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。

巴特西

mybatis源码分析（四）---------------代理对象的生成

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