从 Mybatis Plugin 看 Mybatis 核心组件功能

Mybatis 是一个常用 ORM 持久层框架。其在动态 SQL 执行的基础功能上，提供了插件的功能。Mybatis 插件通过切面编程的方式，无侵入的进行功能扩展。最常见的插件有分页插件（PageHelper）。

Mybatis Plugin 源码分析

mybatis 插件的核心功能代码主要在 "org.apache.ibatis.plugin" 包下，主要包括以下类。

• 注解
  • Intercepts

    // The annotation that specify target methods to intercept.
    // 指定需要拦截的方法的注解
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE)
    public @interface Intercepts {
    /**
    * Returns method signatures to intercept.
    *
    * @return method signatures
    */
    Signature[] value();
    }
    

  • Signature

    /**
    * The annotation that indicate the method signature.
    * 声明被拦截方法的基本信息：类名、方法名、参数列表
    */
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({})
    public @interface Signature {
    
        Class type();
    
        String method();
    
        Class[] args();
    }
    

• 接口
  • Interceptor
    Mybatis 插件中拦截器的公用接口，自定义插件时需要实现该接口的 intercept 方法。

    public interface Interceptor {
    
        Object intercept(Invocation invocation) throws Throwable;
    
        default Object plugin(Object target) {
            return Plugin.wrap(target, this);
        }
    
        default void setProperties(Properties properties) {
            // NOP
        }
    
    }
    

• 功能类

  • Invocation
    Invocation 类中属性与 Signature 注解中的属性一一对应，同时定义 proceed 方法通过反射的方式执行被代理类的方法。

    public class Invocation {
    
        private final Object target;
        private final Method method;
        private final Object[] args;
    
        public Invocation(Object target, Method method, Object[] args) {
            this.target = target;
            this.method = method;
            this.args = args;
        }
    
        public Object getTarget() {
            return target;
        }
    
        public Method getMethod() {
            return method;
        }
    
        public Object[] getArgs() {
            return args;
        }
    
        public Object proceed() throws InvocationTargetException, IllegalAccessException {
            return method.invoke(target, args);
        }
    
    }
    

  • Plugin
    Plugin 类实现 Java 动态代理需要的 InvocationHandler 类，在 wrap 方法返回对应的动态代理类。

    public class Plugin implements InvocationHandler {
    
        private final Object target;
        private final Interceptor interceptor;
        private final Map, Set> signatureMap;
    
        private Plugin(Object target, Interceptor interceptor, Map, Set> signatureMap) {
            this.target = target;
            this.interceptor = interceptor;
            this.signatureMap = signatureMap;
        }
    
        // 当 target 类需要被拦截时，生成动态代理类 
        public static Object wrap(Object target, Interceptor interceptor) {
            Map, Set> signatureMap = getSignatureMap(interceptor);
            Class type = target.getClass();
            Class[] interfaces = getAllInterfaces(type, signatureMap);
            if (interfaces.length > 0) {
            return Proxy.newProxyInstance(
                type.getClassLoader(),
                interfaces,
                new Plugin(target, interceptor, signatureMap));
            }
            return target;
        }
    
        // 确定 method 是否会被 Interceptor 拦截，如果是就执行拦截器中intercept方法
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            try {
            Set methods = signatureMap.get(method.getDeclaringClass());
            if (methods != null && methods.contains(method)) {
                return interceptor.intercept(new Invocation(target, method, args));
            }
            return method.invoke(target, args);
            } catch (Exception e) {
            throw ExceptionUtil.unwrapThrowable(e);
            }
        }
    
        // 获取自定义的 Interceptor 类 Interceptors 注解中的 Signature 注解定义的方法 
        private static Map, Set> getSignatureMap(Interceptor interceptor) {
            Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
            // issue #251
            if (interceptsAnnotation == null) {
            throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
            }
            Signature[] sigs = interceptsAnnotation.value();
            Map, Set> signatureMap = new HashMap<>();
            for (Signature sig : sigs) {
            Set methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
            try {
                Method method = sig.type().getMethod(sig.method(), sig.args());
                methods.add(method);
            } catch (NoSuchMethodException e) {
                throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
            }
            }
            return signatureMap;
        }
    
        private static Class[] getAllInterfaces(Class type, Map, Set> signatureMap) {
            Set> interfaces = new HashSet<>();
            while (type != null) {
            for (Class c : type.getInterfaces()) {
                if (signatureMap.containsKey(c)) {
                interfaces.add(c);
                }
            }
            type = type.getSuperclass();
            }
            return interfaces.toArray(new Class[0]);
        }
    
    }
    

  • InterceptorChain
    Configuration 类中会包含 InterceptorChain 实例， 并在 SqlSessionFactoryBean 类中初始化。

    public class InterceptorChain {
    
        private final List interceptors = new ArrayList<>();
    
        public Object pluginAll(Object target) {
            for (Interceptor interceptor : interceptors) {
            target = interceptor.plugin(target);
            }
            return target;
        }
    
        public void addInterceptor(Interceptor interceptor) {
            interceptors.add(interceptor);
        }
    
        public List getInterceptors() {
            return Collections.unmodifiableList(interceptors);
        }
    
    }
    

  • PluginException

Mybatis Plugin 调用链

Plugin 类通过 wrap 方法生成对应的代理类， wrap 方法的调用链如下：

Configuration
    -> newParameterHandler
        -> interceptorChain.pluginAll
            -> interceptor.plugin
                -> Plugin.wrap(target, this)
    -> newStatementHandler
        -> interceptorChain.pluginAll
            -> interceptor.plugin
                -> Plugin.wrap(target, this)
    -> newExecutor
        -> interceptorChain.pluginAll
            -> interceptor.plugin
                -> Plugin.wrap(target, this)
    -> newResultSetHandler
        -> interceptorChain.pluginAll
            -> interceptor.plugin
                -> Plugin.wrap(target, this)

从整条调用链可以看出 Mybatis 的插件可以增强的功能类有四个：
1. ParameterHandler
2. StatementHandler
3. Executor
4. ResultSetHandler
而这四个类正是 Mybatis 的四大组件，四个类共同完成 JDBC 的相关操作。

Mybatis 四大组件

1. ParameterHandler
   负责将用户传递的参数转换为 JDBC Statement 需要的参数

2. StatementHandler
   封装 JDBC Statement 相关的操作，比如参数的设置、将 Statement 的结果集转换为 List

3. ResultSetHandler
   负责将 JDBC 返回的 ResultSet 结果集转换为 List 类型集合

4. Executor
   MyBatis执行器，是MyBatis 调度的核心，负责SQL语句的生成和查询缓存的维护

Mybatis 的核心架构图如下：

Mybatis.png

Mybatis Plugin 应用实例

了解了 Mybatis Plugin 源码及相关组件功能后， 通过 “执行SQL打印” 的扩展功能再熟悉 Mybatis Plugin 的使用。 完成一个 Mybatis Plugin 的核心是构建一个类实现Interceptor 接口，对于 Spring 应用来说将该实现类注入至IOC容器即可。

@Slf4j
@Intercepts({
        @Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class}),
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class,
                RowBounds.class, ResultHandler.class})})
public class PrintSqlInterceptor implements Interceptor {

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
        Object parameter = null;
        if (invocation.getArgs().length > 1) {
            parameter = invocation.getArgs()[1];
        }
        String sqlId = mappedStatement.getId();
        BoundSql boundSql = mappedStatement.getBoundSql(parameter);
        Configuration configuration = mappedStatement.getConfiguration();
        long start = System.currentTimeMillis();
        Object returnValue = invocation.proceed();
        long time = System.currentTimeMillis() - start;
        printSql(configuration, boundSql, time, sqlId);
        return returnValue;
    }

    private static void printSql(Configuration configuration, BoundSql boundSql, long time, String sqlId) {
        Object parameterObject = boundSql.getParameterObject();
        List parameterMappings = boundSql.getParameterMappings();
        //替换空格、换行、tab缩进等
        String sql = boundSql.getSql().replaceAll("[\\s]+", " ");
        if (parameterMappings.size() > 0 && parameterObject != null) {
            TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
            if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                sql = sql.replaceFirst("\\?", getParameterValue(parameterObject));
            } else {
                MetaObject metaObject = configuration.newMetaObject(parameterObject);
                for (ParameterMapping parameterMapping : parameterMappings) {
                    String propertyName = parameterMapping.getProperty();
                    if (metaObject.hasGetter(propertyName)) {
                        Object obj = metaObject.getValue(propertyName);
                        sql = sql.replaceFirst("\\?", getParameterValue(obj));
                    } else if (boundSql.hasAdditionalParameter(propertyName)) {
                        Object obj = boundSql.getAdditionalParameter(propertyName);
                        sql = sql.replaceFirst("\\?", getParameterValue(obj));
                    }
                }
            }
        }
        logs(time, sql, sqlId);
    }

    private static String getParameterValue(Object obj) {
        String value;
        if (obj instanceof String) {
            value = "'" + obj.toString() + "'";
        } else if (obj instanceof Date) {
            DateFormat formatter = DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT, Locale.CHINA);
            value = "'" + formatter.format(obj) + "'";
        } else {
            if (obj != null) {
                value = obj.toString();
            } else {
                value = "";
            }
        }
        return value.replace("$", "\\$");
    }

    private static void logs(long time, String sql, String sqlId) {
        StringBuilder sb = new StringBuilder()
                .append(" Time：").append(time)
                .append(" ms - ID：").append(sqlId)
                .append(StringPool.NEWLINE).append("Execute SQL：")
                .append(sql).append(StringPool.NEWLINE);
        log.info(sb.toString());
    }

    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    @Override
    public void setProperties(Properties properties0) {
    }
}