mybatis 源码分析(五)Interceptor 详解

本篇博客将主要讲解 mybatis 插件的主要流程,其中主要包括动态代理和责任链的使用;

一、mybatis 拦截器主体结构

在编写 mybatis 插件的时候,首先要实现 Interceptor 接口,然后在 mybatis-conf.xml 中添加插件,

<configuration>
  <plugins>
    <plugin interceptor="***.interceptor1"/>
    <plugin interceptor="***.interceptor2"/>
  </plugins>
</configuration>

这里需要注意的是,添加的插件是有顺序的,因为在解析的时候是依次放入 ArrayList 里面,而调用的时候其顺序为:2 > 1 > target > 1 > 2;(插件的顺序可能会影响执行的流程)更加细致的讲解可以参考 QueryInterceptor 规范 ;

然后当插件初始化完成之后,添加插件的流程如下:

首先要注意的是,mybatis 插件的拦截目标有四个,Executor、StatementHandler、ParameterHandler、ResultSetHandler:

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}

public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
    ResultHandler resultHandler, BoundSql boundSql) {
  ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
  resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
  return resultSetHandler;
}

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}

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);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

这里使用的时候都是用动态代理将多个插件用责任链的方式添加的,最后返回的是一个代理对象; 其责任链的添加过程如下:

public Object pluginAll(Object target) {
  for (Interceptor interceptor : interceptors) {
    target = interceptor.plugin(target);
  }
  return target;
}

最终动态代理生成和调用的过程都在 Plugin 类中:

public static Object wrap(Object target, Interceptor interceptor) {
  Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor); // 获取签名Map
  Class<?> type = target.getClass(); // 拦截目标 (ParameterHandler|ResultSetHandler|StatementHandler|Executor)
  Class<?>[] interfaces = getAllInterfaces(type, signatureMap);  // 获取目标接口
  if (interfaces.length > 0) {
    return Proxy.newProxyInstance(  // 生成代理
        type.getClassLoader(),
        interfaces,
        new Plugin(target, interceptor, signatureMap));
  }
  return target;
}

这里所说的签名是指在编写插件的时候,指定的目标接口和方法,例如:

@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 ExamplePlugin implements Interceptor {
  public Object intercept(Invocation invocation) throws Throwable {
    ...
  }
}

这里就指定了拦截 Executor 的具有相应方法的 update、query 方法;注解的代码很简单,大家可以自行查看;然后通过 getSignatureMap 方法反射取出对应的 Method 对象,在通过 getAllInterfaces 方法判断,目标对象是否有对应的方法,有就生成代理对象,没有就直接反?#38405;?#26631;对象;

在调用的时候:

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  try {
    Set<Method> methods = signatureMap.get(method.getDeclaringClass());  // 取出拦截的目标方法
    if (methods != null && methods.contains(method)) { // 判断这个调用的方法是否在拦截范围内
      return interceptor.intercept(new Invocation(target, method, args)); // 在目标范围内就拦截
    }
    return method.invoke(target, args); // 不在目标范围内就直接调用方法本身
  } catch (Exception e) {
    throw ExceptionUtil.unwrapThrowable(e);
  }
}

二、PageHelper 拦截器分析

mybatis 插件我们平时使用最多的就是分页插件了,这里以 PageHelper 为例,其使用方法可以查看相应的文档 如何使用分页插件,因为官方文档讲解的很详细了,我这里就简单补充分页插件需要做哪几件事情;

使用:

PageHelper.startPage(1, 2);
List<User> list = userMapper1.getAll();

PageHelper 还有很多中使用方式,这是最常用的一种,他其实就是在 ThreadLocal 中设置了 Page 对象,能取到就代表需要分页,在分页完成后在移除,这样就不会导致其他方法分页;(PageHelper 使用的其他方法,也是围绕 Page 对象的设置进行的)

protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal<Page>();
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
  Page<E> page = new Page<E>(pageNum, pageSize, count);
  page.setReasonable(reasonable);
  page.setPageSizeZero(pageSizeZero);
  //当已经执行过orderBy的时候
  Page<E> oldPage = getLocalPage();
  if (oldPage != null && oldPage.isOrderByOnly()) {
    page.setOrderBy(oldPage.getOrderBy());
  }
  setLocalPage(page);
  return page;
}

主要实现:

@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 {

  @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];
      }
      checkDialectExists();

      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)) {
        //判断是否需要进行 count 查询
        if (dialect.beforeCount(ms, parameter, rowBounds)) {
          //查询总数
          Long count = count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
          //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);
          }
        }
        resultList = ExecutorUtil.pageQuery(dialect, executor,
            ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
      } 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 {
      if(dialect != null){
        dialect.afterAll();
      }
    }
  }
}
  • 首先可以看到拦截的是 Executor 的两个 query 方法(这里的两个方法具体拦截到哪一个受插件顺序影响,最终影响到 cacheKey 和 boundSql 的初始化);
  • 然后使用 checkDialectExists 判断是否支持对应的数据库;
  • 在分页之前需要查询总数,这里会生成相应的 sql 语句以及对应的 MappedStatement 对象,并缓存;
  • 然后拼接分页查询语句,并生成相应的 MappedStatement 对象,同时缓存;
  • 最后查询,查询完成后使用 dialect.afterPage 移除 Page对象
posted @ 2019-08-28 14:27 三枣 阅读(...) 评论(...) 编辑 收藏
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