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！