Mybatis之Executor

mybatis-3.4.6.release.

Mybatis之Executor-LMLPHP

图1

Executor是个接口，具体实现是在BaseExecutor和4个子类中。

1.BatchExecutor

BatchExecutor与其它的区别是update方法中，使用的是StatementHandler的batch方法，如下List-1

List-1

public class BatchExecutor extends BaseExecutor {
  ...
  @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;
    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;
  }
 ...

2.ReuseExecutor

ReuseExecutor与其它executor的区别是重用jdbc的Statement，有个map属性，ReuseExecutor的doUpdate、doQuery中，调用prepareStatement时，如下List-2，会根据sql来判断是否已经存在对应的Statement，如果map中已经有了则直接使用，如果不存在，则通过PrepareStatementHandler.prepare方法获取对Statement，之后放入到map中。

List-2

public class ReuseExecutor extends BaseExecutor {
  ...
  private final Map<String, Statement> statementMap = new HashMap<String, Statement>();
  ...
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);
      applyTransactionTimeout(stmt);
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection, transaction.getTimeout());
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
  }

  private boolean hasStatementFor(String sql) {
    try {
      return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();
    } catch (SQLException e) {
      return false;
    }
  }

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

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

3.SimpleExecutor

SimpleExecutor与ReuseExecutor和BatchExecutor不同，即没有重用Statement，在update时也没有使用StatementHandler的batch方法，而是用了update方法。

4.CachingExecutor

CachingExecutor有点特殊，使用了Delete模式，此外用到了Cache，如下List-3

List-3

public class CachingExecutor implements Executor {

  private final Executor delegate;
  private final TransactionalCacheManager tcm = new TransactionalCacheManager();

  public CachingExecutor(Executor delegate) {
    this.delegate = delegate;
    delegate.setExecutorWrapper(this);
  }
...

来看TransactionalCacheManager，如下List-4，通过Cache这个key，得到TransactionalCache，当调用getObject方法时，间接调用TransactionalCache.getObject.

List-4

public class TransactionalCacheManager {

  private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();

  public void clear(Cache cache) {
    getTransactionalCache(cache).clear();
  }

  public Object getObject(Cache cache, CacheKey key) {
    return getTransactionalCache(cache).getObject(key);
  }

  public void putObject(Cache cache, CacheKey key, Object value) {
    getTransactionalCache(cache).putObject(key, value);
  }

  public void commit() {
    for (TransactionalCache txCache : transactionalCaches.values()) {
      txCache.commit();
    }
  }

  public void rollback() {
    for (TransactionalCache txCache : transactionalCaches.values()) {
      txCache.rollback();
    }
  }

List-5

public class TransactionalCache implements Cache {
  private final Cache delegate;
  private boolean clearOnCommit;
  private final Map<Object, Object> entriesToAddOnCommit;
  private final Set<Object> entriesMissedInCache;

  public TransactionalCache(Cache delegate) {
    this.delegate = delegate;
    this.clearOnCommit = false;
    this.entriesToAddOnCommit = new HashMap<Object, Object>();
    this.entriesMissedInCache = new HashSet<Object>();
  }

  @Override
  public Object getObject(Object key) {
    // issue #116
    Object object = delegate.getObject(key);
    if (object == null) {
      entriesMissedInCache.add(key);
    }
    // issue #146
    if (clearOnCommit) {
      return null;
    } else {
      return object;
    }
  }

List-5中，getObject方法中，从委托的cache中getObject如果不是null则返回，不为null则还需要更新统计。

所以mybatis中可以使用二级缓存就是通过这个Cache来实现的。

在没有特殊设置的情况下，mybatis中，如下List-6，默认使用的executor是SimpleExecutor，StatementHandler是RoutingStatementHandler，内部委托给PrepareStatementHandler。

List-6

<select id="findByUsername" resultType="Person" parameterType="Person">
    select * from person where username=#{username};
</select>

如下List-7，没有参数设置，默认情况下使用的还是SimpleExecutor、RoutingStatementHandler——内部委托给PrepareStatementHandler.

List-7

<select id="findAll" resultType="Person">
    select * from person
</select>

Reference

https://github.com/mybatis/mybatis-3/tree/3.4.x

克虏伯