constPool = classFile.getConstPool();

 24:             Annotation tableAnnotation = new Annotation("javax.persistence.Table", constPool);

 25:             tableAnnotation.addMemberValue("name", new StringMemberValue(tableName, constPool));

 26:             // 获取运行时注解属性

 27:             AnnotationsAttribute attribute = (AnnotationsAttribute)classFile.getAttribute(AnnotationsAttribute.visibleTag);

 28:             attribute.addAnnotation(tableAnnotation);

 29:             classFile.addAttribute(attribute);

 30:             classFile.setVersionToJava5();

 31:             //clazz.writeFile();

 32: 

 33:             System.out.println("加强后Entity001:" + clazz.getAnnotation(Entity.class));

 34:             System.out.println("加强后Table002:" + clazz.getAnnotation(Table.class));

 35:             //TODO 当前ClassLoader中必须还没有加载该实体。（同一个ClassLoader加载同一个类只会加载一次）

 36:             c = clazz.toClass();

 37:             System.out.println("加强后toClass-Entity0001:" + c.getAnnotation(Entity.class));

 38:             System.out.println("加强后toClass-Table0002:" + c.getAnnotation(Table.class));

 39:         } catch (Exception e) {

 40:             e.printStackTrace();

 41:         }

 42: 

 43:         return c;

 44:     }

 45:  

 46:     public static void main(String[] args) {

 47:           ClassPoolUtils.tableMapping("com.andy.model.Order", "order1");

 48:     }

执行结果:

 1: 加强前Entity01:@javax.persistence.Entity

 2: 加强前Table02:null

 3: 加强后Entity001:@javax.persistence.Entity

 4: 加强后Table002:@javax.persistence.Table(name="order1")

 5: 加强后toClass-Entity0001:@javax.persistence.Entity(name=)

 6: 加强后toClass-Table0002:@javax.persistence.Table(schema=, catalog=, uniqueConstraints=[], name=order1)