mybatis-3.4.6 缓存介绍

系列

• mybatis-3.4.6 配置介绍
• mybatis-3.4.6 顶层配置解析
• mybatis-3.4.6 子配置解析
• mybatis-3.4.6 mapper解析
• mybatis-3.4.6 SQL执行流程
• mybatis-3.4.6 SqlSession执行过程
• mybatis-3.4.6 缓存介绍
• mybatis-3.4.6 自增主键
• mybatis-3.4.6 foreach 自增主键

开篇

• 这个系列是基于mybatis-3.4.6版本的源码解析，这篇文章主要是梳理mybatis的一级缓存和二级缓存。

• 核心主要是分析cache的使用范围以及对应的SQL执行对缓存的使用。

mybatis多级缓存

• 1、一级缓存: 基于PerpetualCache 的 HashMap本地缓存，其存储作用域为 Session，当 Session flush 或 close 之后，该Session中的所有 Cache 就将清空。

• 2、二级缓存与一级缓存其机制相同，默认也是采用 PerpetualCache，HashMap存储，不同在于其存储作用域为 Mapper(Namespace)，并且可自定义存储源，如 Ehcache。

• 3、对于缓存数据更新机制，当某一个作用域(一级缓存Session/二级缓存Namespaces)的进行了 C/U/D 操作后，默认该作用域下所有 select 中的缓存将被clear。

• 4、从Cache的层面来说，二级缓存在一级缓存之前生效。

mybatis一级缓存

public abstract class BaseExecutor implements Executor {
  // localCache作为mybatis的一级缓存
  protected PerpetualCache localCache;

  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());

    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }

    List<E> list;
    try {
      queryStack++;
      // 先从localCache查询
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        // 如果已经存在则直接返回
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // 缓存不存在就查询DB
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }

    return list;
  }


  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    // 保存到本地缓存
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }
}

• mybatis一级缓存在SimpleExecutor级别的，BaseExecutor作为SimpleExecutor的父类。
• BaseExecutor的localCache用来进行缓存，先查询缓存后查询DB，查询完DB保存到localCache当中。

public class PerpetualCache implements Cache {
  private final String id;
  private Map<Object, Object> cache = new HashMap<Object, Object>();
}

• mybatis一级缓存是通过PerpetualCache来进行存储的。

mybatis二级缓存

public class CachingExecutor implements Executor {

  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    // 通过MappedStatement来进行查询
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        // 缓存不存在的情况下由SimpleExecutor来执行query
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    // 缓存不允许的情况下由SimpleExecutor来执行query
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
}

• 二级缓存在CachingExecutor层被使用。
• 二级缓存是MappedStatement级别的，从MappedStatement当中查询Cache对象。
• 在允许使用二级缓存的场景下优先查询二级缓存，没有的情况下通过SimpleExecutor查询DB。
• 二级缓存是由TransactionalCache进行管理的。

public class TransactionalCache implements Cache {

  private static final Log log = LogFactory.getLog(TransactionalCache.class);

  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>();
  }

  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;
    }
  }

  @Override
  public void putObject(Object key, Object object) {
    entriesToAddOnCommit.put(key, object);
  }
}

• TransactionalCache的entriesToAddOnCommit负载保存二级缓存的数据。

mybatis二级缓存的MappedStatement

public final class MappedStatement {
  // 省略其他变量
  
  // 二级缓存的cache对象
  private Cache cache;
  // 是否使用二级缓存
  private boolean useCache;
}

• MappedStatement内部保存是否使用二级缓存的配置信息。

public class XMLStatementBuilder extends BaseBuilder {

  public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
    // 针对SELECT的命令行
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    // 针对SELECT的命令默认是不刷新cache的
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    // 针对SELECT的命令默认是使用cache的
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }
}

• 在生成MappedStatement对象时候，针对select操作flushCache默认为false，useCache默认是开启。

public class XMLStatementBuilder extends BaseBuilder {

  private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      cacheRefElement(context.evalNode("cache-ref"));
      // 负责解析cache节点
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

  private void cacheElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type", "PERPETUAL");
      Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
      String eviction = context.getStringAttribute("eviction", "LRU");
      Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
      Long flushInterval = context.getLongAttribute("flushInterval");
      Integer size = context.getIntAttribute("size");
      boolean readWrite = !context.getBooleanAttribute("readOnly", false);
      boolean blocking = context.getBooleanAttribute("blocking", false);
      Properties props = context.getChildrenAsProperties();
      // 设置是否使用Cache
      builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
    }
  }
}


public class MapperBuilderAssistant extends BaseBuilder {

  public Cache useNewCache(Class<? extends Cache> typeClass,
      Class<? extends Cache> evictionClass,
      Long flushInterval,
      Integer size,
      boolean readWrite,
      boolean blocking,
      Properties props) {
    Cache cache = new CacheBuilder(currentNamespace)
        .implementation(valueOrDefault(typeClass, PerpetualCache.class))
        .addDecorator(valueOrDefault(evictionClass, LruCache.class))
        .clearInterval(flushInterval)
        .size(size)
        .readWrite(readWrite)
        .blocking(blocking)
        .properties(props)
        .build();
    configuration.addCache(cache);
    currentCache = cache;
    return cache;
  }
}

• XMLStatementBuilder的cacheElement(context.evalNode("cache"))负责创建二级缓存的cache对象。

public class MapperBuilderAssistant extends BaseBuilder {

  public MappedStatement addMappedStatement(
      String id,
      SqlSource sqlSource,
      StatementType statementType,
      SqlCommandType sqlCommandType,
      Integer fetchSize,
      Integer timeout,
      String parameterMap,
      Class<?> parameterType,
      String resultMap,
      Class<?> resultType,
      ResultSetType resultSetType,
      boolean flushCache,
      boolean useCache,
      boolean resultOrdered,
      KeyGenerator keyGenerator,
      String keyProperty,
      String keyColumn,
      String databaseId,
      LanguageDriver lang,
      String resultSets) {

    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
        .resource(resource)
        .fetchSize(fetchSize)
        .timeout(timeout)
        .statementType(statementType)
        .keyGenerator(keyGenerator)
        .keyProperty(keyProperty)
        .keyColumn(keyColumn)
        .databaseId(databaseId)
        .lang(lang)
        .resultOrdered(resultOrdered)
        .resultSets(resultSets)
        .resultMaps(getStatementResultMaps(resultMap, resultType, id))
        .resultSetType(resultSetType)
        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
        // 设置是否使用cache
        .useCache(valueOrDefault(useCache, isSelect))
        .cache(currentCache);

    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
    if (statementParameterMap != null) {
      statementBuilder.parameterMap(statementParameterMap);
    }

    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
  }
}

• configuration.addMappedStatement(statement)负责保存MappedStatement对象。

参考文章

• 源码参考
• mybatis官网介绍
• 深入理解mybatis原理
• Mybatis3.4.x技术内幕
• mybatis 3.x源码深度解析与最佳实践