Mybatis[plus] 源码阅读笔记
调用流程概述：
mybatis的前身是ibatis，原生的ibatis执行curd操作是固定的操作，使用SQLSession接口中的方法进行增删查改，我们先阅读Ibatis部分的代码，后续mybatis[plus]都是在此基础上扩展了MapperProxy以及预设sql语句动态sql条件等封装，我们看一下ibatis的Session核心接口：
public interface SqlSession extends Closeable {
<E> List<E> selectList(String statement);
<K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey);
<T> Cursor<T> selectCursor(String statement);
int insert(String statement);
int update(String statement);
int delete(String statement);
}这个接口里面最终要的四类方法为（selectList、selectMap、selectCursor）、insert、delete、update。
但其实最终执行sql语句的为Executor中的方法
public interface Executor {
int update(MappedStatement ms, Object parameter) throws SQLException;
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
CacheKey cacheKey, BoundSql boundSql) throws SQLException;
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)
throws SQLException;
<E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
}其中分为query和update方法，最后由实际子类的doQuery、doUpdate执行真正的sql语句，其中进一步交给了StatementHandler对象处理
public interface StatementHandler {
Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException;
void parameterize(Statement statement) throws SQLException;
void batch(Statement statement) throws SQLException;
int update(Statement statement) throws SQLException;
<E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException;
<E> Cursor<E> queryCursor(Statement statement) throws SQLException;
BoundSql getBoundSql();
ParameterHandler getParameterHandler();
}可以看到里面最终到达update或者query方法
image-20240611225027427
所以调用逻辑为SqlSession->Executor->StatementHandler，每个SqlSession有一个Executor，每个Executor有一个StatementHandler（configuration中获得的，经过插件的包装，后面会说）；
SqlSession的创建过程：
image-20240611231131172
其中的核心为Configuration，整个ibatis都围绕这这个Configuration类，非常重要负责Executor、ParameterHandler、ResultSetHandler的创建，MapperStatements的保存
一、二级缓存
一级缓存
存在与执行器中，每次创建都会携带，所以每次会话都会有一级缓存的存在：BaseExecutor.java
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}在查询之前，会先去缓存里面查找，其中key值与四个方面有关
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);只有当sql语句、参数、分页、与绑定参数的sql一样才会匹配；
二级缓存
存在与ms中的cache属性当中，只有当configuration中的cacheEnable为true才会创建cacheExecutor
// Configuration.java
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : 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);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
// 默认为true同时ms中的useCache为true，才会实际操作ms中的cache
// CachingExecutor.java
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
// 默认也为true可以看到，只有当cach不为null才会实际操作缓存，在解析mapper.xml的时候才会判断是否设置了cache类型
// XMLMapperBuilder.java
private void configurationElement(XNode context) {
try {
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.isEmpty()) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
cacheRefElement(context.evalNode("cache-ref"));
cacheElement(context.evalNode("cache"));
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
resultMapElements(context.evalNodes("/mapper/resultMap"));
sqlElement(context.evalNodes("/mapper/sql"));
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
}
}
// 类型有很多种