Mybatis 一级缓存

介绍

Mybatis 默认开启一级缓存，一级缓存只是相对于同一个 SqlSession 而言。所以在参数完全一样的情况下，我们使用同一个 SqlSession 对象连续调用同一个 Mapper 方法，往往只执行一次SQL。

读取缓存数据

main 方法

public class Test {
  public static void main(String[] args) throws Exception {
    String resource = "SqlMapConfig.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
    SqlSession session = factory.openSession();
    // 查询方法
    Object obj = session.selectOne("com.dao.UserMapper.findUserById", 2);
    System.out.println(obj.toString());
    session.close();
  }
}

DefaultSqlSession selectOne 方法

  public <T> T selectOne(String statement, Object parameter) {
    // 调用 selectList 方法
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } 
  }

DefaultSqlSession selectList 方法

  public <E> List<E> selectList(String statement, Object parameter) {
    // 调用重载方法
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    MappedStatement ms = configuration.getMappedStatement(statement);
    // 执行器查询
    return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
  }
}

BaseExecutor query 方法

  // 一级缓存器
  protected PerpetualCache localCache;

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                                                                       ResultHandler resultHandler){
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    // 重载方法
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
  }

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                                     ResultHandler resultHandler, CacheKey key, BoundSql boundSql){
    List<E> list;
    // 一级缓存中取数据
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      // 查询数据库
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
    return list;
  }

PerpetualCache getObject 方法

  //一级缓存的本质就是一个 HashMap
  private final Map<Object, Object> cache = new HashMap();

  public Object getObject(Object key) {
    return this.cache.get(key);
  }

缓存数据

BaseExecutor queryFromDatabase 方法

  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);
    }
    // 缓存数据
    localCache.putObject(key, list);
    return list;
  }