Arthas之WatchSql

前言

在使用Arthas排查线上问题的时候，有些时候我们需要查看某些Sql的生成，如果线上没有完备的APM的话，那么如何临时查看呢，前几篇文章我们分析了Mybatis的插件机制，如果你还记得的话，我们可以通过watch这个插件进行查看。

Mybatis核心组件

因为现在大部分公司都是使用的Mybatis，我们就以它举例说明，你可以推导任何的方式，先回顾一下Mybatis的核心组件的执行流程，具体的请参阅我之前的文章，就不挂链接了。

因为一个SqlSession对应一个Executor对象，而Executor是MyBatis的内部SQL执行器，它负责调用StatementHandler操作数据库进行增删改查的具体操作，并把结果集通过ResultSetHandler进行自动映射。我们在自定义拦截器的时候不知道你还记得吗，如下：

/**
 * @author Duansg
 * @date 2022-10-08 11:45 下午
 */
@Intercepts(value = {
    @Signature(type = Executor.class, method = "query",
            args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),
    @Signature(type = Executor.class, method = "query",
            args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})}
)
public class ExampleInterceptor implements Interceptor {
    //.....
}

Executor

Executor的方法如下：

public interface Executor {

  <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;

  // ....
}

其底层都会走到有BoundSql的方法：

 @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
  }

BoundSql

/**
 *    Copyright 2009-2019 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.mapping;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.property.PropertyTokenizer;
import org.apache.ibatis.session.Configuration;

/**
 * An actual SQL String got from an {@link SqlSource} after having processed any dynamic content.
 * The SQL may have SQL placeholders "?" and an list (ordered) of an parameter mappings
 * with the additional information for each parameter (at least the property name of the input object to read
 * the value from).
 * <p>
 * Can also have additional parameters that are created by the dynamic language (for loops, bind...).
 *
 * @author Clinton Begin
 */
public class BoundSql {

  private final String sql;
  private final List<ParameterMapping> parameterMappings;
  private final Object parameterObject;
  private final Map<String, Object> additionalParameters;
  private final MetaObject metaParameters;

  public BoundSql(Configuration configuration, String sql, List<ParameterMapping> parameterMappings, Object parameterObject) {
    this.sql = sql;
    this.parameterMappings = parameterMappings;
    this.parameterObject = parameterObject;
    this.additionalParameters = new HashMap<>();
    this.metaParameters = configuration.newMetaObject(additionalParameters);
  }

  public String getSql() {
    return sql;
  }

  public List<ParameterMapping> getParameterMappings() {
    return parameterMappings;
  }

  public Object getParameterObject() {
    return parameterObject;
  }

  public boolean hasAdditionalParameter(String name) {
    String paramName = new PropertyTokenizer(name).getName();
    return additionalParameters.containsKey(paramName);
  }

  public void setAdditionalParameter(String name, Object value) {
    metaParameters.setValue(name, value);
  }

  public Object getAdditionalParameter(String name) {
    return metaParameters.getValue(name);
  }
}

Arthas Idea Plugin

doQuery的底层在最终查询的时候，都会从boundSql中执行getSql()方法

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    String sql = boundSql.getSql();
    statement.execute(sql);
    return resultSetHandler.handleResultSets(statement);
  }

所以，我们执行watch这个getSql()方法

生成的命令如下

watch org.apache.ibatis.mapping.BoundSql getSql '{params,returnObj,throwExp}'  -n 5  -x 3 

小结

这种方式可以临时用一下，如果是核心业务的话，线上的调用量可能会很频繁，不建议在线上直接使用此命令，在疯狂输出的情况下，你也未必能找到你想要的这条Sql。