【原创】基于注解运行时动态ORM映射

由于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。只是映射的表名不一样而已~项目又使用憨包儿呢特（Hibernate），让我采用原生SQL总感觉不是那么爽，咋办呢？！第一念头就是如果能够动态映射就好啦，也就是现在想查table1只需将实体对应的 table映射为table1即可。咋个实现动态映射呢？！也就是需要动态改变类的注解。又咋个能够动态改变类的注解呢？！本质上修改字节码，重新加载类即可，也就是所谓的字节码增强功能~ 看都看不懂字节码咋个修改呢？！还是需要站在巨人的肩上才行。ASM以及JDK自带的字节码增强都不用，马上想到CGlib,javassist。。。还是痛苦的折腾了良久，为了大家不走我的弯路~ 直接上代码先~

动态映射代码如下:

   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("增强前Entity01:" + clazz.getAnnotation(Entity.class));

  21:              System.out.println("增强前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("增强后Entity001:" + clazz.getAnnotation(Entity.class));

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

  35:              //TODO 当前ClassLoader中必须尚未加载该实体。（同一个ClassLoader加载同一个类只会加载一次）

  36:              c = clazz.toClass();

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

  38:              System.out.println("增强后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:  增强前Entity01:@javax.persistence.Entity

   2:  增强前Table02:null

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

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

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

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

来源：oschina

链接：https://my.oschina.net/u/859556/blog/88585

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