本文是mybatis源码阅读计划的第四篇文章,本文从MapperProxy的invoke方法作为入口，解读Mapper接口的SQL方法执行流程。
本文源码地址为:https://github.com/zouhuanli/MySpringBoot.git。
前面的文章已经解读了mybatis对MapperInterface创建MapperProxy对象，MapperProxy实现了InvocationHandler，代理了Mapper接口的所有方法。
Mapper接口的所有方法都是从MapperProxy#invoke方法开始执行，执行流程如下：
一、MapperProxy与MapperMethod
MapperProxy#invoke方法如下：
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
}
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
跟踪执行流程，继续来到PlainMethodInvoker#invoke方法：
private static class PlainMethodInvoker implements MapperMethodInvoker {
private final MapperMethod mapperMethod;
public PlainMethodInvoker(MapperMethod mapperMethod) {
this.mapperMethod = mapperMethod;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return mapperMethod.execute(sqlSession, args);
}
}
再来到MapperMethod这个类。
public class MapperMethod {
private final SqlCommand command;
private final MethodSignature method;
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//按类型执行SQL方法
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional() && (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ "' attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
}
我们继续进入这行代码”result = sqlSession.selectOne(command.getName(), param);”,这里的sqlSession是sqlSessionTemplate这个对象。 来到sqlSessionTemplate#selectOne这个方法。
@Override
public <T> T selectOne(String statement, Object parameter) {
return this.sqlSessionProxy.selectOne(statement, parameter);
}
这里把所有SQL方法都转发给sqlSessionProxy这个代理对象了。 sqlSessionProxy这个代理对象在这里创建。
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator) {
notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
notNull(executorType, "Property 'executorType' is required");
this.sqlSessionFactory = sqlSessionFactory;
this.executorType = executorType;
this.exceptionTranslator = exceptionTranslator;
this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),
new Class[] { SqlSession.class }, new SqlSessionInterceptor());
}
所以我们来到了SqlSessionInterceptor这个对象。
二、SqlSessionInterceptor
SqlSessionInterceptor源码如下：
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//创建新的SqlSession或者加入已有的SqlSession
SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator
.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
这行代码” Object result = method.invoke(sqlSession, args);“将对应参数转发到普通的DefaultSqlSession去处理。
三、DefaultSqlSession
DefaultSqlSession的SQL方法基本都是转发到Executor去执行，如下。
selectOne方法：
@Override
public <T> T selectOne(String statement) {
return this.selectOne(statement, null);
}
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
}
if (list.size() > 1) {
throw new TooManyResultsException(
"Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
selectList方法：
@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
return selectList(statement, parameter, rowBounds, Executor.NO_RESULT_HANDLER);
}
private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
dirty |= ms.isDirtySelect();
return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
到这里真正执行SQL方法的是Executor。
四、Executor
selectList方法如下,MappedStatement就是之前注册到mappedStatements集合，包含Mapper接口的方法和其对应的SQL语句：
private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
dirty |= ms.isDirtySelect();
return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
继续来到CachingExecutor#query方法：
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler)
throws SQLException {
//获得SQL语句
BoundSql boundSql = ms.getBoundSql(parameterObject);
//缓存的key
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
然后来到这个query方法：
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler,
CacheKey key, BoundSql boundSql) throws SQLException {
Cache cache = ms.getCache();
//先从缓存获取
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;
}
}
//查询数据库
return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
然后是BaseExecutor#query方法：
@Override
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()) {
//清理缓存
clearLocalCache();
}
List<E> list;
try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//从数据库获取
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
//清理缓存
clearLocalCache();
}
}
return list;
}
来到这个方法queryFromDatabase，从数据库查询数据。
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);
}
//存入缓存
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
我们再进入SimpleExecutor#doQuery方法，这个do开头的方法就是真正干活的方法了。
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
//创建StatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler,
boundSql);
//创建PrepareStatement
stmt = prepareStatement(handler, ms.getStatementLog());
//执行SQL语句
return handler.query(stmt, resultHandler);
} finally {
//关闭Statement
closeStatement(stmt);
}
}
这里先创建PrepareStatement，再交给StatementHandler处理。
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
//获得连接
Connection connection = getConnection(statementLog);
//创建PrepareStatement
stmt = handler.prepare(connection, transaction.getTimeout());
//参数化，使用ParameterHandler
handler.parameterize(stmt);
return stmt;
}
创建PrepareStatement成功后，由StatementHandler处理PrepareStatement。
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
//执行语句
ps.execute();
//ResultSetHandler处理执行结果
return resultSetHandler.handleResultSets(ps);
}
可以看到最终也是使用PrepareStatement执行SQL语句，然后由ResultSetHandler处理执行结果，ResultSetHandler主要结果集映射。
到这里，SQL的执行流程就解读完成了。一路追踪下来还是很顺畅的，没有太高的复杂度，笔者本想吐槽mybatis源码很少注释，但是其类设计合理、职责单一，解读执行流程并没有太大的难度。
五、参考材料
1.https://mybatis.org/mybatis-3/zh/
2.Mybatis源码(版本3.5.13)