Mybatis源码分析-一级缓存

为了能提高系统性能，缓存必不可少，mybatis同样也是有缓存的！
分一级缓存和二级缓存，该篇介绍下一级缓存！！
我们回顾下配置阶段，再创建executor执行器时，有个Configuration.newExecutor方法，来看下

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);    }    //默认cacheEnabled为true，那么此时的执行器其实是CachingExecutor，将executor当参数传递给CachingExecutor，装饰了下，增加了缓存功能    if (cacheEnabled) {      executor = new CachingExecutor(executor);    }    executor = (Executor) interceptorChain.pluginAll(executor);    return executor;  }

那么我们再执行Executor.query的时候，也就是说先调用CachingExecutor的query方法

  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {    BoundSql boundSql = ms.getBoundSql(parameterObject);    //cachekey 即缓存的标识    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);  }public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {    if (closed) {      throw new ExecutorException("Executor was closed.");    }    CacheKey cacheKey = new CacheKey();    //StatementId    cacheKey.update(ms.getId());    //自带分页参数数据    cacheKey.update(rowBounds.getOffset());    cacheKey.update(rowBounds.getLimit());    //sql语句    cacheKey.update(boundSql.getSql());    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();    TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();    // mimic DefaultParameterHandler logic    for (ParameterMapping parameterMapping : parameterMappings) {      if (parameterMapping.getMode() != ParameterMode.OUT) {        Object value;        String propertyName = parameterMapping.getProperty();        if (boundSql.hasAdditionalParameter(propertyName)) {          value = boundSql.getAdditionalParameter(propertyName);        } else if (parameterObject == null) {          value = null;        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {          value = parameterObject;        } else {          MetaObject metaObject = configuration.newMetaObject(parameterObject);          value = metaObject.getValue(propertyName);        }        cacheKey.update(value);      }    }    if (configuration.getEnvironment() != null) {      // issue #176//xml中environment的id      cacheKey.update(configuration.getEnvironment().getId());    }    //以上种种，组成一个缓存标识    return cacheKey;  }    public void update(Object object) {    //生成hashcode，具体算法大家可以进去看，主要是分集合和非集合之分    int baseHashCode = object == null ? 1 : ArrayUtil.hashCode(object);     count++;    checksum += baseHashCode;    //扩大count倍    baseHashCode *= count;//加上扩展固定值    hashcode = multiplier * hashcode + baseHashCode;    updateList.add(object);  }

好了 有了key，那么就应该从缓存中获取数据了，

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { ...省略 //获取缓存      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; ...判断是否为空 //为空，则从数据库中获取        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); ...省略    return list;  }  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {    List<E> list;    //标记下    localCache.putObject(key, EXECUTION_PLACEHOLDER);    try {      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);    } finally {    //每次都会移除 PerpetualCache  其实就是放到hashmap中      localCache.removeObject(key);    }    //在put进缓存    localCache.putObject(key, list);    if (ms.getStatementType() == StatementType.CALLABLE) {      localOutputParameterCache.putObject(key, parameter);    }    return list;  }

好了 ，缓存是不是很简单，原来就是放到PerpetualCache类的hashmap中。

最后 session.close();关闭时，将localCache = null;缓存设置为null！！
也就是说 一级缓存的作用体现在同一个session中，一旦关闭，则清空，适合同一个session频繁的调用同样的sql（我想这种情况很少吧！）
而且，这种缓存的做法，默认状态是开启的，自动进行缓存，如果session长时间不关的话，会造成数据量越来越大，造成内存溢出!所以要及时关闭session哦！