5、MyBatis-parameterType 入参封装 Map 流程

以如下入参为例，MyBatis 版本为 3.5.0

public MyUser selectMyUserIdAndAge(Integer id, @Param("user") MyUser user);

打上断点

大致流程

1、进入到 MapperProxy 类的 invoke 方法，执行接口的代理对象中的方法

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 {
            // 代理以后，并不是所有 Mapper 的方法调用时都会调用这个 invoke 方法，如果这个方法是 Object 中通用的方法（toString、hashCode等）则无需执行
            if (Object.class.equals(method.getDeclaringClass())) {
                return method.invoke(this, args);
            } else if (isDefaultMethod(method)) {
                // 如果这个方法的权限修饰符是 public 并且是由接口提供，则执行 invokeDefaultMethod 方法，这里是由代理对象提供
                return invokeDefaultMethod(proxy, method, args);
            }
        } catch (Throwable t) {
            throw ExceptionUtil.unwrapThrowable(t);
        }
        // 去缓存中找 MapperMethod
        final MapperMethod mapperMethod = cachedMapperMethod(method);
        // 执行
        return mapperMethod.execute(sqlSession, args);
    }

    private MapperMethod cachedMapperMethod(Method method) {
        return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
    }

    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();
    }
}

2、进入到 MapperMethod 类的 execute 方法，执行数据库操作

public class MapperMethod {

    private final SqlCommand command;
    private final MethodSignature method;

    public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
        this.command = new SqlCommand(config, mapperInterface, method);
        this.method = new MethodSignature(config, mapperInterface, method);
    }

    public Object execute(SqlSession sqlSession, Object[] args) {
        Object result;
        // 4种情况，insert|update|delete|select，分别调用 SqlSession 的 4 大类方法
        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()) {
                    // 如果有结果处理器
                    executeWithResultHandler(sqlSession, args);
                    result = null;
                } else if (method.returnsMany()) {
                    // 如果结果有多条记录
                    result = executeForMany(sqlSession, args);
                } else if (method.returnsMap()) {
                    // 如果结果是map
                    result = executeForMap(sqlSession, args);
                } else if (method.returnsCursor()) {
                    // 如果结果是游标
                    result = executeForCursor(sqlSession, args);
                } else {
                    // 否则就是一条记录
                    // 装配参数
                    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;
    }
    ...
}

3、进入到 ParamNameResolver 类的 getNamedParams 方法，进行参数封装

public class ParamNameResolver {

    private static final String GENERIC_NAME_PREFIX = "param";

    private final SortedMap<Integer, String> names;

    private boolean hasParamAnnotation;

    public ParamNameResolver(Configuration config, Method method) {
        // 获取参数列表中每个参数的类型
        final Class<?>[] paramTypes = method.getParameterTypes();
        // 获取每个标了 @Param 注解的参数的值,赋值给 name
        final Annotation[][] paramAnnotations = method.getParameterAnnotations();
        // 记录参数索引与参数名称的关系
        final SortedMap<Integer, String> map = new TreeMap<>();
        int paramCount = paramAnnotations.length;

        // 遍历参数保存至 map：（key：参数索引，value：name的值）
        for (int paramIndex = 0; paramIndex < paramCount; paramIndex++) {
            // 是否为特殊参数：RowBounds或ResultHandler
            if (isSpecialParameter(paramTypes[paramIndex])) {
                continue;
            }
            String name = null;
            // 标注了 @Param 注解：name的值为注解的值
            for (Annotation annotation : paramAnnotations[paramIndex]) {
                if (annotation instanceof Param) {
                    hasParamAnnotation = true;
                    name = ((Param) annotation).value();
                    break;
                }
            }
            // 没有标 @Param 注解
            if (name == null) {
                if (config.isUseActualParamName()) {
                    // 全局配置：useActualParamName（jdk1.8）：name为参数名
                    name = getActualParamName(method, paramIndex);
                }
                if (name == null) {
                    // name为map.size()：相当于当前元素的索引，0，1，2，3...
                    name = String.valueOf(map.size());
                }
            }
            map.put(paramIndex, name);
        }
        names = Collections.unmodifiableSortedMap(map);
    }

    private String getActualParamName(Method method, int paramIndex) {
        return ParamNameUtil.getParamNames(method).get(paramIndex);
    }

    private static boolean isSpecialParameter(Class<?> clazz) {
        return RowBounds.class.isAssignableFrom(clazz) || ResultHandler.class.isAssignableFrom(clazz);
    }

    public String[] getNames() {
        return names.values().toArray(new String[0]);
    }

    public Object getNamedParams(Object[] args) {
        final int paramCount = names.size();
        if (args == null || paramCount == 0) {
            // 没有参数
            return null;
        } else if (!hasParamAnnotation && paramCount == 1) {
            // 未使用@Param注解且参数列表只有一个,即 args[0] 参数的值
            return args[names.firstKey()];
        } else {
            // 为参数创建 param+ 索引的格式作为默认参数名称 如：param1  下标从1开始
            final Map<String, Object> param = new ParamMap<>();
            int i = 0;
            // 遍历names集合；{0=arg0, 1=user}
            for (Map.Entry<Integer, String> entry : names.entrySet()) {
                // names集合的 value 作为 key;  names 集合的 key 作为取值的参考args[0]
                // {arg0:args[0],user=args[1]}
                param.put(entry.getValue(), args[entry.getKey()]);
                final String genericParamName = GENERIC_NAME_PREFIX + String.valueOf(i + 1);
                if (!names.containsValue(genericParamName)) {
                    // 额外的将每一个参数也保存到 param 中，使用新的key：param1...paramN,有 @Param 注解可以 #{指定的key}，或者#{param1}
                    param.put(genericParamName, args[entry.getKey()]);
                }
                i++;
            }
            return param;
        }
    }
}

---

https://my.oschina.net/u/3737136/blog/1811654

https://segmentfault.com/a/1190000015977801

https://github.com/tuguangquan/mybatis/tree/master/src/main/java/org/apache/ibatis/binding