Spring FactoryBean 详解

Java框架

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2020-6-1

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引言

在 mybatis-spring 中,我们会使用 MapperFactoryBean 为我们 的mapper(interface)创建代理类。 SqlSessionFactoryBean 通过dataSource成属性创建对应的sqlSessionFactory。两个接口都实现了Spring FactoryBean interface。所以我们就通过这个来理解一下 Spring FactoryBean.

<bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
  <property name="mapperInterface" value="org.mybatis.spring.sample.mapper.UserMapper" />
  <property name="sqlSessionFactory" ref="sqlSessionFactory" />
</bean>

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
  <property name="dataSource" ref="dataSource" />
</bean>

FactoryBean

Customizing instantiation logic with a FactoryBean,Implement the org.springframework.beans.factory.FactoryBean interface for objects that are themselves factories.

简介:
first,FactoryBean 是一个扩展点,主要是扩展创建复杂的对象实现。
second, 实现了FactoryBean接口的类自己就是一个factory,可以创建对象。接口一共三个方法,我们重点看一下其中的 getObject();

Object getObject(): returns an instance of the object this factory creates. The instance can possibly be shared, depending on whether this factory returns singletons or prototypes.

boolean isSingleton(): returns true if this FactoryBean returns singletons, false otherwise.

Class getObjectType(): returns the object type returned by the getObject() method or null if the type is not known in advance.

对照这个过程我们就阅读一下mybatis-spring MapperFactoryBean的源码,这里不再重复,请参考 http://blog.csdn.net/tanqidong1992/article/details/48026491

这里说明一下: sessionSqlTemplate 实现SqlSession, sessionSqlTemplates是线程安全的,所以在FactoryBean getObject的时候就注入了SqlSession,所以完成之后,mapper可以直接使用。

如果不看具体的代理对象生成过程,我们就可以理解了整个对象的创建过程,比如SqlSessionFactoryBean就使用了一个build方法。

FactoryBean 的好处就是简化复杂对象的创建,上面的创建过程绑定了sqlSession,通过代理生成了代理对象,这样使用起来就非常简单了。

Interface 生成代理对象

现在我们来看下为mapper接口生成代理对象的过程:

public class MapperRegistry {

  private Configuration config;
// 为每个mapper类型存储一个对应的MapperProxyFactory
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @SuppressWarnings("unchecked")
  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 {
        //我们直接看这部分代码吧
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
...
}

MapperProxyFactory

public class MapperProxyFactory<T> {

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

  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) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

MapperProxy

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

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

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

参考: https://www.ibm.com/developerworks/cn/java/j-lo-proxy1/

作者:OrangeJoke