1
  private void configurationElement(XNode context) {
2
    try {
3
      //...
4
      cacheElement(context.evalNode("cache")); //解析mapper.xml中的cache标签
5
    } catch (Exception e) {
6
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
7
    }
8
  }
9

10
  private void cacheElement(XNode context) {
11
    if (context != null) { // if hava cache tag 如果有cache标签才执行下面的逻辑
12
      String type = context.getStringAttribute("type", "PERPETUAL");
13
      Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
14
      String eviction = context.getStringAttribute("eviction", "LRU");
15
      Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
16
      Long flushInterval = context.getLongAttribute("flushInterval");
17
      Integer size = context.getIntAttribute("size");
18
      boolean readWrite = !context.getBooleanAttribute("readOnly", false);
19
      boolean blocking = context.getBooleanAttribute("blocking", false);
20
      Properties props = context.getChildrenAsProperties();
21
      builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);//建立mapper缓存
22
    }
23
  }

1
  public Cache useNewCache(Class<? extends Cache> typeClass,
2
      Class<? extends Cache> evictionClass,
3
      Long flushInterval,
4
      Integer size,
5
      boolean readWrite,
6
      boolean blocking,
7
      Properties props) {
8
    Cache cache = new CacheBuilder(currentNamespace)
9
        .implementation(valueOrDefault(typeClass, PerpetualCache.class))
10
        .addDecorator(valueOrDefault(evictionClass, LruCache.class))
11
        .clearInterval(flushInterval)
12
        .size(size)
13
        .readWrite(readWrite)
14
        .blocking(blocking)
15
        .properties(props)
16
        .build();
17
    configuration.addCache(cache);//mapper缓存赋值，如果cache标签为空，不会执行此方法，currentCache为空
18
    currentCache = cache;
19
    return cache;
20
  }

1
  @Override
2
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
3
      throws SQLException {
4
    Cache cache = ms.getCache(); //获取mapper缓存
5
    if (cache != null) {//如果mapper缓存对象不为空 尝试在mapper缓存中获取（没有cache标签此对象就是空）
6
      flushCacheIfRequired(ms);
7
      if (ms.isUseCache() && resultHandler == null) {
8
        ensureNoOutParams(ms, boundSql);
9
        @SuppressWarnings("unchecked")
10
        List<E> list = (List<E>) tcm.getObject(cache, key); //从mapper缓存中获取数据
11
        if (list == null) {
12
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //如果为空，使用delegate查询（BaseExecutor）
13
          tcm.putObject(cache, key, list); // 查询结果保存到mapper缓存
14
        }
15
        return list;
16
      }
17
    }
18
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
19
  }

1
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
2
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
3

4
    List<E> list;
5
    try {
6
      queryStack++;
7
        //localCache SqlSession中的缓存
8
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
9
        //先从SqlSession中的缓存中获取，key是通过sql语句生成
10
      if (list != null) {
11
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
12
      } else {
13
        // 如果缓存中没有 才从数据库查询
14
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
15
      }
16
    } finally {
17
      queryStack--;
18
    }
19
    return list;
20
  }
21

22
  //从数据库读取数据
23
  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
24
    List<E> list;
25
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
26
    try {
27
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
28
    } finally {
29
      localCache.removeObject(key);//将SqlSession中的缓存清除
30
    }
31
    localCache.putObject(key, list);//返回查询结果之前，放入SqlSession中的缓存 刷新
32
    if (ms.getStatementType() == StatementType.CALLABLE) {
33
      localOutputParameterCache.putObject(key, parameter);
34
    }
35
    return list;
36
  }