mybatis源码分析（四）---------------代理对象的生成

在mybatis两种开发方式这边文章中，我们提到了Mapper动态代理开发这种方式，现在抛出一个问题：通过sqlSession.getMapper(XXXMapper.class)来获取代理对象的过程是怎样的？生成的代理对象是通过怎样的方式来调用Mapper接口指定的方法的？

我们根据源码来一步步分析：

首先进入getMapper方法，通过一步步追踪，我们可以进入到MapperRegistry类中的getMapper方法，现在对整个类做分析：

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 * @author Lasse Voss
 */

// MapperRegistry整个类的作用就是注册Mapper接口和生成Mapper接口的代理对象
public class MapperRegistry {
  // 配置文件对象
  private Configuration config;
  // 这是一个HashMap,key是Mapper接口的类型对象，value是MapperProxyFactory
  // 至于MapperProxyFactory是一个创建Mapper接口代理对象的工厂，后面会介绍
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

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

  // 整个就是生成代理对象的方法
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    // 根据Mapper接口的类型对象获取对应的 生成该Mapper接口代理对象的工厂
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null)
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    try {
      // 这个就是生成代理对象的方法 进入该方法
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }
  
  // 注册Mapper接口
  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        // 把Mapper接口的类型对象和MapperProxyFactory对象，放到HashMap中
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  /**
   * @since 3.2.2
   */
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

  /**
   * @since 3.2.2
   */
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  /**
   * @since 3.2.2
   * 通过包名扫描其下的所有Mapper接口
   */
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }

然后进入 mapperProxyFactory.newInstance(sqlSession);这个方法，

// 该类就是用来生成Mapper接口的代理对象的工厂
public class MapperProxyFactory<T> {
  // Mapper接口的class对象
  private final Class<T> mapperInterface;
  // key是Method方法对象，MapperMethod是对Mapper接口中方法的封装，这个MapperMethod值得探究，后面会有讲解
  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) {
    //通过JDK动态代理的方式生成了Mapper接口的代理对象,所以当代理对象调用Mapper接口的方法时，会触发mapperProxy对象中的invoke方法，下面对MapperProxy进行分析 （重点）
    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);
  }

}

贴出MapperProxy类的源码：

// 该类实现了InvocationHandler接口
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;
  }

  // 代理对象调用Mapper接口中的方法时，会触发该方法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);
      }
    }
    // 通过方法名找到MapperMethod，MapperMethod是对方法及其他参数的封装，稍后会提到
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // 此处是对SqlSession调用的封装，在另外一篇文章中，我们提到了Mapper动态代理的方法底层还是通过SQLSession来和数据库交互的，在这里就能看出来了（核心代码就在这个方法中）
    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;
  }

}

进入上面提到的方法，这里只贴出了MapperMethod类的属性和部分方法：

public class MapperMethod {
  // SqlCommand是对方法的全名（方法的全名 = 包名+类名+方法名（也就是Mapper.xml中的id））和操作类型（Mapper.xml中的insert，select，update等）的封装
  private final SqlCommand command;
  // MethodSignature 封装了方法的参数，返回类型等信息
  private final MethodSignature method;

  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
  }
  // 核心逻辑在这里
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    if (SqlCommandType.INSERT == command.getType()) {
      // 对参数做处理
      Object param = method.convertArgsToSqlCommandParam(args);
      // 通过sqlSession.insert(方法全名，param)这种方式来和数据库交互，并对返回的结果做处理，以下都是类似的情况：update,delete,select
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
     // 这个是查询 sqlSession.select()....
    } else if (SqlCommandType.SELECT == command.getType()) {
      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 {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
      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;
  }
}

到这里我们明白了一点：通过Mapper动态代理的方式使用mybatis，其实就是利用了JDK的动态代理特性来生成对象，然后通过反射的方式来调用方法，最终底层还是通过mybatis提供的API sqlSession来和数据库进行交互的。

最后，再对SqlCommand 和 MethodSignature做一个简单的分析：

SqlCommand类有两个属性：name和type分别代表什么呢？通过例子来理解 

 public static class SqlCommand {

    private final String name;  // com.yht.mybatisTest.dao.GoodsDao.selectGoodsById  方法的全名
    private final SqlCommandType type; //SELECT  Mapper.xml文件中节点的类型：SELECT,INSERT,DELETE或者UPDATE

}

MethodSignature类的属性如下：

public static class MethodSignature {

    private final boolean returnsMany; //是否返回多条数据
    private final boolean returnsMap; //是否返回Map数据类型
    private final boolean returnsVoid; // 是否返回void
    private final Class<?> returnType; // 具体的返回的数据对象类型，如：com.yht.mybatisTest.entity.Goods
    private final String mapKey;
    private final Integer resultHandlerIndex;
    private final Integer rowBoundsIndex;
    private final SortedMap<Integer, String> params; // 参数
    private final boolean hasNamedParameters;
}

到此，就结束了。