mybatis四大金刚之executor执行器

Posted 2021-06-29 我爱看明朝

mybatis四大金刚之executor执行器

configuration.newExecutor()所有流程的开始

public class DefaultSqlSession {

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
}

Executor的主流程

mybatis执行器,通过拼接出sql，调用StatementHanlder执行database操作，以及缓存的子类可以重用预处理语句

（本节展示的源码代码，为了帮助同学们，减少干扰理解executor的执行流程省略了一些代码。）

public abstract class BaseExecutor implements Executor{

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    // 省略部分代码
    List<E> list;
    //从db查询
    list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    return list;
  }

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    // doQuery方法的具体实现在BaseExecutor的子类，这里使用了模板方法的设计模式： 
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
     return list;
  }

设计模式 - 模版方法

模板方法： 父类封装整个算法步骤，将某些步骤延迟到子类实现，使得不需要改变算法结构的情况下，重新定义算法中的某些步骤

默认的执行器SimpleExecutor

来看看怎么选的默认处理器

public enum ExecutorType {
  SIMPLE, REUSE, BATCH
}

public class Configuration{
    //默认值
    protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;

  //根据executorTypec返回Executor的子类
  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 {
        //默认的执行器
      executor = new SimpleExecutor(this, transaction);
    }
    // 使用二级缓存
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }
}

简单的执行器，根据对应sql拼接完成后，直接交给StatementHanlder去执行

public class SimpleExecutor extends BaseExecutor{
  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }
}

执行器的类图

可以看到BaseExecutor实现了Executor; SimpleExecutor,ReuseExecutor,BatchExecutor继承了BaseExecutor,
把具体的操作细节方法： doUpdate、doFlushStatements、doQuery、doQueryCursor通过模板方法交给不同的子类实现的。

SimpleExecutor : 简单的执行器
ReuseExecutor : 可重用执行器
BatchExecutor : 批处理执行器

那CachingExecutor这个类是做什么用的呢,我们在后面通过阅读源码同学们就知道具体是做什么的了，我们单单看累的名字记住它是一个缓存执行器。

ReuseExecutor可重用执行器

看名字，我们就知道这个处理器的作用是重复执行，如果已经在当前线程生命周期内查询过了，不需要在重新创建Statement了。我们直接看它的doquery方法，因为它的逻辑和simpleExecutor的区别就在doQuery这几个方法

  
public class ReuseExecutor extends BaseExecutor {

    //存储未预编译的sql于带 "?"
    private final Map<String, Statement> statementMap = new HashMap<String, Statement>();

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.<E>query(stmt, resultHandler);
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
    //如果当前线程存在sql，则获取到map存储的，不需要在创建了
      stmt = getStatement(sql);
      applyTransactionTimeout(stmt);
    } else {
    //没有则创建，并且缓存到map中
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection, transaction.getTimeout());
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
  }

  
  private Statement getStatement(String s) {
    return statementMap.get(s);
  }

  private void putStatement(String sql, Statement stmt) {
    statementMap.put(sql, stmt);
  }

}

注意这里的缓存仅在当前线程内有效。

如何开启的mybatis.xml配置方式是

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<!-- configuration 核心配置文件 -->
<configuration>

    <settings>
        <setting name="defaultExecutorType" value="REUSE"/>
    </settings>
</configuration>

BatchExecutor

批量执行,只对修改操作生效，对查询不生效，只对当前线程中的同修改sql,可以直接复用之前的statement,
使用这个处理器时要和doFlushStatements、commit方法一起使用,当执行commit数据写入数据库

public class BatchExecutor extends BaseExecutor {

 private final List<BatchResult> batchResultList = new ArrayList<BatchResult>();
 private String currentSql;
 private MappedStatement currentStatement;


  @Override
  public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
    final Configuration configuration = ms.getConfiguration();
    final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
    final BoundSql boundSql = handler.getBoundSql();
    final String sql = boundSql.getSql();
    final Statement stmt;
    // 如果sql存在使用之前的statement
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {
      int last = statementList.size() - 1;
      stmt = statementList.get(last);
      applyTransactionTimeout(stmt);
      handler.parameterize(stmt);//fix Issues 322
      BatchResult batchResult = batchResultList.get(last);
      batchResult.addParameterObject(parameterObject);
    } else {
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection, transaction.getTimeout());
      handler.parameterize(stmt);    //fix Issues 322
      currentSql = sql;
      currentStatement = ms;
      statementList.add(stmt);
      batchResultList.add(new BatchResult(ms, sql, parameterObject));
    }
  // handler.parameterize(stmt);
    handler.batch(stmt);
    return BATCH_UPDATE_RETURN_VALUE;
  }

}

BatchExecutor和ReuseExecutor很像区别就在于,ReuseExecutor复用了直接操作数据库，而batch则是最后一起提交写入数据库，可以节省网络通信时间。

如何开启的mybatis.xml配置方式是

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<!-- configuration 核心配置文件 -->
<configuration>

    <settings>
        <setting name="defaultExecutorType" value="BATCH"/>
    </settings>
</configuration>

CachingExecutor

CachingExecutor表示启动二级缓存，默认开启。

public class Configuration{
  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
      //根据配置或默认值创建SimpleExecutor、ReuseExecutor、BatchExecutor执行器
    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);
    }
    //缓存默认开启，创建CachingExecutor
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

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