Mybatis懒加载的实现

因为通过javassist和cglib代理实现的，所以说到底最主要的就是JavasisstProxyFactory类中的invoke方法和里面的load方法。

其实读一读，里面的逻辑就是跟配置中定义的规则一样的
因为github上的mybatis中文版中这部分注释比较少，所以从网上寻找博客，截取了代码注释片段记录下。
JavasisstProxyFactory

public class JavassistProxyFactory implements org.apache.ibatis.executor.loader.ProxyFactory {

 /**
 * 接口实现
 * @param target 目标结果对象
 * @param lazyLoader 延迟加载对象
 * @param configuration 配置
 * @param objectFactory 对象工厂
 * @param constructorArgTypes 构造参数类型
 * @param constructorArgs 构造参数值
 * @return
 */
 @Override
 public Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
 return EnhancedResultObjectProxyImpl.createProxy(target, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
 }

 //省略...

 /**
 * 代理对象实现，核心逻辑执行
 */
 private static class EnhancedResultObjectProxyImpl implements MethodHandler {

 /**
 * 创建代理对象
 * @param type
 * @param callback
 * @param constructorArgTypes
 * @param constructorArgs
 * @return
 */
 static Object crateProxy(Class<?> type, MethodHandler callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {

 ProxyFactory enhancer = new ProxyFactory();
 enhancer.setSuperclass(type);

 try {
  //通过获取对象方法，判断是否存在该方法
  type.getDeclaredMethod(WRITE_REPLACE_METHOD);
  // ObjectOutputStream will call writeReplace of objects returned by writeReplace
  if (log.isDebugEnabled()) {
  log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this");
  }
 } catch (NoSuchMethodException e) {
  //没找到该方法，实现接口
  enhancer.setInterfaces(new Class[]{WriteReplaceInterface.class});
 } catch (SecurityException e) {
  // nothing to do here
 }

 Object enhanced;
 Class<?>[] typesArray = constructorArgTypes.toArray(new Class[constructorArgTypes.size()]);
 Object[] valuesArray = constructorArgs.toArray(new Object[constructorArgs.size()]);
 try {
  //创建新的代理对象
  enhanced = enhancer.create(typesArray, valuesArray);
 } catch (Exception e) {
  throw new ExecutorException("Error creating lazy proxy. Cause: " + e, e);
 }
 //设置代理执行器
 ((Proxy) enhanced).setHandler(callback);
 return enhanced;
 }

  /**
  * 代理对象执行
  * @param enhanced 原对象
  * @param method 原对象方法
  * @param methodProxy 代理方法
  * @param args 方法参数
  * @return
  * @throws Throwable
  */
 @Override
 public Object invoke(Object enhanced, Method method, Method methodProxy, Object[] args) throws Throwable {
  final String methodName = method.getName();
  try {
  synchronized (lazyLoader) {
   if (WRITE_REPLACE_METHOD.equals(methodName)) {
   //忽略暂未找到具体作用
   Object original;
   if (constructorArgTypes.isEmpty()) {
    original = objectFactory.create(type);
   } else {
    original = objectFactory.create(type, constructorArgTypes, constructorArgs);
   }
   PropertyCopier.copyBeanProperties(type, enhanced, original);
   if (lazyLoader.size() > 0) {
    return new JavassistSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs);
   } else {
    return original;
   }
   } else {
    //延迟加载数量大于0
   if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
    //aggressive 一次加载性所有需要要延迟加载属性或者包含触发延迟加载方法
    if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
    log.debug("==> laze lod trigger method:" + methodName + ",proxy method:" + methodProxy.getName() + " class:" + enhanced.getClass());
    //一次全部加载
    lazyLoader.loadAll();
    } else if (PropertyNamer.isSetter(methodName)) {
    //判断是否为set方法，set方法不需要延迟加载
    final String property = PropertyNamer.methodToProperty(methodName);
    lazyLoader.remove(property);
    } else if (PropertyNamer.isGetter(methodName)) {
    final String property = PropertyNamer.methodToProperty(methodName);
    if (lazyLoader.hasLoader(property)) {
     //延迟加载单个属性
     lazyLoader.load(property);
     log.debug("load one :" + methodName);
    }
    }
   }
   }
  }
  return methodProxy.invoke(enhanced, args);
  } catch (Throwable t) {
  throw ExceptionUtil.unwrapThrowable(t);
  }
 }
 }

load方法

/**
  * 执行懒加载查询，获取数据并且set到userObject中返回
  * @param userObject
  * @throws SQLException
  */
 public void load(final Object userObject) throws SQLException {

  //合法性校验
  if (this.metaResultObject == null || this.resultLoader == null) {
  if (this.mappedParameter == null) {
   throw new ExecutorException("Property [" + this.property + "] cannot be loaded because "
     + "required parameter of mapped statement ["
     + this.mappedStatement + "] is not serializable.");
  }

  //获取mappedstatement并且校验
  final Configuration config = this.getConfiguration();
  final MappedStatement ms = config.getMappedStatement(this.mappedStatement);
  if (ms == null) {
   throw new ExecutorException("Cannot lazy load property [" + this.property
     + "] of deserialized object [" + userObject.getClass()
     + "] because configuration does not contain statement ["
     + this.mappedStatement + "]");
  }

  //使用userObject构建metaobject,并且重新构建resultloader对象
  this.metaResultObject = config.newMetaObject(userObject);
  this.resultLoader = new ResultLoader(config, new ClosedExecutor(), ms, this.mappedParameter,
    metaResultObject.getSetterType(this.property), null, null);
  }

  /* We are using a new executor because we may be (and likely are) on a new thread
  * and executors aren't thread safe. (Is this sufficient?)
  *
  * A better approach would be making executors thread safe. */
  if (this.serializationCheck == null) {
  final ResultLoader old = this.resultLoader;
  this.resultLoader = new ResultLoader(old.configuration, new ClosedExecutor(), old.mappedStatement,
    old.parameterObject, old.targetType, old.cacheKey, old.boundSql);
  }

  //获取数据库查询结果并且set到结果对象返回
  this.metaResultObject.setValue(property, this.resultLoader.loadResult());
 }

参考地址：
https://www.cnblogs.com/qixidi/p/10251126.html
https://blog.csdn.net/mingtian625/article/details/47358003

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