Mybatis插件Plugin

Mybatis开源Plugin中最熟知的pagehelper，重点made in China

很多人开始用pagehelper时候，肯定很纳闷，以mysql为例，明明没有加limit语句，为什么打印出来的sql中有了limit语句了，这是怎么回事？？？插件在什么地方给sql加上了limit，为什么又能修改写好的sql，只能从pagehelper的源码中找到疑惑的答案了。

@Intercepts(
    {
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),
    }
)
public class PageInterceptor implements Interceptor {
    //缓存count查询的ms
    protected Cache<CacheKey, MappedStatement> msCountMap = null;
    private Dialect dialect;
    private String default_dialect_class = "com.github.pagehelper.PageHelper";
    private Field additionalParametersField;

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        try {
            Object[] args = invocation.getArgs();
            MappedStatement ms = (MappedStatement) args[0];
            Object parameter = args[1];
            RowBounds rowBounds = (RowBounds) args[2];
            ResultHandler resultHandler = (ResultHandler) args[3];
            Executor executor = (Executor) invocation.getTarget();
            CacheKey cacheKey;
            BoundSql boundSql;
            //由于逻辑关系，只会进入一次
            if(args.length == 4){
                //4 个参数时
                boundSql = ms.getBoundSql(parameter);
                cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
            } else {
                //6 个参数时
                cacheKey = (CacheKey) args[4];
                boundSql = (BoundSql) args[5];
            }
            List resultList;
            //调用方法判断是否需要进行分页，如果不需要，直接返回结果
            if (!dialect.skip(ms, parameter, rowBounds)) {
                //反射获取动态参数
                Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql);
                //判断是否需要进行 count 查询
                if (dialect.beforeCount(ms, parameter, rowBounds)) {
                    //创建 count 查询的缓存 key
                    CacheKey countKey = executor.createCacheKey(ms, parameter, RowBounds.DEFAULT, boundSql);
                    countKey.update("_Count");
                    MappedStatement countMs = msCountMap.get(countKey);
                    if (countMs == null) {
                        //根据当前的 ms 创建一个返回值为 Long 类型的 ms
                        countMs = MSUtils.newCountMappedStatement(ms);
                        msCountMap.put(countKey, countMs);
                    }
                    //调用方言获取 count sql
                    String countSql = dialect.getCountSql(ms, boundSql, parameter, rowBounds, countKey);
                    BoundSql countBoundSql = new BoundSql(ms.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
                    //当使用动态 SQL 时，可能会产生临时的参数，这些参数需要手动设置到新的 BoundSql 中
                    for (String key : additionalParameters.keySet()) {
                        countBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
                    }
                    //执行 count 查询
                    Object countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql);
                    Long count = (Long) ((List) countResultList).get(0);
                    //处理查询总数
                    //返回 true 时继续分页查询，false 时直接返回
                    if (!dialect.afterCount(count, parameter, rowBounds)) {
                        //当查询总数为 0 时，直接返回空的结果
                        return dialect.afterPage(new ArrayList(), parameter, rowBounds);
                    }
                }
                //判断是否需要进行分页查询
                if (dialect.beforePage(ms, parameter, rowBounds)) {
                    //生成分页的缓存 key
                    CacheKey pageKey = cacheKey;
                    //处理参数对象
                    parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
-------------------------------------------------------------------------------------------下面就是加上limit语句的地方（方言通俗就是数据库类型）------------------------------------------------------------------
                    //调用方言获取分页 sql
                    String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
                    BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
                    //设置动态参数
                    for (String key : additionalParameters.keySet()) {
                        pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
                    }
                    //执行分页查询
                    resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
                } else {
                    //不执行分页的情况下，也不执行内存分页
                    resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
                }
            } else {
                //rowBounds用参数值，不使用分页插件处理时，仍然支持默认的内存分页
                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
            }
            return dialect.afterPage(resultList, parameter, rowBounds);
        } finally {
            dialect.afterAll();
        }
    }

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

    @Override
    public void setProperties(Properties properties) {
        //缓存 count ms
        msCountMap = CacheFactory.createCache(properties.getProperty("msCountCache"), "ms", properties);
        String dialectClass = properties.getProperty("dialect");
        if (StringUtil.isEmpty(dialectClass)) {
            dialectClass = default_dialect_class;
        }
        try {
            Class<?> aClass = Class.forName(dialectClass);
            dialect = (Dialect) aClass.newInstance();
        } catch (Exception e) {
            throw new PageException(e);
        }
        dialect.setProperties(properties);
        try {
            //反射获取 BoundSql 中的 additionalParameters 属性
            additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters");
            additionalParametersField.setAccessible(true);
        } catch (NoSuchFieldException e) {
            throw new PageException(e);
        }
    }

}

图中表示对 执行器Executor 的query方法进行拦截，执行器就是真正执行sql的容器

public interface Executor {
  ResultHandler NO_RESULT_HANDLER = null;
  int update(MappedStatement ms, Object parameter) throws SQLException;

   //拦截query方法
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
  List<BatchResult> flushStatements() throws SQLException;
  void commit(boolean required) throws SQLException;
  void rollback(boolean required) throws SQLException;
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  boolean isCached(MappedStatement ms, CacheKey key);
  void clearLocalCache();
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  Transaction getTransaction();
  void close(boolean forceRollback);
  boolean isClosed();
  void setExecutorWrapper(Executor executor);
}

Diagrams图：

    Configuration.class中有newExecutor方法

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
   //根据executorType策略，实例化对应的执行器
    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);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

在Mybatis中可以定义四种插件类型：

ParameterHandler  ResultSetHandler  StatementHandler  Executor

插件就是一个Interceptor比如上面的pageInterceptor

容器在实例化ParameterHander，ResultSetHandler，StatementHandler，Executor时会调用pluginAll方法进行匹配，如果签名一致则返回的就是该对象的代理对象，反之是它本身。通过AOP对方法进行了修改，增强等

自定义简单的插件（打印sql执行时间，监控慢查询）

@Intercepts({ @Signature(type = StatementHandler.class, method = "query", args = { Statement.class, ResultHandler.class}) })
public class SQLInterceptor implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        long startTime = System.currentTimeMillis();
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();
        System.out.println("获取到SQL语句："+sql);
        try {
            return invocation.proceed();
        }finally {
            long endTime = System.currentTimeMillis();
            System.out.println("SQL执行耗时：" + (endTime-startTime) +"ms");
        }
    }