MyBatis源码阅读——MyBatis插件原理

桃扇骨 2022-05-22 00:23 252阅读 0赞

### 前言 ###

MyBatis 允许你在已映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis 允许使用插件来拦截的方法调用包括。

*  Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
 *  ParameterHandler (getParameterObject, setParameters)
 *  ResultSetHandler (handleResultSets, handleOutputParameters)
 *  StatementHandler (prepare, parameterize, batch, update, query)

这些类中方法的细节可以通过查看每个方法的签名来发现，或者直接查看 MyBatis 发行包中的源代码。 如果你想做的不仅仅是监控方法的调用，那么你最好相当了解要重写的方法的行为。 因为如果在试图修改或重写已有方法的行为的时候，你很可能在破坏 MyBatis 的核心模块。 这些都是更低层的类和方法，所以使用插件的时候要特别当心。

### 使用插件 ###

> 功能描述：编写一个MyBatis插件，在执行删除动作的时候，做日志记录

#### 实现过程： ####

编写类DeleteWarningPlugin，实现Interceptor 接口

@Intercepts({@Signature(
            type = Executor.class,
            method = "update",
            args = {MappedStatement.class, Object.class})})
    public class DeleteWarningPlugin implements Interceptor {
    
        @Override
        public Object intercept(Invocation invocation) throws Throwable {
            MappedStatement ms = (MappedStatement) invocation.getArgs()[0];
            System.out.println();
            if ("DELETE".equals(ms.getSqlCommandType().name())) {
                SqlSource sqlSource = ms.getSqlSource();
                RawSqlSource rawSqlSource = (RawSqlSource) sqlSource;
                MetaObject metaObject = SystemMetaObject.forObject(rawSqlSource);
                SqlSource thisSqlSource = (StaticSqlSource) metaObject.getValue("sqlSource");
                MetaObject metaObjectSql = SystemMetaObject.forObject(thisSqlSource);
                String sql = (String) metaObjectSql.getValue("sql");
                System.out.println("系统发生删除操作：" + sql);
            }
            return invocation.proceed();
        }
    
        @Override
        public Object plugin(Object target) {
            return Plugin.wrap(target, this);
        }
    
        @Override
        public void setProperties(Properties properties) {
            for (String key : properties.stringPropertyNames()) {
                System.out.println(key + " " + properties.get(key));
            }
        }
    }

在mybatis-config.xml配置中加入插件

运行调试：

public class Demo6PluginExample {
        public static void main(String[] args) throws IOException {
            String resource = "mybatis/conf/mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            //从 XML 中构建 SqlSessionFactory
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            SqlSession session = sqlSessionFactory.openSession(true);
            try {
                BlogMapper mapper = session.getMapper(BlogMapper.class);
                int result = mapper.delete(25L);
                System.out.println("result: " + result);
            } finally {
                session.close();
            }
        }
    }

调试结果：

someProperty 100
    系统发生删除操作：delete from blog where blog_id = ?
    result: 1

可以看到，插件已经起了效果。

### 插件原理 ###

我们拿Executor 来说，  
![这里写图片描述][70]  
打开源码org.apache.ibatis.session.Configuration->newExecutor()

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        executorType = executorType == null ? defaultExecutorType : executorType;
        executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
        Executor executor;
        if (ExecutorType.BATCH == executorType) {
          executor = new BatchExecutor(this, transaction);
        } else if (ExecutorType.REUSE == executorType) {
          executor = new ReuseExecutor(this, transaction);
        } else {
          executor = new SimpleExecutor(this, transaction);
        }
        if (cacheEnabled) {
          executor = new CachingExecutor(executor);
        }
        //包装插件，executor 在创建过程中被动态代理加强，加强的内容为自定义的插件
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
      }

打开InterceptorChain源码

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

> org.apache.ibatis.session.Configuration->newExecutor() 何时调用？顺序如下  
> SqlSession session = sqlSessionFactory.openSession();  
> DefaultSqlSessionFactory->openSessionFromDataSource()

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          final Executor executor = configuration.newExecutor(tx, execType);
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          closeTransaction(tx); // may have fetched a connection so lets call close()
          throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

# Plugin #

打开Plugin这个类我们可以看到，这其实是一个动态代理

public static Object wrap(Object target, Interceptor interceptor) {
        Map<Class<?>, Set<Method>> 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;
      }

[70]: /images/20220522/6ee9f1408b7a4aa69d15119a456119ba.png