MyBatis的缓存分为一级缓存和二级缓存(全局缓存) ，缓存示意图如下图所示。默认情况下，一级缓存是开启的，且不能被关闭。

  MyBatis的一级缓存(SqlSession级别)

在同一个SqlSession中执行相同的SQL语句时，将查询结果集缓存，第二次查询时直接取缓存，不在操作数据库。注意：一级缓存最多缓存1024条SQL语句。

原理： 当客户端第一次发出SQL查询语句时，MyBatis执行SQL的查询并将查询结果写入SqlSession的一级缓存中，当第二次相同的SQL的查询语句过来时，则直接从缓存中获取数据，具体过程如上图所示。缓存使用的数据结构是Map，其中key是 MapperId+Offset+Limit+SQL+所有的入参。

同一个SqlSession操作相同的SQL语句，如中间有修改操作时，需不要手动flush？

不需要！！！

当同一个SqlSession多次发出相同的SQL语句查询时，MyBatis会直接从缓存中取数据。如果前后两次中间出现过commit操作（修改、添加、删除），则认为数据发生了变化，MyBatis会把SqlSession中的一级缓存区域全部清空，则下一次的SQL语句查询时，需从数据库中查询数据并将查询的结果写入缓存。

1、MyBatis的一级缓存是SqlSession级别的，但是它并不定义在SqlSessio接口的实现类DefaultSqlSession中，而是定义在DefaultSqlSession的成员变量Executor中，Executor是在openSession的时候被实例化出来的，它的默认实现为SimpleExecutor

2、MyBatis中的一级缓存，与有没有配置无关，只要SqlSession存在，MyBastis一级缓存就存在，localCache的类型是PerpetualCache，它其实很简单，一个id属性+一个HashMap属性而已，id是一个名为"localCache"的字符串，HashMap用于存储数据，Key为CacheKey，Value为查询结果

3、MyBatis的一级缓存查询的时候默认都是会先尝试从一级缓存中获取数据的，但是我们看第6行的代码做了一个判断，ms.isFlushCacheRequired()，即想每次查询都走DB也行，将<select>标签中的flushCache属性设置为true即可，这意味着每次查询的时候都会清理一遍PerpetualCache，PerpetualCache中没数据，自然只能走DB

从MyBatis一级缓存来看，它以单纯的HashMap做缓存，没有容量控制，而一次SqlSession中通常来说并不会有大量的查询操作，因此只适用于一次SqlSession，如果用到二级缓存的Mapper级别的场景，有可能缓存数据不断碰到而导致内存溢出。

<insert>、<delete>、<update>最终都会转换为update方法，看一下BaseExecutor的update方法：

public int update(MappedStatement ms, Object parameter) throws SQLException {
  ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
  if (closed) {
   throw new ExecutorException("Executor was closed.");
  }
  clearLocalCache();
  return doUpdate(ms, parameter);
}

clearLocalCache()方法 =====> 所有的增、删、改都会清空缓存，这和是否配置了flushCache=true是无关的。 

namespace+id 其中namespace为Mapper的全量名！

MyBatis的二级缓存(Mapper级别，全局缓存)

跨SqlSession的缓存，Mapper级别的缓存。在Mapper级别的缓存内，不同的SQLSession缓存可以共享。

原理： Mapper级别，Mapper以命名空间为单位创建缓存数据结构，缓存使用的数据结构是Map，其中key是 MapperId+Offset+Limit+SQL+所有的入参。MyBatis的二级缓存是CacheExecutor实现的。CacheExecutor是Executor的代理对象。当MyBatis接收到用户的查询请求时首先会根据Map的Key在CacheExecute缓存中查询数据是否存在，存在则从缓存中取，否则将执行数据库查询操作。

开启二级缓存需要进行相关配置：

1. MyBatis全局配置中启用二级缓存配置 CacheEnable=true；
2. 在对应的Mapper文件中配置Cache节点；
3. 在对应的Select标签中添加useCache=true；

MyBatis中的Cache接口

public interface Cache {

  /**
   * @return The identifier of this cache
   */
  String getId();

  /**
   * @param key Can be any object but usually it is a {@link CacheKey}
   * @param value The result of a select.
   */
  void putObject(Object key, Object value);

  /**
   * @param key The key
   * @return The object stored in the cache.
   */
  Object getObject(Object key);

  /**
   * Optional. It is not called by the core.
   * 
   * @param key The key
   * @return The object that was removed
   */
  Object removeObject(Object key);

  /**
   * Clears this cache instance
   */  
  void clear();

  /**
   * Optional. This method is not called by the core.
   * 
   * @return The number of elements stored in the cache (not its capacity).
   */
  int getSize();
  
  /** 
   * Optional. As of 3.2.6 this method is no longer called by the core.
   *  
   * Any locking needed by the cache must be provided internally by the cache provider.
   * 
   * @return A ReadWriteLock 
   */
  ReadWriteLock getReadWriteLock();

}

  public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    if (closed) throw new ExecutorException("Executor was closed.");
    CacheKey cacheKey = new CacheKey();
    cacheKey.update(ms.getId());
    cacheKey.update(rowBounds.getOffset());
    cacheKey.update(rowBounds.getLimit());
    cacheKey.update(boundSql.getSql());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
    for (int i = 0; i < parameterMappings.size(); i++) { // mimic DefaultParameterHandler logic
      ParameterMapping parameterMapping = parameterMappings.get(i);
      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);
      }
    }
    return cacheKey;
  } 

1、<select>标签所在的Mapper的Namespace+<select>标签的id属性

2、RowBounds的offset和limit属性，RowBounds是MyBatis用于处理分页的一个类，offset默认为0，limit默认为Integer.MAX_VALUE

3、<select>标签中定义的sql语句

即只要两次查询满足以上三个条件且没有定义flushCache="true"，那么第二次查询会直接从MyBatis一级缓存PerpetualCache中返回数据，而不会走DB。

/**
 * @author Clinton Begin
 */
public final class MappedStatement {

  private String resource;
  private Configuration configuration;
  private String id;
  private Integer fetchSize;
  private Integer timeout;
  private StatementType statementType;
  private ResultSetType resultSetType;
  private SqlSource sqlSource;
  private Cache cache;
  private ParameterMap parameterMap;
  private List<ResultMap> resultMaps;
  private boolean flushCacheRequired;
  private boolean useCache;
  private boolean resultOrdered;
  private SqlCommandType sqlCommandType;
  private KeyGenerator keyGenerator;
  private String[] keyProperties;
  private String[] keyColumns;
  private boolean hasNestedResultMaps;
  private String databaseId;
  private Log statementLog;
  private LanguageDriver lang;
  private String[] resultSets;

  private MappedStatement() {
    // constructor disabled
  }

  public static class Builder {
    private MappedStatement mappedStatement = new MappedStatement();

    public Builder(Configuration configuration, String id, SqlSource sqlSource, SqlCommandType sqlCommandType) {
      mappedStatement.configuration = configuration;
      mappedStatement.id = id;
      mappedStatement.sqlSource = sqlSource;
      mappedStatement.statementType = StatementType.PREPARED;
      mappedStatement.parameterMap = new ParameterMap.Builder(configuration, "defaultParameterMap", null, new ArrayList<ParameterMapping>()).build();
      mappedStatement.resultMaps = new ArrayList<ResultMap>();
      mappedStatement.timeout = configuration.getDefaultStatementTimeout();
      mappedStatement.sqlCommandType = sqlCommandType;
      mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
      String logId = id;
      if (configuration.getLogPrefix() != null) logId = configuration.getLogPrefix() + id;
      mappedStatement.statementLog = LogFactory.getLog(logId);
      mappedStatement.lang = configuration.getDefaultScriptingLanuageInstance();
    }

    public Builder resource(String resource) {
      mappedStatement.resource = resource;
      return this;
    }

    public String id() {
      return mappedStatement.id;
    }

    public Builder parameterMap(ParameterMap parameterMap) {
      mappedStatement.parameterMap = parameterMap;
      return this;
    }

    public Builder resultMaps(List<ResultMap> resultMaps) {
      mappedStatement.resultMaps = resultMaps;
      for (ResultMap resultMap : resultMaps) {
        mappedStatement.hasNestedResultMaps = mappedStatement.hasNestedResultMaps || resultMap.hasNestedResultMaps();
      }
      return this;
    }

    public Builder fetchSize(Integer fetchSize) {
      mappedStatement.fetchSize = fetchSize;
      return this;
    }

    public Builder timeout(Integer timeout) {
      mappedStatement.timeout = timeout;
      return this;
    }

    public Builder statementType(StatementType statementType) {
      mappedStatement.statementType = statementType;
      return this;
    }

    public Builder resultSetType(ResultSetType resultSetType) {
      mappedStatement.resultSetType = resultSetType;
      return this;
    }

    public Builder cache(Cache cache) {
      mappedStatement.cache = cache;
      return this;
    }

    public Builder flushCacheRequired(boolean flushCacheRequired) {
      mappedStatement.flushCacheRequired = flushCacheRequired;
      return this;
    }

    public Builder useCache(boolean useCache) {
      mappedStatement.useCache = useCache;
      return this;
    }

    public Builder resultOrdered(boolean resultOrdered) {
      mappedStatement.resultOrdered = resultOrdered;
      return this;
    }

    public Builder keyGenerator(KeyGenerator keyGenerator) {
      mappedStatement.keyGenerator = keyGenerator;
      return this;
    }

    public Builder keyProperty(String keyProperty) {
      mappedStatement.keyProperties = delimitedStringtoArray(keyProperty);
      return this;
    }

    public Builder keyColumn(String keyColumn) {
      mappedStatement.keyColumns = delimitedStringtoArray(keyColumn);
      return this;
    }

    public Builder databaseId(String databaseId) {
      mappedStatement.databaseId = databaseId;
      return this;
    }

    public Builder lang(LanguageDriver driver) {
      mappedStatement.lang = driver;
      return this;
    }

    public Builder resulSets(String resultSet) {
      mappedStatement.resultSets = delimitedStringtoArray(resultSet);
      return this;
    }
    
    public MappedStatement build() {
      assert mappedStatement.configuration != null;
      assert mappedStatement.id != null;
      assert mappedStatement.sqlSource != null;
      assert mappedStatement.lang != null;
      mappedStatement.resultMaps = Collections.unmodifiableList(mappedStatement.resultMaps);
      return mappedStatement;
    }
  }


  public boolean hasNestedResultMaps() {
    return hasNestedResultMaps;
  }



  
  public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.size() <= 0) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }

  private static String[] delimitedStringtoArray(String in) {
    if (in == null || in.trim().length() == 0) {
      return null;
    } else {
      String[] answer = in.split(",");
      return answer;
    }
  }

}

一二级缓存之间的关系 

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);
  }
  // 如果开启二级缓存会使用CachingExecutor对原Executor进行包装  
  if (cacheEnabled) {
    executor = new CachingExecutor(executor);
  }
  // Mybatis插件功能对Executor的增强  
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

MyBatis缓存