FactoryBean简介以及Mybatis-Spring应用

一、BeanFactory和FactoryBean区别？

BeanFactory是工厂类，提供了获取和检索Bean的接口。它代表着Spring的IoC容器，负责Bean实例化以及管理Bean之间的依赖关系。作为Spring框架中最核心的模块，它提供容器的基本规范。

FactoryBean是一个bean，可以作为其他bean的工厂。FactoryBean像其他bean一样在注入到IoC容器中，但是当从IoC容器中获取FactoryBean的时候，实际返回的FactoryBean#getObject()方法返回的对象。如果想获取FactoryBean本身，则需要在bean的名称添加前缀&来获取FactoryBean对象本身（applicationContext.getBean("&" + beanName)）。

二、如何使用FactoryBean？

1. 在了解如何使用FactoryBean之前，先看看FactoryBean接口的定义。

public interface FactoryBean<T> {

    /**
     * 实际返回的bean对象
     */
	@Nullable
	T getObject() throws Exception;

    /**
     * 实际返回bean的class对象
     */
	@Nullable
	Class<?> getObjectType();

    /**
     * 指定bean是否是单例，默认为true（Spring bean默认都是单例）。
     */
	default boolean isSingleton() {
		return true;
	}

}

FactoryBean接口定义了三个方法，其中getObject方法返回bean对象。

1. 接下来通过一个简单的获取加密工具演示如何使用FactoryBean

定义一个FactoryBean类

@RequiredArgsConstructor
public class MessageDigestFactoryBean implements FactoryBean<MessageDigest> {
    // 算法名称
    private final String algorithmName;

    @Override
    public MessageDigest getObject() throws Exception {
        return MessageDigest.getInstance(algorithmName);
    }

    @Override
    public Class<?> getObjectType() {
        return MessageDigest.class;
    }
}

通过上面定义的类，传入不同的算法名称实现获取不同算法的加密类

@Configuration
public class MessageDigestConfiguration {

    @Bean
    public MessageDigestFactoryBean md5() {
        return new MessageDigestFactoryBean("MD5");
    }

    @Bean
    public MessageDigestFactoryBean sha1() {
        return new MessageDigestFactoryBean("SHA-1");
    }

}

这里定义了两种加密算法MD5和SHA-1，这样即可通过applicationContext对象获取不同的加密工具

public class App {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(MessageDigestConfiguration.class);
        MessageDigest md5 = applicationContext.getBean("md5", MessageDigest.class);
        System.out.println(Arrays.toString(md5.digest("test".getBytes())));
        MessageDigest sha1 = applicationContext.getBean("sha1", MessageDigest.class);
        System.out.println(Arrays.toString(sha1.digest("test".getBytes())));
    }

}

// 输出
/*
[9, -113, 107, -51, 70, 33, -45, 115, -54, -34, 78, -125, 38, 39, -76, -10]
[-87, 74, -113, -27, -52, -79, -101, -90, 28, 76, 8, 115, -45, -111, -23, -121, -104, 47, -69, -45]
*/

可以看到，通过同一个FactoryBean类，向IoC容器中注入了两个不同的bean。

如果让FactoryBean#isSingleton方法返回false，那么得到的输出如下

public class App {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(MessageDigestConfiguration.class);

        System.out.println(applicationContext.getBean("md5", MessageDigest.class).hashCode());
        System.out.println(applicationContext.getBean("md5", MessageDigest.class).hashCode());
    }
}

// 输出
/*
768192757
1697752980
*/

可以看出每次通过applicationContext获取的对象都是一个新的对象，从而每个bean都是原型作用域。

三、FactoryBean在Mybatis集成Spring Boot中的应用

使用过Spring Boot的同学都知道，当我们需要扫描Mapper的时候，需要添加@MapperScan注解完成对Mapper对象的扫描，@MapperScan导入MapperScannerRegistrar类完成扫描。

但是Mapper类都是接口，无法被实例化，那么为什么在Spring中能够直接注入Mapper对象呢？

实际上Mybatis是通过FactoryBean对象创建Mapper对象的代理对象，完成Mapper接口的注入。

下面跟随Mybatis-Spring源码了解如何动态创建Mapper对象的实现类。

首先看@MapperScan注解源码

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class) // 导入MapperScannerRegistrar配置类
@Repeatable(MapperScans.class)
public @interface MapperScan {

  String[] basePackages() default {};

}

/**
 * 完成Mapper接口扫描
 */
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {

  private ResourceLoader resourceLoader;

  /**
   * {@inheritDoc}
   */
  @Override
  public void setResourceLoader(ResourceLoader resourceLoader) {
    this.resourceLoader = resourceLoader;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      registerBeanDefinitions(mapperScanAttrs, registry);
    }
  }

  void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry) {
    // 创建scanner对象，扫描Mapper
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

    Optional.ofNullable(resourceLoader).ifPresent(scanner::setResourceLoader);

    Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
    if (!Annotation.class.equals(annotationClass)) {
      scanner.setAnnotationClass(annotationClass);
    }

    Class<?> markerInterface = annoAttrs.getClass("markerInterface");
    if (!Class.class.equals(markerInterface)) {
      scanner.setMarkerInterface(markerInterface);
    }

    Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
    if (!BeanNameGenerator.class.equals(generatorClass)) {
      scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));
    }

    Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
    if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
      scanner.setMapperFactoryBean(BeanUtils.instantiateClass(mapperFactoryBeanClass));
    }

    scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));
    scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));

    // 扫描的包
    List<String> basePackages = new ArrayList<>();
    basePackages.addAll(
        Arrays.stream(annoAttrs.getStringArray("value"))
            .filter(StringUtils::hasText)
            .collect(Collectors.toList()));

    basePackages.addAll(
        Arrays.stream(annoAttrs.getStringArray("basePackages"))
            .filter(StringUtils::hasText)
            .collect(Collectors.toList()));

    basePackages.addAll(
        Arrays.stream(annoAttrs.getClassArray("basePackageClasses"))
            .map(ClassUtils::getPackageName)
            .collect(Collectors.toList()));

    scanner.registerFilters();
    scanner.doScan(StringUtils.toStringArray(basePackages));
  }

  static class RepeatingRegistrar extends MapperScannerRegistrar {
    /**
     * {@inheritDoc}
     */
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
        BeanDefinitionRegistry registry) {
      AnnotationAttributes mapperScansAttrs = AnnotationAttributes
          .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScans.class.getName()));
      if (mapperScansAttrs != null) {
        Arrays.stream(mapperScansAttrs.getAnnotationArray("value"))
            .forEach(mapperScanAttrs -> registerBeanDefinitions(mapperScanAttrs, registry));
      }
    }
  }

}

ClassPathMapperScan通过doScan方法扫描Mapper

  private MapperFactoryBean<?> mapperFactoryBean = new MapperFactoryBean<>();

  @Override
  public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);

    if (beanDefinitions.isEmpty()) {
      LOGGER.warn(() -> "No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
    } else {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

  private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      // 省略其余代码

      // 记录bean的类名（即Mapper接口类名），将Mapper类名传递给MapperFactoryBean作为构造方法参数
      // 这样MapperFactoryBean的getObject方法即可通过动态代理创建Mapper的动态代理对象
      String beanClassName = definition.getBeanClassName();
      definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
      // 修改BeanClass
      definition.setBeanClass(this.mapperFactoryBean.getClass());
    }
  }

ClassPathMapperScan扫描到Mapper类之后，修改BeanClass为MapperFactoryBean

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  // Mapper的class对象，FactoryBean创建该Mapper的动态代理对象
  private Class<T> mapperInterface;

  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }
}

继续跟踪getSqlSession().getMapper(this.mapperInterface)方法

Configuration

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

可以看出来这里是通过MapperProxyFactory对象创建Mapper对象

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    // JDK动态代理
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  /**
   * 创建Mapper对象的代理对象MapperProxy，MapperProxy实现了InvocationHandler接口
   */
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

继续看MapperProxy对象的invoke方法

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

  /**
   * 根据接口定义方法执行SQL，并返回结果
   */
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
    	Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.ofNullable(result);
          }
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName()
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

可以看出Mybatis通过JDK动态代理的方式，创建Mapper接口的代理对象，并通过接口声明的方法查找并执行SQL。