源码分析-Mybatis源码阅读-懒加载机制

源码解析-Mybatis Mybatis, 源码解析 评论
源码解析-Mybatis
01 源码分析-Mybatis源码阅读准备
02 源码分析-Mybatis源码阅读-会话层
03 源码分析-Mybatis源码阅读-执行器
04 源码分析-Mybatis源码阅读-Statement语句处理器
05 源码分析-Mybatis源码阅读-参数处理器
06 源码分析-Mybatis源码阅读-结果集处理器
07 源码分析-Mybatis源码阅读-主键生成器
08 源码分析-Mybatis源码阅读-懒加载机制
09 源码分析-Mybatis源码阅读-游标
10 源码分析-Mybatis源码阅读-类型处理器
11 源码分析-Mybatis源码阅读-缓存
12 源码分析-Mybatis源码阅读-Mapper代理

一 懒加载机制说明

懒加载机制相关代码在org.apache.ibatis.executor.loader包下，主要是针对嵌套结果进行懒加载，在03 源码分析-Mybatis源码阅读-执行器 文章中已经有介绍该包下所有类的相关说明。UML图如下：

image-20210603183533336

二 懒加载机制关键源码解读

ProxyFactory代理工厂接口类源码解读如下：

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/**
 * 代理工厂接口
 * @author Eduardo Macarron
 */
public interface ProxyFactory {

  /**
   * 设置工厂属性
   * @param properties
   */
  default void setProperties(Properties properties) {
    // NOP
  }

  /**
   * 创建一个代理
   * @param target 代理的目标对象
   * @param lazyLoader 要懒加载的属性Map
   * @param configuration 配置
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @return
   */
  Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs);

}

CglibProxyFactory代理工厂源码解读如下

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/**
 * Cglib代理工厂
 * @author Clinton Begin
 */
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
   * @param target 代理的目标对象
   * @param lazyLoader 要懒加载的属性Map
   * @param configuration 配置
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @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);
  }

  /**
   * 创建一个反序列化的代理
   * @param target 代理的目标对象
   * @param unloadedProperties 未加载的属性
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @return
   */
  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);
  }

  /**
   * 创建代理对象
   * @param type 被代理对象类型
   * @param callback 回调对象
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @return 代理对象
   */
  static Object crateProxy(Class<?> type, Callback callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    Enhancer enhancer = new Enhancer();
    enhancer.setCallback(callback);
    // 创建的代理对象 是 原对象的子类
    enhancer.setSuperclass(type);
    try {
      // 获取类中的writeReplace方法
      type.getDeclaredMethod(WRITE_REPLACE_METHOD);
      // ObjectOutputStream will call writeReplace of objects returned by writeReplace
      if (LogHolder.log.isDebugEnabled()) {
        LogHolder.log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this");
      }
    } catch (NoSuchMethodException e) {
      // 如果没找到writeReplace方法，则设置代理类继承WriteReplaceInterface接口，该接口中有writeReplace方法
      enhancer.setInterfaces(new Class[] { WriteReplaceInterface.class });
    } catch (SecurityException e) {
      // nothing to do here
    }
    Object enhanced;
    if (constructorArgTypes.isEmpty()) {
      enhanced = enhancer.create();
    } else {
      Class<?>[] typesArray = constructorArgTypes.toArray(new Class[constructorArgTypes.size()]);
      Object[] valuesArray = constructorArgs.toArray(new Object[constructorArgs.size()]);
      enhanced = enhancer.create(typesArray, valuesArray);
    }
    return enhanced;
  }

  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 {
        // 防止属性的并发加载
        synchronized (lazyLoader) {
          // 被调用的是writeReplace方法
          if (WRITE_REPLACE_METHOD.equals(methodName)) {
            // 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);
            // 存在懒加载属性
            if (lazyLoader.size() > 0) {
              // 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 {
            // 存在懒加载属性且被调用的不是finalize方法
            if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(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
              if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
                // 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);
      }
    }
  }

  private static class EnhancedDeserializationProxyImpl extends AbstractEnhancedDeserializationProxy implements MethodInterceptor {

    private EnhancedDeserializationProxyImpl(Class<?> type, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      super(type, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
    }

    public static Object createProxy(Object target, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      final Class<?> type = target.getClass();
      EnhancedDeserializationProxyImpl callback = new EnhancedDeserializationProxyImpl(type, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
      Object enhanced = crateProxy(type, callback, constructorArgTypes, constructorArgs);
      PropertyCopier.copyBeanProperties(type, target, enhanced);
      return enhanced;
    }

    @Override
    public Object intercept(Object enhanced, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
      final Object o = super.invoke(enhanced, method, args);
      return o instanceof AbstractSerialStateHolder ? o : methodProxy.invokeSuper(o, args);
    }

    @Override
    protected AbstractSerialStateHolder newSerialStateHolder(Object userBean, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      return new CglibSerialStateHolder(userBean, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
    }
  }

  private static class LogHolder {
    private static final Log log = LogFactory.getLog(CglibProxyFactory.class);
  }

}

JavassistProxyFactory代理工厂源码解读如下：

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/**
 * Javassist代理工厂
 * @author Eduardo Macarron
 */
public class JavassistProxyFactory implements org.apache.ibatis.executor.loader.ProxyFactory {

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

  public JavassistProxyFactory() {
    try {
      Resources.classForName("javassist.util.proxy.ProxyFactory");
    } catch (Throwable e) {
      throw new IllegalStateException("Cannot enable lazy loading because Javassist is not available. Add Javassist to your classpath.", e);
    }
  }

  /**
   * 创建一个代理
   * @param target 代理的目标对象
   * @param lazyLoader 要懒加载的属性Map
   * @param configuration 配置
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @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);
  }

  /**
   * 创建一个反序列化的代理
   * @param target 代理的目标对象
   * @param unloadedProperties 未加载的属性
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造方法参数类型列表
   * @param constructorArgs 构造方法参数类型
   * @return
   */
  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);
  }

  static Object crateProxy(Class<?> type, MethodHandler callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {

    ProxyFactory enhancer = new ProxyFactory();
    // 创建的代理对象 是 原对象的子类
    enhancer.setSuperclass(type);

    try {
      // 获取类中的writeReplace方法
      type.getDeclaredMethod(WRITE_REPLACE_METHOD);
      // ObjectOutputStream will call writeReplace of objects returned by writeReplace
      if (LogHolder.log.isDebugEnabled()) {
        LogHolder.log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this");
      }
    } catch (NoSuchMethodException e) {
      // 如果没找到writeReplace方法，则设置代理类继承WriteReplaceInterface接口，该接口中有writeReplace方法
      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 {
      enhanced = enhancer.create(typesArray, valuesArray);
    } catch (Exception e) {
      throw new ExecutorException("Error creating lazy proxy.  Cause: " + e, e);
    }
    ((Proxy) enhanced).setHandler(callback);
    return enhanced;
  }

  private static class EnhancedResultObjectProxyImpl implements MethodHandler {
    /** 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 invoke(Object enhanced, Method method, Method methodProxy, Object[] args) 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 {
        // 防止属性的并发加载
        synchronized (lazyLoader) {
          // 被调用的是writeReplace方法
          if (WRITE_REPLACE_METHOD.equals(methodName)) {
            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) {
              // 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 JavassistSerialStateHolder(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 {
            // 存在懒加载属性且被调用的不是finalize方法
            if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
              // 如果是aggressive，则包括非懒加载属性的读操作也会触发懒加载
              // 同时几个触发方法也会引发所有属性的加载
              if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
                lazyLoader.loadAll();
              } else if (PropertyNamer.isSetter(methodName)) {
                // 调用了赋值方法
                // 得到要赋值的属性名称
                final String property = PropertyNamer.methodToProperty(methodName);
                // 因为被赋值，因此该属性的懒加载操作直接取消，以赋值为准
                lazyLoader.remove(property);
              } else if (PropertyNamer.isGetter(methodName)) {
                // 获取读方法属性
                final String property = PropertyNamer.methodToProperty(methodName);
                // 判断对该属性是否有懒加载
                if (lazyLoader.hasLoader(property)) {
                  // 如果有，就加载下
                  lazyLoader.load(property);
                }
              }
            }
          }
        }
        return methodProxy.invoke(enhanced, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
  }

  private static class EnhancedDeserializationProxyImpl extends AbstractEnhancedDeserializationProxy implements MethodHandler {

    private EnhancedDeserializationProxyImpl(Class<?> type, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      super(type, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
    }

    public static Object createProxy(Object target, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      final Class<?> type = target.getClass();
      EnhancedDeserializationProxyImpl callback = new EnhancedDeserializationProxyImpl(type, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
      Object enhanced = crateProxy(type, callback, constructorArgTypes, constructorArgs);
      PropertyCopier.copyBeanProperties(type, target, enhanced);
      return enhanced;
    }

    @Override
    public Object invoke(Object enhanced, Method method, Method methodProxy, Object[] args) throws Throwable {
      final Object o = super.invoke(enhanced, method, args);
      return o instanceof AbstractSerialStateHolder ? o : methodProxy.invoke(o, args);
    }

    @Override
    protected AbstractSerialStateHolder newSerialStateHolder(Object userBean, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
            List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
      return new JavassistSerialStateHolder(userBean, unloadedProperties, objectFactory, constructorArgTypes, constructorArgs);
    }
  }

  private static class LogHolder {
    private static final Log log = LogFactory.getLog(JavassistProxyFactory.class);
  }

}

ResultLoader结果加载器源码解读：

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/**
 * 结果加载器
 * @author Clinton Begin
 */
public class ResultLoader {

  protected final Configuration configuration;
  /** 执行器 */
  protected final Executor executor;
  /** MappedStatement对象（包含完整的增删改查节点信息） */
  protected final MappedStatement mappedStatement;
  /** 参数对象 */
  protected final Object parameterObject;
  protected final Class<?> targetType;
  /** 对象工厂，创建对象时使用 */
  protected final ObjectFactory objectFactory;
  protected final CacheKey cacheKey;
  /** BoundSql对象（包含SQL语句、参数、实参信息） */
  protected final BoundSql boundSql;
  /** 结果提取器 */
  protected final ResultExtractor resultExtractor;
  /** 当前线程ID */
  protected final long creatorThreadId;

  /** 是否已加载 */
  protected boolean loaded;
  protected Object resultObject;

  public ResultLoader(Configuration config, Executor executor, MappedStatement mappedStatement, Object parameterObject, Class<?> targetType, CacheKey cacheKey, BoundSql boundSql) {
    this.configuration = config;
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.parameterObject = parameterObject;
    this.targetType = targetType;
    this.objectFactory = configuration.getObjectFactory();
    this.cacheKey = cacheKey;
    this.boundSql = boundSql;
    this.resultExtractor = new ResultExtractor(configuration, objectFactory);
    this.creatorThreadId = Thread.currentThread().getId();
  }

  public Object loadResult() throws SQLException {
    // 查询结果列表
    List<Object> list = selectList();
    // 把结果列表转化为指定对象
    resultObject = resultExtractor.extractObjectFromList(list, targetType);
    return resultObject;
  }

  private <E> List<E> selectList() throws SQLException {
    // 初始化ResultLoader时传入的执行器
    Executor localExecutor = executor;
    if (Thread.currentThread().getId() != this.creatorThreadId || localExecutor.isClosed()) {
      // 执行器关闭，或者执行器属于其他线程，则创建新的执行器
      localExecutor = newExecutor();
    }
    try {
      // 查询结果
      return localExecutor.query(mappedStatement, parameterObject, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER, cacheKey, boundSql);
    } finally {
      if (localExecutor != executor) {
        localExecutor.close(false);
      }
    }
  }

  /**
   * 这才是创建一个真的执行器，而ClosedExecutor是假的执行器
   * @return
   */
  private Executor newExecutor() {
    final Environment environment = configuration.getEnvironment();
    if (environment == null) {
      throw new ExecutorException("ResultLoader could not load lazily.  Environment was not configured.");
    }
    final DataSource ds = environment.getDataSource();
    if (ds == null) {
      throw new ExecutorException("ResultLoader could not load lazily.  DataSource was not configured.");
    }
    final TransactionFactory transactionFactory = environment.getTransactionFactory();
    final Transaction tx = transactionFactory.newTransaction(ds, null, false);
    return configuration.newExecutor(tx, ExecutorType.SIMPLE);
  }

  public boolean wasNull() {
    return resultObject == null;
  }

}

ResultLoaderMap要懒加载的属性Map的源码解读：

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/**
 * 要懒加载的属性Map
 * @author Clinton Begin
 * @author Franta Mejta
 */
public class ResultLoaderMap {

  /**
   * 一个对象持有
   * 尚未完成惰性加载的属性map，key为属性名称的大写
   */
  private final Map<String, LoadPair> loaderMap = new HashMap<>();

  public void addLoader(String property, MetaObject metaResultObject, ResultLoader resultLoader) {
    String upperFirst = getUppercaseFirstProperty(property);
    if (!upperFirst.equalsIgnoreCase(property) && loaderMap.containsKey(upperFirst)) {
      throw new ExecutorException("Nested lazy loaded result property '" + property
              + "' for query id '" + resultLoader.mappedStatement.getId()
              + " already exists in the result map. The leftmost property of all lazy loaded properties must be unique within a result map.");
    }
    loaderMap.put(upperFirst, new LoadPair(property, metaResultObject, resultLoader));
  }

  public final Map<String, LoadPair> getProperties() {
    return new HashMap<>(this.loaderMap);
  }

  public Set<String> getPropertyNames() {
    return loaderMap.keySet();
  }

  public int size() {
    return loaderMap.size();
  }

  /**
   * 判断指定属性上是否设置了惰性加载
   * @param property 属性
   * @return
   */
  public boolean hasLoader(String property) {
    return loaderMap.containsKey(property.toUpperCase(Locale.ENGLISH));
  }

  /**
   * 把该对象的一个属性加载出来
   * @param property 属性
   * @return
   * @throws SQLException
   */
  public boolean load(String property) throws SQLException {
    LoadPair pair = loaderMap.remove(property.toUpperCase(Locale.ENGLISH));
    if (pair != null) {
      pair.load();
      return true;
    }
    return false;
  }

  public void remove(String property) {
    loaderMap.remove(property.toUpperCase(Locale.ENGLISH));
  }

  /**
   * 把该对象的所有属性加载出来
   * @throws SQLException
   */
  public void loadAll() throws SQLException {
    // 读出该对象的所有属性集合
    final Set<String> methodNameSet = loaderMap.keySet();
    String[] methodNames = methodNameSet.toArray(new String[methodNameSet.size()]);
    for (String methodName : methodNames) {
      load(methodName);
    }
  }

  private static String getUppercaseFirstProperty(String property) {
    String[] parts = property.split("\\.");
    return parts[0].toUpperCase(Locale.ENGLISH);
  }

  /**
   * 还未加载的属性
   * Property which was not loaded yet.
   */
  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
     * Name of factory method which returns database connection.
     */
    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
     * Object to check whether we went through serialization..
     */
    private final transient Object serializationCheck = new Object();
    /**
     * 输出结果对象的封装
     * Meta object which sets loaded properties.
     */
    private transient MetaObject metaResultObject;
    /**
     * 用以加载未加载属性的加载器
     * Result loader which loads unread properties.
     */
    private transient ResultLoader resultLoader;
    /**
     * 日志记录器
     * Wow, logger.
     */
    private transient Log log;
    /**
     * 用来获取数据库连接的工厂
     * Factory class through which we get database connection.
     */
    private Class<?> configurationFactory;
    /**
     * 未加载的属性的属性名
     * Name of the unread property.
     */
    private String property;
    /**
     * 能够加载未加载属性的SQL的编号
     * ID of SQL statement which loads the property.
     */
    private String mappedStatement;
    /**
     * 能够加载未加载属性的SQL的参数
     * Parameter of the sql statement.
     */
    private Serializable mappedParameter;

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

      /* Save required information only if original object can be serialized. */
      if (metaResultObject != null && metaResultObject.getOriginalObject() instanceof Serializable) {
        final Object mappedStatementParameter = resultLoader.parameterObject;

        /* @todo May the parameter be null? */
        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 {
      /* These field should not be null unless the loadpair was serialized.
       * Yet in that case this method should not be called. */
      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 {
      // 输出结果对象的封装不存在或者输出结果加载器不存在
      if (this.metaResultObject == null || this.resultLoader == null) {
        // 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
       * 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) {
        // 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;
    }
  }

  /**
   * 该执行器唯一作用就是返回自己的状态：已经关闭了
   * 这样，使用者看到关闭了的执行器就会重新创建新的执行器
   */
  private static final class ClosedExecutor extends BaseExecutor {

    public ClosedExecutor() {
      super(null, null);
    }

    @Override
    public boolean isClosed() {
      return true;
    }

    @Override
    protected int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
      throw new UnsupportedOperationException("Not supported.");
    }

    @Override
    protected List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
      throw new UnsupportedOperationException("Not supported.");
    }

    @Override
    protected <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
      throw new UnsupportedOperationException("Not supported.");
    }

    @Override
    protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
      throw new UnsupportedOperationException("Not supported.");
    }
  }
}

AbstractSerialStateHolder抽象串行状态保持器源码解读：

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/**
 * 抽象串行状态保持器
 * 可以序列化，保存了原始对象、原始对象中未加载的属性Map、用来反射生成对象的相关信息
 * @author Eduardo Macarron
 * @author Franta Mejta
 */
public abstract class AbstractSerialStateHolder implements Externalizable {

  private static final long serialVersionUID = 8940388717901644661L;
  private static final ThreadLocal<ObjectOutputStream> stream = new ThreadLocal<>();
  /** 序列化后的对象 */
  private byte[] userBeanBytes = new byte[0];
  /** 原对象 */
  private Object userBean;
  /** 未加载的属性 */
  private Map<String, ResultLoaderMap.LoadPair> unloadedProperties;
  /** 对象工厂，创建对象时使用 */
  private ObjectFactory objectFactory;
  /** 构造函数的属性类型列表，创建对象时使用 */
  private Class<?>[] constructorArgTypes;
  /** 构造函数的属性列表，创建对象时使用 */
  private Object[] constructorArgs;

  public AbstractSerialStateHolder() {
  }

  public AbstractSerialStateHolder(
          final Object userBean,
          final Map<String, ResultLoaderMap.LoadPair> unloadedProperties,
          final ObjectFactory objectFactory,
          List<Class<?>> constructorArgTypes,
          List<Object> constructorArgs) {
    this.userBean = userBean;
    this.unloadedProperties = new HashMap<>(unloadedProperties);
    this.objectFactory = objectFactory;
    this.constructorArgTypes = constructorArgTypes.toArray(new Class<?>[0]);
    this.constructorArgs = constructorArgs.toArray(new Object[0]);
  }

  /**
   * 对对象进行序列化
   * @param out 序列化结果将存入的流
   * @throws IOException
   */
  @Override
  public final void writeExternal(final ObjectOutput out) throws IOException {
    boolean firstRound = false;
    final ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ObjectOutputStream os = stream.get();
    if (os == null) {
      os = new ObjectOutputStream(baos);
      firstRound = true;
      stream.set(os);
    }

    os.writeObject(this.userBean);
    os.writeObject(this.unloadedProperties);
    os.writeObject(this.objectFactory);
    os.writeObject(this.constructorArgTypes);
    os.writeObject(this.constructorArgs);

    final byte[] bytes = baos.toByteArray();
    out.writeObject(bytes);

    if (firstRound) {
      stream.remove();
    }
  }

  /**
   * 对对象进行反序列化
   * @param in 反序列化输入的流
   * @throws IOException
   * @throws ClassNotFoundException
   */
  @Override
  public final void readExternal(final ObjectInput in) throws IOException, ClassNotFoundException {
    final Object data = in.readObject();
    if (data.getClass().isArray()) {
      this.userBeanBytes = (byte[]) data;
    } else {
      this.userBean = data;
    }
  }

  /**
   * 反序列化时被调用，给出反序列化的对象
   * @return 最终给出的反序列化对象
   * @throws ObjectStreamException
   */
  @SuppressWarnings("unchecked")
  protected final Object readResolve() throws ObjectStreamException {
    // 非第一次运行，直接输出已经解析好的被代理对象
    /* Second run */
    if (this.userBean != null && this.userBeanBytes.length == 0) {
      return this.userBean;
    }

    SerialFilterChecker.check();

    // 第一次运行时，反序列化输出
    /* First run */
    try (ObjectInputStream in = new ObjectInputStream(new ByteArrayInputStream(this.userBeanBytes))) {
      this.userBean = in.readObject();
      this.unloadedProperties = (Map<String, ResultLoaderMap.LoadPair>) in.readObject();
      this.objectFactory = (ObjectFactory) in.readObject();
      this.constructorArgTypes = (Class<?>[]) in.readObject();
      this.constructorArgs = (Object[]) in.readObject();
    } catch (final IOException ex) {
      throw (ObjectStreamException) new StreamCorruptedException().initCause(ex);
    } catch (final ClassNotFoundException ex) {
      throw (ObjectStreamException) new InvalidClassException(ex.getLocalizedMessage()).initCause(ex);
    }

    final Map<String, ResultLoaderMap.LoadPair> arrayProps = new HashMap<>(this.unloadedProperties);
    final List<Class<?>> arrayTypes = Arrays.asList(this.constructorArgTypes);
    final List<Object> arrayValues = Arrays.asList(this.constructorArgs);

    // 创建一个反序列化的代理输出，因此还是一个代理
    return this.createDeserializationProxy(userBean, arrayProps, objectFactory, arrayTypes, arrayValues);
  }

  protected abstract Object createDeserializationProxy(Object target, Map<String, ResultLoaderMap.LoadPair> unloadedProperties, ObjectFactory objectFactory,
          List<Class<?>> constructorArgTypes, List<Object> constructorArgs);
}

豆萁程序猿架构师 & Java资深研发

死磕技术,死磕源码.