mybatis源码解读之executor包懒加载功能 

public interface ProxyFactory {

  // 设置工厂属性
  default void setProperties(Properties properties) {
  }

  // 创建代理对象
  Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs);

}

public class CglibProxyFactory implements ProxyFactory {

  private static final String FINALIZE_METHOD = "finalize";
  private static final String WRITE_REPLACE_METHOD = "writeReplace";

  public CglibProxyFactory() {
    try {
      Resources.classForName("net.sf.cglib.proxy.Enhancer");
    } catch (Throwable e) {
      throw new IllegalStateException("Cannot enable lazy loading because CGLIB is not available. Add CGLIB to your classpath.", e);
    }
  }

  // 创建一个代理
  @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);
  }

  // 创建一个反序列化的代理
  public Object createDeserializationProxy(Object target, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    return EnhancedDeserializationProxyImpl.createProxy(target, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
  }
  private static class EnhancedResultObjectProxyImpl implements MethodInterceptor {

    // 被代理类
    private final Class<?> type;
    // 要懒加载的属性Map
    private final ResultLoaderMap lazyLoader;
    // 是否是激进懒加载
    private final boolean aggressive;
    // 能够触发懒加载的方法名“equals”, “clone”, “hashCode”, “toString”。这四个方法名在Configuration中被初始化。
    private final Set<String> lazyLoadTriggerMethods;
    // 对象工厂
    private final ObjectFactory objectFactory;
    // 被代理类构造函数的参数类型列表
    private final List<Class<?>> constructorArgTypes;
    // 被代理类构造函数的参数列表
    private final List<Object> constructorArgs;

    private EnhancedResultObjectProxyImpl(Class<?> type, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      this.type = type;
      this.lazyLoader = lazyLoader;
      this.aggressive = configuration.isAggressiveLazyLoading();
      this.lazyLoadTriggerMethods = configuration.getLazyLoadTriggerMethods();
      this.objectFactory = objectFactory;
      this.constructorArgTypes = constructorArgTypes;
      this.constructorArgs = constructorArgs;
    }

    public static Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      final Class<?> type = target.getClass();
      EnhancedResultObjectProxyImpl callback = new EnhancedResultObjectProxyImpl(type, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
      Object enhanced = crateProxy(type, callback, constructorArgTypes, constructorArgs);
      PropertyCopier.copyBeanProperties(type, target, enhanced);
      return enhanced;
    }

    /**
     * 代理类的拦截方法
     * @param enhanced 代理对象本身
     * @param method 被调用的方法
     * @param args 每调用的方法的参数
     * @param methodProxy 用来调用父类的代理
     * @return 方法返回值
     * @throws Throwable
     */
    @Override
    public Object intercept(Object enhanced, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
      // 取出被代理类中此次被调用的方法的名称
      final String methodName = method.getName();
      try {
        synchronized (lazyLoader) { // 防止属性的并发加载
          if (WRITE_REPLACE_METHOD.equals(methodName)) { // 被调用的是writeReplace方法
            // 创建一个原始对象
            Object original;
            if (constructorArgTypes.isEmpty()) {
              original = objectFactory.create(type);
            } else {
              original = objectFactory.create(type, constructorArgTypes, constructorArgs);
            }
            // 将被代理对象的属性拷贝进入新创建的对象
            PropertyCopier.copyBeanProperties(type, enhanced, original);
            if (lazyLoader.size() > 0) { // 存在懒加载属性
              // 则此时返回的信息要更多，不仅仅是原对象，还有相关的懒加载的设置等信息。因此使用CglibSerialStateHolder进行一次封装
              return new CglibSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs);
            } else {
              // 没有未懒加载的属性了，那直接返回原对象进行序列化
              return original;
            }
          } else {
            if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) { // 存在懒加载属性且被调用的不是finalize方法
              if (aggressive || lazyLoadTriggerMethods.contains(methodName)) { // 设置了激进懒加载或者被调用的方法是能够触发全局懒加载的方法
                // 完成所有属性的懒加载
                lazyLoader.loadAll();
              } else if (PropertyNamer.isSetter(methodName)) { // 调用了属性写方法
                // 则先清除该属性的懒加载设置。该属性不需要被懒加载了
                final String property = PropertyNamer.methodToProperty(methodName);
                lazyLoader.remove(property);
              } else if (PropertyNamer.isGetter(methodName)) { // 调用了属性读方法
                final String property = PropertyNamer.methodToProperty(methodName);
                // 如果该属性是尚未加载的懒加载属性，则进行懒加载
                if (lazyLoader.hasLoader(property)) {
                  lazyLoader.load(property);
                }
              }
            }
          }
        }
        // 触发被代理类的相应方法。能够进行到这里的是除去writeReplace方法外的方法，例如读写方法、toString方法等
        return methodProxy.invokeSuper(enhanced, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
  }
  }

public static class LoadPair implements Serializable {
    private static final long serialVersionUID = 20130412;
  
    // 用来根据反射得到数据库连接的方法名
    private static final String FACTORY_METHOD = "getConfiguration";
    
    // 判断是否经过了序列化的标志位，因为该属性被设置了transient，经过一次序列化和反序列化后会变为null
    private final transient Object serializationCheck = new Object();
   
    // 输出结果对象的封装
    private transient MetaObject metaResultObject;
   
    // 用以加载未加载属性的加载器
    private transient ResultLoader resultLoader;
  
    // 日志记录器
    private transient Log log;
    
    // 用来获取数据库连接的工厂
    private Class<?> configurationFactory;
    
    // 未加载的属性的属性名
    private String property;
   
    // 能够加载未加载属性的SQL的编号
    private String mappedStatement;
  
    // 能够加载未加载属性的SQL的参数
    private Serializable mappedParameter;

    private LoadPair(final String property, MetaObject metaResultObject, ResultLoader resultLoader) {
      this.property = property;
      this.metaResultObject = metaResultObject;
      this.resultLoader = resultLoader;

      if (metaResultObject != null && metaResultObject.getOriginalObject() instanceof Serializable) {
        final Object mappedStatementParameter = resultLoader.parameterObject;

        if (mappedStatementParameter instanceof Serializable) {
          this.mappedStatement = resultLoader.mappedStatement.getId();
          this.mappedParameter = (Serializable) mappedStatementParameter;

          this.configurationFactory = resultLoader.configuration.getConfigurationFactory();
        } else {
          Log log = this.getLogger();
          if (log.isDebugEnabled()) {
            log.debug("Property [" + this.property + "] of ["
                    + metaResultObject.getOriginalObject().getClass() + "] cannot be loaded "
                    + "after deserialization. Make sure it's loaded before serializing "
                    + "forenamed object.");
          }
        }
      }
    }

    public void load() throws SQLException {
      if (this.metaResultObject == null) {
        throw new IllegalArgumentException("metaResultObject is null");
      }
      if (this.resultLoader == null) {
        throw new IllegalArgumentException("resultLoader is null");
      }

      this.load(null);
    }

    /**
     * 进行加载操作
     * @param userObject 需要被懒加载的对象（只有当this.metaResultObject == null || this.resultLoader == null才生效，否则会采用属性metaResultObject对应的对象）
     * @throws SQLException
     */
    public void load(final Object userObject) throws SQLException {
      if (this.metaResultObject == null || this.resultLoader == null) { // 输出结果对象的封装不存在或者输出结果加载器不存在
        // 判断用以加载属性的对应的SQL语句存在
        if (this.mappedParameter == null) {
          throw new ExecutorException("Property [" + this.property + "] cannot be loaded because "
                  + "required parameter of mapped statement ["
                  + this.mappedStatement + "] is not serializable.");
        }

        final Configuration config = this.getConfiguration();
        // 取出用来加载结果的SQL语句
        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 + "]");
        }

        // 创建结果对象的包装
        this.metaResultObject = config.newMetaObject(userObject);
        // 创建结果加载器
        this.resultLoader = new ResultLoader(config, new ClosedExecutor(), ms, this.mappedParameter,
                metaResultObject.getSetterType(this.property), null, null);
      }

      // 只要经历过持久化，则可能在别的线程中了。为这次惰性加载创建的新线程ResultLoader
      if (this.serializationCheck == null) {
        // 取出原来的ResultLoader中的必要信息，然后创建一个新的
        // 这是因为load函数可能在不同的时间多次执行（第一次加载属性A，又过了好久加载属性B）。
        // 而该对象的各种属性是跟随对象的，加载属性B时还保留着加载属性A时的状态，即ResultLoader是加载属性A时设置的
        // 则此时ResultLoader中的Executor在ResultLoader中被替换成了一个能运行的Executor，而不是ClosedExecutor
        // 能运行的Executor的状态可能不是close，这将导致它被复用，从而引发多线程问题
        // 是不是被两次执行的一个关键点就是有没有经过序列化，因为执行完后会被序列化并持久化
        final ResultLoader old = this.resultLoader;
        this.resultLoader = new ResultLoader(old.configuration, new ClosedExecutor(), old.mappedStatement,
                old.parameterObject, old.targetType, old.cacheKey, old.boundSql);
      }

      this.metaResultObject.setValue(property, this.resultLoader.loadResult());
    }

    private Configuration getConfiguration() {
      if (this.configurationFactory == null) {
        throw new ExecutorException("Cannot get Configuration as configuration factory was not set.");
      }

      Object configurationObject;
      try {
        final Method factoryMethod = this.configurationFactory.getDeclaredMethod(FACTORY_METHOD);
        if (!Modifier.isStatic(factoryMethod.getModifiers())) {
          throw new ExecutorException("Cannot get Configuration as factory method ["
                  + this.configurationFactory + "]#["
                  + FACTORY_METHOD + "] is not static.");
        }

        if (!factoryMethod.isAccessible()) {
          configurationObject = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
            try {
              factoryMethod.setAccessible(true);
              return factoryMethod.invoke(null);
            } finally {
              factoryMethod.setAccessible(false);
            }
          });
        } else {
          configurationObject = factoryMethod.invoke(null);
        }
      } catch (final ExecutorException ex) {
        throw ex;
      } catch (final NoSuchMethodException ex) {
        throw new ExecutorException("Cannot get Configuration as factory class ["
                + this.configurationFactory + "] is missing factory method of name ["
                + FACTORY_METHOD + "].", ex);
      } catch (final PrivilegedActionException ex) {
        throw new ExecutorException("Cannot get Configuration as factory method ["
                + this.configurationFactory + "]#["
                + FACTORY_METHOD + "] threw an exception.", ex.getCause());
      } catch (final Exception ex) {
        throw new ExecutorException("Cannot get Configuration as factory method ["
                + this.configurationFactory + "]#["
                + FACTORY_METHOD + "] threw an exception.", ex);
      }

      if (!(configurationObject instanceof Configuration)) {
        throw new ExecutorException("Cannot get Configuration as factory method ["
                + this.configurationFactory + "]#["
                + FACTORY_METHOD + "] didn't return [" + Configuration.class + "] but ["
                + (configurationObject == null ? "null" : configurationObject.getClass()) + "].");
      }

      return Configuration.class.cast(configurationObject);
    }

    private Log getLogger() {
      if (this.log == null) {
        this.log = LogFactory.getLog(this.getClass());
      }
      return this.log;
    }
  }