Mybatis 之构造Mapper (MapperProxy)

布满荆棘的人生 2022-06-08 01:07 191阅读 0赞

我们在代码中使用：

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

查看DefaultSqlSession的getMapper方法：

@Override
      public <T> T getMapper(Class<T> type) {
        return configuration.<T>getMapper(type, this);
      }

接着再看Configuration的getMapper方法：

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

看来还是一个MapperRegistry的对象在处理Mapper：

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);
        }
      }

从代码中我们看到试图从一个叫knownMappers的变量取出MapperProxyFactory。

我们看看这个knownMapper在MapperRegistry中的定义：

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

有getMapper方法，那么必然后addMapper方法：

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 {
            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);
            }
          }
        }
      }

我们在回过头来看getMapper是如何获得Mapper对象的：

1.先获取MapperProxyFactory

2.再调用MapperProxyFactory对象的newInstance方法获得Mapper。

我们看MapperProxyFactory代码：

public T newInstance(SqlSession sqlSession) {
        final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
      }
    
      protected T newInstance(MapperProxy<T> mapperProxy) {
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
      }

这里就是返回的一个代理类实例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;
      }
    
      @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);
      }
    
      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;
      }
    
      @UsesJava7
      private Object invokeDefaultMethod(Object proxy, Method method, Object[] args)
          throws Throwable {
        final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
            .getDeclaredConstructor(Class.class, int.class);
        if (!constructor.isAccessible()) {
          constructor.setAccessible(true);
        }
        final Class<?> declaringClass = method.getDeclaringClass();
        return constructor
            .newInstance(declaringClass,
                MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
                    | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
            .unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
      }
    
      /**
       * Backport of java.lang.reflect.Method#isDefault()
       */
      private boolean isDefaultMethod(Method method) {
        return ((method.getModifiers()
            & (Modifier.ABSTRACT | Modifier.PUBLIC | Modifier.STATIC)) == Modifier.PUBLIC)
            && method.getDeclaringClass().isInterface();
      }
    }

要使用Java的动态代理就必须得实现InvocationHandler接口:

@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);
      }

首先判断代理对象是一个接口还是一个类，显然我们没有对mapper接口进行任何实现，那么它将执行

final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);

生成一个MapperMethod对象，接着调用其execute方法，把sqlSession和参数传递进去，执行Mapper方法。